HI emission and absorption in nearby, gas-rich galaxies II. -- sample completion and detection of intervening absorption in NGC 5156

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by  S. N. Reeves

Mon.Not.R.Astron.Soc.000,1–18(2016) Printed18January2016 (MNLaTEXstylefilev2.2)
H emission and absorption in nearby, gas-rich galaxies
I
II. – sample completion and detection of intervening
absorption in NGC 5156
S. N. Reeves,1,2,3(cid:63) E. M. Sadler,1,2 J. R. Allison,2,3 B. S. Koribalski,3
S. J. Curran,4 M. B. Pracy,1 C. J. Phillips,3 H. E. Bignall,5 and C. Reynolds3
1Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, NSW 2006, Australia
2ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO)
3Australia Telescope National Facility, CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
4School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand
5International Centre for Radio Astronomy Research, Curtin University, Building 610, 1 Turner Avenue, Bentley WA 6102, Australia
6
1
0 Released2016XxxxxXX
2

n
ABSTRACT
a
J We present the results of a survey for intervening 21cm Hi absorption in a sam-
ple of 10 nearby, gas-rich galaxies selected from the Hi Parkes All-Sky Survey
4
1 (HIPASS). This follows the six HIPASS galaxies searched in previous work and
completesourfullsample.Inthispaperwesearchedforabsorptionalong17sight-

] lines with impact parameters between 6 and 46 kpc, making one new detection.
A We also obtained simultaneous Hi emission-line data, allowing us to directly re-
G late the absorption-line detection rate to the Hi distribution. From this we find
the majority of the non-detections in the current sample are because sightline
.
h does not intersect the Hi disc of the galaxy at sufficiently high column density,
p
but that source structure is also an important factor.
-
o The detected absorption-line arises in the galaxy NGC5156 (z = 0.01) at
r an impact parameter of 19 kpc. The line is deep and narrow with an integrated
st opticaldepthof0.82kms−1.HighresolutionAustraliaTelescopeCompactArray
a (ATCA) images at 5 and 8 GHz reveal that the background source is resolved
[ into two components with a separation of 2.6 arcsec (500 pc at the redshift of

1 the galaxy), with the absorption likely occurring against a single component.
v We estimate that the ratio of the spin temperature and covering factor, TS/f,
3 is approximately 950 K in the outer disc of NGC5156, but further observations
5 using VLBI would allow us to accurately measure the covering factor and spin
7 temperature of the gas.
3
0 Key words: galaxies:evolution–galaxies:ISM–radiolines:galaxies–galaxies:
. individual: NGC5156.
1
0
6
1
: 1 INTRODUCTION and Apertif (Oosterloo et al. 2009), it will become pos-
v
sible to conduct the first large, blind absorption-line sur-
Xi Studies of the 21 cm transition of neutral atomic hydro- veys, allowing us to study the evolution of neutral hy-
gen (Hi) provide a unique view of the gas in and around
r drogen over a wide range of cosmic times. FLASH (‘The
a galaxies(seee.g.Hibbardetal.2001;Koribalski&Jerjen First Large Absorption Survey in Hi’) is the planned Hi
2008; Walter et al. 2008). While Hi emission-line stud-
absorption-line survey with ASKAP, and will search for
ies are inhibited by a rapid drop-off in detectability with
Hi absorption along 150,000 sightlines in order to inves-
redshift, the detectability of the Hi absorption-line is es-
tigate the evolution of neutral hydrogen over redshifts
sentially independent of distance (set instead by the flux
z=0.5−1.0.However,ifwewishtoderivephysicalgalaxy
of the background source used to search for absorption).
propertiesfromabsorption-linedata,weneedtoknowthe
This allowsus to study neutral hydrogento muchhigher
expecteddetectionrateofinterveningHiabsorption,and
redshifts than is possible in emission. With the arrival
howthisvarieswithdistancefromthecentreofthegalaxy
of the next generation of radio telescopes, including the
(i.e. impact parameter).
Australian Square Kilometre Array Pathfinder (ASKAP,
Guptaetal.(2010)conductedasystematicstudyto
Johnston et al. 2008), MeerKAT (Booth & Jonas 2012),
addressthisquestion.Usingacombinationofnewobser-
vationsandavailableliteratureresults,theyestimatethat
thedetectionrateofHiabsorptionisaround50percent,
(cid:63) E-mail:[email protected] for impact parameters less than 20 kpc, and integrated
(cid:13)c 2016RAS
2 S. N. Reeves et al.
optical depths greater than 0.1 km s−1. The addition of 2.1 Sample selection
morerecentresults(Borthakuretal.2011,2014;Srianand
Our main sample in this paper consists of 10 radio
et al. 2013; Zwaan et al. 2015) gives a similar overall de-
source-galaxy pairs with impact parameters less than 25
tection rate. While detection rates may differ somewhat
kpc. Radio source-galaxy pairs were selected by cross-
between individual surveys, a common result is that de-
matching galaxies from the HIPASS Bright Galaxy Cat-
tections of Hi absorption at impact parameters greater
alogue (HIPASS BGC, Koribalski et al. 2004) with radio
than 20 kpc appear to be extremely rare.
continuum sources from the 843 MHz Sydney University
Incontrast,detectionsofneutralgasthroughoptical
MolongloSkySurvey(SUMSS,Mauchetal.2003).Inad-
Lyman-α studies have frequently been made at impact
ditiontotheimpactparametercutoff,weimposedamin-
parameters of several tens of kpc, and in some cases ex-
imum (integrated) flux criterion of S > 50 mJy, since
ceeding100kpc(seee.g.Raoetal.(2011)andreferences 843
thefluxofthebackgroundsource(andnottheredshiftof
therein), showing that the gas disc extends well beyond
the galaxy) is what determines the absorption-line sensi-
whatwesee even in21cm observations.This isonly pos-
tivity.Redshiftdataisnotavailablefortheradiosources
sible, however, because the detectability of Lyman-α ab-
in our sample, but we have assumed all of the contin-
sorption does not depend on the spin temperature of the
uumsourcesareathigherredshiftthanthetargetgalax-
gas,meaningthatthesestudiesaresensitivetoveryhigh
ies (i.e. genuine background sources). This is supported
spin temperature gas which would not be detectable in
by redshift analysis of other similar radio source samples
Hi absorption.
