The HIX galaxy survey I: Study of the most gas rich galaxies from HIPASS

3.6 MB·

by  K. A. Lutz

MNRAS000,1–16(2016) Preprint11January2017 CompiledusingMNRASLATEXstylefilev3.0
The HIX galaxy survey I: Study of the most gas rich
galaxies from HIPASS
K. A. Lutz1,2(cid:63), V. A. Kilborn1, B. Catinella3, B. S. Koribalski2, T. H. Brown1,3,
L. Cortese3, H. D´enes2,4, G. I. G. J´ozsa5,6,7 and O. I. Wong3,8
1 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia
2 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia
3 International Centre for Radio Astronomy Research (ICRAR), M468, The University of Western Australia, 35 Stirling Highway,
7
Crawley, WA 6009, Australia
1 4 Research School of Astronomy & Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
0 5 SKA South Africa Radio Astronomy Research Group, 3rd Floor, The Park, Park Road, Pinelands 7405, South Africa
2 6 Rhodes Centre for Radio Astronomy Techniques & Technologies, Department of Physics and Electronics, Rhodes University,

n PO Box 94, Grahamstown 6140, South Africa
a 7 Argelander-Institut fu¨r Astronomie, Auf dem Hu¨gel 71, D-53121 Bonn, Germany
J 8 ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO)

0
1
AcceptedXXX.ReceivedYYY;inoriginalformZZZ

]
A
G ABSTRACT
We present the Hi eXtreme (Hix) galaxy survey targeting some of the most Hi rich
.
h galaxies in the southern hemisphere. The 13 Hix galaxies have been selected to host
p the most massive Hi discs at a given stellar luminosity. We compare these galaxies
o- to a control sample of average galaxies detected in the Hi Parkes All Sky Survey
r (Hipass, Barnes et al. 2001). As the control sample is matched in stellar luminosity,
st we find that the stellar properties of Hix galaxies are similar to the control sample.
a Furthermore, the specific star formation rate and optical morphology do not differ
[ betweenHixandcontrolgalaxies.Wefind,however,theHixgalaxiestobelessefficient

1 in forming stars. For the most Hi massive galaxy in our sample (ESO075-G006, log
v MHI [M(cid:12)] = (10.8±0.1)) the kinematic properties are the reason for inefficient star
7 formationandHiexcess.ExaminingtheAustralianTelescopeCompactArray(ATCA)
4 Hi imaging and Wide Field Spectrograph (WiFeS) optical spectra of ESO075-G006
4 reveals an undisturbed galaxy without evidence for recent major, violent accretion
2 events. A tilted-ring fit to the Hi disc together with the gas–phase oxygen abundance
0 distributionsupportsthescenariothatgashasbeenconstantlyaccretedontoESO075-
.
1 G006 but the high specific angular momentum makes ESO075-G006 very inefficient
0 in forming stars. Thus a massive Hi disc has been built up.
7
1 Key words: galaxies – individual: ESO075-G006, galaxies – kinematics and dynam-
: ics, galaxies – ISM
v
i
X
r
a
1 INTRODUCTION these galaxies host a more massive Hi disc than average.
Twoscenariosareconsidered:thesegalaxiesareeithermore
Over the last two decades large, systematic surveys of the
efficientataccretinggasfromtheirenvironment,ortheyare
atomic hydrogen (Hi) content of galaxies have allowed us
less efficient at converting available gas into stars than Hi
tostudytherelationbetweenstarformation,stellarandHi
poorgalaxies(seealsoHuangetal.2014).Acombinationof
content of galaxies. Scaling relations have shown that the
the two scenarios is also possible.
Hi content of galaxies in the local universe is well corre-
Themechanismsthroughwhichgalaxiescanaccretegas
lated with the galaxies’ stellar content (Haynes et al. 1984;
and replenish their gas reservoir are broadly separated into
Catinella et al. 2010; D´enes et al. 2014; Brown et al. 2015).
Forthiswork,outlierstowardstheveryHirichendofsuch two categories: gas rich mergers (’clumpy accretion’) or ac-
cretion of gas from the halo or the intergalactic medium
scaling relations are examined aiming to investigate why
(’smooth accretion’).
Gas-rich mergers may include both minor and major
(cid:63) E-mail:[email protected] mergers. Major mergers have been found to be a less domi-
(cid:13)c 2016TheAuthors
2 Lutz et al.
nantchannelofgasaccretionbothinobservationalworkand One is keeping recently accreted gas at larger galactocen-
simulations(e.g.vandeVoortetal.2011,Wangetal.2013 tricradiiandlowerdensities(Kim&Lee2013;Forbesetal.
andalsoSancisietal.2008;Sa´nchezAlmeidaetal.2014and 2014), where conditions are less favourable for star forma-
references therein). A series of gas-rich minor mergers may tion. Secondly, a high angular momentum stabilises the gas
contribute to the gas replenishment of galaxies but obser- against Jeans instabilities and subsequent star formation
vations have shown that also this channel does not deliver (Toomre1964;Obreschkowetal.2016).Simulationssuggest
enoughgastoagalaxytosustainstarformation(DiTeodoro thattheangularmomentuminalocalgalaxyisindependent
& Fraternali 2014). of the angular momentum of the host halo (Stevens et al.
In the local universe indirect evidence of cosmological 2016b), but is determined by the merger history and feed-
gas accretion has been found: the integrated metallicity of back(Geneletal.2015;Lagosetal.2016).Furthermore,the
galaxiesislowerthanexpectedinchemicalevolutionmodels angular momentum is thought to constrain the upper enve-
ofaclosedandisolatedgalaxy(vandenBergh1962,Sa´nchez lope of the gas fraction - stellar mass plane (Maddox et al.
Almeida et al. 2014 and references therein). Hence, the gas 2015).
contentofagalaxyneedstobereplenishedwithmetal-poor In this paper we present the Hi eXtreme (HIX) galaxy
gas,i.e.gasoriginatingfromtheintergalacticmedium.This survey.Forthissurveywearetargetinggalaxiesthatcontain
issupportedbycosmologicalhydrodynamicalsimulationsby at least 2.5 times more Hi than expected from their stellar
Dav´e et al. (2013), who among others have shown that the luminosity. In order for these galaxies to accumulate that
cycle of gas inflow, gas outflow and star formation is es- much Hi we expect them to have either gone through a
sential to reproduce scaling relations between two or three recent episode of elevated gas accretion (both smooth and
properties of galaxies, such as the gas content, the stellar clumpy),tobeunabletoformstarsefficientlyfromtheirgas
content, the star formation activity or the metallicity. In reservoir or a combination of these two scenarios.
