11institutetext: 1Netherlands Institute for Radio Astronomy (ASTRON), Oude
Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
2Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV
Groningen, The Netherlands,
3Department of Astronomy, University of Cape Town, Private Bag X3, 7701
Rondebosch, South Africa,
4INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047,
Selargius (CA), Italy,
5Aix-Marseille Univ., CNRS, CNES, LAM, 38 rue Frédéric Joliot Curie, 13338
Marseille, France
6Instituto de Astrofisica, Facultad de Ciencias Exactas,Universidad Andres
Bello, Chile
7LERMA, Observatoire de Paris, PSL research Université, CNRS, Sorbonne
Université, 75104, Paris, France,
8Collège de France, 11 Place Marcelin Berthelot, 75005, Paris, France,
9University of Louisville, Department of Physics and Astronomy, 102 Natural
Science Building, 40292 KY Louisville, USA,
10Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical
Institute (AIRUB), 44780 Bochum, Germany,
11Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg,
Germany,
12Department of Physics and Space Science, Royal Military College of Canada
P.O. Box 17000, Station Forces Kingston, ON K7K 7B4, Canada,
13Department of Physics, Engineering Physics and Astronomy, Queen’s
University, Kingston, ON K7L 3N6, Canada,
14Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomia s/n,
18008 Granada, Spain,
15E.A. Milne Centre for Astrophysics, University of Hull, HU6 7RX, United
Kingdom,
16Department of Physics and Astronomy, University of Manitoba, Winnipeg,
Manitoba R3T 2N2, Canada,
17 Département de physique, Université de Montréal, Complexe des sciences MIL,
1375 Avenue Thérèse-Lavoie-Roux Montréal, Qc, Canada H2V 0B3,
18Laboratoire de Physique et de Chimie de l’Environnement, Observatoire
d’Astrophysique de l’Université Ouaga I Pr Joseph Ki-Zerbo (ODAUO), BP 7021,
Ouaga 03, Burkina Faso

# Possible origins of anomalous H i gas around MHONGOOSE galaxy, NGC 5068

J. Healy  healy AT astron.nl11 W.J.G. de Blok  112233 F.M. Maccagni 1144 P.
Amram 55 L. Chemin 66 F. Combes 7788 B.W. Holwerda 99 P. Kamphuis 1010 D.J.
Pisano 33 E. Schinnerer 1111 K. Spekkens 12121313 L. Verdes-Montenegro 1414 F.
Walter 1111 E.A.K. Adams 1122 B.K. Gibson 1515 D. Kleiner 1144 S. Veronese
1122 N. Zabel 33 J. English 1616 C. Carignan 3317171818

(12 February 2024)

The existing reservoirs of neutral atomic hydrogen gas (H i) in galaxies are
insufficient to have maintained the observed levels of star formation without
some kind of replenishment. This refuelling of the H i reservoirs is likely to
occur at column densities an order of magnitude lower than previous
observational limits ($\mathrm{N}_{\textsc{H$\,$i},\,limit}\sim
10^{19}\,\text{cm}^{-2}$ at 30″ resolution over a linewidth of
$20\,\text{km}\,\text{s}^{-1}$). In this paper, we present recent deep H i
observations of NGC 5068, a nearby isolated star-forming galaxy observed by
MeerKAT as part of the MHONGOOSE survey. With these new data, we are able to
detect low column density H i around NGC 5068 with a $3\sigma$ detection limit
of $\mathrm{N}_{\mathrm{H}\,\textsc{i}}=6.4\times 10^{17}\,\text{cm}^{-2}$ at
90″ resolution over a $20\,\text{km}\,\text{s}^{-1}$ linewidth. The high
sensitivity and resolution of the MeerKAT data reveal a complex morphology of
the H i in this galaxy – a regularly rotating inner disk coincident with the
main star-forming disk of the galaxy, a warped outer disk of low column
density gas ($\mathrm{N}_{\mathrm{H}\,\textsc{i}}<9\times
10^{19}\,\text{cm}^{-2}$), in addition to clumps of gas on the north west side
of the galaxy. We employ a simple two disk model that describe the inner and
outer disks, and are able to identify anomalous gas that deviates from the
rotation of the main galaxy. The morphology and the kinematics of the
anomalous gas suggest a possible extra-galactic origin. We explore a number of
possible origin scenarios that may explain the anomalous gas, and conclude
that fresh accretion is the most likely scenario.

###### Key Words.:

galaxy ISM – galaxy evolution

##  1 Introduction

How galaxies have been able to sustain the observed levels of star formation
is one of the key open questions in galaxy evolution. In order for the current
observed rate of star formation in the local universe to be maintained,
galaxies must refuel the gas reservoirs. How exactly this refuelling takes
place is still an open question, however, it has been shown that minor mergers
alone cannot supply the required amount of gas necessary to sustain the
observed levels of star formation (e.g. Di Teodoro & Fraternali, 2014),
meaning that some form of gas accretion must be ongoing (Larson, 1972; Sancisi
et al., 2008; Ho et al., 2019). There are a number of different ways this
accretion of gas can occur: gas-rich mergers, accretion of recycled gas
(“fountain model” e.g. Fraternali et al., 2002; Melioli et al., 2008, 2009),
or accretion of gas from the cosmic web (van de Voort et al., 2011a; Wetzel &
Nagai, 2015; Cornuault et al., 2018; Ho et al., 2019; Iza et al., 2022).

Simulations have shown that accretion of fresh gas into galaxies generally
comes in two modes (Kereš et al., 2005, 2009; Nelson et al., 2013, 2015; Huang
et al., 2019). In “hot mode” accretion, the gas is shock heated as it collides
with the hydrostatic hot halo near the virial radius, if the gas reaches high
enough densities, it can cool and settle onto the disk. However, in “cold
mode” accretion, the gas is accreted along filaments or in clumps that are not
shock heated. Accretion along filaments has been shown to be more important
when considering how the gas is accreted onto the galaxies (e.g. Kereš et al.,
2009; van de Voort et al., 2011a; van de Voort & Schaye, 2012; Hafen et al.,
2020; Cadiou et al., 2022). Simulations suggest that these filaments have H i
column densities of $10^{17}$ to $10^{18}\,\text{cm}^{-2}$ (van de Voort et
al., 2011a; van de Voort & Schaye, 2012; van de Voort et al., 2019; Ramesh et
al., 2023) which has been impossible to observe with the necessary resolution
until very recently.

Unambiguously identifying accreting gas observationally is extremely difficult
due to the low density of the accreting gas and the numerous different
possible explanations for “accretion-like” features observed in the gas
distribution of nearby galaxies. Deep observations of the H i in nearby spiral
galaxies have identified the presence of extra-planar gas (EPG). This provided
an opportunity to study the connection been the gas in the halo and the galaxy
disk. EPG has now been identified in many spiral galaxies (see review by
Sancisi et al., 2008; Marasco et al., 2019). In edge-on galaxies, such as NGC
891, the EPG is a clearly visible, extended component lagging in velocity
(e.g. Swaters et al., 1997; Oosterloo et al., 2007). In studies of more face-
on galaxies such as NGC 2403, the presence of the EPG is inferred by the
anomalous velocities of the H i (e.g. Fraternali et al., 2001; de Blok et al.,
2014; Li et al., 2023; Veronese et al., 2023). EPG has been shown to account
for $10\text{ to }20\%$ of the total H i mass of the host galaxies (Hess et
al., 2009; Gentile et al., 2013; Vargas et al., 2017; Marasco et al., 2019).
There has been a concerted effort to identify the origin of EPG, particularly
in relatively isolated galaxies (Heald et al., 2011; Gentile et al., 2013).
While fountain models have been used to explain the presence of EPG in many
galaxies (e.g. Li et al., 2023), the fountain models cannot explain all the
observed EPG, suggesting other modes of accretion must also be occurring (e.g.
NGC 2297, Hess et al. 2009).

In order to understand the relationship between the EPG, disk gas, and star
formation, deep, high resolution H i observations in combination with
multiwavelength tracers of past and ongoing star formation are needed for a
representative sample of nearby ($<20\,\text{Mpc}$) galaxies. High resolution
(sub kiloparsec scale) H i observations enable same-scale comparisons between
the H i morphology and kinematics to tracers of star formation activity (e.g.
UV and mid- or near-infrared imaging, molecular gas imaging). The high
sensitivity is necessary to detect the low column density EPG H i gas. The
limitations of previous generations of radio telescopes meant that surveys set
out to tackle these questions had to optimise for either resolution or
sensitivity.

Two of these H i surveys are The H i Nearby Galaxy Survey (THINGS, Walter et
al., 2008) and the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS,
Heald et al., 2011). THINGS is an H i survey with the Very Large Array (VLA)
of 34 nearby ($\text{D}<15\,\text{Mpc}$) gas-rich spiral and dwarf galaxies.
The survey made use of the VLA in B, C, and D configurations, which provided
the high spatial resolution in combination with the high spectral resolution
of the VLA correlator, but H i column density sensitivity was limited to
$\mathrm{N}_{\mathrm{H}\,\textsc{i}}\sim 4.5\times 10^{20}\,\text{cm}^{-2}$ at
$3\sigma$ over $20\,\text{km}\,\text{s}^{-1}$ linewidth at the highest
resolution of $6^{\prime\prime}$ (Walter et al., 2008). Some key results from
THINGS include detailed analyses by Bigiel et al. (2008) and Leroy et al.
(2008) of the correlation between H i and molecular hydrogen
($\mathrm{H}_{2}$) at sub-kpc scales with star formation, showing that star
formation efficiency is driven by the presence of molecular gas. Bigiel et al.
(2008) also confirmed that there is a surface density at which H i saturates,
and the gas is only molecular. While Leroy et al. (2008) found that where the
H i component is the dominant gas phase, star formation efficiency decreases
with increasing radius from the centre of the galaxy. The high resolution
imaging also made it possible to detect radial motions related to the EPG
component in a smaller sub-sample of THINGS galaxies (Schmidt et al., 2016).
Detecting the radial motions of the gas is key to disentangling how gas is
transported from the outskirts to the inner star forming regions of galaxies
with ongoing accretion.