(Condonetal.1998;deZottietal.2010)andofprevious
In Reeves et al. 2015 (hereafter Paper I), we inves-
surveys for intervening absorption in the local Universe
tigated the detection rate of intervening absorption in a
(Darling et al. 2011).
sample of nearby (z < 0.04) gas-rich galaxies, selected
The sample selection described above is identical to
from the Hi Parkes All-Sky Survey (HIPASS, Barnes
thatinPaperI,butwithaslightlyhigherimpactparam-
et al. 2001; Meyer et al. 2004; Wong et al. 2006). By tar-
eter cutoff. In addition, a number of the selected fields
geting nearby galaxies we were also able to map the Hi
contained additional continuum sources which, although
emission,allowingustodirectlyrelatetheabsorption-line
theydonotmeettheabovefluxorimpactparametercri-
detectionratetotheextendedHidistribution.Whilefour
teria, have been included in order to increase the sample
ofthesixsightlinesintersectedtheHidisc,noabsorption-
size.Therefore,intotaloursampleinthispaperconsists
linesweredetected.Weattributedthistothebackground
of 17 sightlines with impact parameters between 6 and
sourcesbecomingresolvedorextended,thusreducingthe
46 kpc, and 843 MHz continuum fluxes of 17-823 mJy.
continuumfluxanddramaticallyaffectingtheabsorption-
Combined with the sightlines searched in Paper I, our
line sensitivity.
fullsampleconsistsof23sightlines.Imagesofthetargets
Our detection rate in Paper I was low compared to
are shown in Figure 1. Hi and optical properties of the
previous surveys (although this is difficult to interpret
target galaxies in our sample are given in Table 1, and
accurately given the small sample size). However, while
properties of the background radio sources, along with
many previous surveys have targeted quasar sightlines,
the angular and linear separations, are given in Table 2.
ours represented an unbiased sample of radio sources (a
Throughout this paper we refer to the objects ob-
mixture of radio galaxies and quasars, in a ratio consis-
served in the pilot sample (Reeves et al. 2015) as sample
tent with the overall radio source population). We there-
A,andthenewobjectsobservedforthispaperassample
fore suggested that the differences in detection rate may
B (see Table 3). In most cases, the results presented in
be (at least partly) due to the differences in source type
this paper refer only to the galaxies of sample B, how-
(since quasars are more likely to remain unresolved, and
ever, where relevant, we consider the combined results of
might therefore be expected to produce a higher detec-
the two samples.
tion rate), and that our results may be more representa-
tive of the expected detection rate for future large, blind
absorption-linesurveys.Inthispaperwebuildonourpre- 2.2 ATCA Hi observations
vious work, expanding our sample with an additional 10
AustraliaTelescopeCompactArray(ATCA)Hiobserva-
galaxies.Withtheexpandedsampleweaimtobetteres-
tions were carried out in 10 × 12 hour periods between
tablish: (i) the influence of background source structure
October 2012 and August 2013, using the 750 m arrays,
andsourcetypeondetectionrate,(ii)theexpecteddetec-
which give an angular scale sensitivity of a few arcsec-
tionrateforfutureblindsurveys,and(iii)howdetection
ondsto∼10-20arcmin.Thetargetsourceswereobserved
rate varies as a function of impact parameter.
in40minutescans,interleavedwith10minutescansofa
Throughout this work we assume a flat ΛCDM cos-
nearby phase calibrator. The ATCA primary calibrator,
mology, with Ω = 0.27, Ω = 0.73, and H = 71 km
M Λ 0
s−1 Mpc−1. All uncertainties refer to the 68.3 per cent PKS1934-638,wasobservedforabout10minutesatboth
thebeginningandendofeachobservationforcalibration
confidence interval, unless otherwise stated.
of the absolute flux scale. Observations were conducted
at night time in order to minimise the effects of solar in-
terference, especially on the shorter baselines.
2 OBSERVATIONS AND DATA REDUCTION
For the frequency setup we have used the Compact
To ensure a uniform sample, and consistency in the de- Array Broadband Backend (CABB, Wilson et al. 2011)
rivedquantities,allaspectsofthesampleselection,obser- 64M-32k configuration, which gives a bandwidth of 64
vations,datareduction,andanalysisareperformedasde- MHz with 32 kHz channels (spectral resolution of ∼6.7
scribed in Paper I, unless otherwise specified. Therefore, km s−1 at the frequency of the redshifted Hi line). The
inordertoavoidunnecessaryrepetition,wedescribeonly noise level achieved in a 12 hour observation (∼2-3 mJy
the most important points in relation to these aspects of per channel) is sufficient to detect absorption in DLA-
theworkhere,andreferthereadertoPaperIforamore strengthsystems(N (cid:38)2×1020cm−2)againsttheback-
HI
detailed description. ground sources we have selected, assuming the source is
(cid:13)c 2016RAS,MNRAS000,1–18
HI in nearby galaxies II 3
Table 1. Hi and optical properties of the target galaxies in our sample. Columns (1)-(6) are the Hi properties as given in the
HIPASSBGC(Koribalskietal.2004).Column(1)istheHIPASSname.Columns(2)and(3)aretheHIPASSJ2000rightascension
and declination. HIPASS positions have typical uncertainties of (cid:54) 1.5 arcmin for signal-to-noise ratios of (cid:62) 10. Columns (4) and
(5)arethesystemicandlocalgroupvelocities.Column(6)istheoptical/IRidentification(Doyleetal.2005).Columns(7)-(9)are
theopticalpropertiesasgivenintheNASAExtragalaticDatabase(NED).Column(7)istheopticalmorphologicalclassification.
Columns(8)and(9)aretheopticalJ2000rightascensionanddeclination.
