11institutetext: Stockholm Observatory, Stockholm University, AlbaNova
University Center, SE-106 91 Stockholm, Sweden
11email<EMAIL_ADDRESS>22institutetext: Department of Space, Earth and
Environment, Chalmers University of Technology, Onsala Space Observatory,
SE-439 92 Onsala, Sweden 33institutetext: Omnisys Instruments AB, Ringvägen
100E, SE-118 60 Stockholm, Sweden 44institutetext: OHB Sweden, PO Box 1269,
SE-164 29 Kista, Sweden

#  Herschel and Odin observations of H2O, CO, CH, CH+, and [N ii] in the
barred spiral galaxy NGC 1365 ††thanks: Herschel is an ESA space observatory
with science instruments provided by European-led Principal Investigator
consortia and with important participation from NASA. Herschel was launched on
May 14, 2009 and decommissioned on June 17, 2013. ††thanks: Odin is a Swedish-
led satellite project funded jointly by the Swedish National Space Board
(SNSB), the Canadian Space Agency (CSA), the National Technology Agency of
Finland (Tekes), the Centre National d’Etudes Spatiales (CNES), France, and
the European Space Agency (ESA). The former Space division of the Swedish
Space Corporation, today OHB Sweden, is the prime contractor also responsible
for Odin operations. Odin was launched on February 20, 2001 and is still in
active operation.

Bar-induced activity in the outer and inner circumnuclear tori

Aa. Sandqvist 11 Å. Hjalmarson 22 B. Larsson 11 U. Frisk 33 S. Lundin 44 G.
Rydbeck 2 2<EMAIL_ADDRESS>
(Received $<$8 July 2020$>$; accepted $<$22 December 2020$>$)

###### Abstract

Context. The Odin satellite is now into its twentieth year of operation, much
surpassing its design life of two years. One of its major astronomical
pursuits was the search for and study of water vapor in diverse regions of the
Solar System and the Milky Way galaxy. The Herschel space observatory was
needed to detect water vapor in external galaxies.

Aims. Our goal is to study the distribution and excitation of water vapor and
other molecules in the barred spiral galaxy NGC 1365.

Methods. Herschel has observed the central region of NGC 1365 in two
positions, and both its SPIRE and PACS observations are available in the
Herschel Science Archive. Herschel PACS images have been produced of the 70
and 160 $\mu$m infrared emission from the whole galaxy, and also of the cold
dust distribution as obtained from the ratio of the 160 to 70 $\mu$m images.
The Herschel SPIRE observations have been used to produce simultaneously
observed maps of the 557 GHz o-H2O, 752 GHz p-H2O, 691 GHz CO($6-5$), 1037 GHz
CO($9-8$), 537 GHz CH, 835 GHz CH+, and the 1461 GHz [N ii] lines (efficiently
probing the warm ionized medium) in the inner bar and circumnuclear torus
region; – however, these observations have no effective velocity resolution.
For this reason Odin has recently observed the 557 GHz ortho-H2O ground state
line in the central region with high (5 km s-1) spectral resolution.

Results.  The emission and absorption of H2O at 557 GHz, with a velocity
resolution of 5 km s-1, has been marginally detected in NGC 1365 with Odin.
The water vapor is predominantly located in a shocked 15″ (1.3 kpc) region
near some central compact radio sources and hot-spot H ii regions, close to
the northeast component of the molecular torus surrounding the nucleus. An
analysis of the H2O line intensities and velocities indicates that a shock-
region is located here. This is corroborated by a statistical image
deconvolution of our SEST CO($3-2$) observations, yielding 5″ resolution, and
a study of our Very Large Array (VLA) H i absorption observations, as well as
comparisons with published interferometric CO observations. Additionally, an
enticing 20″ H i ridge is found to extend south-southeast from the nucleus,
coinciding in position with the southern edge of an O iii outflow cone,
emanating from the nucleus. The molecular chemistry of the shocked central
region of NGC 1365 is analyzed with special emphasis on the CO, H2O and CH,
CH+ results.

Conclusions. The dominating activity near the northeast (NE) torus component
may have been triggered by the rapid bar-driven inflow into the circumnuclear
torus causing cloud-cloud collisions and shocks, leading to the formation of
stellar superclusters and, hence, also to more efficient PDR chemistry, which,
here, may also benefit from cosmic ray focusing caused by the observed aligned
magnetic field. The very high activity near the NE torus component may reflect
the fact that the eastern bar-driven gas inflow into the NE region is much
more massive than the corresponding western gas inflow into the southwest (SW)
region. The H2O and CH+ emissions peak in the NE torus region, but the CO and
CH emissions are more evenly distributed across the whole circumnuclear torus.
The higher energy CO spectral line energy distribution (SLED) is nicely
modeled by a low velocity (10 km s-1) shock, which may as well explain the
required CH excitation and its high abundance in denser gas. The higher
velocity (40 km s-1) shock required to model the H2O SLED in the NE torus
region, paired with the intense UV radiation from the observed massive young
stellar superclusters, may also explain the high abundance of CH+ in this
region. The nuclear H i ridge may have been created by the action of outflow-
driving X-ray photons colliding with ice-covered dust grains. A precessing
nuclear engine, as is suggested by the tilted massive inner gas torus, may be
necessary to explain the various nuclear outflows encountered.

###### Key Words.:

Galaxies: ISM – Galaxies: individual: NGC 1365 – Galaxies: Seyfert – Galaxies:
nuclei

††offprints:
Aage Sandqvist,

##  1 Introduction

NGC 1365 is a prominent barred supergiant spiral galaxy in the Fornax cluster
with a heliocentric velocity of +1632 km s-1 (the velocity with respect to the
Local Standard of Rest, $V_{\rm LSR}=1613$ km s-1, is used in this paper). In
Fig. 1 we present a comparison of a visible-light “true-color” image of NGC
1365 from three exposures taken with the FORS1 camera on ESO’s VLT UT1, along
with an infrared $K$-band image ($0.9-2.5$ $\mu$m) obtained with the HAWK-I
camera on the ESO VLT UT4 telescope (ESO/P. Grosböl). The infrared picture
suggests a smaller stellar bar, or disk, within the radius of the Inner
Lindblad Resonance (ILR) of 30″ (P.A.B. Lindblad et al. lin96 (1996)) that is
roughly perpendicular to the larger bar (as well as elongated along the line
of nodes). Alonso-Herrero et al. (alo12 (2012)) performed a thorough study of
the infrared spectrum ($3.6-500$ $\mu$m) of the central region of NGC 1365
using the Herschel Photodetector Array Camera and Spectrometer (PACS) and
Spectral Photometric Imaging REceiver (SPIRE), as well as Gemini and Spitzer.
From the Herschel Science Archive, we retrieved their 70 and 160 $\mu$m PACS
observations of NGC 1365 and display 70 $\mu$m and 160 $\mu$m images in Fig.
1, where we also indicate the high- and low-frequency beams, respectively, of
the SPIRE spectrometer observations presented in Sect. 2.1. The galaxy
displays a wide range of phenomena indicating activity – including a Seyfert
1.5 type nucleus with strong, broad, and narrow H$\alpha$ lines. Ionized
outflows from the active galactic nucleus (AGN) with velocities up to a few
hundred km s-1 have been observed (e.g., Venturi et al. ven18 (2018) and
references therein). Kristen et al. (kri97 (1997)) used the Hubble Space
Telescope to study the Seyfert nucleus and circumnuclear hot spots in NGC 1365
and found these hot spots to be resolved into a number of bright compact
condensations, which they interpreted as super star clusters (SSC). The
magnetic field in the central region has been mapped at the NRAO VLA with a
resolution of 9″ by Beck et al. (bec05 (2005)). Jörsäter & van Moorsel (jor95
(1995)) used the NRAO VLA to map the H i emission distribution in the arms and
the bar. In this paper, we present the H i absorption results in the galaxy’s
central region, which were not discussed by them. A rapidly spinning
supermassive black hole has been disclosed at the center of NGC 1365 by
Risaliti et al. (ris13 (2013)). At an assumed distance of 18.6 Mpc (Madore et
al. mad98 (1998)), 1″ corresponds to 90 pc. For an extensive review of NGC
1365, see Lindblad (lin99 (1999)).

To set a solid background of observational knowledge, we now provide a rather
detailed introductory review upon which the interpretation of our own
observations can be based. In a flattened rotating proto-galaxy – very likely
created in the potential well of a cosmological dark matter fluctuation
(Persson et al. per10 (2010)) – there exist intrinsic instability modes such
as spiral and bar instabilities, which may be triggered by encounters with
nearby galaxies or by internal “noise” (e.g., P.O. Lindblad lin60 (1960); Lin
& Shu lin64 (1964); Shu et al. shu73 (1973); Sundelius et al. sun87 (1987);
Pfenniger & Norman pfe90 (1990); P.A.B. Lindblad et al. lin96 (1996); P.O.
Lindblad lin99 (1999); Pinol-Ferrer et al. pin12 (2012)). Here the A. Toomre
(too64 (1964)) (in)stability criterion is a useful constraint.

Inside corotation, the bar pattern velocity is slower than the rotational
motion of the gas. According to modeling and observations (e.g., P.A.B.
Lindblad et al. lin96 (1996); P.O. Lindblad et al. polin96 (1996); P.O.
Lindblad lin99 (1999); Sakamoto et al. sak07 (2007); Elmegreen et al. elm09
(2009)), the gas catches up with the bar, and when leaving it on the front
side of the bar experiences a strong shock with a drop of velocity of up to a
few hundred km s-1. This leads to an inflow of gas along the curved leading
dust lanes of the bar (in the eastern bar estimated to be $\approx$22
$\mathrm{M}_{\odot}$/year at an inflow velocity of $\approx$80 km s-1),
creating a rather massive rotating torus inside the ILR. Here the orbit
crowding is expected to cause multiple cloud-cloud collisions and shocks,
leading to cloud coalescing into very massive ($\approx 10^{7}$
$\mathrm{M}_{\odot}$) cloud complexes, as well as to the subsequent birth of
OB stars and massive (several $\times 10^{6}$ $\mathrm{M}_{\odot}$) stellar
superclusters, as is observed in the outer NGC 1365 torus (Elmegreen et al.
elm09 (2009); Galliano et al. gal12 (2012); Fazeli et al. faz19 (2019)). The
aforementioned expectations were based upon the fact that massive OB stars are
born (mainly) in the spiral arms – where orbit crowding and density wave
streaming is causing cloud-cloud collisions and shocks (cf. Roberts & Stewart
rob87 (1987); Rydbeck et al. ryd85 (1985); Aalto et al. aal99 (1999);
Schinnerer et al. sch10 (2010)) – as concluded from the Milky Way
observational result, that the number of giant H ii regions in a specified
volume roughly scales as ¡$n$(H2)¿2, where ¡n(H2)¿ is the mean H2 density in
the same volume (Scoville et al. sco86 (1986)). The physical reason for the
increased stellar formation activity is the increased external pressure
created by the spiral arm molecular cloud collisions, triggering multiple
massive gravitational collapses in these clouds, already residing in their low
internal pressure (low temperature) virial equilibrium. This is a result of
the efficient cooling provided by spectral line radiation from the
interstellar molecules 12CO, 13CO, H2O, and other species – (astro)chemistry
(e.g., Irvine et al. irv87 (1987); van Dishoeck & Blake dis98 (1998); van
Dishoeck et al. dis13 (2013)) and (astro)physics in a necessary collaboration
(Goldsmith & Langer gol78 (1978); Takahashi et al. tak85 (1985); Neufeld et
al. neu95 (1995); see Hjalmarson & Friberg hja88 (1988) for a discussion and
additional references).

Figure 1: Top: Visible-light “true-color” and infrared ($0.9-2.5$ $\mu$m)
images of NGC 1365, obtained with ESO’s VLT (ESO/P. Grosböl). Bottom: The 70
$\mu$m and 160 $\mu$m Herschel PACS images of NGC 1365 (beam sizes
5$\aas@@fstack{\prime\prime}$6 and 11$\aas@@fstack{\prime\prime}$3; Alonso-
Herrero et al. alo12 (2012)) on which are superimposed the high- and low-
frequency beams (20″ and 40″, respectively, dashed circles) of the SPIRE
spectrometer for the two sets of observations (toward the NE and SW positions,
see Sect. 2.1) and the resultant beam (140″$\times$ 160″, solid ellipse) of
the Odin observations. The crosses, from top to bottom (in decreasing
declination), represent the “hot spot” H ii regions L4, L11, L12, L1, L3, and
L2 (Alloin et al. all81 (1981)). The equatorial offsets are with respect to
the optical nucleus, which is marked with a green asterisk.

The very active starburst region in the central parts of the NGC 1365 bar has
been studied in CO molecular lines at high resolutions: the $J=3-2$ CO line
with 5″ effective resolution (Sandqvist san99 (1999); this paper), and the
$J=2-1$ lines of 12CO, 13CO, and C18O with 2″ resolution (Sakamoto et al.
sak07 (2007)). Sandqvist et al. (san95 (1995)) performed VLA aperture
synthesis observations of the radio emission from ionized gas, revealing a
lower level circumnuclear ring of radius $\approx 9\arcsec\ (\approx 800$ pc),
interspersed with compact radio sources, all of which have spectral indices
indicating free-free emission from hot ionized gas, except one, called “F”,
which has the spectral index characteristic of synchrotron radiation (see
Table 2). They also mapped the distributions of the CO $J=1-0$ and $2-1$ lines
using the SEST (Swedish ESO Submillimeter Telescope, now ”retired”). In the
nuclear region, the envisioned synchrotron jet is enshrouded by a fan- or
rather cone-shaped ionized gas outflow, revealed by its visual O iii emission
(Fig. 7(4) and Hjelm & Lindblad hje96 (1996), discussed in detail in the
review paper by Lindblad lin99 (1999)). More recently this kpc-size wide-angle
outflow of ionized gases was studied in detail by Lena et al. (len16 (2016))
and Venturi et al. (ven18 (2018)). Combes et al. (com19 (2019)) present
Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO($3-2$)
with a resolution of $0\aas@@fstack{\prime\prime}1$ in a sample of seven
Seyfert/LINER galaxies which includes NGC 1365, where they estimate a central
AGN (black hole) mass of $4\times 10^{6}$ $\mathrm{M}_{\odot}$ by model
fitting to the velocity field observed inside a radius of
$1\aas@@fstack{\prime\prime}4$. It appears that the rotation axis of the inner
nuclear torus has a position angle (P.A.) and inclination (Incl.) – (P.A.
$\approx 160\degr\pm 10\degr$; Incl. $\approx 63\degr\pm 10\degr$) – which
markedly deviates from the common rotation axis of the spiral arm disk, the
bar, and the outer molecular gas torus of the galaxy (P.A. $\approx 130\degr$;
Incl. $\approx 50\degr$). The orientation of the (symmetry) axes of the
nuclear outflows (the wide-angle bi-conical ionized gas outflow, the
synchrotron radiation radio jet, as well as the narrow CO outflow outlined in
the present paper) all seem to be co-aligned with the galaxy rotation axis.
The tilted rotation axis of the inner nuclear torus, or accretion disk, may
suggest that we are witnessing the action of a precessing nuclear engine, a
matter which will be outlined in our forthcoming discussion. The radius of the
rotating inner torus, possibly a black hole accretion disk, was determined to
be $\approx$0.3″ (26 pc) and its estimated gas mass is $\approx 7\times
10^{6}$ $\mathrm{M}_{\odot}$. Their larger scale mapping, in addition,
confirms the outer rotating circumnuclear molecular gas torus of radius 9″
(800 pc), already known from the CO($3-2$) observations by Sandqvist (san99
(1999)) and the CO($2-1$) SubMillimeter Array (SMA) mapping by Sakamoto et al.
(sak07 (2007)); compare also Fig. 7 of this paper.

The convincing signs of shocks in the NE outer torus region of NGC 1365 have
indeed been observationally reported in terms of vibration-rotation 2 $\mu$m
H2 emission lines, requiring shock excitation and also pronounced Br$\gamma$
line wings, indicative of outflows and shocks emanating from three carefully
studied superclusters (Galliano et al. gal12 (2012)). Fazeli et al. (faz19
(2019)), in their recent VLT SINFONI near-IR bservations of the central 9″
$\times$ 9″ region of NGC 1365, found strong evidence of shocks – in terms of
emission from several 2 $\mu$m vibration-rotation H2 lines, as well as from
the “shock indicator line” $\lambda$1.644 $\mu$m “forbidden” Fe ii – all
emissions being strong in the SW and eastern side of the circumnuclear torus
where the radio sources A, F, and H are situated (See Table 2). Source F may
be especially interesting since it is the one being identified with the
nuclear synchrotron radiation jet and CO outflow discussed in Sects. 4.3.1 and
4.3.2.