The first systematic search for evidence of accretion of cold gas onto
galaxies was the HALOGAS survey, conducted using the Westerbork Synthesis
Radio Telescope, which was designed to detect the low column density H i and
characterise the morphology and kinematics of the detected EPG. The 24 nearby
($\text{D}<20\,\text{Mpc}$) galaxies chosen were moderately inclined or edge-
on as this makes it easier to model and study in detail the morphology and
kinematics of the EPG. The deep observations ($10\times 12\,\text{hours}$ per
galaxy) provided high sensitivity H i maps
($\mathrm{N}_{\mathrm{H}\,\textsc{i}}\sim 1.1\times 10^{19}\,\text{cm}^{-2}$
at $3\sigma$ for a linewidth of $20\,\text{km}\,\text{s}^{-1}$ at a resolution
of $30^{\prime\prime}$, Heald et al. 2011). Studies of the HALOGAS galaxies
have provided much of what is now known about EPG in galaxies as discussed
above. Another one of the important results from HALOGAS has been the
identification of clouds of anomalous gas (Heald & Team, 2014; Kamphuis et
al., 2022), similar to the high velocity clouds (HVC) around the Milky Way
(Wakker & Van Woerden, 1997). Kamphuis et al. (2022) also set important limits
on the rate at which H i can be accreted via these clouds showing that it is
much lower than the global accretion rate which is estimated to be
$0.2\,\text{M}_{\odot}/\text{yr}$ by Sancisi et al. (2008).

The limitations of the previous surveys have been overcome by the arrival of
MeerKAT (Jonas & MeerKAT Team, 2016) where it is no longer necessary to choose
between sensitivity and resolution. The MeerKAT H i Observations of Nearby
Galactic Objects - Observing Southern Emitters (MHONGOOSE; de Blok et al.,
2016, 2020) is one of the MeerKAT Large Survey Projects, having been awarded a
total of $1650\,\text{hours}$ to observe the H i in 30 nearby galaxies
($55\,\text{hours}$ per galaxy). The MHONGOOSE galaxies were selected from an
overlap of galaxies observed as part of both the H i Parkes All Sky Survey
(HIPASS; Barnes et al., 2001; Meyer et al., 2004) and the Survey for
Ionization in Neutral Gas Galaxies (SINGG; Meurer et al., 2006). This gave a
first estimate of the total H i mass of the sources, and ensured the existence
of ultraviolet to infrared observations of the galaxies. All 30 galaxies were
also required to be within $30\,\text{Mpc}$, and were chosen to avoid dense
environments such as the nearby Fornax and Virgo clusters. In this work, we
present the new MeerKAT H i observations of one of the MHONGOOSE galaxies: NGC
5068 (HIPASS J1318-21).

Figure 1: Composite optical/FUV image of NGC 5068. The DECam Legacy Survey
(DECaLS; Dey et al., 2019) $g$,$r$,$z$ provides the RGB colour. The far UV
observed by GALEX as part of the SUNGG Survey (Wong et al., 2016) highlights
the pink star formation regions.

NGC 5068 is a nearby, $d=5.2\,\text{Mpc}$ (Anand et al., 2021), close to face-
on ($i=35.7\pm 10.9^{\circ}$, Lang et al. 2020) barred spiral galaxy
(Rosolowsky et al., 2021). It is an actively star forming galaxy of
intermediate stellar mass ($\text{M}_{\star}=2.29\times
10^{9}\,\text{M}_{\odot}$, Leroy et al. 2019), located well within the scatter
of the so-called star formation “main sequence”. Fig. 1 shows a composite
optical/FUV image of NGC 5068 which highlights the star-forming regions in the
outer regions of the galaxy. Studies of the H i in this galaxy, have up until
now, only made use of observations by single dish telescopes (Koribalski et
al., 2004; Sorgho et al., 2019; Sardone et al., 2021) which lack the
resolution to identify details in the morphology of the H i disk. This work
provides a first look at the high resolution details of the H i distribution
of NGC 5068 while still maintaining an H i column density sensitivity equal to
that of the single dish observations.

This paper is laid out as follows: in Sec. 2, we present the new MeerKAT H i
observations of NGC 5068 and the process by which we calibrated and imaged the
data. In Sec. 3 we discuss the global H i properties of the galaxy, and
compare to previously published observations. A detailed discussion on the
kinematics of the H i follows in Sec. 4. Finally, in Sec. 5, we explore
different possible origins of the anomalous low column density H i gas
detected in NGC 5068.

##  2 MeerKAT observations

NGC 5068 was observed as part of MHONGOOSE in ten $5.5\,\text{hour}$ tracks
(inclusive of time spent on the calibrators) for a total on-source time of
$50\,\text{hours}$. In each track, the primary calibrator, J0408-6545 (rising
tracks) or J1939-6342 (setting tracks), was observed for $10\,\text{min}$. The
secondary calibrator, J1311-2216, was observed every $50\,\text{min}$ for
$2\,\text{min}$. The observations were carried out at night between April 2021
and March 2022 using an average of 62 of the 64 antenna which provide
baselines of $29\text{ to }7800\,\text{m}$. Each MeerKAT dish has a diameter
of $13.5\,\text{m}$ which corresponds to a full width at half max (FWHM) of
the primary beam of $55\,\text{arcmin}$ at $\nu=1420.405\,\text{MHz}$; for
more details see de Blok et al. (2024).

Each track was observed using the narrow-band ($107\,\text{MHz}$) mode of
MeerKAT. With this mode, the observations have a native channel width of
$3.3\,\text{kHz}$ or $0.7\,\text{km}\,\text{s}^{-1}$ at $1420\,\text{MHz}$. We
bin the channels by a factor of two, to a channel width of $6.6\,\text{kHz}$
or $1.4\,\text{km}\,\text{s}^{-1}$, before starting the calibration process.
Due to the shape of the MeerKAT spectral response (van der Byl et al., 2022)
individual $0.7\,\text{km}\,\text{s}^{-1}$ channels are independent, implying
that our binned $1.4\,\text{km}\,\text{s}^{-1}$ channels are as well.

Table 1: General properties of NGC 5068.

$\alpha$, $\delta$ (J2000) |  $13^{\mathrm{h}}18^{\mathrm{m}}54.5^{\mathrm{s}}$ $-21^{\circ}02\arcmin 17\arcsec$
---|---
$v_{sys}$ ($\text{km}\,\text{s}^{-1}$) | $667.8\pm 1.3^{a}$
PA (∘) | $342.4\pm 3.2^{b}$
$i$ (∘) | $35.7\pm 10.9^{b}$
$D_{25}$ ($\arcmin$) | $7.03^{c}$
$d$ (Mpc) | $5.20\pm 0.22^{d}$
$\text{M}_{\star}$ ($\text{M}_{\odot}$) | $2.29\times 10^{9}\,{}^{e}$
SFR ($\text{M}_{\odot}$/year) | $0.275\pm 0.127\,^{e}$

Notes: a calculated from the global profile in this work, b based on optical
properties from Lang et al. (2020), c $B$-band diameter at $25\,\text{mag
/arcsec}^{2}$ isophote from Lauberts & Valentijn (1989), d TRGB from Anand et
al. (2021), e Leroy et al. (2019)

All 10 tracks were calibrated using the same procedure with version 1.0.5 of
the Containerized Automated Radio Astronomy Calibration (CARAcal) pipeline
(Józsa et al., 2020). CARAcal provides a single environment in which to carry
out the usual calibration and reduction steps: flagging of data, cross-
calibration, splitting of the target, self-calibration, and later the
continuum subtraction and spectral line imaging and deconvolution. At each
step, the pipeline produces diagnostic plots which help to identify any issues
that may arise. These are consolidated outside of CARAcal into a calibration
report for each track that is automatically uploaded and stored on a private
GitHub team repository where each report is used to quality check the
observation.

###  2.1 Self calibration and continuum subtraction

We used self-calibration of the continuum to further improve the quality of
the calibration. For this we used a continuum image created from the frequency
range $1390\text{-}1422\,\text{MHz}$, taking care to flag the frequency range
covered by the H i emission from both the Galaxy and the target, NGC 5068.
Four rounds of self-calibration were used, where in each round a progressively
lower signal-to-noise (S/N) threshold is employed to create a mask using the
Source Finding Application (SoFiA, Serra et al., 2015), which thus increases
the number of sources to be included in the progressively better sky model.
The continuum emission of NGC 5068 is bright and extended, and we note the
importance of including all of the diffuse continuum emission of NGC 5068 in
the sky model. Not doing so leads to the appearance of “ghosts” in the data,
mimicking faint H i structures (Grobler et al., 2014; Wijnholds et al., 2016).
The model is also used in the first continuum subtraction step for the H i
spectral line cube. Any residual continuum emission is subtracted using a 1st
order polynomial fit to the line-free channels.