HIPASSName RAHIPASS DecHIPASS vsys vLG OpticalID Morph.Type RAopt Decopt
(J2000) (J2000) (kms−1) (kms−1) (J2000) (J2000)
J0309-41 030942 -410108 955 809 ESO300-G014 SAB(s)m 030937.87 -410149.71
J0316-35 031652 -353219 1570 1436 ESO357-G012 SB(s)d 031652.81 -353225.92
J0541-35 054101 -354243 1264 1035 ESO363-G015 SA(s)d 054100.87 -354227.13
J0319-49 031925 -493634 1028 853 IC1914 SAB(s)d 031925.24 -493559.04
J1415-43 141505 -435806 1875 1684 IC4386/7 SAB(s)dmpec? 141503.07 -435745.23
J0310-53 031002 -532005 1072 893 NGC1249 SB(s)cd 031001.23 -532008.74
J0419-54 041956 -545658 1504 1287 NGC1566 SAB(s)bc 042000.42 -545616.14
J0610-34 061011 -340656 747 505 NGC2188 SB(s)medge-on 061009.53 -340622.33
J1328-48 132841 -485409 2988 2762 NGC5156 SB(r)b 132844.09 -485500.54
J2200-43 220027 -430741 2269 2258 NGC7162A SAB(s)m 220035.74 -430822.45
Opticalpositionreferences:
1Loveday(1996)
2daCostaetal.(1998)
3Lauberts(1982)
4Skrutskieetal.(2006)
5Maddoxetal.(1990)
Table2.Propertiesofthebackgroundcontinuumsourcesusedtosearchforabsorption.Column(1)istheidentification(ID)used
throughoutthiswork.Columns(2)-(6)aretheradiopropertiesasgivenintheSUMSScatalogue(Mauchetal.2003).Column(2)
istheSUMSSname.Columns(3)and(4)aretheSUMSSJ2000rightascensionanddeclination.Columns(5)and(6)arethe843
MHzpeakandintegratedfluxes.Columns(7)and(8)aretheangularandlinearseparationsoftheradiosourceandtheforeground
galaxy.
SourceName∗ SUMSSName RASUMSS DecSUMSS Speak,843 Sint,843 Ang.Sep. Imp.Param.
(J2000) (J2000) (mJybeam−1) (mJy) (arcmin) (kpc)
C-ESO300-G014-1 J030946-410456 030946.24 -410456.6 113.5 114.5 3.5 11.5
C-ESO300-G014-2 J030941-410006 030941.24 -410006.8 110.2 112.1 1.8 6.1
C-ESO357-G012 J031705-353440 031705.10 -353440.3 67.7 71.8 3.4 19.6
C-ESO363-G015-1 J054104-354640 054104.72 -354640.3 67.5 67.5 4.3 18.1
C-ESO363-G015-2 J054100-354634 054100.16 -354634.2 59.9 66.0 4.1 17.4
C-IC1914 J031955-493551 031955.29 -493551.1 210.7 222.9 4.9 17.0
C-IC4386-1 J141517-435922 141517.99 -435922.5 81.7 90.2 3.1 21.5
C-IC4386-2 J141455-440101 141455.11 -440101.8 24.9 25.7 3.6 24.5
C-NGC1249-1 J030954-532339 030954.31 -532339.2 48.5 55.6 3.7 13.3
C-NGC1249-2 J030938-532417 030938.05 -532417.3 27.2 32.9 5.4 19.6
C-NGC1249-3 J030917-531756 030917.15 -531756.9 42.5 48.3 6.9 25.3
C-NGC1566-1 J042015-545345 042015.32 -545345.2 58.1 58.9 3.3 17.3
C-NGC1566-2 J042007-545143 042007.48 -545143.4 35.3 52.6 4.7 24.4
C-NGC2188 J061019-340258 061019.16 -340258.7 95.9 130.9 3.9 8.1
C-NGC5156 J132846-485638 132846.53 -485638.7 790.3 823.2 1.7 18.8
C-NGC7162A-1 J220040-430950 220040.68 -430950.1 46.4 50.8 1.7 15.7
C-NGC7162A-2 J220057-431138 220057.05 -431138.8 15.8 17.4 5.1 46.5
∗ThroughoutthisworkwerefertothecontinuumsourcesbytheIDlistedinColumn(1).Thisisthegalaxyname,with
a ‘C’ (for ‘continuum’) prepended, allowing us to easily identify the galaxy-continuum source pairs. Where there are
multiplesourcesinasinglefieldwehaveaddedasuffix,tonumberthedifferentsources.Inaddition,ifasourceresolves
into multiple components at higher resolution then we add a second suffix (a lowercase letter) to indicate this (e.g.
C-ESO363-G015-2a, C-ESO363-G015-2b). We note that this differs slightly from our convention in paper I, to allow
forthefactthattheremaybemultiplesourcesperfieldinthepresentsample.
unresolved. A summary of the observations is given in behidinganyabsorption.ForNGC5156wewishedtore-
Table 4. observethenewabsorption-linedetectedinthe750array
observations with higher spectral spectral resolution, in
Follow-up observations using the 6 km arrays were
order to spectrally resolve the line profile. The follow-
conductedfortwosources,NGC1566andNGC5156.Our
initialobservationsofNGC1566showedahighHicolumn up observations for NGC5156 were therefore conducted
density along the sightline searched, but no Hi absorp- using the CABB 1M-0.5k configuration, which provides
1 MHz bandwidth with 0.5 kHz spectral resolution (or
tion.Theseobservationsathigherspatialresolutionwere
thereforedesignedtoresolveoutanyremainingHiemis- about 0.1 km s−1 at the frequency of the Hi line).
sionanddeterminewhetherthestrongemission-linecould All data reduction was carried out in miriad (Sault,
(cid:13)c 2016RAS,MNRAS000,1–18
4 S. N. Reeves et al.
Figure 1.SuperCOSMOSB-bandimages(Hamblyetal.2001)ofthesixtargetgalaxies,withSUMSS843MHzradiocontinuum
contours overlaid (Mauch et al. 2003). The SUMSS peak flux is given in the bottom right corner. Radio contours start at 90 per
centofthepeakfluxanddecreasein10percentincrements.
Teuben & Wright 1995) using a purpose-built data re- or absorption. Low resolution cubes (with a synthesised
duction pipeline, and following standard data reduction beam of ∼60 arcsec) were made using natural weighting,
proceduresforcontinuumandspectral-linedata.Foreach andexcludingthebaselinestoantenna6,mediumresolu-
ofthetargetsinoursamplewehaveproducedanHidata tion cubes (∼20 arcsec) were made using natural weight
cube and 1.4 GHz radio continuum image. For the 1M- withtheantenna6baselinesincluded,andhighresolution
0.5k observations of NGC5156, the full spectral resolu- cubes (∼5 arcsec) were made using uniform weighting.
tion of 0.1 km s−1 was higher than we required, so when
Fourier transforming the data we produced a data cube
binned to a resolution of 1 km s−1, which provided an
optimal balance between spectral resolution and signal-
to-noise per channel. Himomentmapswereproducedfromthelowresolu-
As in Paper I, we have produced three sets of data- tion cubes and spectra were extracted from all three sets
cubes and continuum images for the 750 array data, us- ofcubesatthepositionoftheradiocontinuumsource.In
ing different weighting schemes. In doing so we are able thecasethatabackgroundsourcewasresolvedintomul-
tospantherangeofresolutionspossiblewiththe750ar- tiplecomponentsathigherresolution(seeFigureA2),we
rays,andthusoptimiseforthedetectionofeitheremission have extracted a separate spectrum for each component.
