The evolution of HCG 31

22 Pages·2008·2.25 MB·English

A&A430,443–464(2005) Astronomy
DOI:10.1051/0004-6361:20047084 &
(cid:1)c ESO2005 Astrophysics
(cid:1)
The evolution of HCG 31: Optical and high-resolution HI study
L.Verdes-Montenegro1,A.DelOlmo1,M.S.Yun2,andJ.Perea1
1 InstitutodeAstrofísicadeAndalucía,CSIC,Apdo.3004,18080Granada,Spain
e-mail:[email protected]
2 DepartmentofAstronomy,UniversityofMassachusetts,Amherst,MA01003,USA
Received15January2004/Accepted28September2004
Abstract. Here we present the results of our new optical imaging and spectroscopic study and the analysis of new high-
resolution HI images of the Hickson Compact Group HCG 31. Taking advantage of the improved sensitivity and angular
resolutionofthenewopticalandHIimages,wehaveidentifiedanextensivecomplexofstellarandHItidalfeaturesandtheir
kinematics.OurHIstudyshowthatH31AandCarenotanadvancedmergersincetheirvelocityfieldscanbestillseparated
andhavealmostorthogonal orientations.Allofthecurrentsitesofongoingactivestarformationareshowntobeassociated
withthehighestcolumndensitypeakstracedinHI.AnewcompanionA0500−0434located240kpcsouthofthegroupcenter
isalsodiscoveredinHI.Adetailedscenarioforthetidalinteractionsinvolvedandtheoriginsoftheindividualtidalfeatures
areconstructedusingthemorphologyandkinematicsofthetidalfeatures.Thederiveddynamicalmassfortheentiregroupis
about2×1011 M(cid:2),whichisafewtimeslargerthanthesumofthemassesoftheindividualgroupgalaxies.Theultimatefateof
thegroupisthatHCG31isprobablyonitswaytoformasingleHIcloudgroupcontainingallgalaxies.
Keywords.galaxies:individual:HCG31–galaxies:interactions–galaxies:kinematicsanddynamics–galaxies:evolution–
galaxies:structure–radiolines:galaxies
1. Introduction imaging study of HCG 31 and HCG 92 using the OVRO in-
terferometerhasrevealedahighlyperturbeddistributionofthe
Hickson compact groups (HCGs) are highly isolated, dense
moleculargas(Yunetal.1997).Gasstrippingandexhaustion
galaxy systems (Hickson et al. 1992), and therefore tidal in-
by tidally induced star formation were discussed as possible
teractionsare expectedto be continuousanddynamicallyim-
explanations for the CO deficiency. In this paper we are ex-
portant. For this reason they representuniquelaboratoriesfor
aminingtheproposedscenarioforthenatureandevolutionof
studying interactions in extreme environmentsand tidally in-
HCG31usingnewhighspatialandspectralresolutionHImap-
ducedstar formationaswellas morphologicalanddynamical
pingandopticaldata.
evolutionofgalaxiesingeneral.Thisstudyispartofabroader
Fourgalaxies(H31A-D)wereidentifiedbyHickson(1982)
investigation of the evolutionary status of HCGs. Analyzing
in the Palomar Observatory Sky Survey (POSS) as the mem-
the distribution and kinematics of HI emission in a sample
bers of HCG 31. Whether H31A and H31C (Mk 1089) are
of 16 HGCs, we have already established a strong evolution
two separate galaxies or a merger in its final stage has been
of HI content of the groups and individual member galaxies,
thesubjectoffrequentdebatesbecauseoftotheiroverlapping
following a broad evolutionary scenario of increasing HI de-
and irregularmorphologies(see below). H31B is a small spi-
ficiency with the group evolution (Verdes-Montenegro et al.
ral close to H31A and H31C. H31D is a background galaxy
2001, hereafter VM01). Our CO survey of a complete sam-
witharedshiftof∼23000 kms−1(Hicksonetal.1992).Three
ple of HCGs have shown that about 20% of the galaxies are
new emission line objects, H31E, F and G (H31G is also a
deficient in CO when compared with a sample of isolated
Markarian galaxy, MK 1090) were identified by Rubin et al.