###  2.2 H i imaging

The H i cubes were imaged using WSClean (Offringa et al., 2014; Offringa &
Smirnov, 2017) as part of CARAcal. We employed a 3-step iterative strategy to
cleaning the H i data: first, a low-resolution cube was created and cleaned
using a 5$\sigma_{rms}$ clip WSClean auto-mask. From this low-resolution cube,
a mask of the source was created using SoFiA-2 (Westmeier et al., 2021);
second, using the newly created low resolution mask for the cleaning, we
created a new cube at the desired resolution from which an updated clean-mask
was created using SoFiA-2; in the third step, the final H i cubes that are
presented in this work were created and cleaned using the updated mask from
the previous step. In each step, we cleaned the data to $0.5\sigma_{rms}$.
This procedure is described in more detail in the MHONGOOSE paper presenting
the full survey sample (de Blok et al., 2024).

We make full use of MeerKAT’s resolution and sensitivity capabilities by
creating a set of six H i cubes with a $1.5^{\circ}$ field of view (FoV) that
range in resolution ($7\arcsec$ to $90\arcsec$ which correspond to
$0.34\,\text{kpc}$ to $4\,\text{kpc}$). This is done using different
combinations of robust weighting ($r$) and Gaussian tapering ($t$), which
varies the noise and resolution, and thus the H i column density sensitivity
($\mathrm{N}_{\mathrm{H}\,\textsc{i}}\sim 6.8\times 10^{19}\,\text{cm}^{-2}$
to $6.4\times 10^{17}\,\text{cm}^{-2}$ at $3\sigma$ over
$20\,\text{km}\,\text{s}^{-1}$ – see Table 2). The combinations are listed in
the left-most column of Table 2. Using this broad range in sensitivity and
resolution, we can characterise the morphology of the H i. Also listed for
completeness in Table 2 is the resolution (second column), the
$\sigma_{\mathrm{rms}}$ based on $1.4\,\text{km}\,\text{s}^{-1}$ wide channels
(fourth column), $3\sigma$ over $20\,\text{km}\,\text{s}^{-1}$ column density
sensitivity (fifth column), and the column density at S/N$=3$ in the total
intensity maps (sixth column) for each cube.

Table 2: Properties of the H i cubes.

Cube | Resolution | Pixel size | $\sigma_{\mathrm{rms}}^{a}$ |  $\mathrm{N}_{\mathrm{H}\,\textsc{i}}$ ($3\sigma$ over $20\,\text{km}\,\text{s}^{-1}$)c |  $\mathrm{N}_{\mathrm{H}\,\textsc{i}}$ (S/N$=3$)d
---|---|---|---|---|---
| (arcsec $\times$ arcsec) | (arcsec) | (mJy/beam) | ($\text{cm}^{-2}$) | ($\text{cm}^{-2}$)
$r=0.0$, $t=0^{\prime\prime b}$ | $8.1\times 6.9$ | 2 | 0.215 | $6.8\times 10^{19}$ | $9.1\times 10^{19}$
$r=0.5$, $t=0^{\prime\prime}$ | $13.4\times 9.4$ | 3 | 0.169 | $2.3\times 10^{19}$ | $3.1\times 10^{19}$
$r=1.0$, $t=0^{\prime\prime}$ | $26.0\times 17.9$ | 5 | 0.148 | $5.6\times 10^{18}$ | $6.5\times 10^{18}$
$r=1.5$, $t=0^{\prime\prime}$ | $34.3\times 25.6$ | 7 | 0.153 | $3.1\times 10^{18}$ | $3.4\times 10^{18}$
$r=0.5$, $t=60^{\prime\prime}$ | $65.3\times 63.8$ | 20 | 0.241 | $1.0\times 10^{18}$ | $1.1\times 10^{18}$
$r=1.0$, $t=90^{\prime\prime}$ | $93.8\times 91.7$ | 30 | 0.313 | $6.4\times 10^{17}$ | $7.0\times 10^{17}$

Notes: a this is measured per $1.4\,\text{km}\,\text{s}^{-1}$ channel. b $r$
is the robust weighting, and $t$ the Gaussian taper. c based on $3\sigma$
detection in 14 channels. d mean column density at the S/N=3 contour in the
moment 0 map, see Section 3.1 for details.

##  3 Global H i properties

###  3.1 Morphology of the H i disk

Figure 2: Top: Greyscale MeerLICHT $q$-band image and colour with contours
from the six different resolution H i cubes. The contours correspond to
colours in the colourbar which indicate the resolution and the H i column
density at the S/N=3 contour in the intensity maps. Three clouds associated
with the galaxy are labelled A,B,C. Dotted magenta ellipse indicates the
optical extent of the galaxy. The beam for resolution (contour) is shown in
the corresponding colour in the box in the bottom left corner. Bottom: H i
emission combined with Fig. 1. The red, orange, and yellow show moment 0 maps
of the H i emission from the $64.7^{\prime\prime}$ (r05_t60),
$29.6^{\prime\prime}$ (r10_t00), and $7.5^{\prime\prime}$ (r00_t00) resolution
H i cubes respectively. Production of this image followed English (2017),
including the technique of masking in order to combine the data sets and
reveal the optical data’s H ii regions (light pink). Figure 3: Integrated
intensity (moment 0) maps of NGC 5068. In the top panel is the map from one of
the high resolution cubes (r05_t0) at $11\arcsec$, and in the bottom panel is
a map from one of the low resolution cubes (r05_t60) at $64\arcsec$. The
dashed magenta ellipse is centred on the optical centre of the galaxy and
represents the extent of the optical disk. The contours increase as
$\sigma_{S/N=3}\times 2^{n},\,\,n=0,2,4,6,...$. The lowest contour plotted is
the S/N$=3$ contour, $\mathrm{N}_{\mathrm{H}\,\textsc{i}}=3.1\times
10^{19}\,\text{cm}^{-2}$ for the top panel and
$\mathrm{N}_{\mathrm{H}\,\textsc{i}}=1.1\times 10^{18}\,\text{cm}^{-2}$ for
the bottom panel, these S/N$=3$ ($n=0$) values are indicated on the respective
colour bars by the orange line. The red ellipse in the bottom left corner of
each panel represents the beam.  Figure 4: Channel maps taken from the r10_t0
cube that has been smoothed with a 5 channel Hanning window and regridded to
$3\,\text{km}\,\text{s}^{-1}$ channels. Every third
$3\,\text{km}\,\text{s}^{-1}$ channel for a $170\,\text{km}\,\text{s}^{-1}$
range centred on the systemic velocity is shown. The blue contour indicates
3$\sigma_{rms}$, the grayscale contours are $3\sigma_{rms}\times
2^{n},\,\,n=2,4,6,..$. The magenta cross indicates the optical centre of the
galaxy, and dotted magenta ellipse indicates the extent of the optical disk
($D_{25}$) of the galaxy. The green arrows point to “fingers” in the H i. The
orange ellipse in the lower left corner of each panel represents the
$21.6\arcsec$ beam.

The six H i cubes enable us to explore both the details of the dense H i in
the galaxy disk, as well as the low column density EPG. For each cube we
create a suite of moment maps by using SoFiA-2 to identify all detectable H i
emission in the velocity range
($200<v_{rad}\,(\text{km}\,\text{s}^{-1})<1143$) covered by the data cube. We
used the smooth and clip algorithm to identify all signal above a threshold of
$3.5\sigma$ using using all combinations of the following spatial and spectral
smoothing kernels: spatially, these are the original resolution and a spatial
smoothing kernel equal to the synthesised beam; spectrally, we used the
original resolution and kernels of 9 channels ($\sim
12\,\text{km}\,\text{s}^{-1}$), and 25 channels ($\sim
35\,\text{km}\,\text{s}^{-1}$). The source finding yields the H i emission
associated with only the target galaxy, NGC 5068, within a field of view of
$1.5^{\circ}$ which corresponds to a radius of $125\,\text{kpc}$. The primary
beam111The MeerKAT primary beam FWHM is $55\arcmin$ at $z=0$, this entirely
contains the H i of NGC 5068 which spans roughly $30\arcmin$. corrected moment
maps are created from the masked data cubes, however we further limit the
first moment (intensity-weighted mean velocity field) and second moment
(sometimes referred to as the velocity dispersion map) maps to only include
the pixels where the integrated H i column density
($\mathrm{N}_{\mathrm{H}\,\textsc{i}}$) is greater than the $\mathrm{S/N}=3$
threshold. This threshold is determined for each resolution map by using the
S/N map output by SoFiA-2 where $\mathrm{S/N}=\sum
S\,\mathrm{d}v/(\sigma_{rms}\mathrm{d}v\,\sqrt{N_{chan}})$, $S\,\mathrm{d}v$
is the moment zero, $\sigma_{rms}$ is the noise per channel of the H i cube,
$\mathrm{d}v$ is the channel width, and $N_{chan}$ is the number of channels
of the H i cube that contributes to each pixel of the zeroth moment map. This
is only valid under the assumption that the channels are independent which in
this case is true as the observed channels were binned in pairs and no Hanning
smoothing was used. Following Meyer et al. (2017, Eq. 75), we calculate the
$\mathrm{N_{\textsc{H$\,$i},\,S/N=3}}$ threshold by calculating the median
column density of the pixels which are in the range $2.75<\mathrm{S/N}<3.25$
in the S/N map. Despite the differences in how the values are calculated, the
$\mathrm{N_{\textsc{H$\,$i},\,3\sigma}}$ and
$\mathrm{N_{\textsc{H$\,$i},\,S/N=3}}$ values presented in Table 3 column 5
and 6 respectively, are similar although the
$\mathrm{N_{\textsc{H$\,$i},\,S/N=3}}$ values tend to be slightly higher. This
is due to a combination of the smooth and clip algorithm used to identify
emission associated with a detection, and the linking lengths used in SoFiA-2
to add pixels containing emission to the detection mask which results in a
mask that is wider than what would be created using a simple $3\sigma$ clip.
It is worth noting that the column density limits presented in Table 3 are
significantly lower than the previous generation of H i column density limits,
and reflect the excellent quality of these deep MeerKAT data.