(cid:13)c 2016RAS,MNRAS000,1–18
HI in nearby galaxies II 5
Table3.ListofgalaxiesobservedaspartofsamplesAandB 3.2 Radial HI profiles
forthissurvey. In Figure 3 we show the radial Hi profiles of the galax-
ies in our sample. For each galaxy we show two versions
SampleA SampleB
of the profile – the azimuthally averaged profile over the
(Reeves+15) (thiswork)
whole gas disc, and the profile along the axis to the con-
ESO150-G005 ESO300-G014 tinuum source. The ellipse parameters used to derive the
ESO345-G046 ESO357-G012 azimuthally averaged profiles are given in Table 5. For
ESO402-G025 ESO363-G015 fields with multiple background sources, the continuum
IC1954 IC1914 axis profile is just shown for the first source (as listed in
NGC7412 IC4386
Table 2), which was the main target of our observations
NGC7424 NGC1249
(while other sources were simply found by chance in the
NGC1566
samefield,anddidnotnecessarilymeetallofthespecified
NGC2188
selection criteria).
NGC5156
NGC7162A As in Paper I we see a variety of different profile
shapes – from relatively flat profiles as in NGC5156 and
NGC7162A, to much steeper profiles such as NGC2188
andIC1914.Comparingtheazimuthallyaveragedprofiles
Table4.SummaryoftheATCAobservationsforthegalaxies
to the profiles in the direction of the continuum source,
observedinsampleB(thispaper).
we find that the two profiles are almost identical for half
of the galaxies in our sample. Only three galaxies show
Targetgalaxy Obs.Date Array∗ Int.time
dramatically different profiles – IC4386 and NGC1249
(h)
(where the profile towards the continuum source drops
ESO300-G014 2012Oct26 750B 7.73 off much more steeply, as the source is not located along
ESO357-G012 2012Oct30 750B 8.10 themajoraxis),andNGC1566(duetolocalvariationsin
ESO363-G015 2012Oct31 750B 8.51 the Hi distribution from the spiral arms of the galaxy).
IC1914 2012Oct27 750B 8.15 We have also measured the size of the Hi disc (from
IC4386 2013Feb02 750C 8.01
theazimuthally-averagedprofiles)atacolumndensityof
NGC1249 2012Oct29 750B 7.70
N = 2 × 1020 cm−2. The measured Hi disc sizes are
NGC1566 2012Oct28 750B 7.97 HI
presentedinTable5,alongsidetheopticaldiscsizes(R ,
NGC1566(follow-up) 2013Jun10 6C 9.24 25
NGC2188 2012Nov01 750B 8.10 taken from the literature). There is a large range in disc
NGC5156 2013Feb03 750C 7.61 sizes – the Hi discs range from 6.4 to 26.7 kpc, with Hi-
NGC5156(follow-up) 2013Nov12 6A 8.15 to-optical disc ratios of 1.2 to 2.6. Typically though, we
NGC7162A 2013Aug02 750D 7.68 find the Hi disc to be around 10-20 kpc in radius, and
around twice the size of the optical disc.
∗Rangeofbaselinelengths:
750B: 61 – 4500 m; 750C: 46-5020 m; 750D: 31 – 4469 m;
6A:337-5939m;6C:153-6000m
4 HI EMISSION AND ABSORPTION IN
3 HI DISTRIBUTION IN THE TARGET THE SPECTRA TOWARDS THE
GALAXIES BACKGROUND SOURCES
3.1 HI emission maps 4.1 Spectra and continuum images
Hi moment maps (total intensity, velocity, and velocity In Figure A2 we show the 1.4 GHz radio continuum im-
dispersion) for each of the galaxies are presented in Fig- agesofeachofthebackgroundsourcesinoursample,and
ure A1. In general we find fairly symmetric, regular ro- beloweachthespectrumtowardsthatsource(atallthree
tating discs — although some galaxies, such as IC4386 resolutions). As in Paper I, we apply a Bayesian analy-
and NGC2188, show notable asymmetries in their total sis to all spectra, using the multi-nest algorithm (Feroz
intensity and velocity maps, which could indicate recent & Hobson 2008; Feroz, Hobson & Bridges 2009), to de-
interactions. termine objectively whether there is Hi emission and/or
ThemeasuredHimassesaregiveninTable5,along absorption present.
with the HIPASS Hi masses for comparison. We find a The development and testing of this algorithm are
range of Hi masses, between ∼4.0 × 108 and 5.6 × 109 described by Allison, Sadler & Whiting (2012), and the
M (with an estimated uncertainty of 5 per cent). Our application to real spectral-line data by Allison, Sadler
(cid:12)
valuesareconsistentwiththosefromHIPASS–orslightly & Meekin (2014) and references therein. Since then, the
lowersincewithaninterferometerweresolveoutsomeof software has been upgraded and is now capable of fit-
themorediffuseHifluxwhichisdetectablewithParkes. ting simultaneously for a combination of both emission
In Figure 2 we show the Hi distribution overlaid on and absorption. It also now includes a number of addi-
the SuperCOSMOS optical image (Hambly et al. 2001) tionallineprofilessuchasthe‘BusyFunction’(Westmeier
for each galaxy. The radio continuum emission contours et al. 2014), which we used in fitting the double-horned
are also plotted, allowing us to examine the location of emission-line profile seen along the sightline towards C-
thebackgroundsource(s)relativetotheHidisc.Wefind IC4386.Allotherspectrawerefit(orupperlimitscalcu-
thatfiveofthe17sightlinesinoursample(or6ofthe23 lated) assuming a gaussian line-profile.
individualcomponentsseenathigherresolution)intersect Throughout this work we use the statistic referred
theHidiscofthegalaxyatcolumndensitiesabove∼1-2 to as the R-value to give a measure of the significance of
×1020 cm−2 (thelimitofourabsorption-linesensitivity). any detected spectral lines. For a full explanation of this
We discuss this further in Section 6. statistic, and further details on the application of this
(cid:13)c 2016RAS,MNRAS000,1–18
6 S. N. Reeves et al.
Figure 2. SuperCOSMOS B-band images (Hambly et al. 2001) with Hi contours (blue) overlaid. The Hi contour levels are NHI
= 3 × 1019, (1, 2, 5) × 1020, and 1 × 1021 cm−2. The high resolution 1.4 GHz radio continuum emission is also overlaid (red
contours), with an arrow indicating the location of the background continuum source(s) (which are offset from the centre of the
galaxy).Thenumberinthebottomright-handcornershowshowmanyoftheindividualsightlinesintersectthediscatacolumn
densitygreaterthan1×1020 cm−2 (withthenumberofsightlinesatthelowerSUMSSresolution,fromwhichthesesourceswere
selected,giveninbrackets,ifdifferent).Astarindicatesthatthecolumndensityisborderlinealongoneormoreoftheremaining
sightlines. The synthesised beam for the Hi maps (∼60 arcsec) is shown in the bottom left corner, and the synthesised beam for
thecontinuumimages(notshown)is∼5arcsec.