fieldgalaxies(Verdes-Montenegroetal.1998).AdetailedCO
(1990, hereafter R90). Rubin et al. also noted the presence
ofanothersmallgalaxy(H31QorNPM16–04.0219inNED1)
(cid:1) Based on observations made with the VLA operated by with m = 16.24 about 2(cid:4) north of H31A and H31C. Even
B
the National Radio Astronomy Observatory (the National Radio
thoughno redshiftinformationwas available, its membership
Astronomy Observatory is a facility of the National Science
toHCG31wassuggestedbyR90usingitsapparentassociation
Foundation operated under cooperative agreement by Associated
Universities,Inc.)andondatatakenusingALFOSC,whichisowned
by the Instituto de Astrofísica de Andalucía (IAA) and operated at 1 TheNASA/IPACextragalacticdatabase(NED)isoperatedbythe
theNordicOpticalTelescopeunderagreementbetweenIAAandthe Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under
NBIfAoftheAstronomicalObservatoryofCopenhagen. contractwiththeNationalAeronauticsandSpaceAdministration.
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20047084
444 L.Verdes-Montenegroetal.:TheevolutionofHCG31
withinthe HIcloudimagedusingtheVLAbyWilliamsetal. giveninTable1.Atmosphericconditionswerephotometricand
(1991, hereafter W91). Richer et al. (2003) obtained the red- theseeingwasbetween1(cid:4).(cid:4)3to1.(cid:4)(cid:4)5duringtheseruns.
shiftofthisgalaxy,providingafurthersupportforitsmember- The reduction and calibration of the images were carried
shiptothegroup.Thelargescalekinematicsoftheatomicgas outusingstandardtechniques.Theatmosphericextinctionwas
inthelargeHIcloud,uncorrelatedwiththeindividualgalaxies, determined from observations of 11 standard stars from the
was interpreted as an indication of on-goingmergingprocess Landoltlist(1983,1992).Thefluxcalibrationwascarriedout
fortheentiregroupbyW91andothers.Thegroupdisplaysevi- followingthemethoddevelopedbyYoung(1974)andthemag-
denceofvigorousongoingstarformationincludingveryyoung nitudesofthestandardstarsofLandoltweretransformedtothe
globularclustersformed10Myragoinastarburstepisodein- Johnsonsystem for R and I filters throughthe relationsgiven
ducedbytheinteraction,andH31Fappearstobetheyoungest byBessell(1983,1995).Residualsinthestandardstarsinthe
galaxy of the group made of gas-rich material stripped from finalcalibrationwerealwayssmallerthan0.03maginallcolor
the A+C complex(Iglesias-Páramo& Vílchez 1997;Johnson indices. The errors due to variations in the sky were smaller
et al. 1999; Johnson & Conti 2000; O’Halloran et al. 2002; than1%. Oncethe imageswerecalibrated,surfacebrightness
Lopez-Sanchezetal.2004). sensitivity achieved are 24, 24.5, 23.5 and 23 mag/arcsec2 in
InthispaperwereportadetailedstudyofHCG31basedon theCAHA B,V,Rand I-bandrespectively.TheNOT B-band
new optical images obtainedunder good seeing conditionsas imagehasbeensmoothedbyamedianfilteringof4(cid:4)(cid:4)×4(cid:4)(cid:4) for
well as new HI observationsobtained using the VLA. Taking a better sensitivity to extended structures, reaching a surface
advantage of the improvementsin the editing and calibration brightnessof26.6mag/arcsec2.Colorindicesgiveninthispa-
software and new imaging tools in AIPS, we were able to perhavebeencorrectedforGalacticabsorptionusingtheval-
achieve a three fold improvementin the overall sensitivity of uesdeterminedbyBurstein&Heiles(1984)andthereddening
theW91HIdata(seeSect.2)andrevealnewdetailsaboutthe lawfromSavage&Mathis(1979).