The outer much more massive torus is likely be an important reservoir of
matter, feeding the accretion disk of the supermassive black hole (SMBH), but
the inward matter transport process between the outer and inner tori is still
unclear. However, Lena et al. (len16 (2016)), in their study of the nuclear
outflow of ionized gas, also seem to “find kinematic components that may trace
gas which has lost angular momentum and is slowly migrating toward the
nucleus”. Here Fazeli et al. (faz19 (2019)) find “a spiral-shaped molecular
gas structure (at a scale ¡2″) which could indicate inward streaming motions”.
Such kinematic features also appear to be be visible in the ALMA data of
Combes et al. and hopefully will be discussed in their forthcoming study of
gas outflow. Here the observed existence of a narrow relativistic plasma jet
outflow and an accompanying co-aligned molecular gas outflow (jet) from the
nuclear engine may be important. The cold gas expulsion process may be an
upscaled version of the magnetohydrodynamics (MHD) generation of the bi-polar
outflows necessary to solve the angular momentum problem of star formation
(e.g., Königl & Pudritz kon00 (2000); Arce et al. arc07 (2007)), hence
supporting the accretion toward the central AGN. The launching mechanism of
the synchrotron jet as such may be a process closely connected with the
angular momentum loss required for mass accretion onto and growth of the AGN
(e.g., Blandford & Begelman bla99 (1999); Nayakshin nay14 (2014); Garcia-
Burillo et al. gar14 (2014)). For a more initiated and detailed discussion, we
here refer to Aalto et al. (aal16 (2016)).

The submillimeter line spectrum of the Seyfert galaxy NGC 1068 has been
observed with Herschel SPIRE by Spinoglio et al. (spi12 (2012)). While
González-Alfonso et al. (gon10 (2010), gon12 (2012)) find that their Herschel
SPIRE and PACS observations of multiple H2O lines in the ULIRGs (Ultra-
Luminous Infrared Galaxies) Mrk 231, NGC 4418, and Arp 220 were best modeled
by FIR excitation (“radiation pumping”), the SPIRE observations of the early-
stage merger galaxy NGC 6240 lead Meijerink et al. (mei13 (2013)) to conclude
that shock excitation must be the cause of the CO-SLED (Spectral Line Energy
Distribution). Submillimeter H2O lines have been studied in detail in nine
nuclei of actively star-forming galaxies using the Herschel HIFI, SPIRE, and
PACS receivers (Liu et al. liu17 (2017)). That sample did not include NGC 1365
but there are unpublished results which are available in the Herschel SPIRE
and PACS Archive, and they form partial basis of the present paper. However,
SPIRE does not yield spectral resolution capable of resolving any velocity
structure across the galaxy in the signal. Since the 557 GHz (0.54 mm) H2O
line is a sensitive indicator of the existence of photodissociation regions
(PDR) and other physical and chemical processes, leading to enhanced H2O
abundances such as outflows and shocks (e.g., Hjalmarson et al. hja03 (2003),
hja05 (2005); Gerin et al. ger16 (2016)), we have used the Odin satellite to
probe the central region of NGC 1365 with high spectral resolution in this H2O
line.

The outline of this paper reads as follows. After an Introduction, containing
a summary of the current knowledge of NGC 1365 relevant for the present
communication, we present in Sect. 2 observations by the Herschel and Odin
space telescopes. In Sect. 3 our new results from the Herschel PACS and SPIRE
data and the complementary spectrally resolved Odin data are reported together
with support analyses of SEST CO($3-2$) and VLA H i data. In Sect. 4 our
results concerning CO, H2O, CH, CH+, [N ii], and H i are discussed in some
detail. Our Summary and conclusions are provided in Sect. 5. For the
interested reader, we have in Appendices B and C provided shortcut briefings
on the interstellar chemistry of H2O and CH, CH+ in prototype regions of our
own Galaxy, wherein the physical and chemical conditions and processes are
more accurately known, for comparison with observational results in external
galaxies.

##  2 Observations

###  2.1 Herschel SPIRE

The nuclear region of NGC 1365 has been observed by Herschel using the SPIRE
Fourier-transform spectrometer in the SpireSpectroPoint observing mode at two
positions near the two maxima of the central CO torus, NGC 1365 NE: (J2000.0)
$3^{h}33^{m}36\aas@@fstack{s}60,-36\degr 08\arcmin
20\aas@@fstack{\prime\prime}0$
($+2\aas@@fstack{\prime\prime}8,+5\aas@@fstack{\prime\prime}4$), and NGC 1365
SW: (J2000.0) $3^{h}33^{m}35\aas@@fstack{s}90,-36\degr 08\arcmin
35\aas@@fstack{\prime\prime}0$
($-5\aas@@fstack{\prime\prime}7,-9\aas@@fstack{\prime\prime}6$) – the numbers
in parentheses are equatorial offsets from the optical nucleus. These
observations, which are unpublished, were made on 22 August 2010 with total
integration times of 3476 and 5640 seconds for the NE and SW positions,
respectively. The channel resolution of the spectrometer is 1.447 GHz and the
half-power beamwidth (HPBW) is roughly 35″ and 20″ for the low- and high-
frequency parts of the spectra, respectively (Swinyard et al. swi10 (2010);
cf. Fig. 1 of Spinoglio et al. spi12 (2012)). We have retrieved these SPIRE
observations from the Herschel Science Archive.

###  2.2 Odin

The observations of NGC 1365 in the 557 GHz H2O line were performed with the
Odin space telescope in December 2016, March and December 2017, and February
2018. The satellite was commanded toward the galaxy’s optical nucleus located
at $3^{h}33^{m}36\aas@@fstack{s}37,-36\degr 08\arcmin
25\aas@@fstack{\prime\prime}4$ (J2000.0) (Lindblad lin99 (1999)). Position-
switching was performed in right ascension with the OFF-position being
displaced by $-600\arcsec$. The ON-source integration time was 7.4, 51.4,
14.5, and 10.9 hours in December 2016, March and December 2017, and February
2018, respectively. The backend spectrometer was a 1050-channel AOS with a
channel resolution of 1 MHz. The system temperature was about 3500 K. The HPBW
beamwidth of Odin at this frequency is $2\aas@@fstack{\prime}1$ and the beam
efficency is 0.89 (Frisk et al. fri03 (2003)). The nominal pointing accuracy
of Odin is 10″, but due to a systematic pointing error, slightly different
positions were actually observed in the different periods. For the total
averaged H2O profile, this error led to an effective elliptical beam of
140″$\times$ 160″, centered at an equatorial offset from the optical nucleus
of (+10″, $-$2″). This beam is also displayed in Fig. 1.

##  3 Results

###  3.1 Herschel SPIRE and PACS results

From the ratio of the PACS 160 $\mu$m to 70 $\mu$m images of NGC 1365
presented in Fig. 1, we obtain a cold dust color temperature distribution for
the entire galaxy (see Fig. 2) in a manner similar to Alonso-Herrero et al.
(alo12 (2012)) who used the 100 $\mu$m to 70 $\mu$m PACS images for the
central region.

Figure 2: Cold dust color temperature distribution in NGC 1365, obtained from
the ratio of the 160 $\mu$m to 70 $\mu$m Herschel PACS images in Fig. 1. The
contours indicate the 70 $\mu$m intensities from Fig. 1. The crosses, from top
to bottom (in decreasing declination), represent the “hot spot” H ii regions
L4, L11, L12, L1, L3, and L2. The equatorial offsets are with respect to the
optical nucleus, which is marked with a green asterisk.

The Herschel SPIRE apodized spectrum obtained toward the NGC 1365 NE torus
position, corrected to a continuum source size of 14″ in a 40″ beam, is shown
in Fig. 3. A myriad of lines has been detected for the following molecules:
o-H2O and p-H2O, 12CO, CH, NH, CH+, and atoms: [C i] and [N ii]; their
properties are listed in Table 1.

Figure 3: Herschel SPIRE apodized spectrum toward NGC 1365 NE, corrected to a
continuum source size of 14″ in a 40″ antenna beam. The abcissa is the
observed frequency. Semi-extended source calibration is applied according to
the SPIRE Handbook
(http://herschel.esac.esa.int/Docs/SPIRE/spire_handbook.pdf). Table 1:
Parameters of the detected SPIRE spectral lines (Fig. 3), corrected to a
continuum source size of 14″ in 40″ antenna beam

Species | Transition | Frequency | $E_{\rm u}$ | Flux | Flux Error
---|---|---|---|---|---
|  | [GHz] | [K] | ($10^{-17}$ W m-2)
12CO | J = 4-3 | 461.041 | 55.3 | 19.11 | 0.38
12CO | J = 5-4 | 576.268 | 83.0 | 19.27 | 0.38
12CO | J = 6-5 | 691.473 | 116.2 | 15.87 | 0.38
12CO | J = 7-6 | 806.652 | 154.9 | 11.95 | 0.38
12CO | J = 8-7 | 921.800 | 199.1 | 10.16 | 0.38
12CO | J = 9-8 | 1036.91 | 248.9 | 6.86 | 0.75
12CO | J = 10-9 | 1151.99 | 304.2 | 3.57aa$a$Blended CO and H2O | 0.79
12CO | J = 11-10 | 1267.01 | 365.0 | 3.50 | 0.75
12CO | J = 12-11 | 1382.00 | 431.3 | (1.85) | 0.75
12CO | J = 13-12 | 1496.92 | 503.1 |  | 0.75
o-H2O | 110-101 | 556.936 | 60.9 | 1.10 | 0.38
o-H2O | 312-303 | 1097.36 | 249.4 | 2.88 | 0.75
o-H2O | 312-221 | 1153.13 | 249.4 | (1.18)aa$a$Blended CO and H2O | 0.79
o-H2O | 321-312 | 1162.91 | 305.2 | 2.21 | 0.75
o-H2O | 523-514 | 1410.62 | 642.4 |  | 0.75
p-H2O | 211-202 | 752.033 | 136.9 | 1.49 | 0.38
p-H2O | 422-331 | 916.172 | 454.3 | (0.66) | 0.38
p-H2O | 202-111 | 987.927 | 100.8 | 3.27 | 0.75
p-H2O | 111-000 | 1113.34 | 53.4 | (0.92) | 0.75
p-H2O | 422-413 | 1207.64 | 454.3 |  | 0.75
p-H2O | 220-211 | 1228.79 | 195.9 | 2.86 | 0.76
CH | 13/2-11/2 | 532.7 | 25.7 | 1.19 | 0.38
CH | 13/2-11/2 | 536.8 | 25.8 | 1.81 | 0.38
CH | 23/2-13/2 | 1470.7 | 96.3 | 2.53 | 0.75
CH | 23/2-13/2 | 1477.3 | 96.7 | 2.23 | 0.75
NH | 12-01 | 974.48 | 46.8 | 2.82 | 0.76
NH | 11-01 | 999.97 | 48.0 | (1.06) | 0.76
CH+ | 1-0 | 835.137 | 40.1 | 1.47 | 0.38
$[$CI$]$ |  3P1-3P0 | 492.161 | 23.6 | 9.06 | 0.38
$[$CI$]$ |  3P2-3P1 | 809.342 | 62.5 | 15.06 | 0.38
$[$NII$]$ |  3P1-3P0 | 1461.128 | 70.1 | 87.60 | 0.75

We have made map images of the combined NGC 1365 torus NE and SW Herschel
SPIRE observations using the 557 GHz o-H2O and 752 GHz p-H2O lines, as well as
the $J=6-5$ and $9-8$ CO lines, 537 GHz CH and 835 GHz CH+ lines. These images
are shown in Fig. 4 where they are superimposed upon contours of the PACS 70
$\mu$m observations (also seen in Fig. 1), which show the distribution of warm
dust. We note that the NE torus region is much stronger in warm dust emission
than the SW torus peak. The “hot spot” H ii regions L1, L2, L3, L4, L11, and
L12 (Alloin et al. all81 (1981)) are also indicated in these figures as is the
position of the optical nucleus. Their equatorial offsets from the optical
nucleus are listed in Table 2, as are the offsets of the compact radio sources
detected by Sandqvist et al. (san95 (1995)), and the co-located compact MidIR
sources, detected and studied in detail by Galliano et al. (gal05 (2005),
gal08 (2008) and gal12 (2012)) using adaptive optics on the ESO 3.6 m
telescope and VLT. The co-location with CO($2-1$) intensity peaks in the 2″
resolution SMA aperture synthesis mapping by Sakamoto et al. (sak07 (2007)) is
also convincingly illustrated in their Figs. 9 and 10, and is confirmed in the
ALMA CO($3-2$) higher resolution mapping by Combes et al. (com19 (2019)).

The SEST maximum entropy method (MEM) map of the $J=3-2$ CO line with 5″
resolution (Sandqvist san99 (1999)) showed that a central molecular torus
exists around the nucleus with the NE and SW peaks being of similar
magnitudes. This is in clear contrast to the appearance of the 557 GHz o-H2O
and 752 GHz p-H2O lines where the NE torus peak is considerably stronger than
the SW peak, as can be seen in Fig. 4. This is also illustrated in Fig. 6
where the SPIRE profiles for these lines are presented. Since the torus peak
positions NE and SW are separated by only 15″while the beam size is 35″, Fig.
6 in fact tells us that the H2O emissions are very weak in the SW torus peak.

Figure 4: SPIRE maps of 557 GHz o-H2O ($\approx 35\arcsec$), 752 GHz p-H2O
($\approx 35\arcsec$), $J=6-5$ CO ($\approx 30\arcsec$), $J=9-8$ CO ($\approx
20\arcsec$), 537 GHz CH ($\approx 35\arcsec$), and 835 GHz CH+($\approx
35\arcsec$), the beamwidths are in parentheses. The contours are from the PACS
70 $\mu$m observations seen in Fig. 1 and reflect the distibution of warm
dust, the maximum being near the NE torus component. The crosses, from top to
bottom (in decreasing declination), represent the “hot spot” H ii regions L4,
L11, L12, L1, L3, and L2, the latter two being near the SW torus component.
The equatorial offsets are with respect to the optical nucleus which is marked
with a green asterisk.

###  3.2 Odin high spectral resolution H2O results

The Odin observations of NGC 1365 in the 557 GHz H2O line for the four
different periods were integrated yielding a resultant profile with a total
ON-source integration time of 81 hours. In Fig. 5 “Top”, we present the
resultant profile with a channel resolution of 0.54 km s-1 and a linear
baseline subtracted. In the “Upper Middle” part of Fig. 5 we binned the Odin
profile to a velocity resolution of 5 km s-1 and compare it with several
CO($3-2$) profiles – from mapping observations obtained with SEST (Sandqvist
san99 (1999)). The blue profile is the total SEST Odin-beam-convolved
CO($3-2$) profile. Two 5″-resolution SEST statistical image deconvolution
(SID) CO($3-2$) line profiles (see Sect. 3.4), observed close to the NE torus
peak, are also presented in this panel. The green CO profile is in the
direction of $(\Delta\alpha,\Delta\delta)=(+6\arcsec,+8\arcsec)$; the red CO
profile is in the direction of
$(\Delta\alpha,\Delta\delta)=(+7\arcsec,+3\arcsec)$. In the “Lower Middle”
panel of Fig. 5, we binned the Odin H2O profile to a velocity resolution of 30
km s-1 and indicate the 1$\sigma$ rms noise level of 0.9 mK as a red line. Two
emission features are above the 3$\sigma$ level, one near 1450 km s-1, the
other near 1570 km s-1. There may also be an absorption feature with a
2$\sigma$ intensity near 1500 km s-1. It is interesting that near these three
velocities there exist corresponding CO($3-2$) components in the green
profile, presented in the Upper Middle panel of Fig. 5, a profile observed
near the radio contiunuum source G (see Table 2). The intensity of the SPIRE
data from Fig. 3, convolved to an Odin beam, has an intensity similar to the
Odin data convolved to the 1.4 GHz SPIRE resolution. This is shown in Fig. 5
“Bottom” where these two profiles have been superimposed. From the evidence
presented in Fig. 5, we draw the conclusion that Odin has indeed made a
marginal detection of H2O in NGC 1365 – two H2O emission features near 1450
and 1570 km s-1 and a possible absorption feature near 1500 km s-1.

###  3.3 VLA H i observations of the central regions of NGC 1365

We have used the VLA H i data cube of Jörsäter & van Moorsel (jor95 (1995)) to
study the central region of NGC 1365 in greater detail, using both the
emission and absorption data. The total integrated line intensity map is shown
in Fig. 7. Emission is dominant at the H ii region L4, absorption at the
compact radio sources D, E and G, but a marked absorption also is seen at F.
Four velocity-integrated maps, spaced at 100 km s-1, are presented in Fig. 8.
The dominant H i emission is in the ($-$228 to $-$104) km s-1 velocity
interval and originates in the L4 region at the upper left part of that figure
near offsets (+17″,+16″). The major H i absorption is seen in the velocity
interval ($-$104 to 0) km s-1 and comes from the D, E, G region near offsets
(+1″,+7″), as can be seen in Fig. 8. And, finally, Fig. 8 shows that the major
absorption region in the velocity interval of (0 to +104) km s-1 comes from
the F region at offsets near (+3″,$-$3″), and in the velocity interval of
(+104 to +228) km s-1 comes from the region around sources A, L2 and L3, in
the lower right of that H i figure. One may compare these features with the
three velocity components seen in the Odin H2O profile presented in Fig. 5
“Upper Middle”, namely the two emission peaks near 1460 ($-$150) and 1560
($-$50) km s-1 and the absorption component at 1510 ($-100$) km s-1, where the
velocities in brackets are relative to the systemic velocity, all three
originating northeast of the nucleus. The H i position-velocity (P-V) maps are
presented in Appendix A. In Fig. A.1 (for $y=0$″), the map is along the major
axis of NGC 1365, through L4 (wide emission), G (strong absorption), the
nucleus (empty), and finally A and L3 (absorption). The H i map in Fig. A.1
(for $y=+2.5$″) is parallel to the major axis but
$2\aas@@fstack{\prime\prime}5$ southeast of it, thus passing through F – the
absorption close to +50 km s-1 appears at the position of F.

