[table]capposition=top

# Variability in extragalactic class I methanol masers: New maser components
toward NGC 4945 and NGC 253

T. P. McCarthy,1,2 S. P. Ellingsen,1 S. L. Breen,3 M. A. Voronkov,2 X. Chen4,5
and H-h. Qiao5,6

1 School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart,
Tasmania 7001, Australia
2 Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710,
Australia
3 Sydney Institute for Astronomy (SIfA), School of Physics, University of
Sydney, NSW 2006, Australia
4 Center for Astrophysics, GuangZhou University, Guangzhou 510006, China
5 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai
200030, China
6 National Time Service Center, Chinese Academy of Sciences, Xi’An, Shaanxi
710600, China
Email<EMAIL_ADDRESS>
###### Abstract

We have used the Australia Telescope Compact Array (ATCA) to make new
observations of the 36.2-GHz ($4_{-1}\rightarrow 3_{0}$E) methanol transition
toward NGC 4945 and NGC 253. These observations have revealed the presence of
new maser components toward these galaxies, and have provided the first clear
evidence for variability in extragalactic class I methanol masers. Alongside
the new observations of NGC 4945 and NGC 253, we present the results of recent
36.2-GHz methanol maser searches toward 12 galaxies, placing upper limits on
the emission from the 36.2-GHz class I transition and the 37.7-GHz
($7_{2}\rightarrow 8_{1}$E) class II maser line toward these sources. Flux
density values for the 7-mm continuum emission toward these sources are also
reported where applicable. A re-analysis of the published 36.2-GHz methanol
observations of Arp 220 undertaken as part of the search revealed some issues
with previous imaging procedures. The re-analysis, combined with non-
detections in independent follow-up observations suggest that there is no
36.2-GHz methanol emission toward Arp 220 stronger than 3.5 mJy in a 10
$\mbox{km~{}s}^{-1}$ channel (5$\sigma$ upper limit).

###### keywords:

masers – radio lines: galaxies – galaxies: starburst

##  1 Introduction

The properties of galaxies (particularly those that are very distant) are
often unable to be directly measured. Instead, we rely on indirect
measurements through observations of some proxy. Historically, maser emission
has proven to be a powerful tool for both direct and indirect investigation of
various astrophysical phenomena both within our Galaxy and in others (e.g.
star-formation, outflows, black-hole accretion disks; Moran et al., 1999;
Kurtz et al., 2004). Galactic methanol masers, in particular, allow insight
into the various stages of star-formation regions. Their compact and generally
highly luminous nature, allows for conditions or evolutionary stages to be
detected and accurately ascribed to certain locations within these star-
forming regions (e.g. Cyganowski et al., 2009; Cyganowski et al., 2012; Breen
et al., 2013; Voronkov et al., 2014). Extragalactic methanol masers may prove
to be a similarly interesting tool for investigation of external galaxies,
however, at this stage they are a relatively newly discovered and not well
understood phenomenon.

The majority of known extragalactic methanol masers have been detected in the
past five years. Class I methanol masers have been reported in NGC 253, Arp
220, NGC 4945, NGC 1068, IC 342 and NGC 6946 (Ellingsen et al., 2014; Wang et
al., 2014; Chen et al., 2015; McCarthy et al., 2017, 2018b; Gorski et al.,
2018) and class II masers in the Large Magellanic Cloud (LMC) and M31 (Green
et al., 2008; Ellingsen et al., 2010; Sjouwerman et al., 2010). Considering
these extragalactic examples, the class II masers appear to be highly luminous
examples of regular Galactic-type class II masers (Ellingsen et al., 2010),
however, this is not the case with the current detections of class I masers.
Instead, they appear to be associated with large-scale regions of low-velocity
shocks, such as molecular in-fall, within their host galaxies (Ellingsen et
al., 2017; Gorski et al., 2018; McCarthy et al., 2018a).

NGC 253 and NGC 4945 are two nearby, southern, barred spiral galaxies with
starburst nuclei. These sources are considered the prototypical examples of
extragalactic class I methanol masers. The methanol masers in these sources
are associated with the galactic bars of their host. The masers in NGC 253
appear to be associated with the inner bar-nucleus interface, where molecular
in-fall along the bar is interacting with the central starburst zone,
resulting in large-scale low velocity shocks (Ellingsen et al., 2017). The
single maser region in NGC 4945 appears to be located at the interface region
between the galactic bar and the south-eastern spiral arm (Henkel et al.,
2018; McCarthy et al., 2018a).

Currently, there is no direct evidence for variability toward any
extragalactic methanol maser sources. In Galactic sources, variability has
been detected in numerous class II masers (e.g. Goedhart et al., 2003; Olech
et al., 2019). However, the same has not been reported for Galactic class I
sources. Variability in these class II sources has been suggested to be
related to either changes in the pump rate (Araya et al., 2010), or variations
in seed photon flux of the masers (van der Walt, 2011).

In this paper we report the results of new 7-mm observations toward NGC 4945
and NGC 253. Additionally we report upper-limits on the luminosity of 36.2-GHz
class I and 37.7-GHz class II methanol emission toward twelve extragalactic
sources (see Table 1), from data taken with the ATCA over the past few years.
The initial search for extragalactic class I methanol masers was made towards
a sample of luminous extragalactic OH and H2O megamasers (9 of the 12 sources
reported here). These targets were selected before the first detection of an
extragalactic class I methanol maser had been reported. This initial search
was responsible for the first successful detections of 36.2-GHz class I masers
reported in Ellingsen et al. (2014, 2017) and McCarthy et al. (2017). The
remaining three sources (NGC 1097, NGC 1792 and NGC 1808) were observed as
part of a follow up search, and as such selected targets are based on the
general properties of NGC 253 and NGC 4945 (elevated star-formation rates,
barred spirals etc.). We also provide a discussion of the entire sample, both
detections and non-detections, which will allow a more focussed approach to
target selection, allowing a higher success rate for future searches.

Table 1: Details for target sources. Recession velocities have been taken from
the NASA/IPAC Extragalactic Database† (NED). Redshift-independent distances
have been provided where possible, otherwise distances reported are the
luminosity distances provided by NED.

Target | Right Ascension | Declination | $V_{helio}$ | $D$ | Type
---|---|---|---|---|---
Source |  $h$ $m$ $s$ |  ∘ $\prime$ $\prime\prime$ | ($\mbox{km~{}s}^{-1}$) | (Mpc) |
NGC 253 | 00 47 33.10 | –25 17 18.00 | 243 | 3.4[1] | SAB(s)c
IRAS 01417+1651 | 01 44 30.50 | +17 06 05.00 | 8214 | 119 | pair of galaxies
NGC 1097 | 02 46 19.06 | –30 16 29.68 | 1271 | 9.51[2] | SB(s)b, Sy1
NGC 1792 | 05 05 14.45 | –37 58 50.70 | 1211 | 9.6[3] | SA(rs)bc
NGC 1808 | 05 07 42.34 | –37 30 46.98 | 995 | 6.8[3] | (R′)SAB(s)a, Sy2
ESO 558-g009 | 07 04 21.02 | –21 35 19.20 | 7674 | 118 | Sc
IC 2560 | 10 16 18.70 | –33 33 50.00 | 2925 | 32.5[4] | (R′)SB(r)b
IRAS 10173+0829 | 10 20 00.20 | +08 13 34.00 | 14716 | 231 | LIRG
IRAS 11506-3851 | 11 53 11.70 | –39 07 49.00 | 3232 | 30.3[5] | (R′)SAB(r)a
ESO 269-g012 | 12 56 40.50 | –29 34 02.81 | 5014 | 78.8 | S0, Sy2
NGC 4945 | 13 05 27.50 | –49 28 06.00 | 563 | 3.7[6] | SB(s)cd, Sy2
Circinus | 14 13 09.30 | –65 20 21.00 | 434 | 4.21[7] | SA(s)b
NGC 5793 | 14 59 24.70 | –16 41 36.00 | 3491 | 55 | Sb, Sy2
IIZw 96 | 20 57 23.90 | +17 07 39.00 | 10822 | 159 | pair of galaxies

Note: [1]Dalcanton et al. (2009), [2]Bottinelli et al. (1985), [3]Sorce et al.
(2014), [4]Yamauchi et al. (2012), [5]Tully et al. (2016), [6]Tully et al.
(2013) and [7]Tully et al. (2009)