HIdistributioninHCG31.Further,newhigherresolutionob- The B-band NOT image of the group is shown in Fig. 1,
servationsarealsomadeintheCnBconfigurationoftheVLA, wherewehavelabeledallthepreviouslyidentifiedgalaxiesof
andvariousHIfeaturesareexaminedingreaterdetail.We re- the group (A, B, C, G and Q). A long optical tail runs from
port the detection of a new HI companion 15(cid:4) south of the thesouthofH31AandH31CtoH31G.IntheareaofknotsE
groupcenter.
andFidentifiedbyR90wefindsubstructureconsistingofsmall
The observations and data reduction are described in knots that we have labeled as e1 – e5 and f1 – f7 (see inset
Sect.2,andthegroupenvironmentisexaminedinSect.3.We imageinFig.1).OtherweakknotsarealsofoundaroundH31Q
discussourresultsfortheindividualgalaxiesinSect.4andfor (Sect.4.4)andinthewideextendedregionatthenorthofH31A
theintergalacticmaterialinSect.5.Throughoutthispaper,tidal andH31C.
features are referredto as “t-” plus a short identification (e.g.
The V and R-band images of the central group and the
“t-S”forthesoutherntidaltail).AglobaldiscussionofHCG31 B−Icolorindeximageofthecentralgalaxiesofthegroupare
is presented in Sect. 6. We adopt a distance to HCG 31 of
showninFig.2.Inordertoenhancethesmallscalestructures
54.3 Mpc assuming a Hubble constant H =75kms−1Mpc−1
0 a sharpenedR-bandimagewas obtainedbythesubtractionof
and the derived mean heliocentric velocity of 4076 km s−1 a 3(cid:4)(cid:4)×3(cid:4)(cid:4) median filtered image and it is shown in Fig. 2d.
(Sect.3)1.
Onlythemostintensefoundfeaturesaretakenintoaccountfor
the interpretation of the data, and its reality has been always
checkedintheoriginalunsharpenedimage.
2. Observations
Long slit spectra of H31Q and NPM1G -04.0218 (see
2.1.Opticaldata Sect.3)wereobtainedattheNOTtelescopeusingtheALFOSC
spectrograph. Table 1 summarizes the spectra taken for this
We obtained deep CCD images of HCG 31 in the Johnson
study The spectra were reduced using the standard meth-
BVRIfiltersatthe1.5mtelescopeoftheCentroAstronómico
ods. The rms error in the wavelength calibration was less
Hispano-AlemáninCalarAlto(CAHA,Spain).Theseimages
than0.1Å.
with a field of view of 5.(cid:4)4×5(cid:4).4 include the central galax-
ies H31A, H31B, and H31C, as well as H31G to the south,
2(cid:4).4 from the center of the main group. An additional B-band 2.2.VLAHIdata
image with a larger field of view (6.(cid:4)5×6(cid:4).5) includingH31Q,
2(cid:4)tothenorth,wastakenatthe2.5mNordicOpticalTelescope Two complementary sets of HI data are analyzed in this pa-
per. New high resolution VLA HI data have been obtained
(NOT) in La Palma (Spain) with ALFOSC, and this image is
in the CnB configuration in June 1997. HCG 31 was ob-
usedprimarilyforthedescriptionofthegroup(seeFig.1).The
served for 4 hours using 27 antennas with the phase center
galaxyA0500-0434,anewmemberdiscoveredinourHIdata
at α(1950) = 04h59m09s.0, δ(1950) = −04◦19(cid:4)42(cid:4)(cid:4). A veloc-
(seeSect.4.5),liesoutsidetheimagedfield,andanewRimage
ity range between 3704 km s−1 and 4360 km s−1 was cov-
wasobtainedusingtheNOT.Asummaryoftheobservationsis
eredwitha3.125MHzbandwidthinthe2IFmode,identicalto
1 ThecorrectionduetotheVirgoflowisnegligiblesincetheflow W91,butwithafactortwoimprovementinvelocityresolution
direction(l∼276◦, b∼+30◦)isnearlyperpendiculartothedirection (10.6 km s−1). The data were calibrated following the stan-
ofHCG31. dard procedures,and a self-calibration algorithm was used to
L.Verdes-Montenegroetal.:TheevolutionofHCG31 445
Fig.1.B-bandimageofHCG31obtainedatNOT,inalogarithmicgray-scalerepresentation.Higherintensitiesaredarker.Ithasbeensmoothed
witha4(cid:4)(cid:4)×4(cid:4)(cid:4) medianfiltering.Theindividual group membersareidentifiedasA,B,C,D,E,F,G,andQ.TheareaofobjectsEandFis
enlarged in the lower right panel, and the bright emission knots further identified and labeled in the inset. The isophotal contours corre-
spondto20.9,21.8,22.4,22.8,23.4,23.9,24.4,25.0,25.5and25.7magarcsec−2.TheorientationofalltheimagesisNorthupandEastto
theleft.