algorithmtoourdata,wereferthereadertoourprevious resolution spectra. This indicates that more than half of
work(Reevesetal.2015)aswellastheabovereferences. thesightlinesinoursampleintersecttheHidiscatsome
column density. Where an emission-line was detected in
thehigherresolutionspectra,adetectionwasalsoalways
madeatthelowerresolutions(butthereverseisobviously
4.2 HI emission and absorption along the
not always true). Where emission was detected only in
target sightlines
the low resolution spectra the column density of the gas
4.2.1 Hi emission is typically a few × 1020 cm−2 – but for sightlines with
detections in the medium or high resolution spectra, this
Hi emission-lines were detected along 10 of the 17 sight-
impliesthepresenceofgaswithcolumndensitiesofupto
lines in the low resolution spectra, 6 sightlines in the
N = 1021-1022 cm−2.
medium resolution spectra, and 2 sightlines in the high HI
(cid:13)c 2016RAS,MNRAS000,1–18
HI in nearby galaxies II 7
Table 5.DerivedHiparametersofthetargetgalaxies(listedinTable1).Column(1)istheopticalgalaxyID.Column(2)isthe
HimassmeasuredfromtheATCAdata,assumingD=vLG/H0(withanestimateduncertaintyof±5percent).Column(3)isthe
HimassfromHIPASS.TheHIPASSmasseshavebeenre-scaledusingaHubbleconstantofH0 =71kms−1 Mpc−1,aselsewhere
inthiswork,andthemeanuncertaintyontheHIPASSvaluesis15percent(assumingthattheuncertaintyontheintegratedHi
fluxisthedominantsourceoferror).Columns(4)and(5)aretheellipseparameters(ellipticityandpositionangle)derivedfrom
theHimaps,usedtoproducetheazimuthallyaveragedradialprofilesshowninFigure3.Column(6)istheopticaldiscsize(R25,
takenfromtheRC3cataloguedeVaucouleursetal.1991).Column(7)istheHidiscsize,derivedfromtheradialprofiles.Column
(8)istheratiooftheHiandopticaldiscsizes.
log10MHI
ATCA HIPASS Ellipticity PA Ropt RHI RHI/Ropt
(M(cid:12)) (M(cid:12)) (deg) (kpc) (kpc)
ESO300-G014 8.87 8.91 0.49 −9.9 8.3 9.9 1.2
ESO357-G012 9.31 9.25 0.30 −35.7 8.1 14.6 1.8
ESO363-G015 8.87 8.90 0.40 −3.3 5.2 9.3 1.8
IC1914 9.16 9.21 0.45 −68.4 6.6 12.2 1.8
IC4386 9.75 9.85 0.57 −8.3 10.1 26.0 2.6
NGC1249 9.52 9.58 0.57 87.9 8.9 18.9 2.1
NGC1566 9.92 10.04 0.02 135.0 21.8 26.7 1.2
NGC2188 8.64 8.59 0.55 −0.4 4.5 6.4 1.4
NGC5156 9.75 9.79 0.11 −3.4 13.1 22.4 1.7
NGC7162A 9.63 9.78 0.31 35.2 12.1 22.0 1.8
15
) ESO300-G014 ESO357-G012 ESO363-G015
2−
m
c
ms 10
o
at
200
1 5
×
(
HI
N
0
) IC1914 IC4386 NGC1249
2−
m
c
ms 10
o
at
200
1 5
×
(
HI
N
0
) NGC1566 NGC2188 NGC5156
2−
m
c
ms 10
o
at
200
1 5
×
(
HI
N
0
10 20 30 40 10 20 30 40
2)− NGC7162A R(kpc) R(kpc)
m
c
ms 10
o
at
200
1 5
×
(
HI
N
0
0 10 20 30 40
R(kpc)
Figure3.RadialHiprofilesofthegalaxiesinoursample,derivedfromtheATCAHitotalintensitymaps.Theblue(solid)curves
showtheazimuthallyaveragedprofile,andtheblack(dotted)curvestheprofilealongtheaxistowardsthecontinuumsource.The
impact parameter of the continuum source is shown by the dashed vertical line. For fields with multiple background sources, the
continuumaxisprofileisjustshownforthefirstsource(asgiveninTable2),asisthedottedlineindicatingtheimpactparameter
ofthesightline.Thegreyshadedregionindicatessemi-majoraxisofthesynthesisedbeamanddashedhorizontallineindicatesthe
DLAlimit(NHI =2×1020 cm−2).
(cid:13)c 2016RAS,MNRAS000,1–18
8 S. N. Reeves et al.
Table 6 gives the best-fitting parameters for each of K(seeTable7)—however,forsightlineswhereemission
thedetectedemission-lines,aswellasthederivedHicol- wasdetectedinthemediumorhighresolutioncubes(im-
umn density along these sightlines. We note that, since plying column densities of ∼1021-1022 cm−2), we obtain
thebeaminthelowandmediumresolutioncubesismuch lower limits of up to several thousand K.
largerthanthatinthehighresolutioncubes,thenumber Thisisconsistentwiththelowerlimitsofafewhun-
of sightlines with emission-line detections (cid:38)1-2 × 1020 dredKfoundbyCarilli&vanGorkom(1992)fromsimilar
cm−2 is slightly higher than we might predict based on emission- and absorption-line data, as well as estimates
theHishowninFigure2.Thisissimplyaconsequenceof fromrecentLy-αstudieswhichtypicallyfindspintemper-
thefactthatthebeamismuchlargeratlowerresolution, aturesofafewtenstoafewhundredK(Borthakuretal.
and that the emission-lines seen in the extracted spectra 2010,2014;Srianandetal.2013;Kanekaretal.2014).We
represent a weighted average over this beam (or put an- stress, however, that, since our limits are based on non-
other way, the much smaller beam of the high resolution detections, they are not very restrictive, and absorption-
continuum images means we cannot predict the column linedetectionswouldberequiredtoobtainmoreaccurate
density along a given sightline quite as accurately as it estimates of the value of T /f along these sightlines.