###  3.4 Statistical image deconvolution analysis of our $J=3-2$ CO SEST
observations

The SEST CO($3-2$) observations of NGC 1365, published by Sandqvist (san99
(1999)), were performed over an $\approx 120\arcsec\times 60\arcsec$ region
centered on the optical nucleus. A grid spacing of 10″ was used for the outer
parts of the region whereas a grid spacing of 5″ was used for the inner
$\approx 70\arcsec\times 40\arcsec$ region. We have applied a statistical
image deconvolution (SID) analysis (Rydbeck ryd08 (2008)) to the SEST $J=3-2$
CO observations to obtain high resolution (5″) maps at different velocities
and P-V maps. These maps will help us to identify the origins of the different
Odin H2O components. The intensity unit in the maps is antenna temperature.
The main beam efficiency of 0.26 has been applied in the P-V maps to yield
main beam brightness temperature. Examples of CO integrated intensity maps for
the total and selected velocity intervals are shown in Figs. 7 and 8; examples
of CO P-V maps are shown in Fig. A.1.

A comparison of the integrated intensity H i absorption and CO emission maps,
seen in Fig. 7, shows that the main features in both species are oriented in a
northeast-southwest direction along the circumnuclear torus. There is an
additional somewhat enticing feature seen in the H i map, namely, an
approximately 20″-long H i absorption ridge emanating from the nucleus in a
south-southeast direction. This H i ridge may have a CO counterpart as shown
by the low-level extension in the same direction in the CO map. A careful
study of the H i and CO maps in Fig. 8 shows that this ridge has a very wide
velocity dispersion. This ridge will be discussed further in Sect. 4.3.1.

Figure 5: Odin 557 GHz H2O profile observed toward NGC 1365. Top: The
unsmoothed profile with a resolution of 0.54 km s-1. Upper Middle: The Odin
H2O profile binned to a resolution of 5.0 km s-1 (black); the SEST Odin-beam-
convolved CO($3-2$) profile (blue); the green and red SID CO profiles are
observed near the NE torus component (Sect. 3.2). The CO profiles should be
multiplied by a factor of 100. Lower Middle: The Odin H2O profile binned to a
resolution of 30 km s-1. The 1$\sigma$ rms noise level of 0.9 mK is indicated
in red. Bottom: The Odin H2O profile convolved to a 1.4 GHz (SPIRE) resolution
(black); the SPIRE data convolved to an Odin-beam resolution (red).

Figure 6: Herschel SPIRE 557-GHz o-H2O (left) and 752-GHz p-H2O (right) profiles observed toward the NGC 1365 NE (gray) and SW (black) torus positions. Intensity units are W m-2 Hz-1 sr-1. The HPBW beamwidth is 35″ and the separation between the NE and SW positions is 15″. Table 2: Equatorial offsets from the optical nucleus ($3^{h}33^{m}36\aas@@fstack{s}37,-36\degr 08\arcmin 25\aas@@fstack{\prime\prime}4$ ) of hot-spot H ii regions (Alloin et al. all81 (1981)) and compact radio sources (Sandqvist et al. san95 (1995)). Sources |  $\Delta$RA |  $\Delta$Dec | Line | Velocity (Width)aa$a$Velocities and line widths are from Sakamoto et al. (sak07 (2007)) and Galliano et al. (gal12 (2012)).
---|---|---|---|---
| (″) | (″) |  | ( km s-1)
L4 | +17 | +16 |  |
L11 | +10 | +15 |  |
L12 | +2 | +11 |  |
L1 | +4 | +2 |  |
L3 |  $-$6 |  $-$3 |  |
L2 |  $-$5 |  $-$5 |  |
D (= M4) | 0 | +7 | CO($2-1$) | +1560 (70)
|  |  | Br${\gamma}$ | +1555 (145)
|  |  | H2 | +1560 (?)
E (= M5) | +3 | +10 | CO($2-1$) | +1540 (60)
|  |  | Br${\gamma}$ | +1520 (145)
|  |  | H2 | +1530 (?)
G (= M6) | +5 | +7 | CO($2-1$) | +1480 (80)
|  |  | Br${\gamma}$ | +1475 (200)
|  |  | H2 | +1470 (?)
H (= M8?) | +5 | +3 | CO($2-1$) | +1510 (70)
Fbb$b$The steep spectrum of this compact radio source indicates emission from a synchrotron radiation outflow jet originating in the nuclear engine (Sandqvist et al. san95 (1995); Lindblad lin99 (1999)). | +3 |  $-$3 | CO($2-1$) | +1590 (90)
A |  $-$4 |  $-$3 | CO($2-1$) | $\approx 1730-1750$

Figure 7: Panel Number from the Left: (1): VLA H i line total integrated
intensity distribution, emission and absorption, is shown in the central
region of NGC 1365. The circles are hot-spot H ii regions and the squares are
compact radio sources as defined in Table 2. The offsets are with respect to
the optical nucleus, which is marked by a square. (2): The SEST SID CO($3-2$)
total integrated line intensity (antenna temperature – multiply by 3.8 to
obtain main beam brightness temperature). The equatorial offsets are with
respect to the optical nucleus. (3): The SPIRE map of 1461 GHz N ii; the
beamwidth is $\approx 17\arcsec$. The white square marks the area covered in
the H i and CO maps. Remaining features are as defined in Fig. 4. (4): The O
iii emission line region (thick lines), and the VLA 20-cm radio continuum
emission (thin lines) from the radio jet and the radio continuum circumnuclear
ring. The gray scales, ranging from $-0.3$ (black) to $-1.0$ (white), indicate
the 6/20 cm radio spectral index. The cross marks the position of the optical
nucleus (from Sandqvist et al. san95 (1995)).

##  4 Analysis and discussion

###  4.1 Gas distribution, dynamics, and star formation in the central region
of NGC 1365

The distribution of molecular gas in the central region of NGC 1365 can be
understood in the framework of bar-driven gas dynamics, as described in the
Introduction. This implies that the rapid inward gas flows along the bar dust
lanes are transferred into an oval circumnuclear torus of streaming gas clouds
(see Sakamoto et al. sak07 (2007), especially their Fig.12).

####  4.1.1 Cold molecular gas clouds causing intense star formation

According to our Herschel SPIRE mapping (Fig. 4), the ground state o-H2O (110
\- 101) emission at 557 GHz, observed by Odin in NGC 1365 (in the velocity
range $1400-1650$ km s-1), is mainly emanating from within a $10-15$″ size
(i.e., $\approx 1$ kpc) region, including the NE hot spot (of CO, etc.) of the
circumnuclear torus and some very nearby hot spots east thereof. However, we
cannot exclude that some of the water emission in the velocity range
$1400-1500$ km s-1 emanates from the L4 hot spot situated in the leading dust
lane of the eastern bar, about 25″ away from the center (see e.g., Fig. 1),
since this is also the velocity range observed at this position in the
CO($3-2$) line (see Fig. A.1). Our SPIRE map of the 752 GHz line in fact may
support such a contribution, although not the 557 GHz line map (see Fig. 4).

It may seem peculiar that we observe little or no ground state o-H2O emission
from the SW torus region (i.e., in the velocity range $1613-1800$ km s-1),
since the CO($2-1$) as well as the CO($3-2$) emissions are observed to be
similarly strong in the NE and SW torus regions (Sakamoto et al. sak07 (2007);
Sandqvist san99 (1999); Fig. A.1 of this paper). However, in addition to the
lack of sensitivity in our Odin observations, a likely explanation is offered
by the recent mapping of the dust emission at 870 $\mu$m in NGC 1365 usíng
LABOCA (the Large APEX Bolometer Camera; Tabatabaei et al. tab13 (2013)).
These authors estimate that the eastern bar is more massive than the western
one by at least a factor of four. Hence the bar-driven inflow of gas into the
NE torus region may be expected to be much more massive than the similar
inflow into the SW torus region. Tabatabaei et al. also find that the dust
(and presumably the gas) in the western bar is warmer (about 40 K) than that
in the eastern bar (about 20 K) by a factor of two. Even the cold dust
temperature map, presented in our Fig. 2, shows a tendency for the western bar
to be warmer. This temperature difference may indeed explain why the
(optically thick) CO($3-2$) emissions remain similarly strong in the eastern
and western torus and bar regions (Fig. 8 [$-102$ to 0 km s-1] and [$+102$ to
$+228$ km s-1]) in spite of the apparently very different gas masses.

Using the careful SMA aperture synthesis CO (2-1) mapping at 2″ resolution by
Sakamoto et al.(sak07 (2007)), Elmegreen et al. (elm09 (2009)) estimate that
the gas inflow into the NE torus region has a velocity of $\approx 80$ km s-1
and an accretion rate of $\approx 22$ $\mathrm{M}_{\odot}$/yr. They also
estimate a star formation rate (SFR) of $\approx 10$ $\mathrm{M}_{\odot}$/yr
in the central, circumnuclear torus region (using the observed FIR
luminosity). Tabatabaei et al. (tab13 (2013)) find an SFR of $\approx 15$
$\mathrm{M}_{\odot}$/yr in the central 80″ region in a similar way. Based upon
their CO mapping, Sakamoto et al. (sak07 (2007)) and Sandqvist (san99 (1999))
estimated the observed central molecular cloud mass to be $M_{\rm mol}(R\lid
1{\rm kpc})\approx 1\times 10^{9}$ $\mathrm{M}_{\odot}$ and $M_{\rm mol}(R\lid
2{\rm kpc})\approx 5.4\times 10^{9}$ $\mathrm{M}_{\odot}$, respectively,
however relying on very different $X_{\rm CO}$-factors, $0.5\times 10^{20}$
vs. $2.3\times 10^{20}$ (K km s-1)-1. If we instead use a common conversion
value of $1.2\times 10^{20}$ cm-2 (K km s-1)-1 in both cases, as suggested by
Tabatabaei et al., we arrive at $M_{\rm mol}(R\lid 1{\rm kpc})\approx
2.4\times 10^{9}$ $\mathrm{M}_{\odot}$ and $M_{\rm mol}(R\lid 2{\rm
kpc})\approx 2.8\times 10^{9}$ $\mathrm{M}_{\odot}$ – mass estimates likely to
be accurate to within a factor of two.

Our CO($3-2$) P-V diagram (Fig. A.1 [$y=-2.5$″]) nicely demonstrates an
overall solid body rotation (i.e., a rotational velocity increasing linearly
with the distance from the center) of the cicumnuclear molecular cloud torus
region inside $R\lid 1$ kpc, but with considerable velocity dispersion caused
by individual cloud motion, cloud-cloud collisions, shocks and outflows (also
causing increased turbulence in the clouds). For $R$ ¿ 1 kpc we observe the
expected differential rotation. In fact, we may also (in Fig. A.1 [$y=-2.5$″]
see a hint of the NE bar-driven inflow in terms of two velocity components (at
$-150$ and $-110$ km s-1, at a major axis offset of +10″, or 0.9 kpc).
Correcting for a galaxy inclination of 40°, this velocity difference becomes
close to the aforementioned inflow velocity of 80 km s-1. The P-V diagram also
allows us to calculate the dynamical mass $M_{\rm dyn}$ (of gas, stars plus
central AGN) inside a specified radius (using the balance between the
centrifugal force caused by rotation at a velocity $V_{\rm rot}(R_{o})$ and
the gravitational attraction of the mass inside $R_{o}$ – valid in the low-
order approximation of circular motion), namely,

$M_{\rm dyn}$ ($R\lid R_{o}$) = $R_{o}\times V_{\rm rot}^{2}(R_{o})$/G, (1)

where G is the gravitation constant, which in more useful parameters can be
rewritten

$M_{\rm dyn}(R\lid R_{o})\approx 1.2\times 10^{10}$
$\mathrm{M}_{\odot}$$\times R_{o}{\rm[kpc]}\times(V_{\rm rot}/230$)2. (2)

From our P-V-diagrams (Fig. A.1), we may now estimate $V_{\rm rot}\approx 150$
km s-1/sin 40°= 233 km s-1 and $V_{\rm rot}\approx$ 200 km s-1/sin 40°= 311 km
s-1 at $R_{o}=1$ and 2 kpc, and hence calculate dynamical masses of $1.2\times
10^{10}$ $\mathrm{M}_{\odot}$ and $4.4\times 10^{10}$ $\mathrm{M}_{\odot}$,
respectively. Our previously observationally determined gas masses amount to
about 20% of the dynamical masses in the center versus an average of 6% in the
more extended region (including the center). Hence we may see a trend of
increasing gas mass fraction toward the center, but not unexpectedly the
stellar mass dominates.

####  4.1.2 Warm ionized medium, created by the intense star formation

We now move from the discussion of the bar-driven, outer circumnuclear
molecular gas torus, causing the formation of supermassive stellar clusters,
to the creation of the warm ionized medium (WIM), the ISM component
surrounding the molecular cloud ensemble and the stellar superclusters, which
is caused by the intense UV radiation of the multitude of newly formed stars.

Nitrogen has a higher ionization potential (14.5 eV) than does Hydrogen (13.6
eV) and will stay atomic N where hydrogen is atomic H or molecular H2, and
becomes ionized N+ where hydrogen is almost completely ionized H+. The 205
$\mu$m N ii line is easily collisionally excited in warmer gas since its upper
state energy is 70 K and its critical density is only 44 cm-3. Hence it is an
excellent probe of the WIM, consisting of moderately dense H ii regions and
ionized boundary layers of clouds, that is to say, a useful measure of the
star formation rate (SFR) (Langer et al. lan16 (2016); Zhao et al. zha16
(2016)). Our SPIRE map of the 205 $\mu$m N ii line (Fig. 7), observed with a
17″ beam, shows that this emission is extended beyond the SW torus peak and
here is as intense as in the NE peak. The high SFR observed also at and beyond
the SW torus peak provides a nice explanation of the higher dust, gas
temperature (40 vs 20 K) and lower dust, gas mass (by a factor four) in the
western bar region versus the eastern bar, as estimated by Tabatabaei et al.
(tab13 (2013)). This observed mass and temperature asymmetry of the bar may be
understood if the western bar region resides in a, somewhat later, high SFR
stage, where a larger amount of the star forming molecular clouds has been
consumed and where the intense UV radiation from the newly formed stars has
heated the dust.

Figure 8: VLA H i [top] and SEST SID CO($3-2$) [bottom] line intensity maps
integrated over the velocity range indicated in the header [($-228$ to
$-102$), ($-102$ to 0), (0 to +102), (+102 to +228)] km s-1. The CO integrated
intensity is antenna temperature – multiply by 3.8 to obtain main beam
brightness temperature. The equatorial offsets are with respect to the optical
nucleus. The velocity offsets are with respect to the systemic LSR velocity of
1613 km s-1.

###  4.2 Analysis of the Odin and Herschel SPIRE observations

####  4.2.1 Odin results

A Gaussian fit with two emission and one absorption components has been
applied to the Odin H2O profile shown in Fig. 5. The values for $T_{A}$, $V$,
and $\Delta V$ are 7.1 mK, 1456 km s-1, 42 km s-1 and 5.1 mK, 1582 km s-1, 81
km s-1 for the emission components and $-1.5$ mK, 1511 km s-1, 40 km s-1 for
the absorption component. The two emission features appear at velocity offsets
of $-157$ and $-31$ km s-1 with respect to the systemic LSR velocity of 1613
km s-1. The absorption feature has a velocity offset of $-102$ km s-1.
Although the Gaussian fit may be somewhat uncertain – due to the very low
signal-to-noise ratio, and since the lower excitation foreground H2O regions
may absorb not only the thermal dust continuum but also some of the velocity
ranges of the background H2O emission from higher excitation regions – it is
worthwhile to use the velocity information in an attempt to determine the
origin of the these three 557 GHz H2O regions. In the following, we use our
own SEST SID CO($3-2$) and VLA H i results as well as the CO($2-1$) SMA
observations by Sakamoto et al. (sak07 (2007)) and new ALMA observations of
the CO($3-2$) line by Combes et al. (com19 (2019)).

While the large Odin beam is covering most of the bar, the H2O region has
already been limited in extent to the central region by our SPIRE 557 GHz H2O
map presented in Fig. 4. The H2O emission is dominant near the NE torus
component where the aforementioned hot spot L-regions and compact radio
sources gather.