Table 2: Details of the observations for all target sources reported in this paper. The asterisk denotes an observation epoch previously reported in McCarthy et al. (2018a). The details for this epoch have been reproduced here as it is important for the discussion of variability in Section 4.3. Target | Epoch | Array | Flux | Bandpass | Phase | On-source
---|---|---|---|---|---|---
Source |  | Configuration | Calibrator | Calibrator | Calibrator | Time
NGC 253 | 2019 March | H214 | PKS B1934-638 | PKS B1921-293 | 0116-219 | 4.84h
IRAS 01417+1651 | 2014 March | H168 | Uranus | PKS B1921-293 | 0221+067 | 0.50 h
NGC 1097 | 2018 November | H168 | PKS B1934-638 | PKS B1921-293 | 0237-233 | 4.61h
NGC 1792 | 2018 December | H168 | PKS B1934-638 | PKS B1921-293 | 0521-365 | 3.72h
NGC 1808 | 2018 November | H168 | PKS B1934-638 | PKS B1921-293 | 0521-365 | 1.97h
ESO 558-g009 | 2014 March | H168 | Uranus | PKS B1921-293 | 0648-165 | 0.49 h
IC 2560 | 2014 March | H168 | PKS B1934-638 | PKS B1253-055 | 1034-293 | 0.20 h
IRAS 10173+0829 | 2015 August | EW352 | PKS B1934-638 | PKS B1921-293 | 1036+054 | 4.02h
IRAS 11506-3851 | 2014 March | H168 | PKS B1934-638 | PKS B1253-055 | 1144-379 | 0.50 h
ESO 269-g012 | 2014 March | H168 | PKS B1934-638 | PKS B1253-055 | 1322-427 | 0.20 h
NGC 4945 | 2017 June* | H214 | PKS B1934-638 | PKS B1921-293 | j1326-5256 | 1.64h
| 2017 July | H75 | Uranus | PKS B1253-055 | j1326-5256 | 1.92h
| 2019 March | H214 | PKS B1934-638 | PKS B1921-293 | j1326-5256 | 3.93h
Circinus | 2015 August | EW352 | PKS B1934-638 | PKS B1921-293 | 1414-59 | 2.80 h
NGC 5793 | 2014 March | H168 | PKS B1934-638 | PKS B1253-055 | 1510-089 | 0.20 h
IIZw 96 | 2015 August | EW352 | PKS B1934-638 | PKS B1921-293 | 2121+053 | 2.32 h

##  2 Observations

The 7-mm observations reported in this paper were all made using the Australia
Telescope Compact Array (ATCA) across multiple epochs between 2014 and 2019
(see Table 2). We adopted rest frequencies of 36 169.238$\pm 0.001$ MHz
(Voronkov et al., 2014) and 37 703.700$\pm$0.030 MHz (Tsunekawa et al., 1995)
for the class I 36.2- and class II 37.7-GHz transitions of methanol
respectively.

Our observations are single pointings toward the centre of each galaxy (see
Table 1). At 36.2-GHz the FWHM of the ATCA primary beam is 84 arcseconds. This
corresponds to a linear radius from the pointing centre of $\sim$700 pc for
our nearest source (NGC 253 at 3.4 Mpc) and $\sim$47 kpc for our furthest
(IRAS 10173+0829 at 231 Mpc). This range is sufficient for detecting class I
methanol masers at similar spatial offsets from the nucleus of the galaxies as
those reported in previous reported detections.

Either PKS B1921–293 or PKS B1253–055 was utilised as a bandpass calibrator
dependent on the epoch. The 10 minute scans of the target source were
interleaved with 2 minute phase calibrator scans throughout the observations,
with flux density calibration using PKS B1934–638 and Uranus. All sources
observed in each epoch, and their relevant calibrators are listed in Table 2.

The details of the Compact Array Broadband Backend (CABB; Wilson et al., 2011)
configurations for each epoch are detailed in the following subsections.

###  2.1 2014 March

Observations were made on 2014 March 26 and 28 (project code C2879), using the
H168 hybrid array configuration (minimum and maximum baselines of 61 and 192m
respectively). CFB 1M CABB mode was utilised for these observations, resulting
in $2\times 2048$ MHz IF bands, with centre frequencies of 35.3 GHz and 37.3
GHz respectively. The channel width for these IF bands was 1 MHz ($\sim$8.2
$\mbox{km~{}s}^{-1}$ at 36.2-GHz) resulting in 2048 spectral channels for each
IF band. A typical synthesised beam for this hybrid array at 36.2-GHz is
approximately $9^{\prime\prime}\times 5^{\prime\prime}$ (dependent on source
declination).

Dependent on the day, the observing strategy utilised PKS B1934-638 or Uranus
as flux density calibrators, with bandpass calibrated with respect to PKS
B1921-293 or PKS B1253-055 (see Table 2). Scans of target sources were
interleaved with phase calibration scans on nearby appropriate sources.

The weather for this epoch of observation was particularly bad which, when
combined with low on-source times, resulted in bad data quality. We have only
reported detection upper-limits for those sources which we could image to a
sufficient standard. This generally includes sources where we could get
continuum detections (see Table 6) in order to self-calibrate the data and
achieve higher sensitivity. As an example of the data quality, NGC 4945 was
originally observed during this epoch, and the data quality is too poor to
detect the strong 36.2-GHz emission observed toward this source in
observations since. A table of sources that were also observed, but with too
low data quality (RMS over an order of magnitude higher than expected) to draw
any useful conclusions, is included in the appendix (Table A.1).

###  2.2 2015 August

Observations were made on 2015 August 25 and 26 (project code C2879), using an
EW352 east-west array configuration (minimum and maximum baselines of 31 and
352m respectively). The CABB was configured in the CFB 1M/64M hybrid mode,
consisting of $2\times 2048$ MHz IF bands. The first IF band has a channel
width of 1 MHz resulting in 2048 spectral channels, with a centre frequency
for the band of 36.85 GHz. The second IF is $32\times 64$ MHz channels (also
centred on 36.85 GHz) with the ability to define up to 16$\times$2048 channel
zoom bands with a fine resolution of 32 kHz (resulting in a channel width of
$\sim$0.26 $\mbox{km~{}s}^{-1}$ at 36.2-GHz). Two sets of five zoom bands were
combined (‘stitched’) together to cover each of the two lines of primary
interest (36.2-GHz class I and 37.7-GHz class II methanol). A typical
synthesised beam for these 36-GHz EW352 observations is
$26^{\prime\prime}\times 4^{\prime\prime}$.

###  2.3 2017 July

Observations were made on 2017 July 7 (project code C3617), using the ATCA H75
hybrid array configuration (minimum and maximum baselines of 30.6 and 89.2m
respectively). The CABB was configured in CFB 1M-0.5k mode, resulting in
$2\times 2048$ MHz IF bands (with centre frequencies of 36.85 and 32.20 GHz),
each with 1 MHz spectral resolution ($\sim$8.2 $\mbox{km~{}s}^{-1}$ at
36.2-GHz) and the option of configuring 16 zoom bands per IF, with 2048
channels and a fine resolution of 0.5 kHz ($<0.01$ $\mbox{km~{}s}^{-1}$ at
36.2-GHz). We did not use zoom bands for these observations, due to the fine
resolution producing insufficient velocity coverage for extragalactic sources
at this frequency. These H75 array observations at 36.2-GHz had a synthesised
beam size of approximately $27^{\prime\prime}\times 12^{\prime\prime}$.

###  2.4 2018 November and December

Observations were made on 2018 November 28 and 29, and December 3 (project
code C3263) using the H168 hybrid array configuration (minimum and maximum
baselines of 61 and 192m respectively). The CFB 64M-32k CABB configuration was
used, consisting of two $32\times 64$ MHz IF bands (centre frequencies of 32.2
and 36.85 GHz respectively), with both bands additionally allowing up to
16$\times$2048 channel zoom bands to be defined with a fine resolution of 32
kHz. Each target methanol line (36.2-GHz class I and 37.7-GHz class II
methanol), was covered by 4 stitched zoom bands (the 32 kHz channel width
corresponds to $\sim$0.26 $\mbox{km~{}s}^{-1}$ at this frequency). The
synthesised beams of these observations are approximately
$6^{\prime\prime}\times 4^{\prime\prime}$.

It should be noted that due to frequency configuration issues, the flux
density calibration for NGC 1097 required approximation based on levels seen
from the other recent observations. NGC 1097 and NGC 1808 were observed one
day apart and comparison of the flux density values for the bandpass
calibrator used for both these sets of observations (PKS B1921–293) allowed
for an approximate flux density calibration for NGC 1097. Therefore, both RMS
values reported in Table 3 and continuum flux density values in Table 6 have
higher levels of uncertainty ($\sim$40 percent) than all other sources.