The top panel of Fig. 2 shows the $\mathrm{N}_{\mathrm{H}\,\textsc{i}}$
(S/N$=3$) contours of the H i distribution seen in the different resolutions
plotted on the MeerLICHT (Bloemen et al., 2016) $q$-band222The MeerLICHT
$q$-band is a wide-band that covers the $g$ and $r$ bands. image which has a
$5\sigma$ surface brightness limit of $24.61\,\text{mag}/\text{arcsec}^{2}$.
MeerLICHT is a 0.6m optical telescope, located in Sutherland, South Africa,
with a $1.6^{\circ}\times 1.6^{\circ}$ FoV ($\sim 1.7\times$ FWHM of the
MeerKAT primary beam at $z=0$). The bottom panel of Fig. 2 combines H i moment
maps with Fig. 1 and shows spatial relationships between the star-forming
regions and atomic gas structures in the form of H i densities or cavities.
The upper panel shows the extent of the gas at different column density
levels, with the low column density gas extending out to more than 2.5 times
the optical radius, defined by the $B$-band $25\,\text{mag}/\text{arcsec}^{2}$
isophote (Lauberts & Valentijn, 1989), of the galaxy which is approximately
$10\,\text{kpc}$. Fig. 2 also shows finger-like tendrils of H i extending out
from the optical disk at column densities of
$\mathrm{N}_{\mathrm{H}\,\textsc{i}}\sim 3.1\,\text{to}\,9.7\times
10^{19}\,\text{cm}^{-2}$ (green contour). Also of interest are the low-column
density clumps on the north-western side of the galaxy which were identified
by eye. These clumps appear as distinct clouds below H i column densities of
$\mathrm{N}_{\mathrm{H}\,\textsc{i}}\sim 3\times 10^{19}\,\text{cm}^{-2}$ but
become connected to the galaxy H i disk in the $60\arcsec$ (pink) and
$90\arcsec$ (purple) resolution cubes which is likely a resolution effect. The
more prominent clumps have been labelled A, B, C in the top panel of Fig. 2.
While Fig. 2 provides an overall picture of the total H i intensity
distribution, to be able to study the different features, it is necessary to
examine the individual moment 0 maps at the various resolutions.

Figure 3 shows the moment 0 (H i intensity) maps at two different resolutions:
$11\arcsec$ ($0.5\,\text{kpc}$) and $64\arcsec$ ($3\,\text{kpc}$). In the top
panel the high resolution (r05_t0 which corresponds to the robust $(r)=0.5$
and taper $(t)=0\arcsec$ cube) map highlights the clumpy nature of the H i, in
particular the finger-like tendrils and the clouds on the outskirts of the
galaxy. The optical disk is represented by the dotted magenta ellipse measured
as the diameter of the $25\,\text{mag}/\text{arcsec}^{2}$ isophote ($D_{25}$)
in the $B$-band by Lauberts & Valentijn (1989). In the bottom panel of Fig. 3,
the lower resolution (r05_t60) map shows the extent of the low column density
gas which connects the clouds visible in the higher resolution map to the H i
in the galaxy.

The individual channel maps provide a clearer picture of the fingers and
clouds. Fig. 4 shows a selection of channels taken from the $r=1$, no taper
(r10_t0) cube, which has a resolution of $21.6\arcsec$, that has been Hanning-
smoothed with a 5 channel kernel and regridded to a channel width of
$3\,\text{km}\,\text{s}^{-1}$ to increase the signal to noise of the lowest
column density features. This smoothed version of the r10_t0 cube provides a
good balance between resolution and column density sensitivity. We note two
distinct features that are evident in the channel maps: the first being two
“fingers”, one which points out towards the north west (NW) and the other
pointing towards the south east (SE) – both indicated by the green arrows in
the Figure. The second feature is that there appears to be two disks: one
inner disk that has a major axis running approximately NW-SE and is
coincidental with the optical disk, and the second outer disk which appears to
be more elliptical and has a major axis running approximately NE-SW.

The global profile, extracted from the $r=1.0$, $t=90\arcsec$ (r10_t90) cube
using the associated SoFiA-2 mask, is shown in panel (a) of Fig. 5. For each
resolution cube, we determine the mass encapsulated by the mask, the values
are shown in panel (b). It is clear that the H i mass does not change
significantly between the largest four resolutions which suggests that all the
H i mass for the system has been identified. The robust weighting used to
create the lower resolution H i cubes emphasises the shorter baselines making
these measurements more suitable for computing the total H i mass.

From the global profile in Fig. 5, we calculated the total integrated flux,
the line width at 50% ($w_{50}$) and 20% ($w_{20}$) of the peak flux are
measured directly from the profile, and we determine the systemic velocity to
be the velocity at the mid point of the $w_{20}$. These values are listed in
Table 3. While the largest contribution to the flux uncertainty are from the
systematics arising as part of the calibration process, the uncertainties on
the global profile are calculated base on the noise properties of the
corresponding H i cube.

Table 3: Global H i properties of NGC 5068 measured from the global profile. Parameter | Value
---|---
$S_{int}$ | $167.82\pm 0.15\,\text{Jy}\,\text{km}\,\text{s}^{-1}$
$\text{M}_{\textsc{H$\,$i}}$ (total) | $1.07\pm 0.09\times 10^{9}\,\text{M}_{\odot}$
$w_{50}$ | $67.1\pm 0.6\,\text{km}\,\text{s}^{-1}$
$w_{20}$ | $109.1\pm 2.0\,\text{km}\,\text{s}^{-1}$
$v_{sys}$ | $667.8\pm 1.3\,\text{km}\,\text{s}^{-1}$

###  3.2 Comparison with previous observations

Figure 5: (a) Global H i profile of NGC 5068 from MeerKAT (black), HIPASS
(dashed grey; Koribalski et al. 2004), GBT (solid grey and dot-dash grey;
Sorgho et al. 2019; Sardone et al. 2021). The green arrow indicates the
systemic velocity of the galaxy. (b) The H i mass of NGC 5068 measured within
the SoFiA-2 mask for each different resolution cube. The open black square
indicates which cube was used to create the global profile shown in the top
panel.

Here we compare the total H i flux of NGC 5068 with those obtained from
single-dish measurements from the H i Parkes All Sky Survey (HIPASS, Meyer et
al., 2004; Koribalski et al., 2004) and targeted observations using the Green
Bank Telescope (GBT Sorgho et al., 2019; Sardone et al., 2021). Sorgho et al.
(2019) and Sardone et al. (2021) analysed a sub-sample of the MHONGOOSE
galaxies using GBT. Although they use the same GBT spectra, they process the
data differently which resulted in slightly different global profiles and
measurements of the integrated flux for NGC 5068. The total integrated flux
measurements from each survey are listed in Table 4, and the global profiles
are also plotted in Fig. 5(a).

As is evident from Table 4, there are differences in measured fluxes between
the different single-dish measurements. Despite the differences in the
integrated fluxes, the H i global profile line widths ($w_{20}$ and $w_{50}$)
measured for NGC 5068 by Sorgho et al. (2019) and Sardone et al. (2021) from
the GBT observations, and by Koribalski et al. (2004) from HIPASS are
consistent with what we measure from the MeerKAT spectra.

While the shape and amplitude of the Sardone et al. (2021) global profile is
consistent with the MeerKAT spectrum, the quoted integrated flux is
inconsistent. The higher flux density measurement quoted in Sardone et al.
(2021) is due to how the value was calculated: they determined the total flux
density of the galaxy by measuring the flux in annuli with increasing radius
from the galaxy centre, and then summing the flux in all the annuli. This
method of determining the total flux of the galaxy is not the same as the
integrated flux based on the global profile. We recalculated the integrated
flux from the Sardone et al. global profile to be
$S_{\textsc{H$\,$i}}=157.7\,\text{Jy}\,\text{km}\,\text{s}^{-1}$ which is
consistent within 10% of the MeerKAT value.

Given that we do not expect the HIPASS observations of NGC 5068 to missing any
flux, it is therefore curious that there is such a large discrepancy between
the total integrated flux measured from the MeerKAT spectrum and that which is
reported by Koribalski et al. (2004). However, the HIPASS data are known to
underestimate the H i emission of large galaxies as a result of the bandpass
correction method (Barnes et al., 2001) which likely subtracted some of the H
i (see de Blok et al. (2024) for a detailed comparison with all 30 MHONGOOSE
galaxies).

