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Proceedingsofthe7thEuropeanVLBINetworkSymposium Bachiller,R.Colomer,F.,Desmurs,J.F.,deVicente,P.(eds.) October12th-15th2004,Toledo,Spain Continuum EVN and MERLIN observations of Ultra Luminous Infrared Galaxies A.G.Polatidis1 andJ.E.Conway2 1 MaxPlanckInstitutfu¨rRadioastronomie,Bonn,Germany 5 2 OnsalaSpaceObservatory,Sweden 0 0 Abstract.RadioimagingofULIRgalaxiesisidealtoexploretheconnectionbetweenthestarburstandtheAGNphenomenon 2 sinceitisunaffectedbydustobscuration,andprovidestherequiredhighangularresolutiontodistinguishbetweenanAGNand n starburstemission.Wehavemadecombined18cmradiocontinuum,EVNandMERLINobservationsof13ULIRGsthathave a theparsecanddeci-parsecscaleresolutionnecessarytodistinguishbetweenanAGNandsupernovaeremnantsatthecentresof J thesegalaxies,andassessthecontributionofeachtothetotalenergydistribution.Imagesoffourgalaxiesarepresentedhere. 3 1. Introduction present preliminary results from the February 2002 observa- 1 tions. v To date the best studied sample at high resolution of ULIRG 7 ThesourceswereobservedwithsixtelescopesoftheEVN galaxiesistheIRASBrightGalaxysample(hereaftertheBGS 1 (Torun, Medicina, Onsala, Effelsberg, Westerbork, and the sample).Condonetal.(1990;1991)observedover40compact 0 JodrellBank Lovelland Mark 2 telescopesalternatingduring 1 sourceswiththeVLAfrom1.6to8GHz.Lonsdaleetal.(1993, the observations) with a sampling rate of 256 Mbits/s over a 0 L93)madesnapshotVLBIimagesof31sourcesfromCondon 32MHzbandwidth.TheMERLINobservationshada15GHz 5 et al. (1991); of these 21 were detected in VLBI baselines bandwidthinoverlappingfrequencyrange.Duetotherelative 0 greaterthan10Mλ,promptingLondsaleetal.tosuggestthatan / weaknessofthesources,theobservationsweremadeinphase- h AGNwasresponsiblefortheVLBIstructure.However,Smith referencing mode with a cycle of 7 min on the source and 3 p et al. (1998a), in full track observations of Arp 220 showed - minutesonthenearbyphasecalibrator. o thattheVLBIstructureconsistsofradiosupernovaeorsuper- The observations were partially successful. Of particu- r novaeremnants.Followingthis,Smithetal.(1998b)modeled t lar importance was the failure of the Cambridge telescope s the L93 data fitting clustered luminous compact radio super- a which would have provided the common baseline (Lovell - novae,showingthatformostsourcestheVLBIstructurecould : Cambridge) to facilitate the cross-calibration of the two ar- v beexplainedbystarburstactivitywhileforonlyafewsources rays,andthesubsequentreductionoftheMERLINresolution i X thepresenceofanAGNwasinvokedtoaccountfortheparsec to∼250mas.FurthermorethelossoftheLovelltelescopefor scalestructure. r 40% of the run, althoughit was replacedby the Jodrell Bank a We are engaged in a systematic survey of the complete Mark2antenna,resultedinreductionofsensitivityofthearray. sample of the 19 northern (δ > 0) compact ULIRGs in the Consequentlythearrayconsistedofthe6MERLINtelescopes BGSsamplewhichshowedevidencefordetections>1mJyon and6EVNantennaeatthemostforeachsource. atleastonebaselinelongerthan10Mλ. Of these19compact ULIRGssix(IIIZw35,Mrk231,Mrk273,Arp220,NGC7469 andMrk 331)were alreadywellobservedatthe same resolu- 3. Results tion,hencewewereleftwith13objectstoobserve. TheEVNdatawerecorrelatedattheJIVEdataprocessorand