Our CO($3-2$) P-V map in Fig. A.1 ($y=-2\aas@@fstack{\prime\prime}5$) shows a
distinct feature at a velocity offset of $-150$ km s-1, which is close to one
of the H2O emission features velocity of $-157$ km s-1. That occurs at a major
axis offset $x=10$″. This position is very close to the compact radio source G
(see Table 2) and is also the region in the ALMA velocity map where the
velocity offset is most negative, namely $\approx-150$ km s-1. Near the
position of E, the ALMA velocity offset is around $-90$ km s-1, which is
comparable to the Odin H2O absorption feature value of $-102$ km s-1. Near D
the ALMA velocity offset is closer to $-50$ km s-1, which can be compared to
the Odin emission feature value of $-31$ km s-1. The VLA H i maps in Fig. 8
show major absorptions toward G and D in the velocity offset ranges of ($-228$
to $-104$) km s-1 and ($-104$ to 0) km s-1, respectively, agreeing reasonably
well with the above-mentioned comparisons. Our conclusion is thus that the
three Odin H2O features originate in the NE shock region, giving rise to the
violent star formation activity caused by the interactions of the $x_{1}$\-
and $x_{2}$-like streamlines in the inner bar and center, as proposed by
Sakomoto et al. (sak07 (2007)).

An estimate of the expected background continuum antenna temperature,
convolved to the Odin effective beam of 140″ by 160″, can be obtained from the
Herschel SPIRE spectrum seen in Fig. 3. The flux density continuum level at
555 GHz is 4.5 Jy which converts to 1.5 mK antenna temperature, convolved to
the Odin beam.

We now make an effort to estimate H2O abundances based upon our three-
component Gaussian fit to the 557 GHz ground state o-H2O spectrum observed by
Odin. The absorption feature around 1511 km s-1 has a width $\Delta V=40$ km
s-1 and appears to be saturated since the absorption is similar to, or larger
than, the continuum level determined from our Herschel SPIRE spectrum (Fig.
3). A column density estimate for this lower density, lower excitation
foreground gas, or rather a lower limit thereof, then may be obtained from the
relation (derived by Karlsson et al. kar13 (2013)),

$N$(H2O)$=4/3\times 4.89\times 10^{12}\times\tau$(H2O) $\times\Delta V$, (3)

where we assume an optical depth $\tau\approx 2-3$ and an ortho-to-para ratio
OPR $=3$. The resulting total water column density becomes $N$(H2O)
$\gid(5-8)\times 10^{14}$ cm-2. The corresponding H2 column density, in front
of the source(s) of the thermal dust continuum emission (and H2O emission)
being absorbed, may be estimated from the visual extinctions of the mid/near
IR (and radio) sources M4, M5 and M6 (see Table 2), from their observed
Br${\gamma}$/Br${\alpha}$ ratios, estimateded to be $A_{\rm v}=13.5$, 3.2 and
8.5 mag, respectively (Elmegreen et al. elm09 (2009)). If we use an “average”
$A_{\rm v}\approx 10$ mag in the scaling relation found from observations by
the Copernicus satellite (Bohlin et al. boh78 (1978)), that is,

$N$(H2)/$A_{\rm v}=0.94\times 10^{21}$ cm-2, (4)

our estimate would be $N$(H2) $\approx 1\times 10^{22}$ cm-2, with a
considerable uncertainty range (for a discussion and references see e.g.,
Hjalmarson and Friberg 1988). However, this value is in fact about half of the
average total column density found from our SID CO($3-2$) analysis (soon to be
discussed), which is a useful criterion of correctness. Finally, the water
abundance can be estimated to be $X$(H2O) = $N$(H2O)$/N$(H2) $\gid 5\times
10^{-8}$ – an abundance level which is well accommodated by PDR chemistry in
lower density, translucent and diffuse regions (e.g., Hollenbach et al. 2009,
2012; see also our Tables 5 and B.1).

Our previous absorption line analysis must be considered comparatively
accurate – since the method itself relies on the realistic assumption that the
absorbing molecular population is mainly residing in the lowest possible
energy state – although the optical depth of the 557 GHz o-H2O absorption
tentatively observed by Odin is uncertain. In case of the two weak emission
features apparent in the same Odin spectrum, the analysis uncertainty stems to
a large extent from the relatively poor knowledge of sizes, densities, and
temperatures of the emission regions, which by nature also are bound to be
inhomogeneous. Their approximate sizes of $\approx 10$″ are estimated from our
SID CO($3-2$) maps at 5″ resolution (for the relevant velocity ranges shown in
Fig. 8) and the SMA CO($2-1$) maps at 2″ resolution by Sakamoto et al. (sak07
(2007)). Guidance on temperatures and densities is drawn from the analysis of
our multi-transition CO observations by SEST and Herschel SPIRE, presented in
Sect. 4.2.2. Two-component fits to the CO rotation diagram, as well as the CO
SLED, reveal a hot ($T_{\rm kin}\approx 350$ K) component, most likely caused
by shock excitation, together with a lukewarm ($T_{\rm kin}\approx 40$ K)
component having an average H2 density of 104 cm-3. The average H2 column
density of the latter component is estimated to be $6.3\times 10^{21}$ cm-2
across a 10″ $\times$ 30″ region. According to Eq. (4) this corresponds to a
visual extinction $A_{\rm v}\approx 7$ mag. However, the surface filling
factor of CO emission in this region may be less than 100%. The
characteristics of this lukewarm region are very similar to what is expected
from PDR-models for high FUV illumination (such as that in the Orion molecular
cloud; cf. Fig. 7 of Hollenbach et al. hol09 (2009)). This interpretation is
also supported by the best two-component fit of the SED of the thermal dust
emission from the central regions of NGC 1365 – one component at $25\pm 2$ K
and a necessary second one at $55\pm 5$ K, emitted from $\approx 1$% of the
total dust mass (Alonso-Herrero et al. alo12 (2012); Tabatabaei et al. tab13
(2013)).

With this information at hand, we assume a cloud temperature $T_{\rm kin}=55$
K and density $n$(H2) $=10^{4}-10^{5}$ cm-3 in our RADEX (van der Tak et al.
tak07 (2007)) molecular excitation and radiative transfer model fitting of the
557 GHz o-H2O emission features, each having an estimated size of only
$\approx 10$″ in the very large Odin antenna beam ($140\arcsec\times
160\arcsec$, resulting in a beam filling factor of 1/225). Including the beam
filling correction, we get as input parameters $T_{\rm b}\approx 1.6$ K and
$\Delta V=42$ km s-1 for the emission around $-157$ km s-1 and $T_{\rm
b}\approx 1.1$ K and $\Delta V=81$ km s-1 for the emission around $-31$ km
s-1. If we assume an OPR = 3, the corresponding total ortho- and para-H2O
column densities are RADEX modeled to be $N$(H2O) $\approx 15\times 10^{15}$
cm-2 and $19\times 10^{15}$ cm-2, respectively, for $n$(H2) $=10^{5}$ cm-3,
and increasing by an order of magnitude for the lower cloud density of $n$(H2)
$=10^{4}$ cm-3. The comparison H2 column densities may be roughly estimated
from our SID CO($3-2$) maps for the velocity ranges $-200$ to $-100$ km s-1
and $-100$ to 0 km s-1 (Fig. 8) using the scaling relation

$N$(H2) $=1.2\times 10^{20}\times I$(CO) cm-2, (5)

where $I$(CO) is the integrated intensity [in K km s-1] (Tabatabaei et al.
tab13 (2013); for a discussion of the scaling relation method, see e.g.,
Hjalmarson & Friberg hja88 (1988)), resulting in $N$(H2) $\approx 2.3\times
10^{22}$ cm-2 and $3.0\times 10^{22}$ cm-2, respectively, for the two H2O
emission regions. For an average cloud density of 105 cm-3, the resulting H2O
abundance versus H2 can be calculated as $X$(H2O) $=N$(H2O)$/N$(H2) $\approx
6\times 10^{-7}$ for both regions – an abundance increasing by an order of
magnitude in case the average density is 104 cm-3. The lower abundance value
appears to be consistent with PDR modeling results (Hollenbach et al. hol09
(2009), hol12 (2012)), especially since additional ionization support may
exist in terms of cosmic ray focusing by the observed magnetic field alignment
(Beck et al. bec05 (2005)). However, substantial contributions to the ground
state H2O emission, caused by shock excitation and shock chemistry, are also
expected from our modeling of the Herschel SPIRE data presented in Sect. 4.2.2
(see Fig. 9).

####  4.2.2 Herschel SPIRE results

Before entering a detailed presentation of the multi-transition excitation and
radiative transfer analysis of our SPIRE CO and H2O observations (Fig. 3 and
Table 1), it may be helpful to examine the spatial distributions of the 557
GHz ground state o-H2O line ($E_{u}=27$ K in the ortho ladder; $E_{u}=61$ K vs
the para ground state) and the 752 GHz excited state p-H2O line $(E_{u}=137$ K
vs the para ground state), illustrated in Figs. 4 and 6. The two maps show
that both lines are strongest in the NE torus peak area. However, it is
apparent from Fig. 6 that the 557 GHz line has little or no emission at the SW
torus peak, while the 752 GHz excited state transition definitely exhibits
emission here (taking into account the beam size of 35″ and the 15″ separation
of the observed NE and SW positions). Our subsequent analysis of the CO, as
well as the H2O SLEDs, will lead us to the conclusion that shock excitation
and shock chemistry must be main players in case of the higher energy lines.
Therefore, it is useful to know beforehand that such conditions have indeed
been observed in all of the “hot spot” H ii, CO and stellar supercluster
regions in the circumnuclear torus, and also in the presumptive synchrotron
radiation jet source “F” (see our Table 2; VLT SINFONI high resolution near IR
observations by Galliano et al. gal12 (2012) and Fazeli et al. faz19 (2019),
to be discussed in some detail in Sect. 4.4).

We now present an analysis of the Herschel SPIRE apodized spectrum obtained
toward the NE component of the central torus, which was presented in Fig. 3.
As has already been pointed out, the NE component is the dominant one and so
of major interest. It is also the component having a velocity field closer to
that implied by the Odin H2O profile. We shall first treat the CO lines and
then the H2O lines.

The rotation diagram for the CO lines is presented in Fig. 9(Upper left). The
three lowest-level CO lines (blue dots) are obtained from SEST observations
presented by Sandqvist et al. (san95 (1995)), Sandqvist (san99 (1999)), and
Curran et al. (cur01 (2001)), while the remaining higher-level CO lines (red
dots) are from this paper. The results of RADEX analysis applied to these CO
lines (also presented in Fig. 9(Upper left)) show that the high $J$ CO lines
can be best fit by a single kinetic temperature of 400 K and an H2 density of
$n$(H2) $=3\times 10^{3}$ cm-3, yielding a CO column density of $N({\rm
CO})=2.5\times 10^{15}$ cm-2.

A two-temperature RADEX fit to the CO SLED is presented in Fig. 9(Upper
right). The best fit is obtained for an extended cold component with a
temperature of $T_{\rm Ext}=26$ K, density $n$(H2) $=7\times 10^{3}$ cm-3,
yielding a CO column density of $N({\rm CO})=1\times 10^{17}$ cm-2, and a hot
component from a more compact region having a temperature of $T_{\rm Hot}=350$
K, density $n$(H2) $=10^{4}$ cm-3, yielding a CO column density of $N({\rm
CO})=4\times 10^{15}$ cm-2.

Perhaps it would be more realistic to study the NE region as one involving
shocks (see Fig. 9(Lower left)). In this case we have used the C-shock model
of Flower & Pineau des Forêts (flo10 (2010)) where we have scaled their values
to an equivalent (accumulated) source size of 10″- in order to obtain a best
fit. For the low $J$ CO lines, we obtain here a slightly warmer extended
region with a temperature of $T_{\rm Ext}=40$ K and RADEX values of $n$(H2)
$=10^{4}$ cm-3 and $N({\rm CO})=7\times 10^{16}$ cm-2 (represented by the blue
curve) and a slow velocity C-type shock with a shock speed of 10 km s-1 and a
pre-shock density of $n$(H+2H2) $=2\times 10^{4}$ cm-3 (represented by the red
curve). This model yields a CO abundance of $X$(CO) $\approx 10^{-4}$.

Our RADEX modeling fit to the intensities of the lower energy lines yields
$N$(CO) $=1\times 10^{17}$ cm-2 for a homogeneous cloud at a temperature
$T_{\rm kin}\approx 40$ K and density $n$(H2) $=10^{4}$ cm-3. From the mapping
of the CO($2-1$) emission by Sakamoto et al. sak07 (2007)) and our SID
CO($3-2$) deconvolution, it appears that the half power size of the emission
region is only about 30″ $\times$ 10″, which means a beam filling of only 16%
in a 40″ beam-size. The resulting CO column density hence becomes $6.3\times
10^{17}$ cm-2, which for a CO/H2 abundance ratio of $\approx 1\times 10^{-4}$
means an H2 column density $\approx 6.3\times 10^{21}$ cm-2. According to the
scaling relation shown in Eq. (4), this corresponds to a visual extinction of
$\approx 7$ magnitudes. The estimated temperature and density, combined with
this visual extinction, most likely tells us (cf. Hollenbach et al. hol09
(2009), hol12 (2012)) that this CO emission must originate in the extended PDR
surfaces of the molecular complexes, created by the intense UV illumination
from the observed, co-located, newly formed, massive stellar superclusters
(Elmegreen et al. elm09 (2009); Galliano et al. gal12 (2012); Fazeli et al.
faz19 (2019)).

We now turn our attention to the ortho- and para-H2O lines in the SPIRE
spectrum of Fig. 3. RADEX models were performed for two different
temperatures, namely 400 and 1000 K, and model values for density $n$(H2) cm-3
– and column density $N$(H2O) cm-2, respectively, were $10^{3}-3\times
10^{14},10^{4}-3\times 10^{13},10^{5}-3\times 10^{12}$ for o-H2O;
$10^{3}-10^{14},10^{4}-10^{13},10^{5}-10^{12}$ for p-H2O. None of these models
were found to be applicable to the SPIRE H2O data.

We then attempted water line modeling, using the same slow-velocity C-type
shock as above in the CO line analysis, and these results are shown in Fig.
9(Lower right). In this figure the blue and red points and curves represent
o-H2O and p-H2O, respectively. The ortho-to-para ratio is assumed to be 3. A
shock speed of 10 km s-1 and a pre-shock density $n$(H2) $=10^{4}$ cm-3,
nicely matching the SPIRE CO data, do not at all produce a fit to the observed
H2O lines. An improvement of the fit is obtained by increasing the pre-shock
density by an order of magnitude to $n$(H+2H2) $=2\times 10^{5}$ cm-3 and
retaining the same shock speed of 10 km s-1. But the best fit is obtained by
using the original pre-shock density of $n$(H+2H2) $=2\times 10^{4}$ cm-3 and
increasing the shock speed to 40 km s-1. It is, however, still somewhat
difficult to fit all the para-H2O lines. An approximate H2O abundance for this
model is similar to the CO abundance, that is to say, $X$(H2O) = $X$(CO)
$\approx 10^{-4}$. In studies of the SPIRE and PACS results for the ULIRGs Mrk
231, NGC 4418, and Arp 220, it has been found that FIR pumping provides an
excellent fit to the observed H2O line intensities (Gonzalez-Alfonso et al.
gon10 (2010), gon12 (2012), gon14 (2014); cf. Yang et al. yan13 (2013)). The
estimated high H2O abundances of $10^{-6}-10^{-5}$ here were suggested to
result from shock chemistry, (multiple) hot cores, cosmic ray enhancement,
XDRs, and, or “undepleted chemistry” where the icy grain mantles were
evaporated. In our case of NGC 1365 FIR pumping, paired with an enhanced H2O
abundance caused by slow velocity shock sputtering of icy grain mantles
(supplemented by any of the above processes), – all expected from
observational data – this provides a viable alternative to “our poor man” fast
shock modeling. Although no PACS observations of FIR pumping H2O absorption
lines are available in case of NGC 1365, we note that the observed intensity
of the o-H2O ($3_{21}-3_{12}$) line, having an upper state energy of 305 K, is
much higher than can be reached by the shock modeling (see Fig.9(Lower
right)). This strongly suggests that we see the effect of FIR pumping by o-H2O
($2_{12}-3_{21}$) absorption at 75 $\mu$m (Gonzalez-Alfonso et al. gon14
(2014)).

Figure 9: Upper left: Rotation diagram for the CO lines. Blue dots: SEST
observations from the literature, Red dots: SPIRE data, Green dot: SPIRE CO
line blended with a water line. The full, dashed, and dotted lines show
different single temperature RADEX models. Upper right: Two temperature
(RADEX) fits to the CO lines. One extended cold component (blue) with $n$(H2)
= $7\times 10^{3}$ cm-3 and $N$(CO) = $1\times 10^{17}$ cm-2, and one hot
component (red) from more compact regions $n$(H2) = $10^{4}$ cm-3 and $N$(CO)
= $4\times 10^{15}$ cm-2. Lower left: A model consisting of a slightly warmer
extended component with RADEX values of $n$(H2) = $7\times 10^{3}$ cm-3 and
$N$(CO) = $7\times 10^{16}$ cm-2 (blue), and a slow velocity C-type shock with
a shock speed of 10 km s-1 and a pre-shock density of $n$(H +2H2) = $2\times
10^{4}$ cm-3 (red). The C-shock model comes from Flower & Pineau des Forêts
(flo10 (2010)) where we have scaled their values to a 10″-source in order to
obtain a best fit. Lower right: Water lines modeling. Water emission lines
observed by SPIRE – Blue dots: o-H2O, Green arrow: Blended ortho-line, Red
dots: p-H2O. The same slow-velocity C-type shock as for CO but the model with
a shock velocity of 10 km s-1 and a pre-shock density of $n$(H +2H2) =
$2\times 10^{4}$ cm-3 underestimates the line intensities. Increasing the
speed or the pre-shock density will give a better fit but it’s still hard to
fit all para lines.