particularthestellarmass-metallicityrelation(Hughesetal. TheHixandacontrolsampleofgalaxiesareobservedin
2013; Mannucci et al. 2010) is driven by smooth, stochastic Hi with the Australian Telescope Compact Array (ATCA)
accretion:theaccretionofcosmologicalgastriggersstarfor- and optical integral field spectra are obtained with the
mation(increaseofstarformation)anddilutestheinterstel- WiFeS spectrograph for the Hix galaxies. This combined
larmedium(decreaseofthegasphasemetallicity).Further- data set plus publicly available data allow to search for ev-
more, Moran et al. (2012) find that massive galaxies with a idence of gas accretion as well as inefficient star formation
higher Hi content have a steeper drop in gas-phase oxygen as a reason for the Hi excess.
abundance. Putting this in the context of cosmological gas The term Hi excess has been used in many different
accretion,itimpliesthattheoutskirtsofagalaxyarediluted ways. We use the term“Hi excess”to describe a more mas-
by metal-poor inflowing gas. It is, however, suggested that sive Hi content than expected from optical properties.
in the local universe cosmological accretion is less efficient Thisarticleisstructuredasfollow.Insection2,weout-
than at higher redshifts, as the Halogas survey was not linethesampleselectionandthesurveystrategy.Insection
abletodetectthenecessaryamountofHicloudstosustain 3, we present observed and publicly available data as well
star formation (Heald et al. 2011; Heald 2015). asthedatareduction.Section4comparesoursampletothe
In addition to cosmological gas accretion the ’Galactic broadergalaxypopulation.Section5focusesonthemostex-
Fountain’mechanismcanfacilitatetheaccretionofhothalo treme galaxy in our sample, ESO075-G006, and in sections
gas onto the galaxy (Fraternali et al. 2011; Oosterloo et al. 6 and 7 we discuss our findings and conclude.
2007).Inthismodel,thesupernovadrivengalacticfountain ThroughoutthepaperwewillassumeaflatΛCDMcos-
initiallyexpelsmetal-richgasfromthegalacticdiscintothe mology with the following cosmological parameters: H0 =
halo. This now extra-planar gas cools down and falls back 70.0kmMpc−1s−1, Ωm = 0.3. All velocities are used in the
onto the disc. As this happens hot halo gas condenses onto optical convention (cz).
theexpelledgas,coolsandgetsdraggedalongintothedisc.
Thus,thegascontentofthediscincreases(Marinaccietal.
2010, 2011). In Hi observations the expelled, extra-planar
gas can be identified by its lag in rotation velocity. An ex- 2 SURVEY DESCRIPTION AND SAMPLE
ample for this process could be NGC891 (Fraternali et al. SELECTION
2011; Oosterloo et al. 2007), but also several galaxies ob-
2.1 Sample selection
served in the context of the Halogas survey (Heald et al.
2011; Gentile et al. 2013) have shown evidence for lagging Thesampleof Hixgalaxiesisselectedfromaparentsample
extra-planar gas. It is to be noted though that evidence for of1796galaxiesintheSouthernhemisphere.Thisisasubset
sufficient density of hot haloes is scarce (see e.g. Bogda´n of the Hipass catalogue and the Hipass bright galaxy cat-
et al. 2015). alogue (Meyer et al. 2004; Koribalski et al. 2004) including
The alternative is that these very gas rich galaxies are onlyHidetectionswithreliable,singleopticalcounterparts.
inefficientatconvertingtheiravailablegasintostars.Wong ThissamplehasbeencompiledbyD´enesetal.(2014)toob-
et al. (2016) suggest that the Hi based star formation ef- tainscalingrelationsbetweenthestellarandtheHicontent
ficiency (SFE) is in particular related to the stability of a ofgalaxies.UsingtheirscalingrelationbetweentheR-band
disc. An elevated specific angular momentum can be one luminosity as published in Hopcat (Doyle et al. 2005) and
mechanismtostabiliseadisctowardsgravitationalinstabil- theHipassHimass,anHimassispredictedforeachgalaxy
ities (Krumholz & Burkhart 2016; Forbes et al. 2014) and and compared to the Hi mass as measured from the inte-
we therefore consider it as a reason for decreased SFE. An grated 21cm emission line in Hipass.
enhancedangularmomentumhastwoeffectsonthegalaxy. The detailed selection criteria for the Hix sample are:
MNRAS000,1–16(2016)
The HIX galaxy survey I 3
control galaxies were drawn from the same parent sample
11.0
and the selection criteria (ii) and (iii) were the same as
used to define the Hix sample. However, criterion (i) was
10.5 changed to:
]fl10.0 (i) a measured Hipass Hi mass between 1.6 times less
M
and more than what is expected from the scaling relation
[
HI inordertocapturegalaxieswellwithinthescatter(0.3dex).
M 9.5
g
o The ATCA Online Archive2 (ATOA) was searched for
l
9.0 Denes et al. (2014) observationsofthosegalaxiesthatfollowthecontrolsample
Control selectioncriteria.Thisleadto13controlgalaxies,thathave
8.5 HIX beenobservedforatleastonesynthesisinany1.5-kmarray
ESO075-G006
configurationofATCA.InFigure1thesegalaxiesaredrawn
19 20 21 22 23 24 as red circles and lie well within the scatter of the scaling
M [mag] relation. More details and basic properties of the control
R,abs
sample are given in Table 1.
Figure 1. The sample selection: The blue diamonds represent It is to be noted that the control sample is selected to
theHixsample,theredcirclesthecontrolsample.Thelightblue, be normal for an Hi selected sample as e.g. Hipass. This
widediamondmarksthedataofgalaxyESO075-G006.Thegrey implies that the control sample is potentially still Hi rich
shadeinthebackgroundrepresentsthedistributionoftheHipass compared to a stellar mass selected sample.
parentsample.TheblacksolidlinegivestheD´enesetal.(2014)
scalingrelationandtheblackdashedlinemarks,wherethemea-
suredHimassis2.5timeslargerorsmallerthantheexpectedHi
3 OBSERVATIONS, DATA REDUCTION AND
mass.
AUXILIARY DATA
(i) anHipassmeasurementofatleast2.5timesmoreHi 3.1 ATCA observations
thanexpectedfromthescalingrelation.Figure1showsthe
In this work we only present the ATCA observations for
scaling relation by D´enes et al. (2014) between the R-band
ESO075-G006,whichisthemostextremegalaxyinoursam-
absolutemagnitudeandtheHipassHimass,whichwasused ple with respect to (relative) Hi mass and will be further
here. discussedinSection5.TheATCAHidataoftheremaining
(ii) absoluteK-bandmagnitudeMK<−22.0mag,restrict- Hix and control galaxies will be presented in a subsequent
ingthesampletomassivespiralgalaxies.Thisisequivalent
paper.
to a K-band luminosity logL [L ] = 42.8 or a stellar mass
K (cid:12) For the observations of ESO075-G006, the standard
of logM [M ]=9.7.