###  2.5 2019 March

Observations were made on 2019 March 3 (project code C3167), using the H214
hybrid array configuration minimum and maximum baselines of 82 and 247 m
respectively). The CFB 64M-32k CABB configuration was used, consisting of two
$32\times 64$ MHz IF bands (centre frequencies of 36.8 and 38.00 GHz
respectively), with both bands additionally allowing up to sixteen 2048
channel zoom bands to be defined with a fine resolution of 32 kHz. The target
36.2-GHz methanol line was covered by 4 stitched zoom bands (the 32 kHz
channel width corresponds to $0.26$ $\mbox{km~{}s}^{-1}$ at this frequency).
These observations had a synthesised beam of
$6\aas@@fstack{\prime\prime}3\times 4\aas@@fstack{\prime\prime}4$ and
$5\aas@@fstack{\prime\prime}6\times 4\aas@@fstack{\prime\prime}3$ for NGC 4945
and NGC 253 respectively.

The bandpass calibrator PKS B1921-293 was used with flux density calibrated
against PKS B1934-638. A nearby quasar was selected for interleaved phase
calibration scans for each source. Details on these sources can be found in
Table 2.

The weather during this session was not ideal for mm-wavelength observing,
with high ambient temperatures and patchy cloud cover. This results in higher
RMS noise levels than would be expected for our on-source time. However, a
combination of manual flagging (for scans affected by clouds passing through
the line of sight) and self-calibration allows us to achieve a sensitivity
similar to past observations of these sources (McCarthy et al., 2018a;
Ellingsen et al., 2017).

Table 3: Details for the spectral line cubes used to search for methanol spectral lines. Sources highlighted in bold text are those from project C3167, while the rest are from the maser search projects. Velocity ranges and channel widths are identical for the 36.2- and 37.7-GHz spectral lines. The two RMS noise values correspond to the noise for the 36.2- and 37.7-GHz cubes respectively (not applicable to C3167 sources where only 36.2-GHz is observed). RMS noise values are reported post self-calibration where relevant. The RMS values for NGC 1097 (denoted by an asterisk) have a higher level of uncertainty due to less accurate flux density calibration (see Section 2.4).  Target | Velocity | Channel | RMS
---|---|---|---
Source | Range | Width | Noise
| ($\mbox{km~{}s}^{-1}$) | ($\mbox{km~{}s}^{-1}$) | (mJy beam-1)
NGC 253 | 0–500 | 1 | 1.9
IRAS 01417+1651 | 7100 – 9100 | 10 | 1.6/1.6
NGC 1097 | 750 – 1750 | 10 | 0.5*/0.4*
NGC 1792 | 700 – 1700 | 10 | 0.4/0.4
NGC 1808 | 600–1600 | 10 | 0.2/0.2
ESO 558-g009 | 6800 - 9600 | 10 | 2.2/2.2
IC 2560 | 2000 – 3600 | 10 | 6.5/6.7
IRAS 10173+0829 | 13500 – 14900 | 10 | 1.2/1.3
IRAS 11506-3851 | 1900 – 4100 | 10 | 2.2/2.5
ESO 269-g012 | 4000 – 6000 | 10 | 6.6/6.2
NGC 4945 (H214) | 250–850 | 1 | 1.9
NGC 4945 (H75) | 200–1020 | 8.2 | 0.3
Circinus | 0 – 800 | 10 | 0.5/0.6
NGC 5793 | 2500 – 4400 | 10 | 6.4/5.4
IIzw96 | 10000 – 12000 | 10 | 0.6/0.6
Table 4: NGC 4945 36.2-GHz methanol emission details from the 2019 March H214 observations. All reported velocities are with respect to the barycentric reference frame. Uncertainties in the reported flux density values result from the random errors in fitting using the miriad task imfit. Please note the integrated flux density for the continuum emission has units of mJy rather than mJy $\mbox{km~{}s}^{-1}$.  $\\#$ | Label | RA (J2000) | Dec (J2000) | $S_{pk}$ | $S$ | $V_{pk}$ | $V_{\text{Range}}$
---|---|---|---|---|---|---|---
|  |  $h$ $m$ $s$ |  ∘ $\prime$ $\prime\prime$ | (mJy) | (mJy $\mbox{km~{}s}^{-1}$) | ($\mbox{km~{}s}^{-1}$) | ($\mbox{km~{}s}^{-1}$)
1 | M1 | 13 05 28.0 |  $-$49 28 12.67 | 55$\pm 2$ | 500$\pm 40$ | 673 | 630–710
2 | M2 | 13 05 27.1 |  $-$49 28 07.39 | 5$\pm 1$ | 250$\pm 30$ | 490 | 420–560
Table 5: NGC 253 36.2-GHz methanol emission details from the 2019 March H214 observations. All reported velocities are with respect to the barycentric reference frame. Labels here are based on those presented in Ellingsen et al. (2017). Uncertainties in the reported flux density values result from the random errors in fitting using the miriad task imfit.  $\\#$ | Label | RA (J2000) | Dec (J2000) | $S_{pk}$ | $S$ | $V_{pk}$ | $V_{\text{Range}}$
---|---|---|---|---|---|---|---
|  |  $h$ $m$ $s$ |  ∘ $\prime$ $\prime\prime$ | (mJy) | (mJy $\mbox{km~{}s}^{-1}$) | ($\mbox{km~{}s}^{-1}$) | ($\mbox{km~{}s}^{-1}$)
1 | MM6 | 00 47 33.9 |  $-$25 17 11.65 | 25$\pm 2$ | 605.3$\pm 50$ | 207 | 160–260
2 | MM5/MM4 | 00 47 33.7 |  $-$25 17 12.39 | 24$\pm 1$ | 655.9$\pm 50$ | 195 | 140–250
3 | - | 00 47 32.8 |  $-$25 17 22.19 | 5$\pm 2$ | 75.5$\pm 20$ | 300 | 255–345
4 | - | 00 47 32.3 |  $-$25 17 19.06 | 5$\pm 2$ | 69.6$\pm 20$ | 336 | 300–360
5 | MM1/MM2 | 00 47 32.0 |  $-$25 17 28.98 | 30.0$\pm 4$ | 1100$\pm 60$ | 318 | 260–400

###  2.6 Data Reduction

Data were reduced with miriad using standard techniques for the reduction of
7-mm ATCA spectral line data. The data were corrected for atmospheric opacity
and we estimate the absolute flux density calibration to be accurate to better
than 30%. For targets with continuum source detections, multiple iterations of
phase-only self-calibration was performed. Any potential spectral line
emission was isolated from the data by using the uvlin miriad task. This task
subtracts any potential continuum emission by estimating the correlated flux
density on each baseline via polynomial interpolation based on line-free
spectral channels. The velocity ranges and channels widths for our spectral
line cubes varied depending on the source and CABB configuration/frequency
setup that was utilised for the observations. When searching for spectral line
emission, the data were resampled using many different channel widths, in
order to appropriately identify both broad and narrow emission. Typical
channel widths searched were 1, 3, 8, 10 and 20 $\mbox{km~{}s}^{-1}$ as the
native spectral-resolution allows (dependent on the CABB configuration).
Spectral line cubes are produced with a cell (pixel) sizes of $1\times 1$
arcseconds and a Brigg’s visibility weighting robustness parameter of 1.
Details of the final spectral line cubes are listed in Table 3, with image
cube noise levels reported for a consistent channel width of 10
$\mbox{km~{}s}^{-1}$ for ease of comparison.

####  2.6.1 Identification of maser sources

Spectral line cubes were visually inspected for strong maser emission. The
DUCHAMP source finder software (Whiting, 2012) was then used to search each
cube with a $3\sigma$ threshold. Such a low threshold produces many false
positives ($\sim$100), however the vast majority of these are on the outer
edges of our images (which are typically $256^{\prime\prime}\times
256^{\prime\prime}$). Any candidate within 2 times the FWHM of the primary
beam (corresponding to a radius of $\sim$84′′ from the pointing centre at
36.2-GHz) were manually inspected to determine if they were potential maser
components. Generally with data of reasonable quality, very few false
positives (less than two and typically none) were present within this central
region of the image.