S
might appear we can from Figure 2). For the sightlines where the lower limit on T /f is
S
very high, we suggest that this can be explained by a
clumpygasmedium,likethatsuggestedbyBraun(2012).
4.2.2 Hi absorption In this scenario we have high column density clumps
(∼100 pc in size) embedded within a more diffuse gas.
Despite the fact that almost one-third of the sightlines
searchedintersecttheHidiscoftheforegroundgalaxyat Thiswouldresultinahighaveragecolumndensityinthe
column densities above ∼1-2 × 1020 cm−2, we have de- large beam of the lower resolution cubes but, if the nar-
rowsightlinetowardsthebackgroundsourcemissesthese
tectedonlyoneabsorption-lineinoursample.Theabsorp-
small, dense clumps, would still give an absorption-line
tionarisesinthegalaxyNGC5156,abarredspiralgalaxy
non-detection,asobserved.Evidenceforsuchstructuresis
ataredshiftofz=0.01.Thesightlineintersectsthedisc
alsoseeninotherrecentHiabsorption-linestudies(Cur-
atanimpactparameterof19kpc,andtheabsorption-line
has an integrated optical depth of 0.82 km s−1. We note ran et al. 2013; Srianand et al. 2013; Borthakur et al.
2010, 2011, 2014).
thatthisisthehighestimpactparameterofanyinterven-
ingHiabsorption-linedetectedtodate(though,asmen-
tionedinSection1,thisisstillmuchlowerthanwhathas
been seen in optical Lyman-α studies). The background 5 ABSORPTION-LINE DETECTION IN
source,C-NGC5156,isbyfarthebrightestinoursample, NGC5156
witha1.4GHzfluxofaround400mJybeam−1,sogiven
5.1 Initial detection and high-spectral
that we havemade only one detection in our sample it is
resolution follow-up
not surprising that it is along this sightline.
To test for absorption below the noise, we have also Inourinitialobservationswedetectedadeep,narrowHi
stacked the spectra from all of the non-detections to absorption-lineinthegalaxyNGC5156,atanimpactpa-
searchforastatisticaldetectionofabsorptionalongthese rameterof19kpc.Sincethelinewasspectrallyunresolved
sightlines. Despite the large number of sightlines stacked we conducted higher spectral resolution observations us-
wedidnotfindanyevidenceforabsorption.Wecalculate ing the CABB 1M-0.5k configuration (and the 6A array)
a 5-σ upper limit of τpeak (cid:46) 0.015, which corresponds to in order to resolve the line-profile. The follow-up obser-
a column density of NHI (cid:46) 1.5 × 1020. vations confirm the presence of a deep absorption-line,
Table 7 presents the best-fitting parameters for the andprovideaspectrallyresolvedprofileoftheabsorption
detected absorption-line, as well as upper limits for each feature. The absorption-line has a peak optical depth of
ofthenon-detections.Wediscussthedetectedabsorption- 11 per cent, and a width of just 7.6 km s−1, giving it an
lineingreaterdetailinSection5andinvestigatetherea- integrated optical depth of 0.82 km s−1.
sons for the low detection rate in our sample in Section Thetwoabsorption-linespectraarepresentedinFig-
6. ure4.Wenotethat,whilethetwoabsorption-linesappear
tohaveverydifferentline-depths,thisismerelytheeffect
of the lower spectral resolution in the initial observation
4.2.3 Spin temperature of the gas
reducing the apparent depth of the line. We tested this
We can also use the combined emission- and absorption- by convolving the high resolution spectrum to the same
line data to investigate the spin temperature of the gas resolutionastheinitialobservation,andfoundtheobser-
along the sightlines studied. VLBI data is required to vations to be consistent.
resolve the background sources in order to measure the
coveringfactorandthusdetermineanaccuratespintem-
5.2 VLBI continuum observations
perature. However, the ATCA data can still be used to
estimate the ratio of the spin temperature to the cov- Toinvestigatethesmall-scalestructureofthebackground
ering factor (T /f). For the detected absorption-line we radio source in the absorption-line system, a short Very
S
estimate the ratio of the spin temperature and covering LongBaselineInterferometry(VLBI)continuumobserva-
factor is T /f ≈ 950 K (something we would be able to tionofC-NGC5156wasconductedusingtheLongBase-
S
refinewithimprovedVLBIobservations–seeSection5). line Array (LBA). The observations were made as part
Forthenon-detections,wecanstillusethecombined of a 1 × 18 h period on 2012 April 29-30 (along with
data(Hicolumndensityfromtheemission-linedataand a number of other sources which form part of a differ-
theupperlimitontheopticaldepthfromtheabsorption- ent project). We obtained two tracks at different hour
line data) to put some constraints on the value of T /f. angles,withatotalon-sourceintegrationtimeofapprox-
S
We find that the lower limit is typically T /f ∼10-100 imately 65 minutes. Ideally we would have obtained ad-
S
(cid:13)c 2016RAS,MNRAS000,1–18
HI in nearby galaxies II 9
Table 6.Thebest-fittingparametersestimatedforthedetectedHiemission-lines.Columns(1)and(2)arethesightlineandcube
resolutionatwhichthelinewasdetected.Columns(3),(4),and(5)arethevelocity,width,andpeakfluxoftheline.Column(6)
is the integrated Hi line flux. Column (7) is the derived Hi column density. Column (8) is the R-value of the detected line (see
paperI).