(cid:63) (cid:12) ATCA flux and bandpass calibrator PKS1934-638 has also
(iii) declination<−30degforobservabilitywiththeAus-
been observed as the phase calibrator, which was visited
tralian Telescope Compact Array (ATCA).
regularly during the observations. The observations were
Wefurthermoreexcludegalaxiesnearthegalacticplane carried out between November2012 and January2013. On
where the high density of foreground stars complicates the source times are 5.6h, 9.3h and 5.6h in the 1.5D, 750C
photometric measurements. andEW367arrayconfigurationsrespectively.TheCompact
SuperCOSMOSB-bandimages(Hamblyetal.2001a,b) Array Broadband Back end(CABB, Wilson et al.2011)al-
and NED1 have been searched for neighbouring galaxies lowstoobservecontinuumandspectrallinesimultaneously.
within the size of the Hipass spatial resolution element of Thecontinuumbandis2048MHzwide,hasafrequencyres-
15.5arcmin (Barnes et al. 2001). As no galaxies of similar olution of 1MHz and is centred on 2.1GHz. Spectral line
angularsizeandbrightnesshavebeenfound,Hi-excessdue observations cover a 8.5MHz bandwidth with a channel
tosourceconfusioncanbeexcluded.Somegalaxies,however, width of 0.5kHz. This is equivalent to a velocity resolution
are accompanied by small dwarf galaxies. Basic properties of 0.1 km s−1 at 1370MHz, which is the band’s central fre-
of the Hi-excess galaxies are given in Table 1. quency. In this paper we only utilise the spectral line data,
The parent sample for the Hix galaxies is a subset of whichwillbesmoothedtochannelwidthsof4and10kms−1.
Hipass. It is therefore already an Hi selected sample which
isbiasedtowardsHirichsystems.Selectingtheoutliersfrom
3.2 Radio data reduction and analysis
thisparentsampleleadstotheHixgalaxiesbeingthemost
extremegalaxieswithregardstotheir(relative)Himassin The data reduction of the ATCA Hi observations is com-
the Southern hemisphere. pletedusingstandardtasksintheprogrampackagemiriad
(Sault et al. 1995) and the python wrapper mirpy3.
After removing radio frequency interference, a semi-
2.2 Control and comparison samples
automatedpipelineconductsbandpass,fluxand,phasecal-
In order to compare the Hix sample to a sample of average ibration and subtracts a first order polynomial baseline for
Hipass galaxies, a control sample has been compiled. The
2 http://atoa.atnf.csiro.au/
1 http://ned.ipac.caltech.edu/ 3 https://pypi.python.org/pypi/mirpy
MNRAS000,1–16(2016)
4 Lutz et al.
ID RA DEC logM(cid:63)[M(cid:12)] logMHI[M(cid:12)] logMHI/M(cid:63) Vsys [kms−1] D[Mpc] logSFR[M(cid:12)yr−1]
(1) (2) (3) (4) (5) (6) (7) (8) (9)
HIX
ESO111-G014 00:08:18.8 -59:30:56 10.5 10.6 0.093 7786 112 0.33
ESO243-G002 00:49:34.5 -46:52:28 10.5 10.5 −0.014 8885 129 0.16
NGC0289 00:52:42.4 -31:12:21 10.4 10.3 −0.049 1629 23 0.081
ESO245-G010 01:56:44.5 -43:58:21 10.4 10.3 −0.075 5740 82 0.27
ESO417-G018 03:07:13.2 -31:24:03 10.2 10.4 0.14 4746 67 0.18
ESO055-G013 04:11:43.0 -70:13:59 10.1 10.4 0.25 7386 105 0.21
ESO208-G026 07:35:21.1 -50:02:35 9.79 9.89 0.090 2983 40 -0.50
ESO378-G003 11:28:04.0 -36:32:34 9.96 10.2 0.20 3021 41 -0.24
ESO381-G005 12:40:32.7 -36:58:05 10.2 10.4 0.21 5695 80 0.22
IC4857 19:28:39.2 -58:46:04 10.4 10.0 −0.33 4679 67 0.37
ESO461-G010 19:54:04.4 -30:29:03 10.0 10.4 0.40 6709 98 -0.13
ESO075-G006 21:23:29.5 -69:41:05 10.5 10.8 0.29 10609 154 0.46
ESO290-G035 23:01:32.5 -46:38:47 10.4 10.3 −0.11 5881 84 0.012
CONTROL
NGC0685 01:47:42.8 -52:45:42 9.81 9.53 −0.28 1362 18 -0.63
ESO121-G026 06:21:38.8 -59:44:24 10.4 9.95 −0.46 2268 30 0.12
ESO123-G023 07:44:38.9 -58:09:13 10.1 10.0 −0.06 2914 39 -0.54
NGC3001 09:46:18.7 -30:26:15 10.8 10.0 −0.81 2464 32 0.39
ESO263-G015 10:12:19.9 -47:17:42 10.3 9.67 −0.67 2531 33 -0.27
NGC3261 10:29:01.5 -44:39:24 10.8 10.1 −0.76 2564 34 0.26
NGC4672 12:46:15.7 -41:42:21 10.7 10.3 −0.43 3273 45 0.30
NGC5161 13:29:13.9 -33:10:26 10.5 10.1 −0.34 2390 32 0.14
ESO383-G005 13:29:23.6 -34:16:17 10.4 9.75 −0.67 3614 50 -0.18
IC4366 14:05:11.5 -33:45:36 10.7 10.1 −0.61 4615 65 0.54
ESO462-G016 20:23:39.0 -28:16:40 10.1 9.85 −0.23 3055 45 0.073
ESO287-G013 21:23:13.9 -45:46:23 10.5 10.2 −0.34 2697 39 0.18
ESO240-G011 23:37:49.9 -47:43:41 10.9 10.3 −0.55 2840 40 -0.0013
Table1.TheHiextremeandcontrolgalaxysample.Column (1):AnIDforeverygalaxy.Column (2)and(3):RAandDECtakenfrom
2MASS.Column (4):ThestellarmasscalculatedasdescribedinSection3.6.Column (5):TheHimasscalculatedfollowingEquation1
Cusoinlugmtnhe(7m):eaTshuerehdeHlioipcaensstrflicu,xsyasntdemthicevdeilsotacintcyefarosmstaHtiepdasinsC(Moleuymerne(t8a).l.C2o0l0u4m).nV(e6lo):ciTtiheesHheiregaasnmdatshsrofruagchtoiountitshdeepfianpeedraasrefHgIi=venMMH(cid:63)iIn.