###  4.3 Nuclear gas accretion and outflows

The gas inflow processes from the outer torus region onto the inner torus,
nuclear accretion disk, of NGC 1365 were discussed already in the Introduction
of this paper. We have in Table 3 collected a number of relevant parameters
for the galaxy spiral arm disk, the bar, the bar-driven outer torus region,
the inner cold gas torus, the central rapidly spinning SMBH, and the various
observed nuclear outflows We have also included in Table 3 some useful
comparison parameters for the lens-shaped spiral galaxy NGC 1377, seen edge
on, where rather convincing signs of a precessing nuclear bi-polar molecular
outflow were observed at ALMA (Aalto et al. aal16 (2016)).

####  4.3.1 Atomic, molecular and ionic outflows, and a synchrotron radiation
jet from the nuclear engine

It was mentioned already in the Introduction, that among the radio hot spots
of the circumnuclear ring of gas in NGC 1365, ionized by recent star
formation, the outer torus of radius 800 pc, a region “F” with a very steep
radio spectrum was “hiding”, see Fig. 7(4) (Sandqvist et al. san95 (1995)).
This was further illustrated in our Table 2. The envisioned synchrotron
radiation relativistic outflow jet from, and presumably powered by, the
nuclear engine, the black hole and its accretion disk, therefore is expected
to be be oriented along the rotation axis of the inner circumnuclear molecular
gas torus. However, the projected jet orientation instead appears to be along
the similar rotation axes of the galaxy, its prominent bar, and the outer
molecular gas torus, and along the symmetry axis of the wide-angle ionized gas
outflow observed in optical O iii lines, as illustrated in Fig. 7(4)
(Sandqvist et al. san95 (1995); Hjelm & Lindblad hje96 (1996); Sandqvist san99
(1999); Lindblad lin99 (1999); Venturi et al. ven18 (2018); see our Table 3).
This observational fact is very important in our forthcoming scenario
interpretation (Sect. 4.3.2). The observation of a synchrotron radiation jet,
having a de-projected size of some 400 pc, depending upon its somewhat
uncertain inclination, see Table 3, in fact means that we are probing the
radiative acceleration losses in a plasma beam where the electrons are
spiraling at relativistic velocities along a strong co-aligned magnetic field.
Here it is interesting to point out that the observed magnetic field, which
according to the VLA mapping by Beck et al. (bec05 (2005)) is closely aligned
with the bar just in the central regions, is turning its direction toward the
nuclear engine.

Table 3: Physical parameters of NGC 1365 compared with those of NGC 1377. Object | Rotation | or | Radius | Rotation or | Mass |  | Period | Remarks
---|---|---|---|---|---|---|---|---
| Outflow | Axis |  or Size | Outflow Vel. | Dyn.aa$a$This paper, Eq.(2). | Gasbb$b$This paper, Sect. 4.1. |  or Age |
| P.A.[°] | Incl.[°] | [kpc/pc] | [ km s-1] | [$\times 10^{9}$ $\mathrm{M}_{\odot}$] |  | [Myr] |
NGC 1365 |  |  |  |  |  |  |  |
Diskcc$c$Lindblad (lin99 (1999)). | 130 | 50 | 22 |  $\approx$270 | 360 | 13 |  $\approx$500 | H i mass
Barc,dc,dfootnotemark: $c,d$ |  | 50 | 10 | 180 | 13dd$d$Combes et al. (com19 (2019)), who (in their Table 4) list an inner torus tilt, with an uncertainty of $\pm 10\degr$. |  | 340 | Stellar mass
Disk |  | 50 | 10 | 310 | 220 | 9 | 200 | H2 mass
Disk |  |  | 2 | 310 | 44 | 2.8 | 40 |
Disk/torusbb$b$This paper, Sect. 4.1. |  |  | 1 | 230 | 12 | 2.4 | 27 | “Star formation gas ring”,
Outer torus |  $\approx$130 |  $\approx$50 | 800 | 185 | 6 |  | 27 | with solid body rotation
Outer torus |  |  | 500 | 115 | 1.5 |  | 27 | as visualized in Fig. A.1
Nuclear engine: |  |  |  |  |  |  |  |
Inner torusdd$d$Combes et al. (com19 (2019)), who (in their Table 4) list an inner torus tilt, with an uncertainty of $\pm 10\degr$. |  $\approx$160 |  $\approx$63 | 26 |  $\approx$100 | 0.06 | 0.007 | 1.6 | Note torus axis tiltdd$d$Combes et al. (com19 (2019)), who (in their Table 4) list an inner torus tilt, with an uncertainty of $\pm 10\degr$., caused by $\approx$ 30° precession angle
Black Holedd$d$Combes et al. (com19 (2019)), who (in their Table 4) list an inner torus tilt, with an uncertainty of $\pm 10\degr$. |  |  |  |  | 0.004 |  |  | Rapidly spinning SMBH
Nuclear outflows: |  |  |  |  |  |  |  |
Radio Jetc,ec,efootnotemark: $c,e$ |  $\approx$135 |  $\approx$20 |  $\approx$400 | 0.7c? |  |  |  | Table 2; Inclination caused by nuclear engine tilt
CO outflowff$f$This paper, Table 2 & Sect. 4.3.2. |  $\approx$ 135 |  $\approx$20 |  $\approx$400 |  $\approx$70 |  |  | 5.9 | Sect. 4.3.1 & ref d); Inclination by nuclear engine tilt
|  | ($\approx$80) | ($\approx$2200) | ($\approx$23) |  |  | (90) | Unlikely inclination alternative; See Sect. 4.3.2
O iii bi-conegg$g$Hjelm & Lindblad (hje96 (1996)); Lindblad (lin99 (1999)); Venturi et al. (ven18 (2018)). | 130 | 50 | $\pm 2500$ | $\pm 200$ | $-$ | 0.001 |  $\approx$12 | Hollow, wide angle ($\pm 50\degr$) ionized gas “fountain”
H i outflow |  $\approx$160 |  | 2000 | $\pm 200$ |  |  |  $\approx$10 | Fig. 7; Desorption/dissociation from icy grains?
NGC1377 |  |  |  |  |  |  |  |
Nuclear diskhh$h$Aalto et al. (aal12 (2012)). | 15 |  $\approx$0 | 29 | 75 | 0.04 | 0.017 |  |
CO outflowhh$h$Aalto et al. (aal12 (2012)). | 15 |  $\approx$0 | 200 | 140 | $-$ | $0.01-0.05$ | 1.4 | Opening angle $\approx 60-70\degr$
Map areajj$j$Aalto et al. (aal16 (2016)). | 15 |  $\approx$0 | $-$ |  | $-$ | 0.16 |  | Size: $4\arcsec\times 4\arcsec$
Accretion diskjj$j$Aalto et al. (aal16 (2016)). | 15 |  $\approx$0 |  $\approx$10 | 110 | 0.03 | 0.018 |  $\approx$1 | Rotation period
Bipolar CO Jetjj$j$Aalto et al. (aal16 (2016)). | 15 |  $\pm(10-25$)kk$k$in Ref. (j) estimated precession angle of CO jet and nuclear engine. | $\pm 150$ |  $240-850$mm$m$interval depending upon precession angle. | $-$ |  $0.002-0.02$mm$m$interval depending upon precession angle. | $0.2-0.7$ | Precession Period: $0.3-1.1$

According to the CO($2-1$) SMA mapping by Sakamoto et al. (sak07 (2007)),
there is in the outer torus also a “molecular cloud hot spot” co-located with
the synchrotron jet source “F”, which has an intensity peak at a velocity that
is inconsistent with the expected streaming motion in the torus. If the
CO($2-1$) hot spot at F were a “mirror image” of the one at D – moving along
the circumnuclear torus – its line of sight (LOS) velocity, according to Table
2, is expected to be $\approx 1613+(1613-1560)=1666$ km s-1. However, its
observed peak emission LOS velocity is 1590 km s-1, that is, $\approx$ 76 km
s-1 “too low” and “blue shifted” versus the systemic velocity by
$1613-1590=23$ km s-1. This points at the existence of a molecular gas outflow
from the nuclear engine, oriented along the direction of the nuclear
synchrotron radiation jet. The peak emission is LOS blue-shifted by about 25
km s-1 with respect to the systemic velocity. This suggests the presence of a
rather collimated molecular outflow, having a de-projected velocity of about
70 km s-1 in the same direction as the synchrotron jet, and which perhaps is
driven by the same launching process (see Table 3). In fact, also Combes et
al. (com19 (2019)) propose the possibility of a nuclear outflow, observed in
the ALMA CO($3-2$) velocity field, similar to that found for NGC 613 (Audibert
et al. aud19 (2019)). We here compare the CO($2-1$) profile observed at 2″
resolution in radio source F (Fig. 11 of Sakamoto et al. sak07 (2007)),
displaying a pronounced rather narrow peak at 1590 km s-1 on top of a broader
range (1500 – 1750 km s-1) pedestal, with our 5″ resolution CO($3-2$) spectrum
in the same direction. This is shown in Fig. 10, where the arrow indicates a
narrow feature. The weakness of the presumptive molecular jet emission at
$\approx-25$ km s-1, observed at 5″ resolution may suggest that the extent of
this emission is $\lid 2$″, equivalent circular size in the plane of the sky,
that is, indicating a collimated, molecular gas outflow of size 4″ $\times$ ¡
1″.

Figure 10: SID CO($3-2$) profile toward the continnum source “F”. The arrow
indicates a weak and narrow feature, possibly indicative of the molecular
outflow, discussed in Sect. 4.3.1

.

The envisioned $\approx$400 pc size relativistic synchrotron jet and
collimated molecular outflow are enshrouded by a kpc size fan- or, rather,
cone-shaped ionized gas outflow, revealed by its visual O iii emission (Hjelm
& Lindblad hje96 (1996), discussed in considerable detail in the review paper
by Lindblad lin99 (1999)). More recently, this hollow wide-angle
($\approx$100°) outflow of ionized gases has been studied in considerable
detail by Lena et al. (len16 (2016)) and Venturi et al. (ven18 (2018)). From
their optical high spectral resolution mapping of the central 5 kpc
(60″$\times$ 60″) region of NGC 1365, at a spatial resolution of 70 pc
(0.76″), by means of VLT MUSE (multi-unit spectroscopic explorer), paired with
Chandra X-ray data and elaborate analyses, Venturi et al. (ven18 (2018))
revealed the very inhomogeneous and anisotropic structure of the mass outflow
rate and the velocity field of this bi-conical, wide angle, $\pm 2.5$ kpc
size, nuclear outflow of ionized gases, having velocities up to 200 km s-1.
Their Fig. 7, displaying the estimated ionized gas mass outflow rate at 5″
(450 pc) resolution, demonstrates very pronounced radial as well as angular
variations in the range ($10^{-5}-10^{-2}$) $\mathrm{M}_{\odot}$/yr. From a
consideration of the kinetic energy rate and the momentum rate of the extended
outflow of ionized gas and the mass outflow rate available from the AGN,
obtained from their Chandra X-ray data, Venturi et al. argue that the extended
ionized gas outflow must be momentum powered, that is, driven by the X-ray
pressure (i.e., by X-ray photon collisions with the dust component of the
gas). The authors also point out that the expected multiple supernova
explosions (SNe) in the “star formation ring”, the outer torus in our
nomenclature, may also contribute to the powering and temporal variation of
the very extended ($\pm 2.5$ kpc), wide angle ($\pm 50$°), bi-conical nuclear
outflow. This is especially so since the observed outflow extent is as large
as the diameter ($\lid$2 kpc) of the outer torus, while the diameter of the
AGN accretion disk (the inner torus) is only $\approx$50 pc (see our Table 3).
A main reason for the radial variations of the ionized gas may be found in the
observed variable nuclear X-ray emission causing (temporal) variations in the
outflow launching radiation pressure. We return to the likely cause of the
angular variations in Sect. 4.3.2. While SNe-induced multiple “chimney
processes” presumably are in action in the outer torus (as modeled by e.g.,
Norman & Ikeuchi nor89 (1989); Melioli et al. mel09 (2009)), the larger scale
“nuclear fountain” scenario may be the main driver (as modeled by e.g., Wada
wad12 (2012), wad15 (2015); Wada et al. wad16 (2016)).

In Fig. 7(1) it may be seen that there is an $\approx 20\arcsec$ H i
absorption ridge emanating from the nucleus, in a south-southeast direction.
This H i ridge can also be followed in the four individual maps shown in Fig.
8, implying that the ridge has a very wide velocity dispersion ($\approx 400$
km s-1). This H i ridge coincides in position with the southern edge of the O
iii cone and may have arisen by the action of collisions of the outflow-
driving X-ray photons with the dust grains. This interaction most likely would
cause the water ice covering the cold dust grain surfaces to be desorbed and
subsequently cause the water vapor to be dissociated into 2H + O. It is here
interesting to note that, according to Fig. 7 of Venturi et al. (ven18
(2018)), the mass outflow rates of ionized gas are observed to be especially
high along P.A. $\approx$ 135° and $\approx$ 160°, that is, in the direction
of the synchrotron radiation and molecular gas jet as well as along the
elusive H i absorption ridge just discussed. Here it may be interesting to
note that the H i ridge may well be accompanied by a low level wide velocity
CO($3-2$) emission ridge at P.A. $\approx 160$°, as hinted in Fig. 7(2).

The very wide velocity dispersion of the H i absorption ridge, mentioned
above, with both negative and positive velocity components is perplexing, but
not completely surprising. Hjelm & Lindblad (hje96 (1996)) found that the
southeastern edge of their O iii cone had also shown a ”split” structure with
both negative and positive velocity components in a velocity range similar to
the H i ridge (see their observations with slits 6, 7, 10, and 11 in their
Fig. 6). This was interpreted as contributions from both the outflow cone and
the disk. A first approximation to the H i column density, $N$(H i), in the H
i ridge can be obtained from $N$(H i)/$T_{\rm s}=1.823\times
10^{18}\times\tau\times\Delta V$ (cm-2K-1), where $T_{\rm s}$ is the spin
temperature, and the optical depth of the H i absorption is
$\tau=-$ln$[1+(I_{\rm H\,{\sc i}})/(I_{\rm c})]$, $I_{\rm H\,{\sc i}}$ (a
negative number) and $I_{\rm c}$ are the intensities of the H i absorption
line and continuum, respectively, in mJy/beam (Sandqvist san74 (1974)). The
beam for the H i observations is 11$\aas@@fstack{\prime\prime}$6 $\times$
6$\aas@@fstack{\prime\prime}$3, and the continuum has been estimated from Fig.
6 of Sandqvist et al. (san95 (1995)) and corrected to the H i resolution. A
second approximation has been made using the continuum observations in Fig. 7
of Beck et al. (bec02 (2002)), also adjusted to the H i resolution. The
results for two positions in the H i ridge are presented in Table 4.

Table 4: H i column densities in the H i absorption ridge. Source | Offsets | $I_{\rm H\,{\sc i}}$ | $\Delta V$ | $I_{1}$ | $I_{2}$ | $\tau_{1}$ | $\tau_{2}$ | $N_{1}/T_{\rm s}$ | $N_{2}/T_{\rm s}$
---|---|---|---|---|---|---|---|---|---
| ($\Delta\alpha$,$\Delta\delta$) | [mJy/beam] | [ km s-1] | [mJy/beam] | [mJy/beam] |  |  | [cm-2K-1] | [cm-2K-1]
H i ridge top | ($+2\arcsec,-9\arcsec$) | $-0.45$ | 380 | 5.2 | 14.5 | 0.091 | 0.032 | $6.3\times 10^{19}$ | $2.2\times 10^{19}$
H i ridge bottom | ($+7\arcsec,-15\arcsec$) | -0.65 | 340 | 5.2 | 14.5 | 0.13 | 0.045 | $8.3\times 10^{19}$ | $2.8\times 10^{19}$

Using the canonical H i spin temperature of 100 K, we may now estimate an H i
column density for the H i ridge of a few $\times 10^{21}$ cm-2. For the
maximum H i integrated emission region in Fig. 7, located near the position of
the H ii hot spot L4, we determine an H i column density of $6\times 10^{22}$
cm-2, using the conversion factor given by Jörsäter & van Moorsel (jor95
(1995)) – a column density value similar to that found for the +50 km s-1
cloud in the Sgr A complex in the center of our own galaxy (Sandqvist san74
(1974)).