Table1.Summaryofphotometricandspectroscopicobservations.
Observations Telescope Filter/ Galaxies Spatial T Disper. Slit
exp
∆λ(Å) scale((cid:4)(cid:4)/px) (s) (Å/px) ((cid:4)(cid:4))
Broad-band 1.5mCAHA B H31ABCGF 0.32 2400 – –
Images 1.5mCAHA V H31ABCGF 0.32 1500 – –
1.5mCAHA R H31ABCGF 0.32 1800 – –
1.5mCAHA I H31ABCGF 0.32 1200 – –
2.5mNOT B H31ABCGFQ 0.19 1500 – –
2.5mNOT R A0500-0434 0.19 600 – –
Long-slit 2.5mNOT 5820–8330 H31Q 0.19 1800 1.23 1.2
Spectroscopy 2.5mNOT 3000–9000 NPM1G-040218 0.19 800 2.96 1.2
improvetheimagefidelity.Naturalweightingofthedatagives have re-processed the archival VLA HI data originally ob-
asynthesizedbeamof15(cid:4).(cid:4)8×14(cid:4).(cid:4)5,andtheresultingrmsnoise tainedbyW91intheDnCconfiguration.Thisdatasetsuffers
in each channel map is 0.46 mJybeam−1 (1.20 K). At an as- from a serious radio interference problem. After careful edit-
sumeddistanceof54.3MpcandanHIlinewidthof30 kms−1, ing of the data using an improved software, we were able to
the achievedHI mass detectionlimitis 1×107 M(cid:2)/beam.We achieve rms noise level of 0.58 mJybeam−1 (0.89 K with a
446 L.Verdes-Montenegroetal.:TheevolutionofHCG31
Fig.2. a) Isophotal contours corresponding tothe V filterimage, ranging from 18.5 to23.5 mag arcsec−2, withastep of 0.5magarcsec−2.
b)Rimageofthegrouprepresentedasina.c)B−Icolorimageinagray-scalewhereblackisbluerandwhiteisredder.d)SharpenedRimage
obtainedbythesubtractionofa3(cid:4)(cid:4)×3(cid:4)(cid:4) medianfilteredimage.InthelowerrightazoomoftheNEsideofH31BisshownandtheCOemission
fromYunetal.(1997)isoverplotted.
synthesized beam of 21(cid:4).(cid:4)7×18(cid:4).(cid:4)1). This is a three fold im- kinematics(seeSect.3)aswellastoresolvethekinematicsof
provementover the published images in W91, and additional H31F(Sect.5.3).Theemissionthatconnectsthenortheastern
low levelextendedemission featuresare recovered.The stan- sideofthecentralcloudandH31Qwaspreviouslyundetected,
dard VLA calibration procedureis generallyaccurate enough andthissuggeststhisfeatureisclumpy.Someoftheextended
toproduceanabsolutefluxuncertaintyofabout15%. emissionseeninthelowerresolutionmap(Fig.3a)isnotseen
inthenewCnBconfigurationimage,andsomeoftheHIemis-
The velocity integrated HI emission image obtained from sion is quite diffuse and smoothly distributed. The individual
ournewreductionoftheDnCconfigurationdataisshownover- tidalfeaturesdiscussedthroughoutthispaperareidentifiedin
laid on the smoothed B-band image of HCG 31 in Fig. 3a. this figure. The intensity weighted mean radial velocity field
When comparedwith Fig. 7 ofW91, the new HI mapreveals derived from the re-processed DnC and new CnB array data
newdetailssuchasthelowlevelemissionconnectingthecen- are plotted respectively in Figs. 3c and 3d. Our global veloc-
tralcloudcontainingH31A,H31B,andH31Cwiththenorth- ity mapsare consistentwith the one obtainedby Richer et al.