Figure 1: Top: NGC 4945 integrated 36.2-GHz methanol emission (red contours at
10, 25, 50, 70 and 90 per cent of peak 560 mJy $\mbox{km~{}s}^{-1}$ beam-1)
and 7-mm continuum emission (white contours at 2, 10, 30, 50, 70, 90 per cent
of peak 290 mJy beam-1) with background colour map and black dashed contours
of CS $J=2\rightarrow 1$ integrated intensity from Henkel et al. (2018) (2%,
10%, 30%, 50%, 70%, and 90% of the peak of 11.5 Jy km s-1 beam-1). Methanol
and continuum emission data extracted from the 2019 March spectral line cubes
with a synthesised beam size of $6\aas@@fstack{\prime\prime}3\times
4\aas@@fstack{\prime\prime}4$ (white ellipse). Bottom left: Spectrum of
36.2-GHz methanol taken from the location of peak emission at M1 (channel
width of 1 $\mbox{km~{}s}^{-1}$). The vertical dashed line indicates the
systemic velocity of NGC 4945 (Chou et al., 2007). Bottom right: Spectrum of
36.2-GHz methanol emission taken from the location of peak emission in region
M2 (channel width of 12 $\mbox{km~{}s}^{-1}$).

##  3 Results

This paper contains observations from three different projects, two involving
searching for new extragalactic class I methanol maser sources (C2879 and
C3263), and one studying known maser hosts (C3167). In order to facilitate
easier reading we will separate the results based on project goal.

Table 6: Details for the continuum emission from all sources. The horizontal line separates the maser search targets from the new (2019 March) H214 observations of NGC 253 and NGC 4945 (above and below respectively). The flux density values for NGC 1097 (denoted by an asterisk) have a higher level of uncertainty due to less accurate flux density calibration (see Section 2.4). For sources with no detected continuum source, the location of the pointing centre and a 5$\sigma$ upper limits on peak flux density are provided. Uncertainties in the reported flux density values result from the random errors in fitting using the miriad task imfit. Target Source | Right Ascension | Declination | $S_{pk}$ | $S$
---|---|---|---|---
|  $h$ $m$ $s$ |  ∘ $\prime$ $\prime\prime$ | (mJy beam-1) | (mJy)
ESO 269-g012 | 12 56 40.5 | –46 55 34.00 | $<2.3$ | -
IC 2560 | 10 16 18.7 | –33 33 50.00 | $<1.8$ | -
IRAS 11506-3851 | 11 53 11.8 | –39 07 49.18 | 10.0$\pm 0.2$ | 10.7$\pm 0.3$
NGC 5793 | 14 59 24.8 | –16 41 36.70 | 18.8$\pm 0.1$ | 19.3$\pm 0.2$
ESO 558-g009 | 07 04 21.0 | –21 35 19.20 | $<0.6$ | -
IRAS 01417+1651 | 01 44 30.5 | +17 06 05.00 | $<0.7$ | -
Circinus | 14 13 10.0 | –65 20 20.74 | 56.6$\pm 0.6$ | 63$\pm 1$
IIzw96 | 20 57 24.4 | +17 07 40.47 | 5.8$\pm 0.2$ | 5.8$\pm 0.3$
IRAS 10173+0829 | 10 20 00.2 | +08 13 32.07 | 7.1$\pm 0.9$ | 8$\pm 2$
NGC 1097 | 02 46 18.9 | -30 16 28.69 | 8.1$\pm 0.1$* | 8.4$\pm 0.1$*
NGC 1792 | 05 05 14.5 | –37 58 49.87 | 4.4$\pm 0.1$ | 4.4$\pm 0.1$
NGC 1808 | 05 07 42.3 | –37 30 45.64 | 4.5$\pm 0.1$ | 6.2$\pm 0.4$
NGC 253 | 00 47 33.2 |  $-$25 17 17.77 | 214.3$\pm 8$ | 335.7$\pm 20$
NGC 4945 | 13 05 27.4 |  $-$49 28 05.43 | 296$\pm 3$ | 430.8$\pm 7$

###  3.1 Project C3167: the methanol masers of NGC 253 and NGC 4945

####  3.1.1 NGC 4945

The previously reported bright 36.2-GHz methanol maser offset to the south-
east from the nucleus of NGC 4945 was readily detected in the latest 2019
March epoch observation (M1 in Figure 1). In addition to this primary region,
we identify another region of 36.2-GHz emission south-west of the galactic
centre. This emission is offset by approximately 3 arcseconds to the south-
west of the galactic nucleus (M2 in Figure 1). This location corresponds to
the south-west component of 36.4-GHz HC3N and CS ($1-0$) emission reported in
McCarthy et al. (2018a) and the methanol emission covers the same velocity
range. When compared to the primary region, this new emission has a much lower
peak flux density and a significantly broader spectral profile. This emission
is clearly visible in the data prior to any self-calibration, indicating it is
not an artefact of this process.

7-mm continuum emission was detected toward the nucleus of NGC 4945. As in
McCarthy et al. (2018a), this continuum measurement was obtained using the
line-free data in the 36.2-GHz zoom band. The measured flux density is in
agreement with the previously reported measurement. The integrated flux
density agrees with the modelled 7-mm continuum values from Bendo et al.
(2016), which predicts that 75 percent of this 7-mm continuum emission is from
free-free emission.

The flux density values for the methanol and 7-mm continuum emission toward
NGC 4945 are recorded in Table 4 and 6 respectively..

####  3.1.2 NGC 253

We detect the same 36.2-GHz methanol maser regions toward NGC 253 as have been
reported by Ellingsen et al. (2014, 2017), though the resolution of the new
observations does not allow us to resolve all of the individual features at
the locations of the brightest emission. We also identify two tentative new
components of methanol emission (see Figure 2) not formerly detected by
Ellingsen et al. (2017) or Gorski et al. (2017). These components are only
approximately 4$\sigma$ detections, however, they are located at the same
positions and velocities as emission in the 87.9 GHz transition of HNCO
emission (location C and D in figure 1 of McCarthy et al., 2018b). The
previous 36.2-GHz ATCA observations of NGC 253 by Ellingsen et al. (2017) have
a high enough sensitivity (1.1 mJy beam-1 in a 3 $\mbox{km~{}s}^{-1}$
channel), however, their observations make use of more extended array
configurations. Both components have sufficiently high peak flux densities
($\sim$5 mJy) that they should have been detected by previous observations of
NGC 253.

7-mm continuum emission was detected toward the nucleus of NGC 253. Continuum
emission was extracted from the line-free data in the 36.2-GHz zoom band, flux
density values for this continuum emission, along with those for the maser
components are recorded in Table 5.

###  3.2 Projects C2879 & C3263: searching for masers

No additional extragalactic methanol maser sources have been detected in any
of the targets of these projects and RMS noise levels for the 36.2-GHz class I
and 37.7-GHz class II spectral line cubes are listed in Table 3. Results for
NGC 1097 have a higher uncertainty due to flux-calibration issues, as outlined
in Section 2.4.

7-mm continuum emission was detected toward the centre of many of these
galaxies (see Table 6), many of which have no previously reported values in
the literature.

##  4 Discussion

###  4.1 New methanol emission toward NGC 4945

The new methanol emission from region M2 shares overlapping position and
velocities with the south-western CS (1–0) and HC3N (4–3) emission reported in
McCarthy et al. (2018a). This increases our confidence that despite a
relatively low-SNR, the emission at this location is indeed real. Before we
can discuss what this new region of methanol emission means for our
understanding of extragalactic class I methanol masers, we must first justify
that it is reasonable to consider this emission the result of maser processes.

While not as narrow as the linewidths seen toward the primary maser region
(M1), M2 appears to consist of two or more components with FWHM of $\sim$20
$\mbox{km~{}s}^{-1}$. These linewidths are much narrower than those observed
from thermal emission toward the same regions (FWHM of $>60$
$\mbox{km~{}s}^{-1}$; Henkel et al., 2018; McCarthy et al., 2018a), and are
comparable to the masing regions toward NGC 253 (Ellingsen et al., 2017).
Similarly to the main 36.2-GHz methanol masing region, no methanol from the
ground state 48 GHz transition has been reported toward M2 (McCarthy et al.,
2018a).

When investigating the emission at M2 in the three epochs of observation
originally presented in McCarthy et al. (2018a), it should be noted that all
were made in relatively compact ATCA array configurations (2016 Aug, 2017 Jun
and Oct; see Table 7). Looking at these three array configurations we see the
integrated flux density of M2 increasing as the array configuration gets more
compact. This relationship may indicate thermal emission from this region,
however, there are a few important caveats to this interpretation. The first
is that the detections of emission from M2 in these epochs is very marginal,
with only a channel or two above the noise level (see Figure 5). This means
the extracted integrated flux density values from these locations with
marginal detections is less reliable. Secondly, a similar relationship is
observed toward M1, where integrated flux density increases as they array
becomes more compact. Finally, it is hard to determine the true integrated
flux density for M2 from the 2017 July H75 observations, as the emission from
the primary component M1 is also captured within the larger synthesised beam.