(cid:82)
Sightline Cube Velocity(cz) Speak Width(∆cz) Sdv NHI R-value
resolution (kms−1) (mJybeam−1) (kms−1) (mJybeam−1 kms−1) (×1020 cm−2)
C-ESO300-G014-1 Low 1014.7+1.9 11.2+1.3 39.3+5.3 439.4+45.0 1.4+0.1 58.29±0.06
−2.0 −1.2 −4.8 −46.0 −0.1
C-ESO300-G014-2 Low 915.8+0.4 58.4+1.2 45.4+1.1 2650.3+51.3 8.1+0.2 1973.42±0.07
−0.4 −1.2 −1.1 −52.1 −0.2
C-ESO300-G014-2 Medium 911.3+2.1 11.8+1.1 47.1+5.8 555.3+49.9 12.1+1.1 85.14±0.06
−2.0 −1.1 −5.3 −50.7 −1.1
C-ESO300-G014-2 High 920.4+7.3 4.4+1.8 48.8+25.9 214.5+71.6 54.7+18.2 3.92±0.04
−6.7 −1.4 −18.1 −67.0 −17.1
C-ESO357-G012 Low 1508.4+2.8 8.6+1.2 42.0+7.1 359.8+48.7 1.0+0.1 32.71±0.05
−2.8 −1.2 −5.8 −46.2 −0.1
C-IC4386-2 Low 1807.1+3.2 10.6+0.7 108.6+7.4 1149.2+69.6 3.2+0.2 183.38±0.06
−3.2 −0.6 −6.9 −69.3 −0.2
C-NGC1249-1 Low 1040.7+1.6 14.3+1.3 39.9+4.3 571.8+48.6 2.1+0.2 101.08±0.06
−1.6 −1.2 −4.0 −46.4 −0.2
C-NGC1249-1 Medium 1048.0+3.0 5.0+1.4 25.7+8.7 128.6+31.4 2.9+0.7 6.43±0.05
−3.2 −1.3 −6.7 −30.6 −0.7
C-NGC1566-1 Low 1415.0+0.1 142.9+1.9 25.1+0.4 3583.5+43.9 13.9+0.2 5000.22±0.07
−0.2 −2.0 −0.4 −44.9 −0.2
C-NGC1566-1 Medium 1413.2+0.5 34.6+2.1 20.5+1.6 706.9+37.0 15.5+0.8 310.68±0.06
−0.5 −2.0 −1.5 −35.4 −0.8
C-NGC1566-1 High 1415.9+2.4 10.9+1.7 35.3+6.7 383.2+56.8 130.3+19.3 29.26±0.05
−2.4 −1.6 −6.2 −53.7 −18.2
C-NGC1566(JUN13) - 1413.2+2.3 5.5+0.9 28.6+5.0 156.1+24.4 16.6+2.6 18.80±0.05
−2.4 −0.9 −4.4 −24.3 −2.6
C-NGC1566-2 Low 1428.7+0.5 42.0+1.6 30.9+1.4 1297.5+44.0 5.0+0.2 630.20±0.07
−0.5 −1.5 −1.3 −44.8 −0.2
C-NGC1566-2a Medium 1427.3+2.5 8.2+1.2 36.5+6.1 300.6+40.0 6.6+0.9 30.59±0.05
−2.6 −1.1 −5.3 −40.3 −0.9
C-NGC1566-2b Medium 1431.4+1.7 11.5+1.4 30.6+4.5 351.5+39.0 7.7+0.9 54.22±0.06
−1.6 −1.3 −4.3 −38.8 −0.9
C-NGC2188 Low 743.9+4.3 6.8+0.9 83.4+17.1 562.3+73.8 1.5+0.2 54.42±0.05
−4.4 −0.9 −14.8 −71.5 −0.2
C-NGC5156 Low 2933.1+1.7 30.7+3.1 69.7+6.3 2147.7+127.0 6.5+0.4 207.46±0.08
−1.7 −2.5 −5.7 −122.9 −0.4
C-NGC5156 Medium 2942.7+7.0 7.9+1.4 95.1+8.2 728.9+112.6 14.3+2.2 18.90±0.07
−6.9 −1.2 −14.0 −109.4 −2.2
C-NGC7162A-1 Low 2247.7+1.3 22.5+1.1 59.1+3.8 1331.3+63.1 4.6+0.2 372.45±0.06
−1.3 −1.1 −3.5 −61.4 −0.2
C-NGC7162A-1 Medium 2242.7+3.9 5.9+2.2 30.7+17.2 181.7+56.1 4.4+1.3 5.07±0.05
−4.4 −2.0 −10.7 −52.4 −1.3
30
(i)Initialdetection(Feb2013) (ii)Follow-upobservations(Nov2013)
20
10
1)− 0
m
a
e
b 10
y−
J
m
(
S−20
30
−
40
−
50
− 150 100 50 0 50 100 50 0 50
− − v(k−ms−1) − v(k−ms−1)
Figure 4.ComparisonofthelowandhighresolutionHiabsorptionspectraofNGC5156.Whilethetwospectraappeartohave
verydifferentline-depths,wehaveconfirmedthatthisismerelyaneffectofthelowerspectralresolutionofthedataintheinitial
spectrum,andthatthedataarecompletelyconsistent.
ditional cuts, but the LST of the source, and scheduling sitivity of all of the LBA bands, as well as additional
constraints from other targets in our program made this stations for better uv-coverage.
impossible.
We observed with two polarisations (left- and right-
The array used consisted of 8 antennas – the ATCA hand circular polarisation), and a total bandwidth of 64
(6 dishes), Mopra, Parkes, Hobart, Ceduna, Katherine, MHz (2240-2304 MHz). The ATCA, Parkes, Mopra, Ho-
Yaragadee, and Warkworth (New Zealand) – with the bart, and Ceduna observe with a single 64 MHz band,
longest baseline (Yaragadee-Warkworth, ∼5500 km) giv- whileKatherine,Yaragadee,andWarkworthobservewith
ing angular sensitivity down to ∼6 mas. Observations 4contiguous16MHzbands.Thismeansthatbandedges,
wereconductedat2.3GHzwhich,althoughfurtherfrom wherethereisverypoorsensitivity(andwhicharethere-
1.4 GHz than we would have desired, offers the best sen- fore normally flagged out), differ between antennas and
(cid:13)c 2016RAS,MNRAS000,1–18
10 S. N. Reeves et al.
Table 7.Absorption-lineparametersandestimatesofTS/f.Column(1)isthesightlinesearched.Columns(2)-(6)arethevalues
derived from the absorption-line data (3-σ upper limits for the non-detections). Column (2) is the rms-noise level. Column (3) is
the peak 1.4 GHz continuum flux of the background source. Column (4) is the peak optical depth of the line. Column (5) is the
integratedopticaldepth.Fornon-detectionswehavecalculatedtheupperlimitassumingagaussianprofilewithaline-widthof10
km s−1. Column (6) is the absorption-line Hi column density (assuming TS = 100 K and f = 1.0). Columns (7) and (8) are the
relevantemission-linevaluesusedtoestimateTS/f.Column(7)istheemission-lineHicolumndensity.Column(8)istheweighting
scheme from which the emission-line column density was derived. Column (9) is the limit for TS/f calculated from the combined
emission- and absorption-line data. If an emission-line was detected at multiple spatial resolutions, we have calculated a separate
limit for TS/f at each resolution. For the deeper absorption-line observations of C-NGC1566-1 and C-NGC5156 (made in June
2013 and November 2013, respectively) the limit on TS/f is calculated using the emission-line column density derived from the
original(750array)observations,whichwehavedenotedbysquarebrackets.