ernemissionwestofH31Q.ThevelocityintegratedHIimage (2003) from Hα data for H31AC, B and G. Further, our new
fromournewCnBarraydataisshowninFig.3b.Thecentral data can distinguish and separate the velocity field in H31A
areashowsasignificantimprovementduetothehigherspatial andH31C.Thisisconsistentwiththeirvelocitydispersionmap
resolution,andtheextendedemissiontotheeastisalsobetter ofH31ACwherethehighervaluesarereachedintheoverlap-
resolved now. In addition, these new data have allowed us to ping area of the two velocitycomponentsthat we have found
separatetheHIemissionofH31AandCandtodistinguishtheir forH31AandCrespectively.
L.Verdes-Montenegroetal.:TheevolutionofHCG31 447
Fig.3.a)MapoftheHIcolumndensitydistributioninHCG31obtainedfromanewreductionoftheDnCconfigurationdata,superimposed
on aB image median smoothed by 4(cid:4)(cid:4) × 4(cid:4)(cid:4). Thecontours correspond tocolumn densities of 1.4, 2.8, 4.1, 5.5, 6.9, 10.3, 13.8, 17.2, 20.6,
24.1,27.5and31.0×1020 atomscm−2.Thebeamsize(shownontheupperrightcorner)is21(cid:4).(cid:4)7×18(cid:4).(cid:4)1.b)Thesameasa)forthenewVLA
CnBarraydata.Thecontourscorrespondtocolumndensitiesof1.0,2.4,3.8,5.2,6.9,9.6,13.0,16.5,19.9and23.3×1020 atomscm−2.The
beam sizeis15(cid:4).(cid:4)8×14(cid:4).(cid:4)5. The four brightest tidal features areidentified by “t-” plusa short identification according totheir location. The
highestcontoursareplottedinwhiteforclarity.c)MapofthefirstmomentoftheHIradialvelocityinHCG31obtainedfromanewreduction
oftheDnCconfigurationdatashownbothwithiso-velocitycontoursandgray-scale.Thecontoursrangebetween3980and4180 kms−1with
astepof20 kms−1.Thebeamsizeis21(cid:4).(cid:4)7×18(cid:4).(cid:4)1.d)Thesameasc)forthenewCnBarraydata.Thebeamsizeis15.(cid:4)(cid:4)8×14(cid:4).(cid:4)5.Thegray
scaleinpanelsc)andd)representsheliocentricvelocitiesinkms−1asshowninthewedge.
ThevelocitychannelmapsforthehighresolutionCnBcon- confirm the reality of weak features in the CnB array data.
figurationVLAHIdataareshowninFig.4superposedonthe The derived line parameters, velocities and total HI masses
B-bandimage. aresummarizedinTable2togetherwiththedetectionranges.
TheglobalHIspectraarederivedbyintegratingtheemis- ThetotalHIlinefluxdetectedbytheDnCarrayis47%larger
sionintheindividualchannelmapsforbothDnCandCnBcon- thanthesingledishdata.Theobservingtelescopebeam(10.(cid:4)8)
figuration data and are shown in Fig. 5a using a thick and of Williams & Rood (1987) was not large enough to include
thin line respectively. We have also obtained spectra for in- theemissionfromA0500−0434,butthisalonecannotaccount
dividual galaxies and tidal features as shown in Figs. 5b–k. fortheentiredifferencebecauseA0500−0434contributesless
Identificationofthecorrespondingvelocityrangeshasbeenof- than5%ofthetotalemission.SincetheshapesoftheHIpro-
tenproblematicduetosourceconfusion.We havedetermined filesaresimilar,thedifferencemaybeexplainedbetterbythe
themfromthecomplementaryinformationinDnCandCnBar- gain drift and the uncertain flux calibration in the old single
ray data.The higherresolutionCnB data allow betterdistinc- dishdata.Asignificantamountofextendedemissionismissed
tion of overlappingfeatures, but the Dnc data are required to butabout40%oftheDnCHIfluxisrecoveredintheCnBdata.