We have observedthe sources with a combinedMERLIN were thenprocessedwith the AIPSsoftwaresuite. Initialam- and EVN array at 18cm so that we can separate the diffuse plitude calibration and fringe fitting was done with the EVN emissionatMERLINscaleswhichcanbeduetothermalemis- pipelineandwasrefinedlater. sioncombinedwithsynchrotronemissionfromelectronsfrom supernovae diffused into the ISM, from the compact radio emissionwhichcanbeduetoanAGNortosynchrotronemis- 3.1.UGC05101 sionassociatedwithsingleorclusteredradiosupernovaeorsu- pernovaeremnants(SNRs). UGC05101atVLAscalesshowsa∼1.57×1.14arcsec(1110× 810pc,atadistanced=158Mpc)disk–likeemissionelongated roughly east–west, dominated by a central bright component (seenclearlyinCondonetal’s(1991)8.4GHzimage).Atthe 2. Observations MERLIN resolution of 270 mas (Fig. 1) we detect a similar Initiallythefivebrightersourceswereobservedwiththecom- structure measuring 933×533pc (roughly1.3 × 0.75 arcsec). binedMERLINandEVNarray,inFebruary2002.Theremain- ThefluxdensityintheMERLINimageisconsistentwiththat ing 8 sources were observed in May 2003. In this paper we ofCondonetal.(1990)andTheanetal.(2001). 232 A.G.PolatidisandJ.E.Conway:ContinuumEVNandMERLINobservationsofUltraLuminousInfraredGalaxies Fig.1.Left:TheMERLINimageofUGC05101witharestoringbeamof230mas.Thepeakis70.3mJy/bmandthecontours aredrawnat1,2,4,8,16,32x1.8mJy/bm.Right:TheEVNimageofUGC05101restoredwithabeamof22x17.4mas.Thepeak is17.5mJy/bmandthecontoursaredrawnat1,2,4,8,16x1mJy/bm. Inthehighestresolution(15pc)EVNimage∼53%ofthe ratedby450mas(150pc)atthesameorientationastheVLA fluxdensityisrecoveredinanapparentlyS-shapedsourceex- image.Atthe25masresolutionoftheEVNimage,thebrighter tending roughly 128 ×140 parsec. Three major components MERLIN component is resolved in two components (A, B) (A-C),relativelycompact(5-13pc)areembeddedinweakex- separatedby18.4pc,withthebrightestBlocatedatthenorth- tended radio emission, visible in the lower resolution images east.200parsecnortheastofB,wedetecttheresolvedoutcom- thatarenotpresentedhere.ComponentAisclosertothepeak ponentC,whichistheweakercomponentofthedoubleinthe brightnessoftheMERLINimage.Thebrightnesstemperatures MERLINimage. ofcomponentsAandBare∼107Kwhilethatofthemorere- AllthreecomponentsinNGC6286areresolvedwithsizes solvedcomponentCis7106K,whiletheirluminositiesrange between 4 and 10 parsecs, brightness temperatures ranging from51039to1040erg/s. from 1.02 106K (C) to 8.9 106K (B) and luminosities from Lonsdaleetal.(2003)withhigherresolution5GHzglobal 1037 to 1039 erg/s, hence a few times higher than the RSN in VLBI observations find a consistent nuclear structure, while Arp220. theyresolveAandBintwocompactsubcomponentseach. ThefactthattheyareresolvedarguesagainstsingleRSN. These compactcomponentsare ∼50timesmoreluminous Itisplausiblethatoneofthecompactcomponentsmaybedue thanthebrightestradiosupernovae(RSN)inArp220.Theyare to a clumpofsupernovae.Itis also possiblethatthebrightest also resolved and their sizes are too large for single RSN. It componentareduetoAGNactivity. appearsthatthenuclearstructureofUGC05101cannotbeas- sociated with RSN butis rathercaused by an obscuredAGN. 