Table 4: The total integrated flux and linewidth measurements for NGC 5068
from different H i surveys

Survey | Integrated flux | $w_{20}$ | $w_{50}$
---|---|---|---
| (Jy $\text{km}\,\text{s}^{-1}$) | ($\text{km}\,\text{s}^{-1}$) | ($\text{km}\,\text{s}^{-1}$)
HIPASS (Koribalski et al., 2004) | $128.6\pm 12.7$ | 111.0 | 70.0
GBT (Sorgho et al., 2019) | $177.0\pm 0.7$ | – | 69.3
GBT (Sardone et al., 2021) | 191.5a | 108.0 | 67.9
MeerKAT (this work) | $167.82\pm 0.15$ | $109.1\pm 2.0$ | $67.1\pm 0.6$

Notes: a $S_{\textsc{H$\,$i}}=157.7\,\text{Jy}\,\text{km}\,\text{s}^{-1}$
measured from the global profile assuming a gain of $1.86\,\text{K/Jy}$ from
Sardone et al. (2021)

##  4 Kinematics of the H i

Figure 6: Velocity field (moment 1) for NGC 5068. Pixels with an H i column
density below $\mathrm{N}_{\mathrm{H}\,\textsc{i}}=5.9\times
10^{18}\,\text{cm}^{-2}$ (S/N=3) are masked. The colourbar in the bottom of
the image indicates the velocity of the gas, the grey contours are spaced
$10\,\text{km}\,\text{s}^{-1}$ apart with the black contour indicating the
systemic velocity, $v_{sys}=667\,\text{km}\,\text{s}^{-1}$. The dotted magenta
ellipse is centred on the optical centre of the galaxy and represents the
optical size of the galaxy. Figure 7: Position-velocity diagram extracted with
a width of $21.6\arcsec$ which corresponds to the size of the beam. The slice
is extracted along optical major axis. The inset on the top right of the
figure shows the velocity field from Fig. 6, the grey line running from the
magenta star to the filled circle represent the path along which the PV slice
was extracted, the magenta star and circle are located in the lower left and
upper right corners indicating the direction of the slice, these symbols are
repeated lower left and right corners of the main figure. The horizontal
dashed green line indicates the systemic velocity, while the light green
filled circles show the moment 1 velocity at each position along the major
axis. The dashed vertical magenta lines correspond to the edge of the optical
disk represented by the dotted magenta ellipse in the inset. The blue and
orange contours in the colour bar correspond to the blue and greyscale
contours on the PV slice. Figure 8: Same as Fig. 7 but at an angle of
$224^{\circ}$ through the outer disk. Figure 9: Moment 2 map for NGC 5068 at
$21.6\arcsec$ resolution. Pixels with an H i column density below the S/N=3
threshold of $\mathrm{N}_{\mathrm{H}\,\textsc{i}}=5.9\times
10^{18}\,\text{cm}^{-2}$ are masked. The colourbar in the bottom of the image
indicates the moment 2 velocity of the gas, the thin grey contours are spaced
$5\,\text{km}\,\text{s}^{-1}$ apart. The dotted magenta ellipse is centred on
the optical centre of the galaxy and represents the optical size of the
galaxy. Figure 10: Top left: Position-velocity (PV) diagram through the high
moment 2 region of NGC 5068. The blue and orange lines in the grayscale
colourbar correspond to the blue and grayscale contours. The green dashed line
indicates the systemic velocity. Top right: moment 2 map of the galaxy with
the black line tracing the path along which the PV slice was extracted,
starting from the blue star and ending at the magenta triangle. Bottom: H i
line profiles extracted at different locations along the PV slice, the
extracted locations are indicated by the magenta arrows in the PV diagram. The
location of each spectrum is also indicated by the labelled open circles on
the moment 2 map. Green arrows at the bottom of the line profiles indicate the
systemic velocity of the galaxy, while the black arrows point to the moment 1
velocity.

In Fig. 6, we present the intensity weighted mean velocity field (moment 1)
from the r10_t0 cube at $21.6\arcsec$. For this analysis, we use the r10_t0
cube as it provides a good combination of resolution and H i column density
sensitivity. The systemic velocity ($v_{sys}$, listed in Table 3) is measured
as the central velocity of the global profile – this is a reasonable
assumption given the symmetry of the global profile. The $v_{sys}$ is
indicated by the green arrow in Fig. 5, and by the thick black contour in Fig.
6. The H i kinematics presented in Fig. 6 are rather complex, but there
appears to be at least three main components to the velocity field: (1) a
regularly rotating inner disk with a mass measured within the
$\Sigma_{\textsc{H$\,$i}}=1\,\text{M}_{\odot}\,\text{pc}^{-2}$ contour of
$\mathrm{M_{disk}}=9.7\times 10^{8}\,\text{M}_{\odot}$; (2) a separate,
inclined warped “disk” which extends to larger radii than the optical disk,
that has an H i mass of $\mathrm{M_{outer}=8.9\times
10^{7}}\,\text{M}_{\odot}$, calculated as all the H i outside the inner disk
(including the 3rd component); (3) the north/north western quadrant which is
home to the clumpy clouds identified in Fig. 2 which make up on average
$8.9\times 10^{4}\,\text{M}_{\odot}$ each, with the most massive being C at
$1.6\times 10^{5}\,\text{M}_{\odot}$.

Table 5: H i mass measurements of the different components of the H i disk of NGC 5068. Component |  H i mass ($\text{M}_{\odot}$)
---|---
M${}_{\mathrm{\textsc{H$\,$i},\,total}}$ | $1.07\times 10^{9}$
Mdisk | $9.7\times 10^{8}$
Mouter | $8.9\times 10^{7}$
Average Mclumps | $8.9\times 10^{4}$
$\text{M}_{\textsc{H$\,$i}}$ (residual clumpy gas) | $2.6\times 10^{7}$

The first component is the regularly rotating main inner disk that is
coincident with the optical body represented by the magenta ellipse in Fig. 6.
We find that the
$\Sigma_{\textsc{H$\,$i}}=1\,\text{M}_{\odot}\,\text{pc}^{-2}$ contour (which
is roughly equivalent to $1.2\times 10^{20}\,\text{cm}^{-2}$, the second
lowest contour in the top panel of Fig. 3) neatly encircles this region of the
velocity field. This contour
($\Sigma_{\textsc{H$\,$i}}=1\,\text{M}_{\odot}\,\text{pc}^{-2}$) is usually
used to determine the H i diameter of galaxies (e.g. Wang et al., 2016), a
parameter that has a very tight correlation with the H i mass. The contour is
also roughly $20\text{ to }50\%$ larger than the optical disk (see top panel
of Fig. 3), which is consistent with other measurements of late type galaxies
(Bosma, 2016). Compared to the total H i mass of the system (see Table 5), the
inner disk ($M_{\mathrm{disk}}=9.7\times 10^{8}\,\text{M}_{\odot}$) clearly
makes up the majority ($\sim 90\%$) of the H i mass of the galaxy.

A position velocity (PV) diagram extracted along the optical major axis (see
Table 1) is presented in Fig. 7. The PV slice is extracted using the width of
the beam (21.6″) from the r10_t0 cube that has been Hanning smoothed and
regridded to $3\,\text{km}\,\text{s}^{-1}$. The PV slice shows that while in
the region corresponding to the optical disk (denoted by the vertical magenta
lines) there is regular rotation as seen in the velocity field (Fig. 6). The
light green circles overlaid on the PV slice in Fig. 7 indicate the moment 1
velocity in each line of sight. Anomalous gas is clearly visible in the PV
slice, indicated by the regions of gas deviating from the regular rotation and
extending to higher and lower velocities. The sudden dip in velocity on the
approaching side of the galaxy at roughly $-1.4\arcmin$ offset from the centre
has similar characteristics to the H i holes identified in NGC 6946 (Kamphuis
& Sancisi, 1993; Boomsma et al., 2008), and M31 and M33 (Deul & den Hartog,
1990). While the kinematics of the feature are similar to the NGC 6946 holes,
there is no obvious hole in the morphology of the H i. Thus it is more likely
that the feature in the PV diagram is indicative of some kind of expanding
feature or shell or a hole that has not blown out yet which is why there is no
corresponding feature in the intensity map. Features such as holes or shells
in the H i distribution are thought to be caused by energetic processes such
as supernovae or stellar winds. Since this dip in the PV slice coincides with
emission in the continuum and far-UV, both tracers of ongoing and recent star
formation, it is likely the aforementioned processes are responsible for the
feature. Another noteworthy feature of the PV slice is that there is clearly
low column density extraplanar gas distributed throughout the disk.

The second component is comprised of the low column density gas
($\mathrm{N}_{\mathrm{H}\,\textsc{i}}<10^{19}\,\text{cm}^{-2}$) at radii
larger than the inner disk. The gas in this component appears to have disk-
like kinematics with a position angle of $\sim\\!224^{\circ}$. In Fig. 8 we
present a PV diagram through the centre of NGC 5068 at this angle which shows
clearly the rotation of this component at larger radii than the inner disk
(represented by the vertical magenta lines in the figure). The elliptical
geometry of this component suggests that it is more inclined than the inner
disk – approximately $i\sim 53^{\circ}$. A closer look at the kinematics in
Fig. 6 show that the outer regions are twisted in an S-shape which suggests a
warp. Deep $r$\- ($\mu_{3\sigma}=26.5\,\text{mag}/\text{arcsec}^{2}$) and
$g$-band ($\mu_{3\sigma}=27.2\,\text{mag}/\text{arcsec}^{2}$) imaging from the
DECam Legacy Survey (DECaLS, Dey et al., 2019) Data Release 10 (DR10) shows
that there is no stellar counterpart associated with this gas.