theopticalconvention(cz).Column (8):Luminositydistancecalculatedfromthesystemicvelocityincolumn7convertedtotheGalactic
Standard of Rest frame and assuming our standard cosmology. Column (9): The star formation rate measured as described in Section
3.6.
each array configuration separately. All available observa- maps are regridded to the resolution of the SuperCOSMOS
tions from different array configurations are then combined optical image.
in the Fourier transformation. For the detailed analysis of The resulting ATCA data cubes are then further anal-
the Hi properties of ESO075-G006 in Section 5, two differ- ysedwithstandardmiriadtasks,SAOImageDS9,TiRiFiC
ent data cubes were produced. The cube for the moment (Jo´zsa et al. 2007) and python scripts 4.
analysis has been combined by using a Brigg’s robust pa-
rameter of 0.5 and a channel width of 4kms−1 to maximise
3.3 WiFeS observations
sensitivity, the cube for the tilted ring fit with a robust pa-
rameter of 0.0 and a velocity channel width of 10kms−1 to ESO075-G006 has also been observed with the Wide Field
maximise spatial resolution. As the longest baselines with Spectrograph (WiFeS, Dopita et al. 2007) integral field
Antenna6 are in most cases too long to pick up signal unit(IFU)mountedontheSidingSpringObservatoryANU
from diffuse and extended Hi and merely add noise to the 2.3mtelescope.ESO075-G006hasbeenobservedwiththree
data, all baselines with Antenna6 are excluded. For the Hi pointings towards the north-western edge, the centre and
data of ESO075-G006, this leads to synthesised beam sizes the south-eastern edge of the disc. WiFeS has been setup
of 52.4arcsec by 41.2arcsec (rob=0.5) and 40.6arcsec by with R = 3000 gratings in both the red and blue arm in
25.3arcsec (rob=0.0) and a noise level of 1.81mJybeam−1 combination with the dichronic set at 560nm. For reliable
and1.2mJybeam−1 respectively.Thelowestcolumndensity skylinesubtraction,galaxydatahavebeentakeninthenod-
limit in the moment analysis (on rob=0.5 data) assumes a and-shufflemode.Everynightofobservationshasbeencom-
3σdetectionoveravelocitywidthof12kms−1andamounts pleted with standard calibration images including bias, sky
to 70mJybeam−1kms−1 = 0.4·1020cm−2 = 0.3M(cid:12)pc−2. Mo- anddomeflatfieldandwireimagingforcentringtheslitlets
mentmapshavebeencreatedwiththemiriadtaskmoment, as well as NeAr and CuAr arc lamp spectra for wavelength
including a 3σ clipping. Moment 1 and 2 maps are further
maskedtothelowestcolumndensitycontourandallmoment
4 https://www.python.org/
MNRAS000,1–16(2016)
The HIX galaxy survey I 5
Pointing RA DEC PA texp (i) The Hipass survey data are used to calculate the Hi
(1) (2) (3) (4) (5) mass (Barnes et al. 2001; Meyer et al. 2004; Zwaan et al.
Centre 21:23:28.7 -69:41:02 165◦ 1800s 2004).
South 21:23:30.7 -69:41:31 75◦ 3600s (ii) The AllWISE data release (Cutri et al. 2013) of the
North 21:23:26.9 -69:40:32 75◦ 1800s WISE mission (Wright et al. 2010) provides imaging in the
four mid-infrared WISE bands: W1 (3.4µm), W2 (4.6µm),
Table2.DetailsforthethreeWiFeSpointingsonESO075-G006.
W3(12µm),W4(22µm).Thefluxofgalaxiesineachbandis
Column (1):anIDforthepointing.Column (2)and(3):thecen-
measured from images obtained from the ICORE co-adder5
tral coordinates. Column (4): the rotator angle of the spectro-
graph.Column (5):thetotalon-sourceexposuretime. (Masci & Fowler 2009) and converted into magnitudes fol-
lowing the prescription in Cutri et al. (2013).
(iii) ThecatalogueofextendedsourcesintheTwoMicron
calibration. Two to three times a night a standard star All Sky Survey (2MASX, Skrutskie et al. 2006) provides
for flux calibration has been observed. The observations of magnitudesintheJ,H andK-bandandprecisecoordinates
ESO075-G006 have been conducted in August 2014, July of the centre of the stellar disc.
and October 2015 under photometric conditions. For more (iv) TheHipassopticalcounterpartcatalogue(Hopcat,
details of the three pointings see Table 2. Doyle et al. 2005) provides Bj, R and I-band magnitudes,
opticalpositionanglesandopticalsemi-majortosemi-minor
axis ratios from SuperCOSMOS (Hambly et al. 2001a,b).
3.4 WiFeS data reduction and analysis HopcatalsostatestheGalacticextinctionmeasuresE(B−V)
atthelocationofeachgalaxyasestimatedbySchlegeletal.
Childress et al. (2014) provide the fully automated Py-
(1998).
WiFeSpipelineforWiFeSdata.Thispipelineincludesbad
(v) Optical imaging used in this work is obtained from
pixelrepair,biasanddarkcurrentsubtraction,flatfielding,
the SuperCOSMOS (Hambly et al. 2001a,b) web page6.
wavelengthcalibration,skysubtraction,fluxcalibrationand
(vi) The ESO-LV catalogue (Lauberts & Valentijn 1989)
data cube creation. The resulting data cubes are 70 by 25
provides both isophotal diameters and diameters at 90 per
pixels (35 by 25arcsec) in size.
cent B light
PyWiFeSisrunoneachobservedgalaxydatacubein-
(vii) Near- and far ultraviolet (NUV, FUV) photometry
dividually.Foreachpointing,datacubesaremedianstacked
has been measured from the Galaxy Evolution Explorer
afterwards. This way small spatial offsets between observa-
imaging(GALEX,Martinetal.2005;Morrisseyetal.2007).
tions can be taken into account. This procedure results in NUV images are available for all except one Hix galaxy
twodatacubesforeachpointing:onecontainingtheredhalf
(ESO208-G026)andexceptfourcontrolgalaxies(NGC685,
of the spectrum and the other one the blue half.