Hiabsorption(highresolutionspectra) Hiemission Combined
(cid:82)
Sightline σchan Speak,1.4 τpeak τdv NHI(abs) NHI(em) Cube TS/f
(mJybeam−1) (mJybeam−1) (percent) (kms−1) (×1020 cm−2) (×1020 cm−2) resolution (K)
C-ESO300-G014-1 2.56 66.7±1.8 <0.12 <1.22 <2.2 1.4+0.1 Low >61
−0.1
C-ESO300-G014-2 2.74 53.5±1.4 <0.15 <1.63 <3.0 8.1+0.2 Low >274
−0.2
C-ESO300-G014-2 2.74 53.5±1.4 <0.15 <1.63 <3.0 12.1+1.1 Medium >406
−1.1
C-ESO300-G014-2 2.74 53.5±1.4 <0.15 <1.63 <3.0 54.7+18.2 High >1841
−17.1
C-ESO357-G012-1a 2.35 21.4±1.5 <0.33 <3.49 <6.4 1.0+0.1 Low >15
−0.1
C-ESO357-G012-1b 2.05 5.6±0.6 <1.11 <11.70 <21.3 1.0+0.1 Low >5
−0.1
C-ESO363-G015-1 2.36 39.4±0.6 <0.18 <1.90 <3.5 - - -
C-ESO363-G015-2a 2.37 12.4±1.2 <0.57 <6.09 <11.1 - - -
C-ESO363-G015-2b 2.44 11.4±1.0 <0.64 <6.81 <12.4 - - -
C-IC1914-1a 2.46 58.4±1.0 <0.13 <1.34 <2.4 - - -
C-IC1914-1b 2.33 38.9±1.0 <0.18 <1.91 <3.5 - - -
C-IC4386-1 2.48 81.1±0.6 <0.09 <0.97 <1.8 - - -
C-IC4386-2 2.46 20.0±0.4 <0.37 <3.91 <7.1 3.2+0.2 Low >45
−0.2
C-NGC1249-1 2.36 16.2±0.6 <0.44 <4.64 <8.5 2.1+0.2 Low >25
−0.2
C-NGC1249-1 2.36 16.2±0.6 <0.44 <4.64 <8.5 2.9+0.7 Medium >34
−0.7
C-NGC1249-2 2.34 14.1±0.5 <0.50 <5.28 <9.6 - - -
C-NGC1249-3a 2.47 8.2±0.5 <0.90 <9.54 <17.4 - - -
C-NGC1249-3b 2.46 7.5±0.6 <0.99 <10.46 <19.1 - - -
C-NGC1566-1 2.48 23.9±0.8 <0.31 <3.30 <6.0 13.9+0.2 Low >231
−0.2
C-NGC1566-1 2.48 23.9±0.8 <0.31 <3.30 <6.0 15.5+0.8 Medium >258
−0.8
C-NGC1566-1 2.48 23.9±0.8 <0.31 <3.30 <6.0 130.3+19.3 High >2164
−18.2
C-NGC1566-1(JUN13) 1.31 32.0±0.9 <0.12 <1.30 <2.4 [13.9+0.2] Low >584
−0.2
C-NGC1566-1(JUN13) 1.31 32.0±0.9 <0.12 <1.30 <2.4 [15.5+0.8] Medium >655
−0.8
C-NGC1566-1(JUN13) 1.31 32.0±0.9 <0.12 <1.30 <2.4 [130.3+19.3] High >5484
−18.2
C-NGC1566-2a 2.57 13.3±0.5 <0.58 <6.16 <11.2 5.0+0.2 Low >45
−0.2
C-NGC1566-2a 2.57 13.3±0.5 <0.58 <6.16 <11.2 6.6+0.9 Medium >59
−0.9
C-NGC1566-2b 2.21 5.2±0.3 <1.27 <13.42 <24.5 5.0+0.2 Low >21
−0.2
C-NGC1566-2b 2.21 5.2±0.3 <1.27 <13.42 <24.5 7.7+0.9 Medium >32
−0.9
C-NGC2188-1a 2.55 13.0±0.6 <0.59 <6.25 <11.4 1.5+0.2 Low >13
−0.2
C-NGC2188-1b 2.31 8.8±0.0 <0.79 <8.37 <15.3 1.5+0.2 Low >10
−0.2
C-NGC5156 3.12 388.6±4.7 0.15+0.40 0.96+0.15 1.7+0.3 6.5+0.4 Low 370
−0.05 −0.11 −0.2 −0.4
C-NGC5156 3.12 388.6±4.7 0.15+0.40 0.96+0.15 1.7+0.3 14.3+2.2 Medium 822
−0.05 −0.11 −0.2 −2.2
C-NGC5156(NOV13) 3.29 398.0±6.7 0.11+0.01 0.82+0.04 1.5+0.1 [6.5+0.4] Low 430
−0.01 −0.04 −0.1 −0.4
C-NGC5156(NOV13) 3.29 398.0±6.7 0.11+0.01 0.82+0.04 1.5+0.1 [14.3+2.2] Medium 954
−0.01 −0.04 −0.1 −2.2
C-NGC7162A-1 2.43 34.2±0.8 <0.21 <2.25 <4.1 4.6+0.2 Low >113
−0.2
C-NGC7162A-1 2.43 34.2±0.8 <0.21 <2.25 <4.1 4.4+1.3 Medium >106
−1.3
C-NGC7162A-2 1.72 13.6±0.6 <0.38 <4.01 <7.3 - - -
thesemustthereforebetreatedseparatelywhenreducing leaved the observations of the target source (3.5 mins)
the data. withobservationsofanearby,brightphasecalibrator(1.5
mins) to allow phase referencing to be performed, if nec-
Severalobservationsofbright‘fringefinders’(includ- essary.
ing1921-293,the‘main’fringefinder)weremadethrough-
out the observation, to allow for calibration of station The data were correlated at Curtin University of
clock delay and rate offsets. Since we did not know how Technology using the DiFX-2 software correlator (Deller
bright the source would be at this resolution, we inter- et al. 2007, 2011) and standard continuum data parame-
(cid:13)c 2016RAS,MNRAS000,1–18