The differences in the position angle and inclination between this component
and the inner disk suggest that this outer disk is not as a result of gas
being swept out of the inner disk, but rather it has a separate origin. Given
the kinematics and the geometry of the outer disk relative to the inner disk,
we describe it as a warped inclined outer disk which could possibly be a
warped polar disk. Characteristically polar disks are highly inclined relative
to the inner disk and the position angles can also be very different to that
of the inner disk – see the prototypical examples of NGC 4650A (Arnaboldi et
al., 1997) and NGC 660 (Gottesman & Mahon, 1990; van Driel et al., 1995).
Without proper modelling (see Deg et al. under review, for detailed modelling
of polar disk galaxy candidates), the polar disk scenario is difficult to
confirm. Throughout the rest of this work we will primarily refer to this
component as the outer disk.

The northern region of the galaxy containing the clouds marked A,B,C in Fig. 2
and Fig. 6 makes up the third component. The kinematics of this region are not
consistent with either the inner or outer disks, and thus we consider them a
separate component.

###  4.1 High moment 2 ring

In Fig. 9, we present the second moment map at $21.6\arcsec$ resolution for
NGC 5068. The moment 2 can be used as a measure of the linewidth of the H i
along each line of sight. In the case that the line profiles are Gaussian,
then the moment 2 represents the velocity dispersion. Normally the velocity
dispersion across the star forming disk is on average $\sim 10\text{ to
}15\,\text{km}\,\text{s}^{-1}$ (e.g. Leroy et al., 2008). The median moment 2
value within the optical disk region (indicated by the dashed magenta ellipse
in Fig. 9) is $\sigma_{v}\sim 18\,\text{km}\,\text{s}^{-1}$. Outside the
optical disk, there are regions where the moment 2 values are significantly
higher ($\sigma_{v}>25\,\text{km}\,\text{s}^{-1}$) than in the inner disk.
There are a number of processes that can add energy to the interstellar medium
of galaxies, and thus drive turbulence leading to an increase in the velocity
dispersion. Many of these processes are associated with star formation
activity, such as stellar feedback and supernova explosions (e.g. Tamburro et
al., 2009; Krumholz & Burkhart, 2016). Since the high moment 2 values in NGC
5068 are found outside the stellar disk, it is likely that something other
than star formation related processes is responsible for the high moment 2
values.

In order to investigate the source of the high moment 2 values, we take a PV
slice 1 beam wide ($21.6\arcsec$) through the ring of high moment 2 values on
the outside edge of the optical disk region, where values range from
$\sigma_{mom2}\sim 22\text{ to }35\,\text{km}\,\text{s}^{-1}$. The PV diagram
is presented in the top panel of Fig. 10. It is clear from this PV diagram, as
well as the major axis PV diagram in Fig. 7, that there is low column density
gas that spans a wide velocity range. It is probable that this broad low
column density component is present throughout the H i disk of this system but
is hidden in the moment 2 map due to much brighter narrow components,
particularly in the inner disk region. However, this is not likely the cause
of the high moment 2 ring on the edge of the optical disk.

In Fig. 10 we show four line profiles (P1 to P4) extracted at different points
along the PV slice. These profiles show that there are multiple distinct
overlapping components along the line of sight. This is particularly obvious
in P2 and P4. Given that we already know that there are multiple components
making up the overall H i disk for NGC 5068, it is likely that the multi-
component profiles responsible for the high moment 2 ring are as a result of a
superposition of the inner and outer disks. This could also explain the offset
between the peaks of the different components in P2 and P4.

Figure 11: Same as Fig. 4. The red contours ($0.5\sigma^{data}_{rms}\times
2^{n},\,n=0,2,4,..$) represent the inner and outer disk models generated with
Galmod. Figure 12: Left: H i gas surface density of the clumpy gas not
contained in the model overlaid on an GALEX FUV image of NGC 5068. Right: the
velocity field of the clumpy gas overlaid on the MeerKAT continuum image, the
colour shows difference in moment 1 velocity of the clumpy gas and the total
velocity field shown in Fig. 6.

###  4.2 Modelling the H i kinematics

In order to separate out the inner and outer disks from the third component so
that we can investigate the origin of the H i in this system, it is necessary
to create a model that describes the morphology and kinematics of the two
disks. Modelling the velocity field of NGC 5068 using a tilted ring (Rogstad
et al., 1974) model is not a trivial task, especially considering the
degeneracy of parameters such as position angle, inclination, and rotation
velocity of almost face-on galaxies such as this one (Józsa et al., 2007).

As discussed in the previous section, there are at least three components to
the total H i disk of this galaxy, however using a version of the Gipsy (Allen
et al., 1985; van der Hulst et al., 1992) task Galmod as implemented in
3dBarolo (di Teodoro & Fraternali, 2015), we have built a simple toy model
describing the rotation of the inner and outer disks. For the inner disk, we
used the inclination and position angle listed in Table 1 as measured from
near-infrared imaging. The inclination of the outer disk was set to a constant
of $53^{\circ}$ which was estimated from the geometry of the outer H i
contours.

We experimented with varying the velocity dispersion – using the average
velocity dispersion in the different annuli, as well as different constant
values ($6\text{ to }20\,\text{km}\,\text{s}^{-1}$), but found it had no real
impact on the final model. Thus the velocity dispersion was set to a constant
$15\,\text{km}\,\text{s}^{-1}$ for both the inner and outer disks as this is
similar to the median value of the moment 2 map. The position angles for the
warped outer disk, and the rotation velocities for both the inner and outer
disk were derived from fits of the parameters to the data using the fitting
task, 3dFit, in 3dBarolo. The centre for both the inner and outer disk was set
to the optical centre which is consistent with the centre of the H i.

The resulting model is shown by the red contours overplotted on the channel
maps in Fig. 11. The flux of the model is normalised by that of the data, and
is therefore not an independent parameter. The threshold of the lowest red
contour in Fig. 11 is chosen to match the $3\sigma_{rms}$ (blue) contour of
the data. We note that this simple model describes the two disks fairly well
apart from the fingers noted in Fig. 4 and the clouds (A,B,C) to the N/NW of
the galaxy which we did not include in the model.

In order to find the emission that is not well described by the model, we
blank the data where the model is greater than $0.5\sigma_{rms}$ or greater,
this value corresponds to the lowest red contour in Fig. 11 which was chosen
to match the $3\sigma_{rms}$ data (blue) contour. This gives a “residual” data
cube from which we can create the usual moment maps of the clumpy gas – the
gas that is not described by either disk model. We impose two further
restrictions on what we consider as part of the clumpy gas. The first
requirement is to only include high signal-to-noise features in the residual
maps, we do this by creating a S/N map using a linewidth of
$20\,\text{km}\,\text{s}^{-1}$. All pixels in the moment 0 map with S/N$<$5
are excluded. The second requirement is that the values of the corresponding
moment 2 map must be $\mathrm{mom2}<40\,\text{km}\,\text{s}^{-1}$ – this is to
exclude pixels where the emission could be due to wings of the line not being
properly modelled. It would be interesting to see if any of the clumpy gas is
related to star formation. We therefore compare the moment maps of the
anomalous gas with GALEX FUV observations and radio continuum observations.
Both are known to trace emission related to recent or ongoing star formation.
The FUV image is taken from the Survey for Ultraviolet emission in Neutral Gas
Galaxies (SUNGG, Wong et al., 2016) which collated new and archival targeted
observations for a sample of nearby galaxies. The H i surface density of the
clumpy gas is shown overlaid on a GALEX FUV image of NGC 5068 in the left
panel of Fig. 12, and the associated velocity field is overlaid on the MeerKAT
continuum image in the right panel of Fig. 12.

From both panels of Fig. 12, we can see that spatially, the clumpy gas is
mostly located outside of the main star forming disk, which suggests that this
gas is likely not currently involved with any star formation activity. The
total mass of this clumpy gas is $\mathrm{M_{clumpy}}=2.6\times
10^{7}\,\text{M}_{\odot}$ (see Table 5). Since the this gas was not included
in the model, it is clearly not rotating with either the inner disk or the
outer disk. The velocity field does show a velocity gradient in the different
clumps, however it is unclear what the cause of the gradient may be. However
it is evident that the clumpy gas on the northern side of the galaxy is
responsible for driving the peculiar morphology of the velocity field in that
region – see Fig. 6.

##  5 Origin of the anomalous gas

In the previous section, we have separated out the three components that
comprise the H i of this system: 1–the inner disk that is spatially coincident
with the optical disk; 2– the outer disk which has a kinematic warp, and due
to its geometry relative to the inner disk could be considered a polar disk;
3–the clumpy gas that is not described by either disk component. In this
section we discuss a number of scenarios that could explain the origin of the
peculiar behaviour of the H i in this system.

###  5.1 Interactions with nearby group galaxies

Figure 13 shows the eight spectroscopically confirmed galaxies within a
$5^{\circ}$ radius of NGC 5068. NGC 5068 was identified as part of a loose
group of galaxies (Pisano et al., 2011), PGC 46400 (Kourkchi & Tully, 2017).
Karachentsev et al. (2017) measured the distances to NGC 5068 ($5.2\pm
0.2\,\text{Mpc}$), LEDA 44681 ($7.2\pm 0.3\,\text{Mpc}$), UGCA 320
($6.03\,\text{Mpc}$), and UGCA 319 ($5.75\,\text{Mpc}$) using the redshift-
independent tip of the red giant branch method. They conclude that UGCA 320
and UGCA 319 are an isolated pair that are dynamically separate from NGC 5068
and LEDA 44681.