448 L.Verdes-Montenegroetal.:TheevolutionofHCG31
Fig.4. Channel maps of the 21 cm line radiation obtained with the VLA CnB configuration, superimposed on the B-band image median
smoothedby4(cid:4)(cid:4)×4(cid:4)(cid:4).Heliocentricvelocitiesareindicatedineachpanel.Contourscorrespondto−3.0,3.0,6.1,11.8and17.9K,andtherms
noiseofthemapsis1.2K.Thesynthesizedbeam(15(cid:4).(cid:4)8×14(cid:4).(cid:4)5)isplottedintheupperleftpanel.
L.Verdes-Montenegroetal.:TheevolutionofHCG31 449
Fig.4.continued.
Fig.5.HIlineprofilesfortheindividualHIfeatures.ThicklinesshowthespectraderivedfromtheDnCarraydatawhilethethinlinesshowthe
spectrafromtheCnBarraydata.Panelb)alsodisplaystheweakemissioninthebridgejoiningH31AandH31B(namedast-C,seeSect.4.1).
TheHIspectrumobtainedatthelocationofH31Fisshowninpanelj).
450 L.Verdes-Montenegroetal.:TheevolutionofHCG31
Table2.HIemissionparameters.
Object Va Velocityrange ∆vb20 ∆v50c Tmaxd log(M(HI)/M(cid:2))e
(kms−1) (kms−1) (kms−1) (kms−1) (K)
H31A 4090 4021–4180 169.2 169.2 13.6 9.30
H31B 4122 4042–4222 190.6 126.3 13.6 9.28(g)
t-C 4095 4074–4095 52.9 21.3 6.3 8.00
H31C 3984 3936–4021 85.8 64.3 20.0 9.20
H31G 4005 3937–4064 128.7 42.9 20.1 9.28
H31Q 4090 4053–4138 84.9 42.4 9.8 8.95
A0500-0434 3952 3852–4053 233.1 233.1 9.5 9.04
t-E 4080 4042–4138 107.5 63.2 6.8 9.41
t-S 4016 3936–4116 190.8 126.8 20.5 9.81
H31F 3968 3937–4011 74.6 63.8 19.1 8.78
t-NE 4111 4085–4117 84.9 42.4 7.9 8.48
t-NW 4048 4000–4064 85.0 63.3 18.3 9.59
TotalDnC 4059 232.7 148.3 20.5 10.37
TotalCnB 4053 3937–4222 222.3 158.6 20.5 9.99
Totalsingledishf 4041 212 147 0.2 10.20
a Centralvelocityat20%ofthepeakemissionintheintegratedspectraobtainedfromtheDnCarraydata,exceptforH31Fandt-C,only
resolvedintheCnBarray.
b Linewidthmeasuredat20%ofthepeakemissioninthesamedataindicatedina.
c Linewidthmeasuredat50%ofthepeakemissioninthesamedataindicatedina.
d Peaktemperaturemeasuredinthesamedataindicatedina.
e M(HI)=2.36×105×D(Mpc)2×S ∆V(Jy kms−1),whereD=54.3MpcandS ∆Vistheintegratedemissioninthesamedataindicated
HI HI
ina.
f ObtainedfromWilliams&Rood(1987).
g The calculated mass includes the emission from t-C unresolved with the DnC-array data. The t-C mass is given below based on the
CnBdata.
Fig.6.a)1.4GHzratiocontinuum emissionassociatedtothecentralareaof HCG31derivedfromtheline-freechannelsoftheCnBarray
data.Thecontourscorrespond to1.1,2.1,3.2,4.8,8.0,14.3and27.1K(rmsnoise0.37K)andaresuperposed ona Bimageofthegroup.