NGC3001andNGC3261,ESO123-G023).NUV countsare
One stellar population model is constructed for each
measured from intensity maps and converted into magni-
pointing by median stacking the spectra of all spaxels in
tudes using GALEX’s flux calculator 7.
onepointing.Thestellarpopulationfitisconductedonboth
the red and blue cube together using the pPXF method The WISE and GALEX photometry is remeasured in
andcomplimentaryPythonscriptbyCappellari&Emsellem elliptical apertures, where the semi major axis is of the size
(2004). The stellar population model library is taken from oftheESO-LV25magarcsec−2 isophotalradiusandtheaxis
Vazdekis et al. (2010). ratio is taken from Hopcat. Stars are masked out. The lo-
For the measurement of the emission line strength the calskybackgroundisestimatedfromdedicatedbackground
datacubesoftheredandbluehalfofthespectrumareanal- apertures and subtracted either with funtools8 (GALEX)
ysed independently. Each individual data cube is Voronoi or as prescribed in the supplement (WISE).
binned following Cappellari & Copin (2003); Cappellari The 2Mass data for the Hipass parent sample are re-
(2009).TheVoronoibinningisconstructedsuchthatthesig- trieved from the VizieR catalogue access tool, which also
naltonoiseratiointhewavelengthrangearoundtheO[III] took care of reliable cross-matching.
(blue)orN[II]line(red)reaches50.Theerrormapnecessary
for the binning is constructed from the variance data cube
asgivenbythePyWiFeSpipeline.ThesizeoffinalVoronoi 3.6 Estimating galaxy properties
binsarebetween1and200spaxels.IneachVoronoibinthe
The Hi mass is calculated from the published Hipass inte-
stellar continuum model as fitted to the entire pointing is
grated 21cm emission signal by:
subtracted.EmissionlinesarefittedwithGaussiansineach
Voronoibin.Thisresultsinaredshiftmeasurementfromthe 2.356·105
Hα line and line strengths that can be used to estimate the MHI[M(cid:12)]= 1+z ·(D[Mpc])2·FHI[Jykms−1] (1)
gas-phase oxygen abundance.
with z being the galaxy’s redshift, D the distance to the
The Voronoi bins of the red and the blue arm are dif-
galaxy in Mpc and F the integrated flux density in units
ferent. So each pixel is assigned the value of the entire bin HI
of Jy km s−1. The error estimation closely follows the error
and metallicities are calculated on a pixel by pixel basis.
3.5 Public Data 5 http://irsa.ipac.caltech.edu/applications/ICORE/
6 http://www-wfau.roe.ac.uk/sss/index.html
Inadditiontotheobserveddata,publiclyavailabledataare 7 http://asd.gsfc.nasa.gov/archive/galex/FAQ/counts background.html
also used: 8 https://github.com/ericmandel/funtools
MNRAS000,1–16(2016)
6 Lutz et al.
ESO111-G014 ESO243-G002 NGC0289 ESO245-G010
25kpc 25kpc 25kpc 25kpc
ESO417-G018 ESO055-G013 ESO208-G026 ESO378-G003
25kpc 25kpc 25kpc 25kpc
ESO381-G005 IC4857 ESO461-G010 ESO290-G035
25kpc 25kpc 25kpc 25kpc
Figure 2.PanelofSuperCOSMOS Bj-bandpoststampimagesofallHixgalaxies.Allimagesare3arcminby3arcmininsizeexceptfor
the image of NGC289, which is 6arcmin by 6arcmin in size. The scale bar in the bottom right corner indicates 25kpc at the distance of
thegalaxy.TheopticalimageofESO075-G006isshownfurtherdowninSection5.
estimation of F as suggested by Koribalski et al. (2004): Where GALEX photometry is not available, the SFR pre-
HI
scriptionbyCluveretal.(2014),whichisbasedsolelyonthe
S
S/N= (cid:112) peak (2) luminosityintheW3-band,isused.TheirIMFisconsistent
rms2+(0.05·S )2
peak withaChabrier(2003)IMFandthecomparisonofSFRses-
timated with the both methods does not reveal systematic
4 (cid:113)
∆F = · S ·F ·dv (3) offsets. ESO075-G006 is not detected in the W3-band. Its
HI S/N peak HI SFR does therefore not include a mid-IR component.
where S is the peak intensity in the spectrum, rms the Thegas-phaseoxygenabundanceisestimatedwiththe
peak
rootmeansquareofthedatacubeanddvthevelocityreso- O3N2andN2methodusingtheHα,Hβ,[OIII]λ5007˚Aand
lution. Using Gaussian error propagation the error of F is [NII]λ6584˚AemissionlinesasdescribedbyPettini&Pagel
HI
thenpropagatedandcombinedwiththeerrorofthedistance (2004).
to estimate the error of the Hi mass.
AllstellarmassesfortheHix,controlandHipasssam-
ples were estimated following equation (3) in Wen et al.
(2013), whichassumes a linear relationship between the log
of the stellar mass and the log of the Galactic extinction
corrected 2Mass K-band luminosity.
4 THE HIX SAMPLE IN CONTEXT
BaryonicmassesforHixandcontrolsamplearethesum
of atomic and stellar mass. The atomic gas mass is the Hi InthissectiontheHixgalaxysampleispresentedandcom-
mass increased by 35percent to include Helium. pared to the control sample, the parent sample and other
Star formation rates (SFR) for the Hix, control and Hi rich samples, where appropriate data are available.
HipasssamplesareestimatedusingtheGALEX NUV lumi- Asafirstoverview,Figure2showstheopticalimagesof
nosity in combination with the WISE W3-band luminosity allHixgalaxiesexceptESO075-G006,forwhichtheoptical
toaccountfordustobscuredstarformation.Weusethepre- image is shown Section 5. All Hix galaxies are spiral galax-
scription in equation (1) and (2) in Saintonge et al. (2016) ies. One of them shows clear signs of interaction (ESO245-
assuming a Chabrier (2003) initial mass function (IMF). G010), the remaining galaxies appear quite regular.
MNRAS000,1–16(2016)
The HIX galaxy survey I 7
4.1 HI mass and stellar mass
1.0
The relation between gas fraction (defined as M /M ) and
HI (cid:63)
stellar mass is well known and used here to test the Hix 0.5
selectioncriteriaandcomparetheHixgalaxiestothecontrol
0.0
and parent sample as well as other samples.
catedFiagbuorvee3thsheo1wσs tshcaisttreerlaotfiotnh.eMpaorsetnHtisxamgapllaex,iweshailreetlhoe- / MHI 0.5 HGAIPSASSS
control galaxies populate the Hipass scatter. The two Hix g M 1.0 HIghMass
galaxies that are located within the grey shaded area are o Lemonias et al. 2014
ESO208-G026 and IC4857. They have been selected to be l 1.5 BLUEDISK
HIGHz
Hi excess galaxies according to the R-band scaling relation
Control
2.0
of D´enes et al. (2014). On the gas mass fraction vs. stel- HIX
lar mass plane, however, they lie within the scatter of the ESO075-G006
2.5
Hipass parent sample and thus, their Hi-richness on that 9.0 9.5 10.0 10.5 11.0 11.5
scale is not as obvious as for the other Hix galaxies. In a
log M [M ]
future analysis of spatially resolved Hi data of these galax- fl
ies, we will determine whether these two galaxies are true
Figure 3. The Hi gas mass fraction as a function of the stellar
Hix galaxies, more“normal”Hipass galaxies or something
mass. The blue diamonds present the Hix sample and red cir-
in between.