The nearest two galaxies, 2MASX J13292099-2110452 and LEDA 169678, have a
projected separation of $405\,\text{kpc}$ and a velocity separation of
$17\,\text{km}\,\text{s}^{-1}$ and $87\,\text{km}\,\text{s}^{-1}$
respectively. Despite these two close neighbours, NGC 5068 is classified as an
isolated galaxy using the criteria of Verdes-Montenegro et al. (2005): there
are no other galaxies with diameters between $1/4$ and 4 times the diameter of
the target that lie within a radius that is 20 times the diameter of the
potential neighbour. This is based on the assumption that for a galaxy with a
diameter of $25\,\text{kpc}$, an interloper of similar mass moving at a “field
velocity” of $150\,\text{km}\,\text{s}^{-1}$ would take $3\,\text{Gyr}$ to
close a separation of 20 times the interlopers diameter ($\sim
500\,\text{kpc}$)333Gas orbital time of NGC 5068 is $\sim 1\,\text{Gyr}$,
meaning that any gas would have settled following such an interaction..

The right panels of Fig. 13 show the composite $giz$ images of the group
galaxies (excluding NGC 5068) as observed by DECaLS DR10. The $5\times
5\,\text{arcmin}^{2}$ colour images show a collection of blue galaxies that
are significantly smaller on the sky than NGC 5068 which has a semi-major axis
of $7.03\arcmin$ (see Table 1). Over the redshift range of these galaxies,
$1\arcsec$ ranges from $40\,\text{pc}$ to $57\,\text{pc}$. Based on the
optical colours, sizes, and morphologies of this collection of galaxies, it is
not likely even the closest two neighbours discussed above have ever
interacted with NGC 5068.

Figure 13: Nearby galaxies around NGC 5068. The galaxies marked with blue
squares or orange circles were identified as group members with NGC 5068 by
Pisano et al. (2011) and Kourkchi & Tully (2017). Left panel: sky distribution
of the galaxies, the circles represent regions of different sizes used to
determine isolation or identify group members. The R1t and R2t circles are
taken from Kourkchi & Tully (2017) and are used to group sources together. The
AMIGA (Verdes-Montenegro et al., 2005) isolation radius for NGC 5068 is
represented by the dotted cyan circle. For reference, also plotted is the
MeerKAT field of view (dashed green circle), and the brown dashed circle
represents a radius of $300\,\text{kpc}$. The colour $giz$ images on the right
are $5\times 5\,\text{arcmin}^{2}$ cutouts taken from DECaLS DR10.

###  5.2 Stripped gas from a passing neighbour

What if the anomalous gas is the result of a gas rich unidentified neighbour
passing by NGC 5068 and losing all of its gas to NGC 5068? In such scenario
the gas may have been stripped off the neighbour which continued on past NGC
5068, and part of the gas settled into the potential of NGC 5068 as the outer
disk with the remainder of the gas remaining as clumps beyond the outer disk.
We do not believe this to be a likely origin of the anomalous gas. Below we
discuss why the scenario is improbable using simple assumptions based on the
mass of the H i in the outer disk.

If the interaction had taken place, it would have been more than
$1\,\text{Gyr}$ in the past as the outer gas has already completed at least
one orbit as it has settled into a disk-like component within the potential of
NGC 5068.

Given the H i mass of the outer gas ($\text{M}_{\textsc{H$\,$i}}=8.9\times
10^{7}\,\text{M}_{\odot}$), the stellar mass of the neighbour would be roughly
$4.45\times 10^{7}\,\text{M}_{\odot}$, this assumes
$\text{M}_{\textsc{H$\,$i}}/\text{M}_{\star}=2$, which is consistent with the
Small Magellanic Cloud (Stanimirović et al., 1999) and other dwarf galaxies
(Huang et al., 2012). The stellar mass ratio between the two galaxies is
$1:50$, which means that the neighbour would not likely leave an imprint on
NGC 5068 as a result of the interaction.

Using the $\text{M}_{\textsc{H$\,$i}}$–$D_{\textsc{H$\,$i}}$ relation from
Wang et al. (2016, Eq. 2), and assuming the diameter of the optical disk is
$0.8\times D_{\textsc{H$\,$i}}$, we determine the neighbour is roughly
$D_{opt}\sim 4\,\text{kpc}$ ($\sim 1.4\arcmin$ at $5.2\,\text{Mpc}$). With a
conservative stellar mass to light ratio of 0.6, we calculate the mean
$r$-band surface brightness of the neighbour to be $\mu_{r}=23.7\,\text{mag
arcsec}^{-2}$. If this neighbour passed by NGC 5068 with a maximum velocity of
$v=100\,\text{km}\,\text{s}^{-1}$, it would be located within a
$100\,\text{kpc}$ ($36\arcmin$) radius after $1\,\text{Gyr}$. The $5\sigma$
detection threshold in the DECaLS $r$-band image is $\mu_{r}=26.3\,\text{mag
arcsec}^{-2}$ which means that the neighbour would be visible in the DECaLS
data, however there is no likely candidate in the DECaLS imaging.

###  5.3 Minor merger with a gas rich galaxy

Since there is no neighbour in our search area, what if NGC 5068 underwent a
merger with a gas-rich low mass satellite? Given the H i mass difference
between the inner and outer disks, this would suggest a merging ratio of 1:10.
Such an event would have to have taken place more than $1\,\text{Gyr}$ ago as
the outer H i has settled in the potential of NGC 5068. Minor mergers are
usually defined as where the lower mass galaxy is more than four times less
massive than the larger galaxy. Some studies (Lotz et al., 2008; Conselice,
2006) have suggested that the timescales for minor mergers can vary from
$\tau\sim 0.3\text{ to }1\,\text{Gyr}$ assuming an initial separation of
$30\,\text{kpc}$. More recently, Conselice et al. (2022) have shown using
hydronamical simulations that the timescale of mergers is also dependent on
both redshift and mass ratio of the merging galaxies. Based on the Conselice
et al. (2022) study, the merger timescale for a $z\sim 0$ merger with a mass
ratio of 1:10, could range between $\tau\sim 1.7\text{ to }2\,\text{Gyr}$,
however this assumes that the galaxies have
$\text{M}_{\star}>10^{9}\,\text{M}_{\odot}$. Given that the H i in the NGC
5068 system has already settled, it is likely that if a minor merger had
occurred it would have happened more than $2\,\text{Gyr}$ ago. Despite the
mass difference between the constituent galaxies, minor mergers are known to
have an impact on the morphology and kinematics of the larger galaxy (e.g.
Kazantzidis et al., 2009; Qu et al., 2011; Martin et al., 2018; Ghosh et al.,
2022).

The optical morphology of NGC 5068 is undisturbed, and does not appear to have
undergone any recent interactions. However it is possible that in such a
scenario the main stellar disk of NGC 5068 was undisturbed. Mergers are
thought to be one of the origin scenarios of polar disks (e.g. Stanonik et
al., 2009, and references therein). While there is no clearly observable
stellar component within the available deep optical imaging from DECaLS
associated with the outer disk, it is possible that the outer disk could be
caused by a minor merger and the associated stellar disk has been stretched
out below detection limits. This scenario, however, does not explain the
clouds which are a largely located on the northern side of the galaxy.

If all the anomalous gas (all H i gas not associated with the inner disk) from
NGC 5068 could be traced back to one event such as a minor merger, given the
time that has likely elapsed ($>2\,\text{Gyr}$), one would expect that the
clouds would have settled into a more symmetric distribution around the
galaxy. While we cannot conclusively rule out a minor merger as a possible
origin scenario given that this scenario does not neatly explain all the gas,
we do not do assign this scenario a high probability of being the origin of
the anomalous gas.

###  5.4 Fountain triggered accretion

Galactic fountains can be an important mechanism by which gas is recycled
within a galaxy system (see, for example, Shapiro et al., 1976; Fraternali &
Binney, 2006): gas is expelled from the main disk by processes associated with
star formation such as supernovae and solar winds. The expelled gas mixes with
the hotter gas in the halo and then the mixed gas cools and condenses before
it is re-accreted onto the galaxy. This fountain driven accretion of gas means
that the accreted gas contains both the original expelled gas, and gas from
the halo, the net flow of the gas is into the galaxy. Given that this process
arises as a result of activity in the stellar disk, the accreted gas is likely
to be coincident with the stellar disk.

Simulations suggest that gas accreted onto the galaxy subsequent to mixing
with gas ejected from the galaxy disk through fountain processes are likely to
have a higher metal content than primordial gas (see review by Almeida et al.,
2014; Brook et al., 2014; Tumlinson et al., 2017) which is thought to occur
along filaments into the galaxy. Thus looking at the gas phase metallicity of
the star forming regions on the edge of the galaxy disk, particularly where
the anomalous gas and the star forming regions overlap, may provide some
insight into the origin of gas (Howk et al., 2018).