In support of our own forthcoming scenario proposal in Sect. 4.3.2, we now
will turn to the very interesting SMA and ALMA results presented by Aalto et
al. (aal12 (2012), aal16 (2016)). Their CO($2-1$) SMA observations at a
resolution of $\approx$ 0.6″ of the lenticular spiral galaxy NGC 1377, at a
distance of 21 Mpc where 1″ $\approx$102 pc, – very similar to the parameters
of NGC 1365 – revealed an extended molecular outflow and an orthogonal nuclear
disk – outflow extent $\approx$200 pc, opening angle of $60\degr-70\degr$,
outflow velocity $\approx$140 km s-1 and mass $\approx(1-5)\times 10^{7}$
$\mathrm{M}_{\odot}$; dynamical versus molecular nuclear disk masses $\approx
4\times 10^{7}$ $\mathrm{M}_{\odot}$ versus $1.7\times 10^{7}$
$\mathrm{M}_{\odot}$).

From their analysis of the ALMA CO($3-2$) observations at 0.2″ resolution, and
maximum recoverable structure size of 5″, Aalto et al. (aal16 (2016))
discovered a very collimated molecular jet, counter-jet system of size $\pm
150$ pc, displaying velocity reversals along the jets with a swing of $\pm
150$ km s-1. This was interpreted and satisfactorily modeled as a precessing
bi-polar jet system, having a precession angle $\approx 10-25$° around a
direction along the plane of the sky, precession period $\approx$ 0.3 – 1.1
Myr, outflow velocity $\approx$ 240 – 850 km s-1, and outflow mass $\approx$
(0.2 – 2)$\times 10^{7}$ $\mathrm{M}_{\odot}$. The outflow rate was estimated
to be $\approx$10 $\mathrm{M}_{\odot}$/yr – a value they considered to be very
uncertain. The molecular mass in the entire $4\arcsec\times 4\arcsec$ map was
estimated to be $16\times 10^{7}$ $\mathrm{M}_{\odot}$. The rotational
velocity of an accretion disk of size $\approx$10 pc was estimated to be
$\approx$110 km/s, which implies a rotation period of $\approx$1 Myr. This in
fact provides a lower limit to the jet precession period, which must be larger
than the rotation period of its launching accretion disk. The more relevant
physical parameters have been collected in our Table 3, for comparison with
our scenario in NGC 1365.

Aalto et al. (aal16 (2016)) propose that their elusive, gaseous precessing
jet, counter-jet system is likely to be powered by faint bipolar “radio jets”,
that is, by relativistic plasma outflows suffering from energy losses by
synchrotron radiation, as was described earlier, or alternatively by an
accretion disk-wind similar to those found toward proto-stars. The latter case
also includes discussions of analogous MHD models of precessing bi-polar
outflows, powered by the accretion onto a proto-star – a jet precession which
has been observed, for example, in L1551 IRS5 and IRAS 16293-2422 (Fridlund &
Liseau fri94 (1994); Kristensen et al. kri13 (2013)). Aalto et al. (aal16
(2016)) also carefully discuss the various scenarios which may cause the
observed molecular jet precession, with at-the-point references. Their favored
alternatives are: i) misalignment between the spin orientation of the SMBH and
the rotation axis of the surrounding accretion disk, or ii) an accretion
inflow of gas having misaligned angular momentum.

####  4.3.2 Tilted massive inner torus of cold gas – a precessing nuclear
engine?

Scrutinizing the various observed and calculated physical parameters collected
in Table 3, we note two problematic issues where nature, however, has provided
elegant and efficient solutions. The first one is the “angular momentum
problem” at the formation of a compact object – in our case a central black
hole, while a very similar problem appears in proto-stellar formation – as
illustrated by the rotation time column. While the rotation time of the galaxy
disk falls in the range $500-40$ Myr, the bar-induced outer torus, having a
radius of $400-1000$ pc and a cold gas mass of $\approx 2\times 10^{9}$
$\mathrm{M}_{\odot}$, is rotating almost as a solid body with a revolution
time of 27 Myr. This outer torus is encircling a much smaller inner torus,
accretion disk of radius 26 pc and estimated cold gas mass of $\approx 7\times
10^{6}$ $\mathrm{M}_{\odot}$ and a rotation time of $\approx 1.6$ Myr – in its
turn surrounding a rapidly spinning SMBH having a mass of $\approx 4\times
10^{6}$ $\mathrm{M}_{\odot}$ (Combes et al. com19 (2019); Risaliti et al.
ris13 (2013)). To overcome the “angular momentum problem” nature has found a
solution here to the energy balance problem in terms of MHD launching of a
relativistic plasma outflow, which is observable because of its synchrotron
radiation loss, and a co-existing collimated molecular gas (CO) outflow. It
has also launched the directly observable X-ray emission from the nuclear
engine, which in addition is causing the observed very extended wide angle
ionized gas outflow by the action of its radiation pressure.

With these facts in mind, we arrive at the second problem in case of NGC 1365.
While the symmetry axis of the hollow wide angle ionized gas outflow appears
to coincide with the rotation axis of the galaxy disk and the outer torus
(P.A. = 130°; Incl. = 50°), the rotation axis of the inner torus, black hole
nuclear engine (P.A. = 160°$\pm$10°; Incl. = 63°$\pm$10°) according to the
model fit of CO($3-2$) ALMA data by Combes et al. (com19 (2019)), deviates by
$\approx$ 30°$\pm$10° from the common direction. The direction of the
synchrotron radiation jet, collimated molecular gas outflow is observationally
known only in terms of its P.A. $\approx$ 135°. However, to allow the
previously outlined powering scenario of the synchrotron radiation jet,
collimated CO outflow, the rotation axis of the nuclear engine must have
agreed with the outflow direction at an earlier time. If we consider these
facts, the only realistic alternative seems to be that the inner torus is
precessing, at an assumed precession angle of $\approx$ 30°, around the common
rotation axis of the galaxy disk and the outer torus. The precession period
must be larger than the inner torus rotation period of 1.6 Myr and presumably
be lower than the rotation period of the outer torus, 27 Myr. The inclination
of the synchrotron radiation, CO jet in this scenario must be $\approx$
50°$\pm$30°, that is, Incl. = 20°, or 80°, where the former case results in an
outflow size of 400 pc, a reasonable outflow velocity of 70 km s-1 and an
outflow age of $\approx$ 6 Myr. The other alternative must be considered
unlikely because of the resulting very large size and age (see Table 3). Since
the rotation axis of the accretion disk must have precessed (in the same
direction as the rotation of the outer and inner tori) by about 75% of its
period from its launching of the synchrotron jet, molecular outflow to its
current position, we may now estimate the precession period of the nuclear
engine to be about 8 Myr, which is well below the rotation period of the outer
torus, 27 Myr.

This scenario solves not only the launching of the synchrotron, CO jet but may
also help to explain the angular mass loss rate inhomogeneity of the wide
angle ionized gas outflow observed by Venturi et al. (ven18 (2018)). In
addition the proposed inner torus precession scenario provides a natural
solution of the problem that the bi-conical outflow is covering a very wide
sector (P.A. $\approx$ 80° $-$ 180°) even at a radius of 2.5 kpc, which
corresponds to a launch time of $25-50$ Myr at an estimated outflow velocity
of $100-50$ km s-1 and is hollow, that is, with little or no outflow near the
symmetry axis (cf. Hjelm and Lindblad hje96 (1996)). This is not well
accommodated by state of the art theoretical modeling of nuclear outflow
fountains (e.g., Wada wad15 (2015)). A precession angle of $\approx$ 25 $-$
30° and a bi-conical outflow sector of $\pm$25° should here be able to do the
required job of supporting the observed ionized outflow, created during
several precession periods.

At first sight a precession angle as large as 20° $-$ 40° may seem
unrealistic. However, the ALMA observations of NGC 1377 by Aalto et al. (aal16
(2016)), paired with their careful analysis, lead to a required accretion disk
precession angle of 10° $-$ 25°. The physical reason for the suggested inner
torus precession in NGC 1365 could be “an accretion inflow of gas having
misaligned angular momentum”, which is the previously mentioned second
alternative for NGC 1377 of Aalto et al. (aal16 (2016)), since the outer torus
of NGC 1365 is asymmetric and very inhomogeneous in its mass distribution. As
shown in Table 3, the accretion tori of NGC 1365 and NGC 1377 have similar
sizes, rotation times and cold gas masses.

###  4.4 Discussion of the H2O emissions in NGC 1365 versus those in other
galaxies

The outer circumnuclear torus region and the nuclar region contain all the
ingredients necessary to locally raise the gas-phase water abundance to levels
which are several orders of magnitude higher than the quiescent, cold cloud
value of $8\times 10^{-10}$ versus H2, assuming an ortho-to-para ratio of 3.
Here most H2O is likely to reside as water ice on cold grain surfaces (e.g.,
van der Tak et al. tak10 (2010)).

We have in Table 5 collected the results from our spatially unresolved but
spectrally resolved Odin observations of the 557 GHz ground state o-H2O line.
There are also the results from spectrally unresolved, but spatially partly
resolved, Herschel SPIRE multi-transition H2O and CO mapping observations, for
comparison with: i) Odin ground state o-H2O and o-H${}_{2}^{18}$O results for
our Galactic center region, Sgr A (Karlsson et al. kar13 (2013)), ii) Herschel
HIFI, SPIRE, and PACS results for the merger NGC 6240 (with galaxy-wide
shocks; Meijerink et al. mei13 (2013), Liu et al. liu17 (2017)), and iii) the
results from a detailed radiative transfer and excitation analysis of a sample
of other galaxies extensively observed by Herschel HIFI, SPIRE, and PACS (Liu
et al. liu17 (2017); Gonzalez-Alfonso et al. gon10 (2010), gon12 (2012)).

In addition to the comparisons between galaxies provided in Table 5, we have
in an Appendix B included a Table B.1 comparing our estimated H2O abundances
in NGC 1365 with more detailed Sgr A results, as well as with observed H2O
abundances in a number of well-known types of Galactic molecular cloud
regions, where also detailed chemical models are established. For the
interested reader a short summary of current chemical models of water
formation and destruction is also included in this Appendix.

Table 5: Comparison of NGC 1365 and other galaxies.

Object | Region | Density | Gas / Dust | H2O abundance | Comments
---|---|---|---|---|---
|  | (cm-3) | Temperature (K) | vs H2 ($\times 10^{-8}$) |
NGC 1365 | absorptionaafootnotemark: $a$ | $10^{2}-10^{4}$ |  $\approx 20$ccfootnotemark: $c$ | $5-8$ | A
(Barred | emissiona,ba,bfootnotemark: $a,b$ | $10^{4}-10^{6}$ |  $40-60/55$ccfootnotemark: $c$ |  $6-600$j,kj,kfootnotemark: $j,k$ | A
spiral) | shocksbbfootnotemark: $b$ |  $10^{4}-10^{5}$ddfootnotemark: $d$ |  $350/22-32$ccfootnotemark: $c$ |  $200-10,000$nnfootnotemark: $n$llfootnotemark: $l$ | B
Sgr Aee$e$Sandqvist et al. (san95 (1995)). | absorption |  $10^{2}-10^{3}$iifootnotemark: $i$ |  $100-30$iifootnotemark: $i$/15mmfootnotemark: $m$ | 3 | C
| emission |  $10^{4}-10^{5}$fffootnotemark: $f$ | 80fffootnotemark: $f$/$20-30$mmfootnotemark: $m$ | $2-7$ | D
| line wings | $3\times(10^{4}-10^{5})$ |  $80-160$/$20-30$mmfootnotemark: $m$ | $80-570$ | E
NGC 6240 | warm emission | $10^{6}$ | $60-70/60-70$ | 10 | F
(Mergerg,hg,hfootnotemark: $g,h$) | shocks |  $5\times(10^{4}-10^{6})$ddfootnotemark: $d$ | $120-400/20-30$ | 10 | G
Galaxy | absorption | $10^{3}-10^{5}$ | $20-200/20-30$ | $0.1-10$ | H, J
samplehhfootnotemark: $h$ | cold emission | $10^{3}-10^{5}$ | $20-30/20-30$ | $0.1-1$ | H
| warm emission | $10^{5}-10^{6}$ | $40-70/40-70$ | $1-10$ | H
| hot emision | $10^{5}-10^{6}$ | $100-200/100-200$ | $100-1000$ | K

(a) This paper – Odin observations of ground state o-H2O (see text). (b) This
paper – Herschel SPIRE mapping observations plus modeling (see text). (c) Dust
temperatures from Alonso-Herrero et al. (alo12 (2012)) and Tabatabei et al.
(tab13 (2013)). (d) Pre-shock density; The post-shock density becomes an order
of magnitude higher. (e) from Odin observations of ground state o-H2O and
o-H${}_{2}^{18}$O, assuming an ortho-to-para ratio of 3 (Karlsson et al. kar13
(2013)). (f) from Walmsley et al. (wal86 (1986)). (g) from Herschel SPIRE
oservations (Meijerink et al. mei13 (2013)). (h) from Herschel HIFI, SPIRE and
PACS observations (Liu et al. liu17 (2017)). (i) from the Sandqvist et al.
(san15 (2015)) Herschel HIFI 487 GHz search for O2. (j) the abundance scales
$\approx$ inversely with cloud density (k) lower abundances may be accomodated
by PDR models, while the high ones indicate shocks (see Comments). (l) fast
shock model abundance (Flower & Pineau de Forêts flo10 (2010)) in an effective
accumulated 10″ size area (see Comments). (m) from Sandqvist et al. (san08
(2008)). (n) from slow shock chemistry, in case FIR pumping dominates
(Gonzalez-Alfonso et al. (gon10 (2010), gon12 (2012), gon14 (2014)).

####  4.4.1 Comments to Table 5

NGC 1365:

A) Odin observes very weak ground state o-H2O emission from dense molecular
cloud complexes located in the circumnuclear torus region in the velocity
range $1400-1650$ km s-1 – an emission which in a velocity range around 1500
km s-1 suffers from absorption caused by the low excitation, foreground cloud
envelopes of the cloud core emission plus the thermal dust continuum
background. Taking into account the source size, approximately 15″, and
location of maximum emission apparent from the Herschel SPIRE mapping, we find
that the integrated H2O intensities observed by Odin and Herschel agree very
well, and that the velocity range of the Odin emission is as expected for
emission from the NE torus region.

The existence of water absorption by lower excitation cloud envelopes most
likely can be ascribed to the PDR chemistry, including grain surface
reactions, in action in such regions. The water emission from the massive
complexes of cloud cores, co-located with super starburst clusters and H ii
regions residing in the NE torus region of NGC 1365 (cf. Elmegreen et al.
elm09 (2009)), is bound to originate from a combination of PDR chemistry,
including grain surface reactions, quiescent warm molecular cloud ion-molecule
chemistry, and also hot cores. There is possibly influence by an increased
ionization level caused by cosmic ray focusing by the aligned magnetic field,
as mapped by Beck et al. (bec05 (2005)), and locally also by X-ray emission
from the star forming superclusters. Outflow and shock chemistry may also
contribute to the ground state o-H2O emission (see next Comment).

B) The CO and H2O emissions from the higher energy states appear to be best
explained by shock excitation. While the CO-SLED can be nicely matched by a
two-component model using $40-60$ K for the lower energy lines and requiring a
low velocity ($\approx 10$ km s-1) shock at a pre-shock density of $10^{4}$
cm-3 to reach the observed intensities of the higher energy lines. The
H2O-SLED seems to require a high velocity ($\approx 40$ km s-1) shock at a
pre-shock density of about $10^{4}$ cm-3, using the shock model by Flower &
Pineau de Fôrets (flo10 (2010))( cf. Comment G for the merger NGC 6240). There
is probably a requirement also for some radiative Far IR (FIR) excitation,
which was used to model the Herschel SPIRE observations of H2O in the ULIRGs
Mrk 231, NGC 4418 and Arp 220 by González-Alfonso et al. gon10 (2010), gon12
(2012), as discussed in Sect. 4.2.2.

If FIR excitation were dominating, the required H2O abundance would be a
factor of $10-100$ times lower than the fast shock (40 km s-1) model abundance
of $10^{-4}$ versus H2 (Gonzalez-Alfonso et al. gon10 (2010), gon12 (2012),
gon14 (2014); Flower & Pineau de Forêts flo10 (2010)). This is an abundance
level achievable by low velocity shock chemistry including shock release of
icy grain mantles. However, no PACS observations of the FIR pumping water
vapor absorption lines are available in case of NGC 1365.

Sgr A:

C) This is absorption in the low excitation $-30$ km s-1 foreground spiral
arm. The PDR chemistry, including grain surface reactions, is the likely
explanation for the observed water abundance. However, the possible Herschel
HIFI detection of the 487 GHz O2 line in this spiral arm tells us that a
($\approx 20$ km s-1) Galactic spiral arm density wave shock also may be in
action (cf. Sandqvist et al. 2015).

D) The emissions from the warm and dense +20 km s-1, +50 km s-1, and CND
(circumnuclear disk, torus) cloud cores are resulting from PDR chemistry,
including grain surface reactions, quiescent cloud ion-molecule chemistry, and
possibly hot cores.