clesthecontrolsample.ThelightbluediamondisESO075-G006,
For comparison, Figure 3 also shows the running av- which will be discussed in more detail further below. Blue and
erage of the GASS sample (data release 3, Catinella et al. red errorbars in the top right corner give the median errorbars
2010,2012,2013).Non-detectionsareincludedasupperlim- fortheHixandcontrolsamplerespectively.Thedarkgrey,solid
its. GASS is a stellar mass selected sample and as such not line presents the running average of the Hipass parent sample
biased towards Hi rich systems as it is the case for Hipass. along with the 1σ scatter as the grey shaded region. The light
grey,dashedlineandshadedarearepresentstherunningaverage
The running average of the GASSsample is therefore lower
than for the Hipass parent sample and even our control and1σscatteroftheGASSsample.Opensymbolspresentother
surveysof(Hirich)galaxies:HighMass(squares),Lemoniasetal.
sample appears Hi rich compared to GASS. This further
(2014)(circles),HIGHz(asterisks)andBluedisk(crosses).
emphasises that the Hix galaxies are among the most Hi
rich galaxies in the local universe.
WefurthercomparetheHixsampletoothersurveysof SimilartotheHixsurvey,theHighMasssurveyaimsto
Hi rich galaxies: HighMass (Huang et al. 2014), a sample selectthemostHiextremegalaxiesfromanHiselectedsur-
by Lemonias et al. (2014), Hi-Monsters (Lee et al. 2014), vey(inthiscasefromAlfalfa).Tocomparethetwosample
HIGHz (Catinella & Cortese 2015) and Bluedisk (Wang selections, we use the 2Mass stellar masses for our Hipass
etal.2013).ForHighMassandLemoniasetal.(2014)sample parentsample.Theseareonlyavailablefor1475outof1796
galaxies, SFRs and stellar masses were calculated the same galaxies. We further cut this sample down to galaxies, that
way as for the Hix galaxies. For the HIGHz and Bluedisk (a) are located at dec<−30deg, (b) are more massive than
galaxiesthosevaluesweretakenfromtherespectivepublica- ourstellarmasscut(M <−22.0mag)and(c)areneitherin
K
tions.Forallsamples,distancesandHimasseswereadopted closeproximitytotheGalacticplane(forgoodphotometry)
from the respective publications. nor a massive neighbouring galaxy (exclude source confu-
HighMass and Lemonias et al. (2014) galaxies have sion).ApplyingtheHighMasssampleselectiontothisparent
been selected from Alfalfa (Giovanelli et al. 2005) using sample yields 22galaxies. These 22galaxies include all Hix
thelogf -logM [M ]scalingrelation.Thus,bothsamples galaxies except for the two galaxies that are also located in
HI (cid:63) (cid:12)
are also located above the 1σ scatter of Hipass. While the theHipass1σscatterinFigure3(ESO208-G026,IC4857).
Lemonias et al. (2014) sample focusses on early type, AGN Theremaining11galaxiesinthis“Hipass-HighMass”sample
hostinggalaxies,HighMassgalaxiesarespirals.HI-Monsters do not appear in the Hix survey because they are not out-
are HI massive galaxies (logM [M ]>10.3) fromAlfalfa, lierstotheD´enesetal.(2014)scalingrelation.Thismeans,
HI (cid:12)
i.e. very similar to HighMass. The HI-Monsters sample in- when estimating their Hi mass from their R-band luminos-
cludesinadditionaselectionoflowsurfacebrightness(LSB) ity,theestimatedHimassisnot2.5timeslargerthantheir
galaxieslikeMalin1.HIGHzareasampleofthehighestred- measuredHimass.ThreeoftheHighMasslikegalaxiesthat
shiftHIdetectionsofsinglegalaxies.TheBluedisksample arenotinHixhavelargestellarmasses(logM(cid:63)[M(cid:12)]≈11.0).
has been selected to be Hi rich for their morphology and There are, however, no differences found, when comparing
starformationactivity.Figure3showsthattheseBluedisk thedistributionofHimassandHimassfractionoftheHix
galaxies lie well within the 1σ scatter of Hipass. like galaxies and HighMass like galaxies above our stellar
In being Hi massive and spiral galaxies, the HighMass mass cut.
(and Hi-Monsters) sample galaxies appear to be the most
similar to the Hix sample.
4.2 Star formation activity
As none of the Hix galaxies is a LSB galaxy and most
of the Hi-Monsters have larger stellar masses than the Hix In order to further characterise the sample of Hix galaxies,
galaxies,thereislittleoverlapbetweenthesestudies.Where we investigate their star formation activity and compare it
both samples overlap, they host comparably massive Hi to the control sample.
reservoirs at a given stellar mass. InFigure4,thescatterplotshowstherelationbetween
MNRAS000,1–16(2016)
8 Lutz et al.
the star formation efficiency (SFE) defined as SFR divided
by Hi mass, and stellar mass for the Hix and control sam-
9.0
ples.Thisdefinitionhasbeenusedbeforebye.g.Wongetal.
(2016) and Schiminovich et al. (2010). We obtain a linear
9.5
fits to the data with a Monte Carlo method. This method
]
includes creating 1000 random samples that have the same 1r− 10.0
meanandstandarddeviationinstellarmassandSFEasthe y
[
sample, to which the line is fitted. To each of these random E
F 10.5
samples, a line is then fitted with a standard least squares S
g
fit. The final slope and interception of the fit is then mean lo 11.0
of the results of the 1000 random samples.
The average specific star formation rates of Hix 11.5 Schiminovich et al (2010)
(logsSFR[yr−1] = (−10.3 ± 0.2)) and control sample Wong et al (2016), D
(logsSFR[yr−1]=(−10.4±0.2))arecomparable,i.e.atagiven 12.0 Wong et al (2016), P
stellarmassbothsamplesformstarsatasimilarrate.How- 9.5 10.0 10.5 11.0 11.5
ever,asHixgalaxiesaremoreHirichthanthecontrolsam- log M [M ]
ple, their SFE are systematically lower as can be seen in fl
Figure 4. Figure 4. The star formation efficiency as a function of stellar
IC4857 is the only Hix galaxy that forms stars at mass. Symbols are as in Figure 3. The red, solid line is a fit to
similar efficiency as the control sample. ESO123-G023 and theHixdataandtheblue,solidlineafittothecontroldata.The
ESO240-G011 are the least efficient control galaxies. black,dottedlineistheaverageSFEforlogM(cid:63)/M(cid:12)>10galaxies
The model by Wong et al. (2016) is calibrated on found by Schiminovich et al. (2010) and the grey, dotted lines
SINGGgalaxies(HIselected,Meureretal.2006)andcomes aremodelpredictionsbyWongetal.(2016).Forcomparison,the
intwoflavours:TherelativecontributionsbyHiandH are light blue, dashed lines are fits to the HighMass and Lemonias
2 etal.(2014)samplesrespectively(coloursasfortheirsymbols).