NGC 5068 has been observed by a number of multi-wavelength surveys, but two
surveys in particular have looked at the gas-phase metallicity of the star
forming regions in NGC 5068: PHANGS-MUSE (Emsellem et al., 2022; Williams et
al., 2022) and TYPHOON (M. Seibert et al. in prep; Grasha et al., 2022). Both
Williams et al. (2022) and Grasha et al. (2022) show that the metallicity of
the gas in NGC 5068 cannot be explained by the simple radial model that
describes the negative metallicity gradient seen in star-forming galaxies.
Grasha et al. (2022, Fig. 3) in particular show that the H ii regions on the
northern side of the galaxy are more metal poor than regions at the same
radius on the southern side of the galaxy.

Given the distribution of the star forming regions throughout NGC 5068 (see
the background FUV and continuum images in Fig. 12), if fountain-driven
accretion was responsible for the anomalous gas in NGC 5068, it is plausible
that the distribution of the clumpy gas not explained by the disks would be
more equally distributed around the galaxy. We conclude that while it is very
likely that there are fountain-processes ongoing in NGC 5068, it is not the
likely source of the anomalous H i gas.

###  5.5 Accretion of gas along a filament

The final scenario that we consider is that the anomalous gas is a result of
gas accretion external to the galaxy halo. How this accretion occurs, and what
the observational signatures of such accretion look like are still unclear.
Simulations have shown that in galaxies the size of NGC 5068, accretion is
expected to occur along filaments, condensing into clumps and clouds as it
nears the galaxy disk (Kaufmann et al., 2006; van de Voort et al., 2011b;
Wetzel & Nagai, 2015; Cornuault et al., 2018; Iza et al., 2022).

Several observational signatures have been suggested as proof of these
accretion modes. Due to the gas condensing as it reaches the H i disk, it is
expected that the gas is accreted in clouds or clumps. Clouds or clumps with
anomalous velocities have been observed around the Milky Way, these clouds,
are known as intermediate (IVC) and high (HVC) velocity clouds (see review by
Wakker & Van Woerden, 1997; Wakker et al., 1999, and references therein). The
clouds around the Milky Way have been shown to trace the flow of accreting
gas, both as a result of extragalactic accretion (Wakker et al., 2007, 2008;
Peek et al., 2008), and re-accretion of gas relating to a galactic fountain
(McClure-Griffiths et al., 2013; Marasco et al., 2022).

The left panel of Fig. 12 shows that the anomalous gas associated with NGC
5068 is clumpy, it should be noted that majority of this clumpy gas is very
low column density
($\mathrm{N}_{\mathrm{H}\,\textsc{i}}<10^{19}\,\text{cm}^{-2}$). As discussed
in Section 3, the clouds on the north western side of the galaxy range in H i
mass from $\text{M}_{\textsc{H$\,$i}}\sim 10^{4}\,\text{M}_{\odot}$ to
$\text{M}_{\textsc{H$\,$i}}\sim 10^{5}\,\text{M}_{\odot}$ which is consistent
with the H i masses of the IVC/HVC complexes around the Galaxy (Wakker et al.,
2008). The velocity field of the anomalous gas relative to the galaxy velocity
field (see Fig. 6) is shown in the right panel of Fig. 12. The velocities of
the anomalous gas show that the clouds coincident with the star-forming disk
deviate from the overall rotation of the main H i disk.

In Section 5.3, we mentioned how mergers are one of the formation scenarios of
polar disks. More recently, it has been shown that accretion could also
trigger the formation of a polar disk, and this case an associated stellar
component would not be expected (Macciò et al., 2006; Brook et al., 2008;
Stanonik et al., 2009). Another feature present in polar disks when accretion
is considered the origin, is kinematic warps (e.g. Brook et al., 2008). While
the origin of the warps are not yet clear, it has been shown that they can be
explained as the result of skewed angular momentum of gas accreting due to
cosmic infall (Ostriker & Binney, 1989; Jiang & Binney, 1999; Sánchez-Salcedo,
2006; Rand & Benjamin, 2008).

Both the morphology and the kinematics of the anomalous gas in combination of
the warped (possibly polar) outer disk of NGC 5068 point to ongoing accretion.
It is interesting to note to that some of the clumpy gas is coincident with
the optical disk right where there is some of the brightest star formation on
the northern ridge of the galaxy. These star forming regions also have some of
the lowest gas phase metallicity in the maps presented in Grasha et al.
(2022).

Putting all of this together, we propose the following scenario: gas is being
accreted along a filament that is aligned with the north/north western edge of
the galaxy. This gas condenses into the clumps/clouds when it gets close to
the galaxy disk, over time this accretion has given rise to what is now
observable as the outer disk. Within this scenario, it is possible that not
all the clouds feed the outer disk, but some are pulled into the star-forming
disk enhancing the the star formation on the northern side of galaxy.

##  6 Summary

In this paper we have presented new MeerKAT observations of the H i in NGC
5068 taken as part of the MeerKAT Large Survey Project, MHONGOOSE. The
combination of the impressive sensitivity and resolution of these data have
revealed a number of interesting features in the H i disk of NGC 5068. We have
identified three separate components to the total H i disk: the inner disk,
the outer disk which shows signatures of a kinematic warp and given the
geometry could be a polar disk, and lastly the clumpy region to the north
western side of the galaxy. We constructed a model that contained a regularly
rotating inner disk plus a more inclined warped outer disk that described the
majority of the gas kinematics adequately. Using the model, we were able to
isolate a significant amount of clumpy gas that was not well explained by the
model of the disks. This clumpy gas accounts for $\sim 2\%$ of the total H i
mass of the system and also contains the clumps to the north/north west of the
galaxy.

We explored a number of different possible origin scenarios for the anomalous
gas:

  * (a)

Interaction with the nearby group galaxies – the projected distances between
the galaxies make this an unlikely scenario.

  * (b)

Stripped gas from a passing neighbour – assuming the neighbour survived the
passage, we find no evidence for its existence.

  * (c)

A minor merger with a gas rich neighbour – we cannot conclusively exclude this
scenario, but do not believe it is the most likely scenario as it does not
explain the presence of the clumpy gas on only one side of the galaxy.

  * (d)

Fountain triggered accretion – accretion is likely, but this scenario would
suggest a more metal enrichment than what is observed in the outer star
forming regions, and for the clumpy gas distribution to follow the locations
star forming regions.

  * (e)

Accretion of gas along a filament – this scenario neatly explains the
kinematics and morphology of all the anomalous H i.

While the last scenario is the most likely based on the data we have presented
in this paper, further detailed modelling of the gas is needed to understand
if and how the gas is transported between the inner and outer disks.
Unambiguous detection of cold mode accretion of gas onto galaxies will require
the detection of H i gas at column densities of
$\mathrm{N}_{\mathrm{H}\,\textsc{i}}\sim 5\times 10^{18}\,\text{cm}^{-2}$ or
lower, and at cloud scale resolution in combination of other tracers of cold
gas such as Lyman-$\alpha$ (van de Voort & Schaye, 2012; Ao et al., 2020;
Kacprzak, 2017). Simulations and some recent observations of high redshift
galaxies suggest that Lyman-$\alpha$ emission traces cold gas being accreted
onto the galaxies (e.g. Dijkstra & Loeb, 2009; Daddi et al., 2021). However,
with the currently available facilities, the ongoing MHONGOOSE survey provides
the best chance of detecting and studying in detail, the low column density H
i that could be linked to cold gas accretion.

###### Acknowledgements.

We acknowledge useful discussions on the interpretation of this data with
members of the MHONGOOSE team. JH thanks Nathan Deg for helpful discussions on
polar ring galaxies. We thank the MeerLICHT team for their help with the
MeerLICHT imaging, in particular: Paul Vreeswijk, Danielle Pieterse, Steven
Bloemen, Paul Groot, and Patrick Woudt. Thanks to Filippo Fraternali and Thijs
van der Hulst for useful discussions about the interpretation of the H i in
this galaxy. This project has received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020 research and innovation
programme grant agreement no. 882793, project name MeerGas. PK acknowledges
financial support by the German Federal Ministry of Education and Research
(BMBF) Verbundforschung grant 05A20PC4 (Verbundprojekt D-MeerKAT-II). DJP and
NZ are supported through the South African Research Chairs Initiative of the
Department of Science and Technology and National Research Foundation. KS
acknowledges support from the Natural Sciences and Engineering Research
Council of Canada (NSERC). BKG acknowledges the financial support of the
European Union’s Horizon 2020 Research and Innovation Programme (ChETEC-INFRA
– Project no. 101008324). LVM acknowledges financial support from the grant
CEX2021-001131-S funded by MCIN/AEI/ 10.13039/501100011033, from the grant
PID2021-123930OB-C21 funded by MCIN/AEI/10.13039/501100011033, by “ERDF A way
of making Europe” and by the European Union. LC acknowledges the financial
support from the Chilean Agencia Nacional de Investigación y Desarrollo
through the grant Fondecyt Regular 1210992. This paper makes use of MeerKAT
data. The MeerKAT telescope is operated by the South African Radio Astronomy
Observatory, which is a facility of the National Research Foundation, an
agency of the Department of Science and Innovation. This research made use of
Astropy,444http://www.astropy.org a community-developed core Python package
for Astronomy (Robitaille et al., 2013; The Astropy Collaboration et al.,
2018). Part of the data published here have been reduced using the CARACal
pipeline, partially supported by ERC Starting grant number 679627 “FORNAX”,
MAECI Grant Number ZA18GR02, DST-NRF Grant Number 113121 as part of the ISARP
Joint Research Scheme, and BMBF project 05A17PC2 for D-MeerKAT. Information
about CARACal can be obtained online under the URL:
https://caracal.readthedocs.io.

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