The Nature of the Activity in Hickson Compact Groups of Galaxies

arXiv:astro-ph/9709098v1 10 Sep 1997ThenatureoftheactivityinHicksoncompactgroupsofgalaxies.

RogerCoziol1,Andr´eL.B.Ribeiro2

ReinaldoR.deCarvalho3

and

HugoV.Capelato2Received–2–

ABSTRACT

Wepresenttheresultsofthespectralclassificationofthe82brightest

galaxiesinasampleof17compactgroups.WeverifythattheAGNsare

preferentiallylocatedinthemostearly–typeandluminousgalaxiesofthe

groups,asisusuallyobservedinthefield.ButtheseAGNsalsoappearto

besystematicallyconcentratedtowardsthecentralpartsofthegroups.Our

observationssuggestacorrelationbetweenactivitytypes,morphologiesand

densitiesofgalaxiesinthecompactgroups.Thisisconsistentwithascenarioin

whichgalaxiesofcompactgroupsevolvebyinteractingwiththeirenvironment

andarecurrentlyinaquietphaseoftheiractivity.

Subjectheadings:galaxies:Compactgroups–galaxies:Seyfert–galaxies:

LINERs–galaxies:interactions–3–

1.Introduction

OneoftheimportantaspectsofthestudyoftheHicksoncompactgroupsofgalaxies

(HCGs;Hickson1982)residesintheattractivepossibilityofassessingtheeffectsofstrong

interactionsonthemorphologyandstellarcontentofgalaxies(Hicksonetal.1992).The

availabledataontheHCGshoweverseemstopresentmanycontradictionswithafast

mergingevolutionscenario,suggestingthatwestilldonotfullyunderstandthenature

ofthesesystems.Toshednewlightonthisproblemanewspectroscopicsurveyoffaint

galaxiesintheregionsofHicksoncompactgroupswasrecentlyundertakenbydeCarvalho

etal.(1997;seealsoRibeiroetal.1997)todeterminethekinematicalstructureofthe

groups.ThesestudiesshowthatHCGsexhibitavarietyofdynamicalconfigurationsas

opposedtothepreviousviewthattheyareallisolatedandhighdensitystructures.

SpectroscopicobservationsofgalaxiesaroundHCGsnotonlyallowtoestablishhow

isolatedthesestructuresarefromotherstructuresbutalsoyieldvaluableinformationon

thenatureoftheactivityoftheindividualgroups.Previousstudiesontheactivityofthe

galaxiesincompactgroupshaveledtocontradictoryconclusions.Recently,forexample,

Sulentic&Raba¸ca(1993)andVegnugopal(1995)contesttheclaimbyHicksonetal.

(1989a)thatthefarinfraredemissionisenhancedincompactgroups.Radioobservations

(Menon,1992,1995),opticalspectroscopyandimaging(Rubinetal.1991)allsuggest

thattidalinteractionsandmergersbetweencompactgroupgalaxiesdidhappeninthe

past.However,manygalaxiesinthegroupsseemtobenormalanditisnotclearwhich

phenomenon,eitherstarburstorAGN,isthemainsourceofactivityobservedinthe

groups.OneextremeexampleofactivitiesencounteredinacompactgroupisHCG16,

whichincludesoneSeyfert2galaxy,twoluminousLINERgalaxiesand3starburstgalaxies

(Ribeiroetal1996).Butinasampleof17groups,HCG16istheonlyoneofitskind,

whichsuggeststhatingeneraltheactivityinthegroupsisnotprominent.–4–

Inthiscontributionwepresenttheresultsofourclassificationoftheactivitytypesof

asampleofluminousgalaxiesfrom17HCGs.Weshowthatasignificantfractionofthese

galaxiesdisplayAGNactivity.Moreover,50%oftheAGNpopulationinthegroupsas

studiedhereare“low–luminosity”AGNs(LLAGNs)–thatis,faintAGNs,eitherSeyfert2

orLINER,whicharehiddenbehindthestrongstellarcontinuumoftheirhostgalaxy.We

alsoshowthattheAGNpopulationalwaysappearssegregatedtowardsthecompactcores

(whichgenerallyencompassmostofHickson’soriginalcompactgroups,Ribeiroetal.1997)

whereasstarburstgalaxiestendtobedistributedintheirexternalparts.