disentangled using a prescription either dependent on the
stellar surface mass density (D) or the hydrostatic pressure
(P). While the hydrostatic pressure model is the preferred
model by Wong et al. (2016), the stellar surface mass den-
sity model better describes the SFEs of the control sample. in a volume of 400,000Mpc3 one expects to find only one
TheHixgalaxiesformstarslessefficientlythansuggestedby galaxy to hosts an as massive Hi disc as ESO075-G006.
bothmodels.Allgalaxiesformstarslessefficientlythanthe In this section the optical morphology, the Hi proper-
averagevaluefoundbySchiminovichetal.(2010).Itistobe ties and kinematics, the optical spectra and the local en-
noted though that the scatter of their data is three orders vironment of ESO075-G006 will be discussed. This section
ofmagnitudes.TheSFEsofallHixandcontrolgalaxiesare aimstounderstandthereasonfortheHiexcessinthemost
largerthan10−10.75yr−1.BelowthisSFE,Schiminovichetal. extremeHixsamplegalaxy.Similaranalysisfortheremain-
(2010) consider a galaxy passive for their Hi reservoir. ingHixgalaxiesandacomparisontothecontrolsamplewill
Linear fits to the SFEs of the HighMass and Lemonias be presented in a future paper.
et al. (2014) galaxies are also shown in Figure 4. In partic-
ular the linear fit to the HighMass galaxies is very close to
that of the Hix sample further emphasising the similarity
of the two samples. Lemonias et al. (2014) galaxies tend to
5.1 Optical morphology of ESO075-G006
be less efficient than the Hix galaxies. The HIGHz sample
is the most efficient at forming stars from their massive Hi The SuperCOSMOS B-band image of ESO075-G006 is
j
reservoirs. shown in panel (a) of Figure 6. ESO075-G006 hosts a bar
To summarise, Hix galaxies form stars less efficiently and seems to be accompanied by a dwarf galaxy located to
than the control sample. As outlined in the introduction, the north-west. In the optical an inner ring and an outer
one reason for decreased SFE can be the increased angular pseudo ring is visible as the RC3 morphological classifica-
momentum of a galaxy. In the following section we will dis- tionindicates.TheinnerringisalsovisibleinGALEX NUV
cuss ESO075-G006, for which the resolved ATCA Hi data images as a star forming ring (see grey scale image in Fig-
will be analysed and the increased angular momentum hy- ure 10, exposure time: 207s). As both the SuperCOSMOS
pothesis will be tested. and GALEX NUV imaging is relatively shallow, faint spi-
ral arms or an XUV disc can not be excluded. There is no
GALEX FUV image available. The stellar mass is calcu-
lated as logM [M ]=10.5 and the ESO-LV catalogue gives
(cid:63) (cid:12)
a 25magarcsec−2 isophotal radius of 30.2arcsec=21.7kpc.
5 THE MOST EXTREME: ESO075-G006
As some of the most Hi massive galaxies known, e.g.
ESO075-G006 hosts the most massive Hi disc of the Hix Malin 1 (Pickering et al. 1997), are low surface brightness
galaxies with an Hi mass of log M [M ] = (10.8±0.1) as (LSB)galaxies,ESO075-G006isagoodcandidateforaLSB
HI (cid:12)
measured in Hipass. This measured Hi mass is 2.6 times galaxy. However, when using the central K-band surface
larger than expected from its Hopcat R-band magnitude. brightness as indicator (Monnier Ragaigne et al. 2003), the
An Hi mass of almost 1011M is rare. According to the Hi valueof17.41magarcsec−2 ofESO075-G006isbrighterthan
(cid:12)
mass function by Martin et al. (2010) based on Alfalfa, the threshold to LSB discs.
MNRAS000,1–16(2016)
The HIX galaxy survey I 9
rantswithopeninganglesof90degcentredoneitherasemi-
0.10
major or semi-minor axis. This allows for profiles along the
ATCA, robust=0.5
semi-major axis in north-western and south-eastern direc-
0.08 HIPASS
TiRiFiC tion.Thetwoprofilesalongthemajoraxisarealsopresented
] in Figure 7(a). Within the errors of the measurements, the
y 0.06
[J approachingandrecedingsidesshowthesameradialcolumn
ty densitybehaviour,againpointingtowardsaregularHidisc.
si 0.04
n The size of an Hi disc is commonly quantified through
e
D R , which is the radius at which the column density of
Flux 00..0002 tWhHeaInHgiedtroapl.s(t2o0114M,(cid:12)2p01c−62).hBarvoeeilfsou&ndRhReHeI(t1o99c7o)rraenladtelatreer-
markably well with the Hi mass of the disc. This relation
0.02 indicates that the average Hi column density in settled Hi
discs is constant from galaxy to galaxy. For ESO075-G006
10200 10400 10600 10800 11000 R = (74±4)kpc is measured from the azimuthally aver-
HI
VHEL [km s−1] aged radial profile in Figure 7(a) and corrected for beam
smearing following Wang et al. (2014, 2016). The measured
Figure5.ComparisonofHispectraasmeasuredfromtheHipass value is then compared to the expected RHI,exp that is cal-
data cube (light blue), the ATCA data cubes (dark blue for ro- culated from equation (2) in Wang et al. (2014, 2016). We
bust=0andmediumblueforrobust=0.5)andtheTiRiFiCmodel findRHI,exp=(74±18)kpc.Themeasuredandexpectedvalue
datacube(reddashedline,seeSection5.3.2). are in agreement within the errors. Hence, the average Hi
column density of ESO075-G006 is normal.
5.2 HI morphology and radial column density
profiles
5.3 HI kinematics
The two Hi data cubes for ESO075-G006 have been pro- In this section we investigate the Hi kinematics in detail
duced as explained in Section 3. In this section we use the
searchingforevidenceofrecentgasaccretione.g.radialmo-
robust=0.5, dv=4kms−1 data cube to analyse the distribu-
tions or vertical gradients indicating a galactic fountain.
tion of Hi in ESO075-G006.