The emission from this new methanol region also displays evidence of
variability. This is the best evidence that the emission does not result from
a thermal process. This variability is discussed in more detail in Section
4.3, as it is the first compelling evidence for variability in extragalactic
class I methanol masers.

Due to the broad spectral profile of the emission and the larger velocity
range, the integrated flux density of M2 is only approximately 50% lower than
the primary masing region, despite a much lower peak flux density (by a factor
of $\sim$10). This integrated intensity is approximately 5 orders of magnitude
higher than that of a typical Galactic style 36.2-GHz emission, and over an
order of magnitude (a factor of $\sim$40) higher than the combined 36.2-GHz
emission from the Milky Way CMZ (Yusef-Zadeh et al., 2013). As this region is
unresolved by our synthesised beam ($6.3\times 4.4$ arcseconds), we can put an
upper limit on its angular size. This angular size corresponds to a linear
size of $113\times 79$ pc at the 3.7 Mpc distance of NGC 4945 (likely a vast
over-estimate based on variability timescale, see Section 4.3). This implies
we have over 40 times the 36.2-GHz emission coming from an area on the sky
which is at most 30% larger than the CMZ would be at this distance.

Considering all these factors together, we conclude that the most reasonable
explanation is that this emission is masing. McCarthy et al. (2018a) also put
an upper limit of 6 mJy on the presence of any 44-GHz methanol emission from
NGC 4945. This means the 36.2-GHz methanol emission from this region is
unlikely to be resulting from the cumulative effect of Galactic-style class I
masers, as otherwise we would expect to also see evidence of 44.1-GHz maser
emission (Voronkov et al., 2014).

###  4.2 Tentative new methanol emission in NGC 253

The angular resolution of our observations does not allow us to separate the
methanol emission into the individual components as reported in previous
interferometeric studies (Ellingsen et al., 2017; Gorski et al., 2017; Chen et
al., 2018). However, despite this we observe two new components of emission
(labelled 3 and 4 in Figure 2). These components are only $4\sigma$
detections, however, their location and velocity strongly correlate with the
components of HNCO emission reported in McCarthy et al. (2018b). Ellingsen et
al. (2017) observe a close correlation between the location of 36.2-GHz
methanol masers and thermal emission from HNCO, a low-velocity shock tracer.
This indicates that despite being a marginal detection, these components are
likely real.

Comparison of our new observations with past ATCA observations of the 36.2-GHz
methanol masers toward NGC 253 (Ellingsen et al., 2014, 2017), the whole
spectrum (combining all regions) is more similar to what is reported in the
original detection paper. The ‘intermediate resolution’ observations displayed
in figure 1 of Ellingsen et al. (2017) shows significantly higher peak flux
density values then what we see from our observations or their past
observations (Ellingsen et al., 2014). However, upon re-analysis of the data,
this appears to be an error (with flux density values out by a factor of 2) in
how the spectrum has been extracted from their ‘intermediate resolution’
spectral line cube (and it does not affect the spectra taken from the combined
array cube in figure 2 of Ellingsen et al. (2017)), and the data is consistent
across all 3 epochs.

Figure 2: All data in this figure is from the 2019 March epoch, in the ATCA
H214 array configuration. Field image: NGC 253 integrated 36.2-GHz methanol
emission (black contours 2.5, 10, 25, 50, 70, and 90 per cent of the 1364 mJy
$\mbox{km~{}s}^{-1}$ beam-1 peak) and 7-mm continuum emission (white contours
2.5, 10, 30, 50, 70, and 90 per cent of the 215 mJy beam-1 peak) with
background image of integrated CO $J=2\rightarrow 1$ emission from Sakamoto et
al. (2011) on a logarithmic scale. Magenta plus signs indicate the peak
components of the methanol emission and have been labelled 1 through 5 moving
from high right ascension to low. Spectra: Surrounding spectra from the
36.2-GHz spectral line cube at the location of each magenta plus sign.
Synthesised beam size for our observations is
$5\aas@@fstack{\prime\prime}6\times 4\aas@@fstack{\prime\prime}3$ (white
ellipse). Spectra 1, 2 and 5 have a channel width of 1 $\mbox{km~{}s}^{-1}$,
while 3 and 4 have 10 $\mbox{km~{}s}^{-1}$ channel width in order to better
show the velocity range of the weaker emission from these regions. Figure 3:
NGC 253 integrated 87.9 GHz HNCO emission (black contours 15, 30, 50, 70, and
90 per cent of the 1.9 Jy $\mbox{km~{}s}^{-1}$ beam-1) and the 7-mm continuum
emission (white contours 2.5, 10, 30, 50, 70, and 90 per cent of the 215 mJy
beam-1 peak) with background image of integrated CO $J=2\rightarrow 1$
emission from Sakamoto et al. (2011) on a logarithmic scale. Magenta plus
signs indicate the peak components of the 36.2-GHz methanol emission and have
been labelled 1 through 5 moving from high right ascension to low. Synthesised
beam for the 87.9-GHz HNCO emission is approximately
$6\aas@@fstack{\prime\prime}4\times 4\aas@@fstack{\prime\prime}9$. Figure 4:
Black dots indicate the detectable peak emission levels for each target source
in the maser search projects. Red stars represent the peak luminosity levels
in each of the four sources with detected 36.2-GHz class I methanol masers.
All peak emission levels are given for a 10 $\mbox{km~{}s}^{-1}$ velocity
channel. Peak or integrated luminosity values for this figure (and throughout
the rest of the paper) are given with units of Jy kpc2 (or Jy
$\mbox{km~{}s}^{-1}$ kpc2 for integrated luminosity), which is the flux-
density value multiplied by the squared distance to the source (in kpc).

Figure 5: Spectra of 36.2-GHz emission at M2 toward NGC 4945 from each
individual observation epoch (two epochs presented in this paper and the three
from McCarthy et al. (2018a)). Vertical scales have all been set to the same
range for ease of comparison. Note for the 2017 Jul H75 spectrum, the emission
at $\sim$670$\mbox{km~{}s}^{-1}$ is due to the larger synthesised beam
capturing emission from the main maser component in NGC 4945. The vertical
dashed line indicates the systemic velocity of NGC 4945 (Chou et al., 2007).
Figure 6: The spectrum from the M2 methanol maser region in NGC 4945 using the
combined data from the three epochs reported in McCarthy et al. (2018a).

###  4.3 Evidence for variability in extragalactic methanol masers

We currently have five epochs of observations of the 36.2-GHz methanol
transition toward NGC 4945, three of which were originally reported in
McCarthy et al. (2017) and McCarthy et al. (2018a), with the remaining 2
reported in this paper. This allows us to compare the methanol emission over a
time baseline of approximately 3.5 years. Tabulated flux density values for
the primary methanol maser component (M1), newly discovered central region
(M2) and 7-mm continuum emission can be found in Table 7.

The detection of the new emission at M2 in NGC 4945 is the first strong
evidence for variability in an extragalactic class I methanol maser source.
This is most evident when comparing the two observations made using the H214
array configuration, one in 2019 March and the other in 2017 June (see Figure
5 for spectra and Table 7 for flux density values). Due to the difficulties of
comparing emission observed with different array configurations, the
comparison of these two H214 epochs will be the main focus of this section. We
see a 330 percent increase in integrated flux density across the velocity
range of the new component (420–560 $\mbox{km~{}s}^{-1}$) when comparing these
two epochs (see Table 7). We also see evidence of this emission in the much
lower resolution (synthesised beam size of $26.6\times 12.0$ arcseconds) H75
array data from 2017 July, which is bracketed by the higher resolution
observations in both 2017 June and October, neither of which show significant
emission. This may indicate that emission from this region was much more
diffuse during this period, with it becoming significantly more compact
sometime prior to the most recent observation. However, if we assume a time-
scale of 21 months for the variability (based on the time period between the
H214 observations), this puts an upper limit on the size of the emitting
region of 21 light-months ($\sim$0.53 pc). At a distance of 3.7 Mpc, this
corresponds to an angular size of 0.03 arcseconds, meaning the emission would
be compact enough to be detectable on any ATCA baseline. It is therefore more
likely that the change in luminosity of the region is instead related to an
excitation change. Combining all data from the three intermediate resolution
observations reported in McCarthy et al. (2018a) (see Figure 6), we can
clearly see this emission at M2 (despite the the individual epochs showing no
clear evidence) with a peak flux density of $\sim 3$ mJy and integrated flux
density of 185 mJy $\mbox{km~{}s}^{-1}$. This further supports the
interpretation that the variability is a result of an excitation change,
rather than the emission simply being more diffuse during these earlier
epochs.