ShownasthickcontoursistheCOemissionfromYunetal.(1997).Thesynthesizedbeam(15(cid:4).(cid:4)7×14(cid:4).(cid:4)4)isplottedintheupperleft.b)1.4GHz
continuumemissionimageofH31GisshownsuperposedonaB−Icolorimage.Radiocontinuumcloselyfollowstheredder(lighter)B−Icolor
regions.Thecontourscorrespondto1.6,2.6,3.7,4.8,5.8,6.9and7.9K(1σ =0.85K).Thesynthesizedbeam(15(cid:4).(cid:4)1×7(cid:4).(cid:4)5)isplottedinthe
upperleft.
2.3.1.4GHzcontinuum the continuumimage presented by W91. The new continuum
mapisproducedusingnaturalweightingofthe uvdatain or-
der to maximize the sensitivity, and a synthesized beam of
The 1.4 GHz radio continuum distribution has been obtained
15.(cid:4)(cid:4)7×14(cid:4).(cid:4)4 and an rms noise of 0.37 K (0.14 mJybeam−1)
from the line free channels in both data sets, and the result-
are achieved. The peak of the continuum emission is
ingcontinuummap,Fig.6a,isasignificantimprovementover
L.Verdes-Montenegroetal.:TheevolutionofHCG31 451
located at the intersection of galaxies H31A and H31C, un-
likethecontinuumimageshownbyW91,whoidentifiedH31C
as the radio continuum source. The radio continuum peak
is also shifted with respect to the CO emission, plotted as
thick contours reproduced from Yun et al. (1997). An ex-
tension in the direction of H31F and G is also evident. A
new continuum source is detected at the position of H31B
at a level of 1.1 ± 0.2 mJy, nearly coincident with the op-
tical center. The radio continuum image for H31G has been
produced using robust weighting of the uv data in order to
to better resolve the continuum structure(synthesized beam
of 15.(cid:4)(cid:4)1×7(cid:4).(cid:4)5). It was marginally detected by W91, and is
clearly detected now (Fig. 6b), peaking on the opposite side
ofthebluestregioninthisgalaxy.Thetotaldetected1.4GHz
continuum fluxes for H31AC, H31B and H31G are 22 ± 3,
2.1±0.3,and3.3±0.5mJy,respectively.While the 1.4GHz
fluxforH31ACaswellasH31Garewithintheobservedscat-
ter in the well knownFIR-radiocorrelation(0.26in dex;Yun
et al. 2001), H31bshows some excessradio continuumemis-
sion.Galaxieshostingradio-quietAGNstypicallyshowradio-
Fig.7.AdiagramoftheenvironmentofHCG31.Filledcirclesiden-
excess of a factor 2–4 (see Yun & Carilli 2002), and the ra-
tifythemembersofthegroupandgalaxieswithsimilarredshift.The
dio excess seen in H31b is consistent with the presence of a
galaxieswithoutredshiftdataarerepresentedbyopencircles,andthe
radio-quietAGN.
centerofAbell531ismarkedwithacross.Thedottedcircleindicates
theVLAprimarybeam.Galaxieswithaclearlydifferentredshiftfrom
thegroupareomitted.
3. ThemembersandenvironmentofHCG31 4. Thegalaxies
The three brightest members of HCG 31 (H31A, H31B 4.1.H31AC
andH31C)arelocatedwithinadiameterofonly14kpc(0.(cid:4)9).
H31A and H31C have a disruptedappearancenot only in the
Adding H31G increases the group diameter to 32 kpc (R90),
opticallight,butalsoinHI,withatleastthreevelocitycompo-
and H31Q expands the group diameter further to 66 kpc.
nentsoverlapping.Hencetheintensityweightedmeanvelocity
We determine a mean heliocentric velocity for the group of
oftheHIemissionisconfusingandpossiblyevenmisleading.
4076 kms−1,a velocitydispersionof60 kms−1 andamean
Nevertheless, we were able to separate the HI gas associated
separationbetweengalaxiesof10kpc,basedontheopticalve-
with the individual galaxies H31A and C using the HI mor-
locities of H31A, H31B, H31C (Hickson 1993), H31G (R90)
phologyandkinematicsasdescribedbelow.
andH31Q(Richeretal.2002).
