75 kpc trails of ionized gas behind two Irr galaxies in A1367

In a 6h Halpha exposure of the N-W region of the cluster of galaxies A1367 we discovered a 75 kpc cometary emission of ionized gas trailing behind two Irregular galaxies. The Halpha trails correspond in position and length with tails of syncrotron radiatio

ToappearinApJL

APreprinttypesetusingLTEXstyleemulateapj

75KPCTRAILSOFIONIZEDGASBEHINDTWOIRRGALAXIESINA1367

G.Gavazzi1,A.Boselli2,L.Mayer3,J.Iglesias-Paramo2,J.M.V´ lchez4&L.Carrasco5,6

Universit`adegliStudidiMilano-Bicocca,P.zzadellescienze3,20126Milano,Italy

Laboratoired’AstrophysiquedeMarseille,TraverseduSiphon,F-13376,MarseilleCedex12,France

3DepartmentofAstronomy,UniversityofWashington,Seattle

4InstitutodeAstrof´ sicadeAndaluc´ a,CSIC,Apdo.3004,18080,Granada,Spain

5InstitutoNacionaldeAstrof´ sica,OpticayElectr´onica,ApartadoPostal51.C.P.72000Puebla,Pue.,M´exico

6ObservatorioAstron´omicoNacional,UNAM,ApartadoPostal877,C.P.22860,EnsenadaB.C.,M´exico

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ToappearinApJL

arXiv:astro-ph/0111085v1 5 Nov 2001

ABSTRACT

Ina6hHαexposureoftheN-WregionoftheclusterofgalaxiesA1367wediscovereda75kpccometaryemissionofionizedgastrailingbehindtwoIrregulargalaxies.TheHαtrailscorrespondinpositionandlengthwithtailsofsyncrotronradiation.AtthegalaxysideoppositetothetailsthetwogalaxiesshowbrightHIIregionsalignedalongarcs,wherethestarformationtakesplaceattheprodigeousrateof～1M⊙yr 1.Fromthemorphologyofthegalaxiesandofthetrailingmaterial,weinferthatthetwogalaxiesaresu eringfromrampressureduetotheirhighvelocitymotionthroughtheclusterIGM.Weestimatethat～109M⊙ofgas,probablyionizedinthegiantHIIregions,issweptoutformingthetails.Thetailscrosseachotheratsome100kpcfromthepresentgalaxylocation,indicatingthatamajortidaleventoccurredsome～5×107yrago.Weexcludethatmutualharassmentproducedtheobservedmorphologyandweshowwithnumericalsimulationsthatitcouldhavemarginallyaidedrampressurestrippingbylooseningthepotentialwellofthegalaxies.

Subjectheadings:galaxies:clusters:individual(A1367)—galaxies:irregular—galaxies:evolution—

galaxies:intergalacticmedium—methods:N-Bodysimulations

1.INTRODUCTION

CGCG97-073and97-079(Zwickyetal,1961-68)areIrregularmemberstotheclusterofgalaxiesA1367.TheyhaveamongthehigheststarformationratefoundinIr-regulargalaxies:～1M⊙yr 1asderivedfromtheirHαluminosity～1.5×1041ergsec 1(Kennicutt,Bothun&Schommer,1984;Gavazzietal.1998).ThestarformationtakesplaceinbrightHIIregionsdistributedalongcurvedpathsonthegalaxyperipheryfacingtheclustercenter(seeFig.Bothgalaxiesaresigni cantlyanemicontheop-positeside.RadiocontinuumobservationswiththeVLA(Gavazzi&Ja e1985,Gavazzi&Ja e1987,Gavazzietal.1995),revealedthe”head-tail”appearenceoftheradiosourceassociatedwiththesetwogalaxies.Thetailsex-tendupto75kpc(assumingH0=75kms 1Mpc 1)onthesideoppositetothebrightHIIregions.Observationsinthe21cmlineofHI(Gavazzi1989;Dickey&Gavazzi1991)revealedthatbothgalaxieshaveaslightlyde cientHIcontent,displacedinthedirectionmarkedbythera-diocontinuumtails,asopposedtotheirH2contentwhichappearsnormalinallrespects(Bosellietal.1994).Theseasymmetriessuggestthatthetwogalaxiesareexperienc-ingrampressureduetotheirhighvelocitymotionthroughtheIGM.Abadietal.(1999),Murakami&Babul(1999)andQuilisetal.(2000)performedhigh-resolutionhydro-dynamicalsimulationsofgalaxiessubjecttorampressurestrippinginrichclusters.Inlessthan1GyrtheirgalaxymodelsloseallthegasasaresultoframpressureandviscousstrippingwhenthedensityoftheIGMandthetransitvelocityareashighasintheComacluster.Ex-tendedgaseoustailsformandthegasisshockedatthe

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leadingedgeofthegalaxies,considerablyenhancingitsdensity,thusmostlikelyleadingtointensestarformation,asobservedin97-073orinNGC4522(Kenney&Koop-mann1999)intheVirgocluster.

Thesephenomenamightcontributesigni cantlytotheen-richmentoftheintergalacticmediuminclusters,whichremainsanunsettledissue(e.g.Madau,Ferrara&Rees2001;Mori,Ferrara&Madau,2001;Silich,etal.2001;Recchietal.2001).

Whiletakingdeep(onehourexposure)HαimagesoftheclusterA1367withtheWideFieldCameraattheIsaacNewtonTelescope(LaPalma),weserendipitouslydiscoveredalowsurfacebrightnessHαemissiontrailingbehind97-079,spatiallycoincidentwiththeradiocontin-uumtail.Thisemissionwastoofainttobemeasuredandrequiredcon rmation.Wethustookmoreobserva-tions1usingnarrow-bandHα lters:1hwiththeNOTand4hwiththeSPMtelescopes.Eachindividualobservationcon rmedtheexistenceofthetrailingHα.Theresultingstacked6hexposureimage,whichwepresentinthisLet-ter,issu cientlydeeptoallowarobustdeterminationofthe uxinthetailof97-079andrevealedanevenfaintertrailbehind97-073.ThecometaryHαtrailsdiscoveredinA1367arethemostextendedlow-brightnessHαemissionfeatureseverdetected.OtherprominentexamplesarethetheMagellanicstream(Weiner&Williams,1996)andthecloudinLeo(Reynoldsetal.1986).

2.HαOBSERVATIONS

Weobservedthe eldcenteredonCGCG97-073and97-079usingthreetelescopes:the2.5mIsaacNewtonTele-1

onobservationstakenwiththeINT,WHT,NOTandSPMtelescopes.TheINT,WHTandNOTareoperatedbytheI.N.G.atthe

SpanishObservatoriodelRoquedeLosMuchachosoftheI.A.C.TheSPMtelescopebelongstoO.A.NdeMexico.

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