E) These H2O line wings, not suffering from self-absorption, associated with
the +20 km s-1, +50 km s-1, and CND clouds must be caused by shock chemistry
as well as shock desorption, sputtering from icy grain mantles.

NGC 6240:

F) This is emission from warm cloud complexes in the merger interface region.

G) This is most likely emission from “galaxy-wide shocks”. Meijerink et al.
(mei13 (2013)) found that the CO-SLED, observed by Herschel SPIRE, is best
modeled by a ($\approx 10$ km s-1) shock excitation and a pre-shock density of
$5\times 10^{4}$ cm-3, leading to a post-shock density of $4\times 10^{5}$
cm-3. They also point out that the 2 $\mu$m H2 emission, observed at the
merger central region, requires a shock in the velocity range $47-16$ km s-1
for a pre-shock density of $5\times 10^{5}-10^{7}$ cm-3. See also the H2O SLED
analysis by Liu et al. (liu17 (2017)) and the discussion of NGC 6240 in their
Appendix.

Other galaxies observed by HIFI, SPIRE and PACS aboard Herschel Space
Observatory:

H) These results were based upon multi-transition CO and H2O SLEDs as well as
dust SED observations by Herschel HIFI, SPIRE, and PACS, paired with extensive
radiative transfer and excitation modeling, including collisional as well as
radiative excitation, see Liu et al. (liu17 (2017)).

J) Similarly to what is visualized by the Odin 557 GHz ground state o-H2O
spectrum of NGC 1365, emission plus absorption is often seen in the observed
Herschel HIFI ground state o-H2O spectra, and are even more frequent in the
Herschel HIFI 1113 GHz ground state p-H2O spectra (van der Tak et al. tak16
(2016); Liu et al. liu17 (2017)). It may be interesting to note that the 1113
GHz line is not clearly visible and not claimed to be detected in our NGC 1365
SPIRE data.

K) Only for the most infrared luminous galaxies Mrk 231, NGC 4418, and Arp
220, these compact hot gas, dust components were required to fit the
observational data. Here the high temperature, possibly caused by shocks,
leads to efficient release of H2O from the icy grain surfaces into the gas
phase, hence explaining the high gas phase abundance of water. In addition,
X-ray emission from the AGN, or from a nuclear star burst, may provide a
contribution by XDR chemistry. FIR excitation appears to be very important in
these galaxies (Liu et al. liu17 (2017); Gonzalez-Alfonso et al. gon10 (2010),
gon12 (2012)).

####  4.4.2 Some conclusions concerning the H2O emission and absorption
regions in galaxies

The ortho and para ground state rotational transitions of H2O at 557 and 1113
GHz often exhibit sharp absorption features caused by lower excitation
foreground gas – where the water vapor content of $\approx 10^{-9}-10^{-8}$
versus H2 is resulting from PDR chemistry – intersecting broad velocity
emission from the denser molecular cloud ensemble – caused by a mixture of PDR
and “hot core” chemistries and possibly also is influenced by shock sputtering
and shock chemistry. These processes all rely upon the release of icy grain
mantles and result in average H2O abundances of $\approx 10^{-8}-10^{-7}$.

The rotationally excited state H2O emissions, observed by Herschel HIFI and
SPIRE, appear to be best explained by a combination of FIR and shock
excitation, where FIR excitation is likely to be dominating in the ULIRGs.
Shock chemistry and shock release of icy grain mantles are the likely
processes causing the estimated very high H2O abundances of $\approx
10^{-6}-10^{-4}$, with possible influences from “hot core chemistry” in
regions of very extensive star formation.

The FIR H2O lines at shorter wavelengths, observed by Herschel PACS, all have
very high critical densities for collisional excitation and therefore are
bound to be “FIR pumped”. The very high H2O abundances of $10^{-6}-10^{-5}$,
estimated from these observations, are likely to result from efficient release
of icy grain mantles, caused by varying combinations of “hot cores” near
massive young stars, shock sputtering, and XDR chemistry caused by intense
X-ray emission, as well as shock chemistry – all depending upon the
environmental conditions in the galaxy in question.

###  4.5 Methylidyne radical, CH, and its ion CH+ – probable signatures of
shocks and intense UV-illumination in the NGC 1365 circumnuclear torus region

We here enter a discussion of the presence of CH as well as CH+ in the
circumnuclear torus region, as is revealed by the SPIRE spectrum (Fig. 3 and
Table 1) and by our CH and CH+ maps, shown in Fig. 4. While an emission line
at 835.1 GHz nicely matches the $J=1-0$ rotational transition of CH+ in its
${}^{1}\Sigma$ electronic ground state, CH in its ${}^{2}\Pi$ electronic
ground state appears in terms of two emission doublets, at 532.7/536.8 GHz and
1470.7/1477.4 GHz, classified as the ($1^{+}_{3/2}$ –¿
$1^{-}_{1/2}/1^{-}_{3/2}$ –¿ $1^{+}_{1/2}$) and ($2^{-}_{3/2}$ –¿
$1^{+}_{3/2}/2^{+}_{3/2}$ –¿ $1^{-}_{3/2}$) transitions, respectively (with
quantum states denoted $N_{J}^{parity}$. More details are given in Appendix C,
and an energy level diagram can be found in Rangwala et al. ran14 (2014)).
These CH transitions are caused by a step-wise internal energy decay of the CH
molecular population from the $\Lambda$-doubled second lowest ($N=2,J=3/2$)
rotationally excited state (at $E_{u}\approx 96.5$ K) to the $\Lambda$-doubled
lowest ($N=1,J=3/2$) rotationally excited state (at $E_{u}\approx 25.7$ K),
and subsequently to the $\Lambda$-doubled ${}^{2}\Pi$ ($N=1,J=1/2$) CH ground
state. Here the rather obvious, and simplest, contributing excitation
mechanism would be “direct radiative pumping” causing population transport
from the $N=1,J=1/2$ ground state $\Lambda$-doublet state to the $N=2,J=3/2$
rotationally excited $\Lambda$-doublet state by means of CH absorptions of the
thermal dust emission at 2006.8/2010.8 GHz ($\approx 149$ $\mu$m), that is,
close to the thermal emission maximum of dust at a temperature of $\approx 20$
K, as is observed in NGC 1365 (Figs. 1 and 2). However, no proof in terms of
such observed 149 $\mu$m CH absorptions is available in NGC 1365, but appears
to have been observed in the ($L_{\rm FIR}\approx
10^{12}$$\mathrm{L}_{\odot}$) ULIRG, Arp 220, by Herschel PACS, as reported in
the SPIRE and HIFI based CH analysis by Rangwala et al. (ran14 (2014)).

Arp 220 is a late-stage merger with two closely located counter-rotating disks
and also was included in the multi-line H2O analysis by Liu et al. (liu17
(2017)), where it together with Mrk 231 required a hot and dense gas component
to fit the observed H2O-SLED (cf. our Table 5 and its comments). Stepwise
collisional CH excitation may also be contributing processes, which then would
require the high gas densities and temperatures resulting from the shocks
which are observed to be in action in the many hot spots of the outer
circumnuclear torus of NGC 1365 (Galliano et al. gal12 (2012); Fazeli et al.
faz19 (2019)) and here were crucial for the H2O and CO excitation (see Sect.
4.2.2). Extensive shocks and star formation-induced outflows also are likely
to be present in Arp 220, as well as in the IR bright ($L_{\rm FIR}\approx
2\times 10^{11}$$\mathrm{L}_{\odot}$) Seyfert 2 galaxy NGC 1068 (Spinoglio et
al. spi12 (2012); Liu et al. liu17 (2017)), which will be our forthcoming CH,
CH+ comparison galaxy .

The CH assignment of the striking 532.7/536.8 GHz line doublet requires some
carefulness, since unfortunately the laboratory frequencies of the HCN($6-5$)
and HCO+($6-5$) lines at 531.7 and 535.1 GHz are almost overlapping,
considering the poor SPIRE spectral resolution and the broad galaxy spectral
lines. However, there are no visible signs of the HCN($7-6$) and HCO+($7-6$)
lines in our NGC 1365 SPIRE spectrum, which limits their contribution to our
CH doublet to be at most about 20 %. This is consistent with the comparatively
small contributions from HCN and HCO+ found by Rangwala et al. (ran14 (2014))
in their complementary Herschel HIFI observations of a number of galaxies,
including Arp 220.

From the Herschel SPIRE line intensities listed in Table 1, we estimate the
upper state column densities of the 533/537 GHz and 1471/1477 GHz CH doublets
to be $23\times 10^{12}$ cm-2 and $15\times 10^{12}$ cm-2, for an assumed,
accumulated source size of 14″. We can then determine an excitation
(population distribution) temperature for these states to be $T_{\rm
ex}\approx 75$ K. This allows us to calculate the total column density in the
four lowest rotational states to be $\approx 8\times 10^{13}$ cm-2, under the
assumption of a common excitation temperature. The estimated excitation
temperature is in fact a lower limit since the 533/537 GHz doublet may contain
some HCN/HCO+ emission and is at least as high as the kinetic temperature of
the quiescent molecular clouds, which suggests influences of FIR pumping
and/or efficient collisional excitation in the observed shock regions.

In the same way, we estimate the upper ($J=1$) state column density of CH+ to
be $1.6\times 10^{12}$ cm-2, for an assumed source size of 14″ (Fig. 4). The
minimum total CH+ column density is estimated to be $\approx 3.5\times
10^{12}$ cm-2 for excitation temperatures in the range 40 to 100 K (cf.
Falgarone et al. 2010b ). However, in view of the intense UV-illumination from
newly formed stars, the CH+ column density may well be an order of magnitude
larger, approaching that of CH. The reason is formation by the reaction
between C+ and vibrot-excited H2, which also causes a large enhancement of the
rotationally excited CH+ states, as has been observed in the Orion Bar (Nagy
et al. nag13 (2013), nag17 (2017); see also Appendix C). In the case of NGC
1365 we have to rely on the observed existence of multiple vibrot H2 lines in
the hot spots of the outer torus region, and the existence of strong UV
radiation in these regions of intense star formation (Galliano et al. gal12
(2012); Fazeli et al. faz19 (2019)). For these reasons we may estimate the
CH/CH+ abundance ratio to be $\lid 23$.

From the simultaneously observed SPIRE maps (Fig. 4), it appears that the H2O
and CH+ emissions peak in the NE torus region while the CO and CH emissions
are more evenly distributed across the circumnuclear torus. The higher energy
CO-SLED is nicely modeled by a low velocity (10 km s-1) shock, which also may
satisfactorily explain the required CH excitation as well as its high
abundance in denser gas (see Appendix C). The higher velocity (40 km s-1)
shock, possibly required to model the H2O-SLED in the NE torus region, paired
with the intense UV radiation from the observed massive young stellar
superclusters (Galliano et al. gal12 (2012)) may explain the previously
discussed possible overabundance of CH+ (Gerin et al. ger16 (2016); Godard et
al. god19 (2019); see also Appendix C).

With these uncertainties in mind, we now proceed to estimate the CH and CH+
abundances. Here a comparison H2 column density is available from our multi-
transition SPIRE-CO modeling in Sect. 4.2.2. Correcting the model value
$N$(CO) $\approx 4\times 10^{15}$ cm-2 for the beam filling of a 14″ source
size in a 40″ antenna beam, and assuming a CO/H2 abundance ratio of $10^{-4}$,
we arrive at an average of $N$(H2) $\approx 4\times 10^{20}$ cm-2 for the
shocked gas regions of the outer torus. The CH and CH+ abundances then become
$X$(CH) $\approx 2\times 10^{-7}$ and $X$(CH+) $\approx 10^{-8}$. In the
unlikely case that the emissions were originating in the quiescent molecular
clouds, these abundances would be lower by a factor of about 30.

Only in NGC 1068, out of the four prototypical AGN or star burst dominated
galaxies (NGC 1068, Arp 220, M82 and NGC 253) selected for Herschel CH
observations by Rangwala et al. (ran14 (2014)), did both the CH and the CH+
lines appear in emission, similar to our case of NGC 1365. In the other
galaxies, CH+ was observed in absorption, presumably from lower excitation
foreground gas, while CH was observed in emission (from higher density
regions). Our IR luminous comparison galaxy, NGC 1068 ($L_{\rm FIR}\approx
2\times 10^{11}$ $\mathrm{L}_{\odot}$), is exhibiting a number of similarities
with NGC 1365 ($L_{\rm FIR}\approx 5\times 10^{10}$ $\mathrm{L}_{\odot}$),
that is, a circumnuclear torus – a prominent “star forming molecular cloud
ring” of radius $1-1.5$ kpc versus $\approx 0.8$ kpc in NGC 1365 – and a
smaller circumnuclear disk – CND of radius $100-150$ pc versus $\approx 26$ pc
in NGC 1365 – as discussed in more detail by Spinoglio et al. (spi12 (2012))
and Liu et al. (liu17 (2017)).

The CH abundance estimated in NGC 1068 by Rangwala et al. (ran14 (2014)), from
the 533/537 GHz doublet alone, is $\approx 10^{-8}$ versus H2. The CH/CH+
abundance ratio is found to be $\approx 23$, just as in NGC 1365, however in
both cases somewhat uncertain values because of lacking observations of the
molecular excitation. The NGC 1068 abundances were nicely accommodated by XDR
physics, chemistry in the CND, driven by the X-ray emission from the AGN. A
similar inner torus generated XDR emission of CH and CH+ is likely to exist
also in NGC 1365. However, here the dominant parts of the CH and CH+ emissions
are located to the UV-illuminated shock regions of the outer torus, as is
strongly indicated by our observed emission maxima at the NE torus FIR and
molecular gas mass peak. In the absence of Herschel Space Observatory,
improved knowledge of the CH and CH+ molecular population distributions in NGC
1365, as well as in NGC 1068, could here be achieved by spectrally resolved
Sofia limited mapping observations of the CH doublet at 2 THz and the CH+
($J=2-1$) transition at 1669 GHz.

In this context the results from Herschel PACS observations of multiple OH+,
H2O+, and H3O+ absorption lines in the ULIRGs, NGC 4418 and Arp 220, by
Gonzalez-Alfonso et al. (gon13 (2013)) are useful to contemplate, since the
importance of an observed molecular population distribution is emphasized. It
turned out that in the single case of Arp 220 the pure inversion, metastable
state lines of H3O+ revealed a very high population distribution temperature
of $\approx 500$ K, indicating hot gas or “formation pumping”. There, excess
energy from the H3O+ formation process leaves the resulting molecule in a high
rotational temperature molecular state distribution). The population
distributions of OH+ and H2O+ did not show such an anomaly. These results are
very important for the understanding of the molecular formation processes in
question. However, no OH+, H2O+ or H3O+ signals can be identified in our NGC
1365 SPIRE spectrum (Fig. 3 and Table 1).

In the light of our current discussion of CH and CH+ observations and their
modeling interpretations, and including the results from the very recent
modeling of UV irradiated molecular shocks by Godard et al. (god19 (2019)),
inspired by the ALMA discovery of CH+($1-0$) emission and absorption lines
from a number of lensed star burst galaxies at a redshift of $\approx 2.5$
(Falgarone et al. fal17 (2017)) – we are inclined to believe that the CH and
CH+ emission lines observed from the circumnuclear torus region of NGC 1365
are signposts of shock action in this region, presumably penetrated by intense
UV light and molecular outflows caused by the observed extensive star
formation.

##  5 Summary and conclusions

We have used the Odin satellite to observe the central region of the barred
spiral galaxy NGC 1365 in the 557 GHz H2O line. After a total of 81 hours of
ON-source integration time, we have obtained a marginal detection of this
water vapor line at a velocity resolution of 5 km s-1. We have combined these
observations with Herschel PACS and SPIRE observations of two positions in the
galaxy’s nucleus, obtained from the Herschel Science Archive, and produced
simultaneously observed SPIRE images of the distribution of o-H2O, p-H2O, CO,
CH, CH+, and N ii.

The water vapor emission is predominantly located in the shocked 15″ (1.3 kpc)
region of the northeastern (NE) component of the molecular torus surrounding
the nucleus. Here, several compact radio sources, hot-spot H ii regions, as
well as co-located very massive molcular cloud complexes, and young stellar
superclusters have been observed – all of which triggered by the rapid bar-
driven inflow into the circumnuclear torus, causing cloud-cloud collisions and
shocks. The two ground state H2O emission components, detected in the Odin
spectrum, have approximate velocities of –150 and –40 km s-1 with respect to
the systemic velocity of the galaxy, while an intersecting absorption appears
at a velocity of –100 km s-1, which agrees with kinematic studies of the bar-
driven circumnuclear torus gas flow in the vicinity of the NE component. For
the absorbing gas region, we find an H2O fractional abundance of $\gid 5\times
10^{-8}$, which is well accomodated by PDR chemistry. In the presumably denser
emission regions, the H2O abundance appears to increase but may still be
understood in terms of PDR chemistry taking place in this region of strong UV
radiation from the young super star clusters. However, part of the ground
state H2O emission may also be caused by shock excitation and chemistry, as
will be discussed next.