The angular size of 0.03 arcseconds also allows us to put a lower limit of
$\sim$6500 K on the brightness temperature of the emission from M2 based on
the peak spectral channel (channel width 10 $\mbox{km~{}s}^{-1}$), or
$>3\times 10^{5}$ K integrated over a 140 $\mbox{km~{}s}^{-1}$ velocity range.
This brightness temperature provides further evidence that this emission is
the result of a non-thermal process, as discussed in Section 4.1.

The brighter region to the south-east of the nucleus (M1 in Figure 1) also
shows some evidence of variability. The multiple spectral components reported
in McCarthy et al. (2018a) are all present without any measurable shift in
their velocities. When directly comparing the emission from the south-eastern
bright region to the 2017 June observations in the same array configuration
(H214), we observe a decrease in peak flux density of $\sim$6 percent and
increase in integrated flux density of $\sim$30 percent. The integrated flux
density of this primary region is also significantly higher ($\sim$15 percent)
than that observed using a more compact array configuration (H168) in 2017
October.

Our previous 2017 June H214 observations, when compared to other 7-mm
observations toward NGC 4945, had lower continuum values than expected
(McCarthy et al., 2018a). However, here we observe the same values of peak and
integrated flux density for the 7-mm continuum source as reported from the
previous H214 observations of NGC 4945 (see Table 7 for values). This suggests
that instead of the 2017 June continuum levels being anomalously low, the 2015
August EW367 continuum levels may be higher than expected. This is likely
related to the poor uv-coverage (highly elongated beam) of these observations
preventing accurate imaging (flux density values are extracted from images
using the imfit miriad task). The consistency of the 7-mm continuum
measurements between these two H214 observations increases the confidence in
our flux density calibration, which strengthens our comparisons of the maser
emission between these two epochs.

It can be difficult to measure variability in the weak millimetre spectral
line emission from these extragalactic sources. It is difficult to isolate
what is variability in the source, and what is variability caused by differing
array configurations, weather and uv-coverage between observations. With that
said, the increased integrated flux density observed from M2 in our recent
observations (2019 March) when compared to past observations (in particular
the 2017 Jun H214 epoch) is far too large to be the result of weather or
systematic effects.

The tentative new components detected toward NGC 253 also may indicate
variability in this emission. The previous observations of NGC 253 at 36.2-GHz
(Ellingsen et al., 2017) have appropriate sensitivity to detect these
components, however, these observations use a higher-resolution east-west
oriented array without complete uv-coverage. The resulting synthesised beam of
those observations makes determining component separation along the north-east
to south-west axis difficult, which is required to resolve the individual
south-western components.

The mechanism driving such significant variation in these masers is mostly a
mystery, especially because at this stage we do not understand the nature of
the variability. It could be that it is a periodic phenomenon, or instead we
may be observing the creation of new masers at these locations which will
eventually stabilise. In Galactic sources, class I methanol masers are
observed to exist for long periods of time, remaining relatively stable (Kurtz
et al., 2004). This stability is consistent with the expected variability
time-scales for saturated class I masers, where the low-velocity shocks have
long relaxation times (15 years; Leurini et al., 2016). However, it should be
noted that long-term monitoring of class I methanol maser sources has not been
undertaken to date, and our understanding of variability in these Galactic
class I masers is lacking. Another collisionally pumped maser species is
water, which is observed to be highly variable in both Galactic and
extragalactic sources (e.g. Haschick & Baan, 1990; Felli et al., 2007;
McCallum et al., 2007). These water masers trace more energetic environments,
with higher velocity shocks (Hollenbach et al., 2013). It may be that
extragalactic class I methanol masers also trace higher-energy environments
than their Galactic counterparts, though this is largely speculation as all
modelling of the class I methanol lines is based on our understanding of
Galactic sources (Sobolev et al., 2007; McEwen et al., 2014; Leurini et al.,
2016). In order to confirm variability in these extragalactic class I methanol
masers and develop our understanding of the phenomenon, constant and
consistent monitoring is required. This monitoring should ideally be made with
the same spatial resolution in order to accurately quantify any variability
observed toward these sources.

Epoch | Flux density source (mJy) | Angular Scale
---|---|---
| M1 | M2 | Continuum | Phase Cal. | Min/Max (arcsec).
2015 Aug (EW352) | 27.9 (289) | 2.1 (44) | 313 (414) | 3328 | 3.6/55.8
2017 Jun (H214) | 27.4 (305) | 2.3 (65) | 236 (348) | 1369 | 4.4/20.7
2017 Jul (H75) | 21.3 (450) | 4.4 (279) | 452 (447) | 935 | 12.0/55.8
2017 Oct (H168) | 30.4 (346) | 2.8 (71) | 385 (489) | 1141 | 5.0/27.9
2019 Mar (H214) | 25.7 (397) | 4.3 (215) | 246 (356) | 1110 | 4.4/20.7
Table 7: Table of flux density values from each epoch for the emission at M1,
M2 and the continuum for NGC 4945, including the phase calibrator for these
observations. Peak values are given, with integrated values given in
parentheses. The peak values at both region M1 and M2 are given with a channel
size of 10 $\mbox{km~{}s}^{-1}$. It should be noted that these flux density
values have an uncertainty of approximately 30% due to flux density
calibration and continuum subtraction (see Section 2.6). We have left the
random fitting errors off these values in order to facilitate easier reading,
in general these random errors are around 5%.

###  4.4 Non-detection in Arp 220 follow-up

The weather during the 2014 March observations, was quite poor (particularly
on March 28). During the processing of the data from this epoch we found that
certain imaging parameters were causing spurious emission to appear in the
spectral line cubes of targets with relatively low on-source time (and
therefore, high noise). This realisation prompted re-analysis of the Arp 220
data published in Chen et al. (2015), as the data reported in that paper was
from this observing session, and has not been confirmed in any subsequent
observations. Upon reanalysing with more robust imaging parameters we find a
similar spectral profile to that which is reported in Chen et al. (2015),
however, at much lower flux density than that reported ($\sim$6 mJy peak
compared with $\sim$25 mJy). The RMS noise in the cube is 4.3 mJy, so we can
not be confident that this is real emission. Additionally, we see no evidence
for the reported 37.7-GHz emission (Chen et al., 2015) in the re-analysed
data. We have also reduced data from an ATCA follow-up observation in 2014
November (EW367 array configuration), with much longer integration times (4.8
hours). Analysis of these data does not show any 36.2-GHz methanol emission
toward the central region of Arp 220 comparable to that reported in Chen et
al. (2015). Based on these follow up observations, we put a $5\sigma$ upper
limit on emission from the 36.2-GHz methanol line of 3.5 mJy. A recent
36.2-GHz search by Henkel et al. (submitted) targetted Arp 220 with the
Effelsberg 100m telescope and report no detection of 36.2-GHz emission (with
an upper limit of 4.2 mJy).

###  4.5 Sample discussion

When considering the non-detections toward most of our targets, it is
important to note that the observations conducted under project C2879 were
also responsible for discovering the first extragalactic methanol masers.
Hence at the time the observations were made there was no information on the
luminosity, nor likely environments to host extragalactic class I methanol
masers. This results in many of the target sources not having a high enough
sensitivity to detect methanol masers of comparable luminosity to those
observed toward our known hosts (see Figure 4). This was not a problem for the
subsequent searches, which uses much higher sensitivity levels, based on the
currently known extragalactic class I maser sources.

Of the 12 sources for which we report 36.2-GHz methanol upper limits (C2879
and C3263 targets), only four have sensitivity levels adequate for detecting
masers with the same luminosity as those detected toward NGC 4945, with the
sensitivity of the majority over an order of magnitude too low (see Figure 4).
When combining the non-detections with detections for analysis of the sample,
the majority of the time we will include only these sources with appropriate
sensitivity levels. This means our primary sample for discussion will include:
NGC 4945, NGC 253, NGC 6946, NGC 1068, NGC 1097, NGC 1792, NGC 1808, IC 342
and the Circinus Galaxy.