Theorganizationofthepaperisasfollows.InSection2wedefineoursampleof

compactgroupgalaxiesandexplainthecriteriathatweusedtodoourclassification.

WealsopresentthecharacteristicsoftheLLAGNsinoursampleandshowtheresults

oftemplatesubtraction,whichisanessentialpartoftheclassificationscheme.Section

2.1presentstheresultsofourclassificationofalltheemissionlinegalaxiesfollowingtwo

diagnosticdiagramsoflineratios.Abriefdiscussiononthelow–luminositynatureofthe

LLAGNsfollowsinSection2.2.InSection3,wediscussthespatialandmorphological

distributionoftheactivegalaxiesinthecompactgroups.Weconclude,inSection4,with

abriefdigressiononthesignificanceofourfindingsfortheunderstandingofthenatureof

compactgroups.

2.SpectralclassificationoftheAGNscandidates

Thespectrapresentedinthispaperarepartofasampleof316galaxiesintheregions

of17HCGswhichwereobservedbydeCarvalhoetal.(1997).Thespectraweretaken

atthe4mCTIOtelescopeusingtheARGUSfiber–fedspectrograph.Thedetailsofthe

instrumentalsetupanddatareductionarediscussedbydeCarvalhoetal.(1997).Inthis

sample,82galaxies(ofwhichonly67arekinematicallyassignedtothegroups:seede–5–

Carvalhoetal.1997)havespectrawithsufficientlyhighS/Ntoallowaproperclassification

oftheactivitytype.Ofthesegalaxies,28(34%)presentonlyabsorptionlinesand54(66%)

presentbothemissionandabsorptionlines.Inthisarticlethenon-emissionlinegalaxies

areconsiderednon–active.Acompleteanalysisoftheabsorptionfeaturesinthesegalaxies

willbepublishedelsewhere(Cozioletal.inpreparation).Thisarticleisdedicatedtothe

classificationoftheemission–linegalaxies.

Luminousemission–linegalaxiesareusuallyeasytoidentifyandclassify.These

galaxiesareeitherstarburstorAGNgalaxies.Formanygalaxiesinoursample,however,

theusualclassificationcriteriacannotbeapplieddirectly,sincetheydonotshowany

emissionlines(i.e.withinthelimitsofourobservations)withtheexceptionofthe

[NII]λ6584line.Figure1showsthespectraforthesegalaxies.Thegreatsimilarityofthe

spectrasuggeststhateitherallthesegalaxiesareofsimilarmorphologicaltypeortheyare

dominatedbythesamestellarpopulations.Traceofionizedgazinearly–typespiralsand

ellipticalswasalreadyobservedbeforeanditisgenerallysuggestedthatamildSeyfertor

LINER,thatisalow–luminosityAGN,couldresideinthenucleiofallthesegalaxies(see

Filippenko&Sargent1985,Phillipsetal.1986;hereafterPJDSB).Giventhesimilarity

ofthephenomenonobservedhere,wewillprovisionallycallthesubsampleofthegalaxies

displayedinfigure1theLLAGNscandidates.ThebasiccharacteristicsoftheLLAGN

candidatesarepresentedinTable1.Thenumbersincolumns1and2followthenumbering

usedindeCarvalhoetal.(1997)andthelettersincolumn3correspondtothegalaxies

intheHickson(1982)list.RedshiftsdeterminedbydeCarvalhoetal.(1997)arelistedin

column4.TheBluminosities,astakenfromHicksonetal.(1989),arelistedincolumn

5.Theabsolutemagnitudes,presentedincolumn6,weredeterminedassumingH0=75

kms−1Mpc−1.Incolumn7wegivealsothemorphologiesofthegalaxies,asreportedby

MendesdeOliveira&Hickson(1994).