Model studies of the multi-transition SPIRE CO and H2O observations toward the
NE component reveal the presence of a slow-velocity C-type shock with a shock
speed of 10 to 40 km s-1 and an H2 pre-shock density of 104 cm-3, where the
lower velocities are sufficient to model the observed CO spectral line energy
distribution (SLED). The observed H2O SLED requires the higher shock
velocities, which are also sufficient to drive the vibrational excitation of
H2. In addition, FIR pumping may be required to satisfactorily explain the H2O
SLED. A model abundance versus H2 of 10-4 was reached for CO as well as for
H2O. If the H2O excitation were dominated by FIR pumping, the required H2O
abundance is lower by a factor of $10-100$ and is most likely caused by slow
velocity shock chemistry (as discussed in Sect. 4.2.2 and Appendix B ).

While the H2O and CH+ emissions seem to be concentrated near the NE torus
component, the CO and CH emissions appear to be more evenly distributed
between the NE and the SW torus component, as revealed by the SPIRE images.
These relative distributions may reflect the fact that the eastern bar-driven
gas inflow into the NE torus region is much more massive than the
corresponding gas inflow from the western bar into the SW torus region. In the
case of CH we estimate an abundance of $2\times 10^{-7}$. The estimated CH+
abundance of $\approx 10^{-8}$ in the NE torus region is likely to be a lower
limit because of (here unconfirmed) formation via the abundant vibrationally
and rotationally excited H2 in this shock and UV irradiated region. Such a CH+
abundance enhancement has indeed been observed in the Orion Bar (See Appendix
C).

We have made a statistical image deconvolution (SID) of our single dish SEST
observations of the CO($3-2$) emission in the nuclear region of NGC 1365,
yielding an angular resoltuion of 5″, complementing recent ALMA observations.
These SID observations yield dynamical masses of $1.2\times 10^{10}$
$\mathrm{M}_{\odot}$ and $4.4\times 10^{10}$ $\mathrm{M}_{\odot}$ inside radii
of 1 and 2 kpc, respectively, where the gas mass amounts to $\approx 20$% in
the inner region and $\approx 6$% in the region inside 2 kpc.

We discuss a co-located collimated CO jet, a relativistic radio jet and a
wide-angle inhomogeneous ionized O iii gas cone outflow from the nuclear
engine. Our VLA H i absorption analysis reveals an H i ridge extending from
the nucleus in a south-southeast direction, coinciding with the southern edge
of the O iii cone. With an H i column density of a few $\times 10^{21}$ cm-2,
this ridge may have been created by the action of outflow-driving X-ray
photons colliding with dust grains, causing water ice from the dust grains to
be desorbed and dissociated into 2H + O. A precessing nuclear engine appears
to be required to accomodate the various outflow components, and is also
consistent with the ALMA observations by Combes et al. (com19 (2019)) of a
tilted massive inner gas torus or accretion disk.

###### Acknowledgements.

First of all we wish to thank Per Olof Lindblad for many valuable discussions
and his numerous comments on the manuscript of this paper. We also wish to
thank the anonymous referee for the careful reading of the manuscript and the
many constructive comments, which much improved our paper. For valuable
contributions in the late-stage preparation of the Odin satellite observing
program and its successful operation, we wish to thank H.-G. Floren at
Stockholm Observatory, M. Battelino and B. Jakobsson at OHB Sweden, G. Persson
at Onsala Space Observatory and S.-O. Silverlind at the Swedish Space
Corporation in Esrange.

## References

  * (1) Aalto, S., Hüttemeister, S., Scoville, N. Z., & Thaddeus, P. 1999, ApJ, 522, 165
  * (2) Aalto, S., Muller, S., Sakamoto, K, et al. 2012, A&A, 546, A68
  * (3) Aalto, S., Costagliola, F., Muller, S., et al. 2016, A&A, 590, A73
  * (4) Alloin, D., Edmunds, M. G., Lindblad, P. O., & Pagel, B. E. J. 1981, A&A, 101, 377
  * (5) Alonso-Herrero, A., Sanchez-Portal, M., Ramos Almeida, C., et al. 2012, MN, 425, 311
  * (6) Arce, H. G., Shepherd, D., Gueth, F., et al. 2007, Molecular Outflows in Low- and High-Mass Star-forming Regions, in Protostars and Planets V, ed. B. Reipurth, D. Jewitt, & K. Keil, (University of Arizona Press, Tucson), 245
  * (7) Armijos-Abendano, J., Martin-Pintado, J., Requena-Torres, M. A., et al. 2019, arXiv1902.05098
  * (8) Audibert, A., Combes, F., García-Burillo, S., et al. 2019, A&A, 632, A33
  * (9) Beck, R., Shoutenkov, V., Ehle, M., et al. 2002, A&A, 391, 83
  * (10) Beck, R., Fletcher, A., Shukurov, A., et al. 2005, A&A, 444, 739
  * (11) Bergin, E. A., Melnick, G. J., & Neufeld, D. A. 1998, ApJ, 499, 777
  * (12) Black, J. H., & Dalgarno, A. 1973, ApL, 15, 79
  * (13) Blandford, R. D., & Begelman, M. C. 1999, MNRAS 303, L1
  * (14) Bohlin, R. C., Savage, B. D., & Drake, J. F. 1978, ApJ, 224, 132
  * (15) Combes, F., Garcia-Burillo, S., Audibert, A., et al. 2019, A&A, 623, A79
  * (16) Curran, S. J., Polatidis, A. G., Aalto, S., & Booth, R,. S. 2001, A&A, 368, 824
  * (17) Elmegreen, B. G., Galliano, E., & Alloin, D. 2009, ApJ, 703, 1297
  * (18) Falgarone, E., Godard, B., Cernicharo, J., et al. 2010a, A&A, 521, L15
  * (19) Falgarone, E., Ossenkopf, V., Gerin, M., et al. 2010b, A&A, 518, L118
  * (20) Falgarone, E., Zwaan, M. A., Godard, B., et al. 2017, Nature, 548, 430
  * (21) Farquhar, P. R. A., Millar, T. J., & Herbst, E. 1994, MNRAS, 269, 641
  * (22) Fazeli, N., Busch, G., Valencia-S., M., et al. 2019, A&A, 622, 128
  * (23) Flower, D. R. & Pineau des Forêts, G. 2003, MNRAS, 343, 390
  * (24) Flower, D. R. & Pineau des Forêts, G. 2010, MNRAS, 406, 1745
  * (25) Flower, D. R., Le Bourlot, J., Pineau des Forêts, G., & Cabrit, S. 2003, MNRAS, 341,70
  * (26) Fridlund, C.V.M.; & Liseau, R. 1994, A&A, 292, 631
  * (27) Frisk, U., Hagström M., Ala-Laurinaho, J., et al. 2003, A&A, 402, L27
  * (28) Galliano, E., Alloin, D., Pantin, E., et al. 2005, A&A, 438, 803
  * (29) Galliano, E., Alloin, D., Pantin, E., et al. 2008, A&A, 492, 3
  * (30) Galliano, E., Kissler-Patig, M., Alloin, D., & Telles, E. 2012, A&A, 545, A10
  * (31) Garcia-Burillo S., Combes, F., Usero, A., et al. 2014, A&A, 567, A125
  * (32) Gerin, M., de Luca, M., Goicoechea, J. R., et al. 2010, A&A, 521, L16
  * (33) Gerin, M., Neufeld, D. A., & Goicoechea, J. R. 2016, ARA&A, 54, 181
  * (34) Godard, B., & Cernicharo, J. 2013, A&A, 550, A8
  * (35) Godard, B., Pineau des Forêts, G., Lesaffre, P., et al. 2019, A&A, 622, A100
  * (36) Goldsmith, P. F., & Langer, W. D. 1978, ApJ, 222, 881
  * (37) González-Alfonso, E., Fischer, J., Isaak, K., et al. 2010, A&A, 518L, 43
  * (38) González-Alfonso, E., Fischer, J., Graciá-Carpio, J., et al. 2012, A&A, 541, A4
  * (39) González-Alfonso, E., Fischer, J., Bruderer, S., et al. 2013, A&A, 550, A25
  * (40) González-Alfonso, E., Fischer, J., Aalto, S., & Falstad, N. 2014, A&A, 567, A91
  * (41) Herbst, E., & Leung, C. M. 1986a, MNRAS, 222, 689
  * (42) Herbst, E., & Leung, C. M. 1986b, ApJ, 310, 378
  * (43) Hjalmarson, Å.; & Friberg, P. 1988, ASIC, 241, 65
  * (44) Hjalmarson, Å., Sume, A., Elldér, J., et al. 1977, ApJS, 35, 263
  * (45) Hjalmarson, Å., Frisk, U., Olberg, M., et al. 2003, A&A, 402, L39
  * (46) Hjalmarson, Å., Bergman, P., Biver, N., et al. 2005, AdSpR, 36, 1031
  * (47) Hjelm, M., & Lindblad, P.O. 1996, A&A, 305, 727
  * (48) Hollenbach, D., Kaufman, M. J., Bergin, E. A., & Melnick, G. J. 2009, ApJ, 690, 1497
  * (49) Hollenbach, D., Kaufman, M. J., Neufeld, D., Wolfire, M., & Goicoechea, J. R. 2012, ApJ, 754, 105
  * (50) Irvine, W. M., Goldsmith, P. F., & Hjalmarson, Å. 1987, ASSL, 134, 561
  * (51) Jörsäter, S., & van Moorsel, G. A. 1995, AJ, 110, 2037
  * (52) Karlsson, R., Sandqvist, Aa., Hjalmarson, Å., et al. 2013, A&A, 554, A141
  * (53) Kristen, H., Jörsäter, S., Lindblad, P. O., & Boksenberg, A. 1997, A&A, 328, 483
  * (54) Kristensen, L. E., Ravkilde, T. L., Field, D., Lemaire, J. L., & Pineau des Forêts, G. 2007, A&A 469, 561
  * (55) Kristensen, L. E., Klaassen, P.D., Mottram, J.C., Schmalzl, M., & Hogerheijde, M. R. 2013, A&A, 549, L6
  * (56) Königl, A., & Pudritz, R. E. 2000, Disk Winds and the Accretion-Outflow Connection, in Protostars and Planets IV, ed. V. Mannings, A. P. Boss, & S. S. Russell, (University of Arizona Press, Tucson), 759
  * (57) Langer, W. D., Goldsmith, P. F., & Pineda, J. L. 2016, A&A, 590, 43
  * (58) Lena, D., Robinson, A., Storchi-Bergmann, T., et al. 2016, MNRAS, 459, 4485
  * (59) Lin, C. C., & Shu, F. H. 1964, ApJ, 140, 646
  * (60) Lindblad, P. O. 1960, Stockholms observatoriums annaler, 21, bd 3
  * (61) Lindblad, P. A. B., Lindblad, P. O., & Athanassoula, E. 1996, A&A, 313, 65
  * (62) Lindblad, P. O., Hjelm, M., Högbom, J., et al. 1996, A&AS, 120, 403
  * (63) Lindblad, P. O. 1999, A&AR, 9, 221
  * (64) Liu, L., Weiss, A., Perez-Beaupuits, J. P., et al. 2017, ApJ, 846:5
  * (65) Madore, B. F., Freedman, W. L., Silbermann, N., et al. 1998, Nat, 395:3, 47
  * (66) Meijerink, R. & Spaans, M. 2005, A&A, 436, 397
  * (67) Meijerink, R., Spaans, M., & Israel, F. P. 2007, A&A, 461, 793
  * (68) Meijerink, R., Kristensen, L. E., Weiss, A., et al. 2013, ApJ, 762L, 16
  * (69) Melioli, C., Brighenti, F., D’Ercole, A., & de Gouveia Dal Pino, E. M. 2009, MNRAS, 399, 1089
  * (70) Melnick, G. J., Tolls, V., Neufeld, D. A., et al. 2008, ApJ, 683, 876
  * (71) Melnick, G. J., Tolls, V., Neufeld, D. A., et al. 2010, A&A, 521, L27
  * (72) Mitchell, G. F. 1984, ApJS, 54, 81
  * (73) Morris, P.W.; Gupta, H.; Nagy, Z.; et al. 2016, ApJ, 829, 25
  * (74) Nagy, Z.; van der Tak, F.F.S.; Ossenkopf, V.; et al. 2013, A&A, 550, A96
  * (75) Nagy, Z.; Choi, Y.; Ossenkopf-Okada, V.; et al. 2017, A&A, 559, A22
  * (76) Nayakshin, S. 2014, MNRAS, 441,1380
  * (77) Neill, J. L., Wang, S., Bergin, E. A., et al. 2013, ApJ, 770, 142
  * (78) Neufeld, D. A., Lepp, S., & Melnick, G. J. 1995, ApJS, 100, 132
  * (79) Nilsson, A., Hjalmarson, Å., Bergman, P., & Millar, T. J. 2000, A&A, 358, 257
  * (80) Norman, C. A., & Ikeuchi, S. 1989, ApJ, 345, 372
  * (81) Olofsson, A. O. H., Olofsson, G., Hjalmarson, Å., et al. 2003, A&A, 402, L470
  * (82) Persson, C. M., Olofsson, A. O. H., Koning, N., et al. 2007, A&A, 476, 807
  * (83) Persson, C. M., Olberg, M., Hjalmarson, Å., et al. 2009, A&A, 494, 637
  * (84) Persson, C. M., Maoli, R., Encrenaz, P., et al. 2010, A&A, 515, A72
  * (85) Pfenniger, D., & Norman, C. 1990, ApJ, 363, 391
  * (86) Pinol Ferrer, N., Lindblad, P. O., & Fathi, K. 2012, MNRAS, 421, 1089
  * (87) Rangwala, N., Maloney, P. R., Glenn, J., et al. 2014, ApJ, 788, 147
  * (88) Risaliti, G., Harrison, F. A., Madsen, K. K., et al. 2013, Nature, 494, 449
  * (89) Rodgers, S. D., & Charnley, S. B. 2001, ApJ, 546, 324
  * (90) Roberts, W. W. Jr., & Stewart, G. R. 1987, ApJ, 314, 10
  * (91) Rydbeck, G. 2008, ApJ, 675, 1304
  * (92) Rydbeck, G., Hjalmarson, Å., & Rydbeck, O.E.H. 1985, A&A, 144, 282
  * (93) Rydbeck, O. E. H., Elldér, J., & Irwine, W. M. 1973, Nature 246, 466
  * (94) Rydbeck, O. E. H., Kollberg, E., Hjalmarson, Å., et al. 1976, ApJS, 31, 333
  * (95) Sakamoto, K., Ho, P. T. P., Mao, R.-Q., Matsushita, S., & Peck, A. B. 2007, ApJ, 654, 782
  * (96) Sandqvist, Aa. 1974, A&A, 33, 413
  * (97) Sandqvist, Aa. 1999, A&A, 343, 367
  * (98) Sandqvist, Aa., Jörsäter, S., & Lindblad, P. O. 1995, A&A, 295, 585
  * (99) Sandqvist, Aa., Larsson, B., Hjalmarson, Å., et al. 2008, A&A, 482, 849
  * (100) Sandqvist, Aa., Larsson, B., Hjalmarson, Å., et al. 2015, A&A, 584, A118
  * (101) Schinnerer, E., Weiß, A., Aalto, S., & Scoville, N. Z. 2010, ApJ, 719, 1588
  * (102) Scoville, N. Z., Sanders, D. B., & Clemens, D. P. 1986, ApJ, 310, L77
  * (103) Shu, F. H., Milone V., & Roberts, W.W.Jr. 1973, ApJ, 183, 819
  * (104) Snell, R. L., Howe, J. E., Ashby, M. L., et al. 2000, ApJ, 539, L101
  * (105) Spinoglio, L., Pereira-Santaella, M., Busquet, G., et al. 2012, ApJ, 758, 108
  * (106) Stacey, G. J., Lugten, J. B., & Genzel, R. 1987, ApJ, 313, 859
  * (107) Sundelius, B., Thomasson, M., Valtonen, M.J., & Byrd, G.G. 1987, A&A, 174, 67
  * (108) Swinyard, B. M., Ade, P., Baluteau, J.-P., et al. 2010, A&A, 518, L4
  * (109) Tabatabaei, F. S., Weiss, A., Combes, F., et al. 2013, A&A, 555, A128
  * (110) Takahashi, H., Holenbach, D.J., & Silk, J. 1985, ApJ, 292, 192
  * (111) Toomre, A. 1964, ApJ, 139, 1217
  * (112) Turner, B. E. 1988, ApJ 329, 425
  * (113) Turner, B. E., & Zuckerman, B. 1974, ApJ, 187L, 59
  * (114) van der Tak, F. F. S., Black, J. H., & Schöier, F.L. 2007, A&A, 468, 627
  * (115) van der Tak, F. F. S., Marseille, M. G., Herpin, F., et al. 2010, A&A, 518, L107
  * (116) van der Tak, F. F. S., Weiss, A., Liu, L., & Güsten, R. 2016, A&A, 593, A43
  * (117) van Dishoeck, E. F., & Blake, G. A. 1998, ARA&A, 36, 31
  * (118) van Dishoeck, E. F., Herbst, E., & Neufeld, D. A. 2013, ChRv, 113, 9043