H31A.– The brightest knots of this galaxy align in the
WehavesearchedforneighborsofHCG31ina1Mpcra- E-W direction, with fainter emission extending to the north
dius (1◦) in the CfA catalog (Huchra et al. 1993) and NED – (a B-bandimage is shown in Fig. 8a). These bright knots are
see Fig. 7. The inner0.5 Mpc contains1 galaxywith redshift prominentinthesharpenedR-bandimage(Fig.2d)constructed
and magnitude similar to the HCG 31 members whose dis- by subtractinga 3(cid:4)(cid:4)×3(cid:4)(cid:4) medianfiltered image.Two of these
coveryis reportedin thispaper(Sect. 4.5,A0500-0434).One knotsareembeddedinfaintemissionorientatedwithPA∼88◦,
other galaxy with no previously known redshift is found 28(cid:4) the brightest one with a roundshape characteristic of a small
(442 kpc)to the north(NPM1G -04.0218),butour own NOT bulge.TheB−Icolorimage(Fig.2c),showsadistinctredfea-
spectrum shows it to be a background galaxy (z = 0.087). ture(B−I = 1.4)with PA = 65◦. Itscolorsare muchredder
Between the projecteddistance of 0.5 and1 Mpc, NGC 1729 than those of H31C (see below) and suggest that it is tracing
is at the same redshift as the group (v = 3644 km s−1, thediskofH31A.
r
mB = 13, 0.91 Mpc to the NW), and thirty additional galax- Approximately 3 × 109 M(cid:2) of atomic gas are associated
ies are known, including the three known to be in the back- with the main body of H31A, generally correlating well with
ground. The 18 faint galaxies with no known redshifts clus- theopticalfeatures.Thechannelmapspeakonthetwobright-
ter around Abell cluster 531 (z = 0.94), and they are likely estopticalknots(Fig.4).Anarc-likebridge(t-C)joinsH31A
the clustermembers.ThereforeA0500-0434(see Sect.4.5)is andB(V =4074−4095 kms−1),whilethetwotidalfeaturest-
probabletheonlygalaxydynamicallyassociatedtothegroup, Eandt-NEappeartoconnectwithH31A.WeshowinFig.5b
located 239 kpc south of the group center and similar in red- the global HI profiles obtained from the DnC and CnB array
shiftandsizetoeachmemberofHCG31.Fromtheseconsid- data, including the weak emission corresponding to t-C ob-
erations,weconcludethatHCG31islocatedinalowdensity tainedfromtheCnBarraydata.Thecorrespondingparameters
region. aregiveninTable2.
452 L.Verdes-Montenegroetal.:TheevolutionofHCG31
Fig.8. a), c) and e) present respectively the HI column density (CnB-array data) of H31A (V = 4021−4180 km s−1), H31C (V =
3937−4021 kms−1)andH31B(V = 4042−4222 kms−1)overplottedona B-bandimageinalogarithmicgrayscale.Thecontoursare1.0,
2.4, 3.8, 5.2, 6.9, 9.6, 13.0, 16.5, 19.9 and 23.3×1020 atoms cm−2 for H31A, and 0.9, 1.9, 3.8, 5.7, 7.5, 9.4, 11.3, 13.2, 15.1, 17.0, and
18.8×1020 atomscm−2 forH31C andB.Thedashed linesindicatethePAof88◦,20◦ and40◦ usedfor thepositionvelocitycutplottedin
Figs.9a–cforH31A,H31CandH31Brespectively.b),d)andf)showthefirst–ordermomentoftheradialvelocityfieldofH31A,H31Cand
H31Bwherebothiso-velocitycontours(H31A:3950to4020 kms−1,step15 kms−1;H31C:3950to4020 kms−1,stepof7 kms−1;H31B:
4050to4190 kms−1;step10 kms−1)andgray-scaleareshown,togetherwithasketchofthesharpenedRimageofFig.2d.Thescalegoesas
inthewedgewherethenumbersindicateheliocentricvelocitiesinkms−1.Thebeamsizeis15(cid:4).(cid:4)8×14(cid:4).(cid:4)5andisplottedintheupperright.