Considering the general properties of the galaxies with detections we see
three common factors: They are all barred, spiral galaxies with elevated
levels of star-formation toward their nucleus (when compared to the Milky
Way). However, it is important to note that searches for extragalactic
methanol masers have been biased toward these types of sources based on the
first detections in NGC 253 and NGC 1068 (Ellingsen et al., 2014; Wang et al.,
2014). As of yet no statistically complete search for these masers has been
undertaken, and as such, it is hard to determine the most relevant properties
for future target selection. The observations for project C3263, reported in
this paper, used the above properties to target nearby southern sources.
Despite that, the three sources observed so far as part of that project (NGC
1097, NGC 1792 and NGC 1808) have not shown any sign of methanol maser
emission. The only other source observed with appropriate sensitivity is the
Circinus Galaxy, which is a spiral galaxy with no bar, however it does show
evidence of molecular outflows from its type 2 Seyfert AGN.

In order to determine the most efficient way, in terms of target selection and
telescope time, to search for extragalactic 36.2-GHz methanol masers, we must
determine the luminosity levels of a ‘typical’ region. The currently reported
extragalactic 36.2-GHz methanol masers (in NGC 4945, NGC 253, NGC 6946 and IC
342) all have varying peak and integrated luminosity levels. In this
discussion we consider the peak and integrated flux density from a single
emission component at our resolution (rather than the combined emission across
the whole source) as these are what will be detected when searching using
interferometers like the VLA or the ATCA. Peak luminosities (in a 10
$\mbox{km~{}s}^{-1}$ channel) for the brightest region in each source vary
from $1.1\times 10^{6}$ Jy kpc2 in IC 342 to $5.2\times 10^{6}$ Jy kpc2 toward
NGC 4945, with integrated luminosities ranging from $2.8\times 10^{7}$ Jy
$\mbox{km~{}s}^{-1}$ kpc2 in IC 342 to $1.6\times 10^{8}$ Jy
$\mbox{km~{}s}^{-1}$ kpc2 toward NGC 253. It must be noted that the flux
density values for IC 342 and NGC 6946 are from observations with much higher
spatial-resolution than the ATCA observations (VLA C-configuration) of NGC
4945 and NGC 253 ($\sim$1 arcsec compared to $\sim$5 arcsec). We calculate an
average peak luminosity of $3.0\times 10^{6}$ Jy kpc2 across these 4 sources.
With a sensitivity of 0.5 mJy (in a 10 $\mbox{km~{}s}^{-1}$ channel), a
5$\sigma$ detection of a maser with this luminosity can be made out to a
distance of $\sim$34 Mpc. Assuming the ability to self-calibrate off a
continuum source, this sensitivity can be achieved with about 4 hours of on-
source time with the ATCA in an intermediate resolution array configuration
(H168 – EW367) or 45 minutes of on-source time with VLA D-configuration (VLA
D-configuration will allow detection of any extended emission, such as that
seen toward NGC 253). A 10 $\mbox{km~{}s}^{-1}$ channel width is appropriate
for searching for these masers as this will allow multiple, above noise level,
consecutive channels across the FWHM of the spectral profile for the vast
majority of known extragalactic 36.2 GHz methanol sources. The only exception
to this is the spectral profile of the M1 region in NGC 4945, which has a FWHM
of $\sim$20 $\mbox{km~{}s}^{-1}$. Though this source has a much higher peak
luminosity then the other sources, so the ability to image with narrower
channel widths will still allow flexibility for detecting these sources.
However, it should be noted that our analysis of optimal search parameters for
these 36.2-GHz masers is based off a sample size of four sources, and as such
is susceptible to small number statistics.

###  4.6 Galaxy-wide star-formation as a predictor for class I methanol masers

Within our Galaxy, methanol masers (both class I and II) provide useful sign-
posts for the location of star-formation regions (Caswell, 1997; Kurtz et al.,
2004; Voronkov et al., 2006; Breen et al., 2010, 2011). It was initially
hypothesised that these extragalactic class I masers may be useable in the
same manner, identifying starburst galaxies at cosmologically interesting
distances (Chen et al., 2016). However, that speculation was largely
predicated on the more luminous emission reported toward Arp 220. Reanalysis
showing no 36.2-GHz methanol emission stronger than 3.5 mJy (peak luminosity
of $\sim$2$\times 10^{10}$ Jy kpc2) toward Arp 220, means that at present the
known extragalactic methanol sources are not readily detectable at
cosmologically interesting distances with current generation facilities. Even
considering only the sources that meet our detection threshold in Figure 4, we
have four sources with star-formation rates (SFR) at the same level or higher
than those observed toward NGC 253 or NGC 4945 with confident non-detections
for 36.2-GHz class I methanol masers.

When considering known examples of extragalactic class I methanol masers, we
see that locally, there is no clear link to star-formation rates in the
regions where they are detected. The M1 region toward NGC 4945 and the masers
toward NGC 253 are all likely associated with giant molecular clouds in these
sources, however, these regions do not necessarily display enhanced star-
formation rates when compared to the rest of the nuclear region (Ellingsen et
al., 2017; McCarthy et al., 2018a). M1 in NGC 4945 may be associated with the
star-formation from Knot B, though even if this is correct, we do not observe
class I maser emission from other regions in NGC 4945 with enhanced star-
formation levels. Instead of tracing current star-formation, based on our
current understanding of the environments of these masers (dense-gas and low-
velocity shocks), we believe they may indicate regions that are soon to
experience higher levels of star-formation, essentially the class I maser
emission is associated with the triggering of the star-formation.

The presence, or lack-thereof, of class I methanol masers may not directly
infer information about the star-formation rate of its host, however, it can
still provide useful information about the mechanism triggering the star-
formation. The morphological features that these class I masers have been
observed toward (Ellingsen et al., 2017; Gorski et al., 2018; McCarthy et al.,
2018a) can be related to the star-formation in a galaxy and, therefore, the
class I masers (and their association with low-velocity shocks) can help
identify mechanisms that are providing the gas fueling these starburst regions
(Ellingsen et al., 2017).

##  5 Conclusions

We present the results of new observations toward NGC 4945 and NGC 253,
detecting new components of methanol emission toward both galaxies. Evidence
of variability, especially toward the new region in NGC 4945, indicates that
this emission is likely the result of a maser process. This is the first
reported example of variability in an extragalactic class I methanol maser and
we recommend continued monitoring of these sources in order to both confirm
and further quantify this variability.

We provide flux density upper-limits for the 36.2-GHz class I and 37.7-GHz
class II methanol transitions toward 9 galaxies, observed as part of the
original extragalactic class I methanol searches which detected NGC 253 and
NGC 4945, and 3 more sources observed more recently. In the majority of
sources from the older observations (C2879), sensitivity levels are not
sufficient in order to rule out maser emission of the same order as is
observed toward NGC 4945 or NGC 253.

We also report the results of a re-analysis of the ATCA data on Arp 220. We do
not find any evidence of the 36.2-GHz methanol maser emission toward this
source. It appears a combination of poor data quality and sub-optimal imaging
procedures may have resulted in the original reported detection. We place a
$5\sigma$ upper limit on the 36.2-GHz line of 3.5 mJy.

## Acknowledgements

We thank the referee for the valuable comments which have helped improve the
manuscript. The ATCA is part of the Australia Telescope which is funded by the
Commonwealth of Australia for operation as a National Facility managed by
CSIRO. This research has made use of NASA’s Astrophysics Data System Abstract
Service. This research has made use of the NASA/IPAC Extragalactic Database
(NED), which is operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics and
Space Administration. This research also utilised aplpy, an open-source
plotting package for python hosted at http://aplpy.github.com. This research
made use of Astropy, a community-developed core Python package for Astronomy
(Astropy Collaboration et al., 2013).