Figure 5 shows the spectrum as taken from the ATCA
datacubeandfromtheHipassdata.Theyagreewithinthe
5.3.1 2D moment analysis
noise level. The double horn is close to symmetric indicat-
ing that the approaching a receding half of the disc con- Panel (b) of Figure 6 shows the velocity field of ESO075-
tainapproximatelythesameamountofgas.Theintegrated G006.Theiso-velocitycontoursshowthetypicalspider-web
fluxdensityofESO075-G006measuredfromtheATCAdata pattern of regularly rotating discs. Radial motions can in-
amounts to 12.8Jykms−1. Using Equation 1 this translates duce S-shaped iso-velocity contours in the central parts of
intologM [M ]=10.8±0.1,whichisinagreementwiththe the galaxy. This, however, is not seen and is a first indica-
HI (cid:12)
Hipass measurement of logM [M ] = 10.8±0.2 and indi- tion that radial motion are not playing a major role in the
HI (cid:12)
cates that the ATCA data cube is not missing any diffuse velocity field of ESO075-G006.
Hi. Figure6(d)showsthevelocitydispersionoftheHidisc
TheHicolumndensitycontoursoverlaidontheoptical of ESO075-G006. The low velocity dispersion in the outer
SuperCOSMOS image are shown in Figure 6(c). The loca- parts,especiallyalongthemajoraxisofthediscfurtherindi-
tionsofmaximumcolumndensityintheHidiscarefoundto catesaregularlyrotatingdisc.Theincreaseofthemeasured
thenorthandsouthofthecentreandalignwiththeridgeof velocity dispersion towards the centre of the disc is due to
the central bar. Furthermore, they coincide with the outer- projection effects.
mostspiralarmsasvisibleinNUV andfaintlyvisibleinthe
optical.Theapproachingandrecedinghalfappearverysimi-
lar,i.e.thediscissymmetric.Noobvioussignsofinteraction 5.3.2 3D kinematic analysis
andtailsarevisible.InthecentreofthedisctheHicolumn
To further explore the kinematic properties of ESO075-
densitydropstoapproximately1M pc−2.Thisisacommon
(cid:12) G006, a tilted ring model is fitted to the robust=0,
featureofHidiscsandhintstomoleculargasinthecentreof dv=10km s−1 Hi data cube using TiRiFiC (Jo´zsa et al.
ESO075-G006 (Bigiel & Blitz 2012). The distance between
2007). For the fit 7rings, each 30arcsec wide, are defined.
thegeometricalcentreoftheHidiscandthe2MASXcoordi-
In addition two half discs are defined, each centred on the
natesofthestellardisccentreislessthan10arcsec,whichis
major axis with an opening angle of 180degrees. This way
smallerthanthespatialresolutionelement.Therefore,both theapproachingandtherecedingsideoftheHidisccanbe
coincide. In conclusion, the Hi morphology points towards
fittedindependently.However,onlythesurfacebrightnessis
a well settled disc.
allowed to vary between the half discs, all other quantities
Radial profiles of the Hi column density are obtained
arefittedtofullrings.Avarietyofmodelswerefittedtothe
from the moment 0 map by measuring the median column
data:
density in co-centric elliptical annuli. In Figure 7(a) an az-
imuthallyaveragedprofileisshown.Toaccountforpossible (i) a flat rotating disc.
discasymmetries,theannuliwerealsodividedinfourquad- (ii) a warped disc with purely rotational motions.
MNRAS000,1–16(2016)
10 Lutz et al.
-69°39' -69°39'
10.7 ]
1−
0) 0) m s
0 0
0 0 k
J2 J2 30
c ( 42' c ( 42' 10.6 [1
De De nv
co
p
Vo
10.5
45' 45'
24m 21h23m 24m 21h23m
RA (J2000) (a) RA (J2000) (b)
100
90
-69°39' -69°39' 80
70
) )
J2000 J2000 5600 1m s]−
( (
Dec 42' Dec 42' 40 [kσ
30
20
10
45' 45'
24m 21h23m 24m 21h23m
RA (J2000) (c) RA (J2000) (d)
Figure 6. (a) The optical SuperCOSMOS Bj-band image of ESO075-G006. (b) The velocity field (moment 1 map) of ESO075-G006
solid and dashed contours are separated by 20km s−1and indicate the receding and approaching side respectively. The thick black
contourisdrawnat10613kms−1indicatingthesystemicvelocity.(c)Thecolumndensity(moment0)mapofESO075-G006overlaidon
the SuperCOSMOS optical B-band image. Hi column density contours are at (0.3, 1.3, 2.3, 3.3)M(cid:12)pc−2. (d) The velocity dispersion
(moment2)mapofESO075-G006.Contoursareseparatedby15kms−1.Theredellipsesinthebottomleftcornersofthemomentmaps
indicatethesynthesisedbeamsize.Allmomentmapsarecreatedfromtherob=0.5cube.
(iii) discs with lagging thick discs, i.e. Galactic Fountain plest model with a flat rotation curve, which is the warped
mechanism. disc.Inthefollowing,thismodelispresentedanddiscussed.
(iv) flatandwarpeddiscswithradiallyconstantorvary- The rotation curve is shown in Figure 7(b). Fitting a
ing radial velocity components. rotation curve of the functional form:
(cid:34) (cid:32) (cid:33)(cid:35)
(v) rotating discs with a combination of radial velocity −r
v (r)=v · 1−exp (4)
components and vertical velocity gradients. rot flat l
flat
(Leroy et al. 2008 and references therein) yields a rotation
All models return similar values for surface brightness, velocity of v =(201±6)kms−1.
flat
systemic and rotation velocity, inclination (range), position Themodelsuggestsawarpininclination,whichisdrop-
angle (range) and kinematic centre. The flat disc model is pingfrom41to36degbetweenradiiof90and120arcsec.The
not able to produce a flat rotation curve at large radii but radialvariationofinclination,i.e.thewarpisshowninFig-
requires a sharp drop in the rotation velocity by 30km s−1. ure 7(c). This is a warp at the lower end of the warp scale
Thismodelisthereforeexcluded.Morecomplicatedandex- found e.g. in the THINGS galaxies (Schmidt et al. 2016).
otic models (models (iii), (iv) and (v)) do not trace emis- Thepositionangleisheldconstantwithradiusandisfitto
sion in channel maps and position–velocity diagrams bet- 306deg (see again Figure 7(c)).
ter than a warped disc with purely rotational motions. The Furthermore, TiRiFiC finds a velocity disper-
dataarelimitedbysensitivityandspatialresolution.Theef- sion σ of 11.2km s−1and a systemic velocity of
fectsinvestigatedinmodels(iii),(iv)and(v)canstilloccur 10613km s−1. The kinematic centre of the disc is lo-
in ESO075-G006 but we are not able to detect evidence in cated at 21h23(cid:48)29.5(cid:48)(cid:48) −69◦41(cid:48)06(cid:48)(cid:48), which is the same
favourofthem.WeapplyOccam’sRazorandselectthesim- location as the 2MASX centre of the stellar disc.
MNRAS000,1–16(2016)