## References

  * Araya et al. (2010) Araya E. D., Hofner P., Goss W. M., Kurtz S., Richards A. M. S., Linz H., Olmi L., Sewiło M., 2010, ApJ, 717, L133
  * Astropy Collaboration et al. (2013) Astropy Collaboration et al., 2013, A&A, 558, A33
  * Bendo et al. (2016) Bendo G. J., Henkel C., D’Cruze M. J., Dickinson C., Fuller G. A., Karim A., 2016, MNRAS, 463, 252
  * Bottinelli et al. (1985) Bottinelli L., Gouguenheim L., Paturel G., de Vaucouleurs G., 1985, A&AS, 59, 43
  * Breen et al. (2010) Breen S. L., Ellingsen S. P., Caswell J. L., Lewis B. E., 2010, MNRAS, 401, 2219
  * Breen et al. (2011) Breen S. L., Ellingsen S. P., Caswell J. L., Green J. A., Fuller G. A., Voronkov M. A., Quinn L. J., Avison A., 2011, ApJ, 733, 80
  * Breen et al. (2013) Breen S. L., Ellingsen S. P., Contreras Y., Green J. A., Caswell J. L., Stevens J. B., Dawson J. R., Voronkov M. A., 2013, MNRAS, 435, 524
  * Caswell (1997) Caswell J. L., 1997, MNRAS, 289, 203
  * Chen et al. (2015) Chen X., Ellingsen S. P., Baan W. A., Qiao H.-H., Li J., An T., Breen S. L., 2015, ApJ, 800, L2
  * Chen et al. (2016) Chen X., Ellingsen S. P., Zhang J.-S., Wang J.-Z., Shen Z.-Q., Wu Q.-W., Wu Z.-Z., 2016, MNRAS, 459, 357
  * Chen et al. (2018) Chen X., Ellingsen S. P., Shen Z.-Q., McCarthy T. P., Zhong W.-Y., Deng H., 2018, ApJ, 856, L35
  * Chou et al. (2007) Chou R. C. Y., et al., 2007, ApJ, 670, 116
  * Cyganowski et al. (2009) Cyganowski C. J., Brogan C. L., Hunter T. R., Churchwell E., 2009, ApJ, 702, 1615
  * Cyganowski et al. (2012) Cyganowski C. J., Brogan C. L., Hunter T. R., Zhang Q., Friesen R. K., Indebetouw R., Chandler C. J., 2012, preprint,  (arXiv:1210.3366)
  * Dalcanton et al. (2009) Dalcanton J. J., et al., 2009, ApJS, 183, 67
  * Ellingsen et al. (2010) Ellingsen S. P., Breen S. L., Caswell J. L., Quinn L. J., Fuller G. A., 2010, MNRAS, 404, 779
  * Ellingsen et al. (2014) Ellingsen S. P., Chen X., Qiao H.-H., Baan W., An T., Li J., Breen S. L., 2014, ApJ, 790, L28
  * Ellingsen et al. (2017) Ellingsen S. P., Chen X., Breen S. L., Qiao H.-H., 2017, MNRAS, 472, 604
  * Felli et al. (2007) Felli M., et al., 2007, Astronomy & Astrophysics, 476, 373–664
  * Gao et al. (2016) Gao F., et al., 2016, ApJ, 817, 128
  * Goedhart et al. (2003) Goedhart S., Gaylard M. J., van der Walt D. J., 2003, MNRAS, 339, L33
  * Gorski et al. (2017) Gorski M., Ott J., Rand R., Meier D. S., Momjian E., Schinnerer E., 2017, ApJ, 842, 124
  * Gorski et al. (2018) Gorski M., Ott J., Rand R., Meier D. S., Momjian E., Schinnerer E., 2018, ApJ, 856, 134
  * Green et al. (2008) Green J. A., et al., 2008, MNRAS, 385, 948
  * Haschick & Baan (1990) Haschick A. D., Baan W. A., 1990, ApJ, 355, L23
  * Henkel et al. (2018) Henkel C., et al., 2018, A&A, 615, A155
  * Hollenbach et al. (2013) Hollenbach D., Elitzur M., McKee C. F., 2013, preprint,  (arXiv:1306.5276)
  * Jensen et al. (2003) Jensen J. B., Tonry J. L., Barris B. J., Thompson R. I., Liu M. C., Rieke M. J., Ajhar E. A., Blakeslee J. P., 2003, ApJ, 583, 712
  * Kurtz et al. (2004) Kurtz S., Hofner P., Álvarez C. V., 2004, ApJS, 155, 149
  * Leurini et al. (2016) Leurini S., Menten K. M., Walmsley C. M., 2016, A&A, 592, A31
  * McCallum et al. (2007) McCallum J. N., Ellingsen S. P., Lovell J. E. J., 2007, MNRAS, 376, 549
  * McCarthy et al. (2017) McCarthy T. P., Ellingsen S. P., Chen X., Breen S. L., Voronkov M. A., Qiao H.-h., 2017, ApJ, 846, 156
  * McCarthy et al. (2018a) McCarthy T. P., Ellingsen S. P., Breen S. L., Henkel C., Voronkov M. A., Chen X., 2018a, MNRAS, 480, 4578
  * McCarthy et al. (2018b) McCarthy T. P., Ellingsen S. P., Breen S. L., Voronkov M. A., Chen X., 2018b, ApJ, 867, L4
  * McEwen et al. (2014) McEwen B. C., Pihlström Y. M., Sjouwerman L. O., 2014, ApJ, 793, 133
  * Moran et al. (1999) Moran J. M., Greenhill L. J., Herrnstein J. R., 1999, Journal of Astrophysics and Astronomy, 20, 165
  * Olech et al. (2019) Olech M., Szymczak M., Wolak P., Sarniak R., Bartkiewicz A., 2019, MNRAS, 486, 1236
  * Sakamoto et al. (2011) Sakamoto K., Mao R.-Q., Matsushita S., Peck A. B., Sawada T., Wiedner M. C., 2011, ApJ, 735, 19
  * Sjouwerman et al. (2010) Sjouwerman L. O., Murray C. E., Pihlström Y. M., Fish V. L., Araya E. D., 2010, ApJ, 724, L158
  * Sobolev et al. (2007) Sobolev A. M., et al., 2007, in Chapman J. M., Baan W. A., eds, IAU Symposium Vol. 242, Astrophysical Masers and their Environments. pp 81–88 (arXiv:0706.3117), doi:10.1017/S1743921307012616
  * Sorce et al. (2014) Sorce J. G., Tully R. B., Courtois H. M., Jarrett T. H., Neill J. D., Shaya E. J., 2014, MNRAS, 444, 527
  * Tsunekawa et al. (1995) Tsunekawa S., Ukai T., Toyama A., Takagi K., 1995, Department of Physics, Toyama Univerity, Japan
  * Tully & Fisher (1988) Tully R. B., Fisher J. R., 1988, Catalog of Nearby Galaxies
  * Tully et al. (2009) Tully R. B., Rizzi L., Shaya E. J., Courtois H. M., Makarov D. I., Jacobs B. A., 2009, AJ, 138, 323
  * Tully et al. (2013) Tully R. B., et al., 2013, AJ, 146, 86
  * Tully et al. (2016) Tully R. B., Courtois H. M., Sorce J. G., 2016, AJ, 152, 50
  * Voronkov et al. (2006) Voronkov M. A., Brooks K. J., Sobolev A. M., Ellingsen S. P., Ostrovskii A. B., Caswell J. L., 2006, MNRAS, 373, 411
  * Voronkov et al. (2014) Voronkov M. A., Caswell J. L., Ellingsen S. P., Green J. A., Breen S. L., 2014, MNRAS, 439, 2584
  * Wang et al. (2014) Wang J., Zhang J., Gao Y., Zhang Z.-Y., Li D., Fang M., Shi Y., 2014, Nature Communications, 5, 5449
  * Whiting (2012) Whiting M. T., 2012, MNRAS, 421, 3242
  * Wilson et al. (2011) Wilson W. E., et al., 2011, MNRAS, 416, 832
  * Yamauchi et al. (2012) Yamauchi A., Nakai N., Ishihara Y., Diamond P., Sato N., 2012, PASJ, 64, 103
  * Yusef-Zadeh et al. (2013) Yusef-Zadeh F., Cotton W., Viti S., Wardle M., Royster M., 2013, ApJ, 764, L19
  * van der Walt (2011) van der Walt D. J., 2011, AJ, 141, 152

Table A.1: Details of the 2014 March sources excluded due to poor data
quality. Recession velocities have been taken from the NASA/IPAC Extragalactic
Database† (NED). Redshift-independent distances have been provided where
possible, otherwise distances reported are the luminosity distances provided
by NED.

Target | Right Ascension | Declination | $V_{helio}$ | $D$
---|---|---|---|---
Source |  $h$ $m$ $s$ |  ∘ $\prime$ $\prime\prime$ | ($\mbox{km~{}s}^{-1}$) | (Mpc)
MRK 1029 | 02 17 03.57 | +05 17 31.40 | 9076 | 133
NGC 1052 | 02 41 04.80 | –08 15 21.00 | 1510 | 18.0[1]
J0350–0127 | 03 50 00.35 | –01 27 57.70 | 12322 | 185
UGCA 116 | 05 55 42.63 | +03 23 31.80 | 789 | 10.3[2]
NGC 5765b | 14 50 51.50 | +05 06 52.00 | 8333 | 126[3]
TXS 2226–184 | 22 29 12.50 | –18 10 47.00 | 7520 | 108

Note: [1]Jensen et al. (2003), [2]Tully & Fisher (1988), and [3]Gao et al.
(2016)