3.3.Mrk266 ThisisconsistentwithImanishietal.’s(2003)detectionofFe Kα line emission in X-rays, though the X-ray spectrum was Mrk266isasystemofinteractinggalaxies(atadistanced=113 measured with an aperture of 8 arcsec (5.6 kpc) which cov- Mpc) showing an interaction region in the middle and tidal ersthe wholeradiosourceandhenceitis noteasy to orrelate tails.Mazzarellaetal.(1988)classifythenorthernasaLINER thepositionoftheX-raysourceandtheradioemission.Multi- andthesoutherngalaxyasaSeyfert2type.VLAandMERLIN frequencyobservationsareneededtofurtherexplorethispos- imagesshowatleastthreeco-linearcomponentsofwhichthe sibility. outer are associated with the two interacting galaxiesand the centralmaybearegionofsynchrotronemissionstimulatedby the collision of the two galaxies with the interaction region. 3.2.NGC6286 With increasing resolution the two southern components be- NGC6286isanedge-onspiralgalaxy(atadistanced=73Mpc) come resolved while the northern (which is the target of our in an interacting pair with NGC 6285 ( 1’.5 , 30 kpc) clas- EVNobservations)remainscompact. sifiedasaLINER.Condonetal.(1990)interpretthe1.6GHz In our MERLIN image (Fig. 2), the northern component VLAimage,showinga20”(6.8kpc)resolvedsourceelongated hasafluxdensityof21mJy,consistentwiththeimageofThean alongtheopticalaxisasstarburstinduced. et al. (2001) and a spectral index α1.6−8GHz=0.93. The higher Our MERLIN image (Fig. 2) recovers37 mJy (∼ 25% of resolution EVN observations (beam=21mas, 107 pc) show a the VLA flux density) and consists of two-componentssepa- relativelycompact(2×2pc)component(3.4mJy) andarc-like A.G.PolatidisandJ.E.Conway:ContinuumEVNandMERLINobservationsofUltraLuminousInfraredGalaxies 233 Fig.2.TopLeft:TheMERLINimageofNGC6286witharestoringbeamof300mas.Thepeakis25.7mJy/bmandthecontours aredrawnat1,2,4,8,16,32x0.6mJy/bm.TopRight:TheEVNimageofNGC6286restoredwithabeamof22.7x18.77mas.The peakis7.6mJy/bmandthecontoursaredrawnat1,2,4,8,16x0.39mJy/bm.BottomLeft:TheMERLINimageofMrk266with arestoringbeamof534x200mas.Thepeakis13.2mJy/bmandthecontoursaredrawnat1,2,4,8,16,32x0.3mJy/bm.Bottom right:TheEVNimageofMrk266restoredwithabeamof27x18mas.Thepeakis4.2mJy/bmandthecontoursaredrawnat 1,2,4,8,16x0.12mJy/bm. emission which culminates in a more extended (18×14pc) 3.4.NGC2623 weaker (1.7 mJy) subcomponent ∼41pc, while a third weak component is visible ∼100pc further north. Their brightness NGC 2623 is a LINER galaxy (at a distance d=74.138 Mpc) temperaturerange from2.3 107K to 3.3 105K and their lumi- whose optical appearance suggests a late state of merging. A nosityfrom41038to8.21038erg/s. single brightcomponentis seen at the centre of the galaxy at nearinfraredimages(thenucleusisheavilyobscuredattheV band).Liparietal(2004)findevidencefortwostarburstevents, acircumnuclearburstwithanageof1Gyrandacompactnu- CombiningourEVNimagewitharchivalMERLIN5GHz clearstarburstwithanageof10Myr. data,wefindthatthebrightest,compactcomponenthasasteep spectrumofα5−1.6GHz=0.681,whileweakerandmoreextended Condonetal.(1991)detectedawellresolvedstructurewith componenthasarisingspectrumofα5−1.6GHz=-0.023. the VLA at 8 GHz indicating a starburst origin for the radio 234 A.G.PolatidisandJ.E.Conway:ContinuumEVNandMERLINobservationsofUltraLuminousInfraredGalaxies Fig.3.Left:TheMERLINimageofNGC2623witharestoringbeamof220mas.Thepeakis27.4mJy/bmandthecontoursare drawnat1,2,4,8,16,32x1.2mJy/bm.Right:TheEVNimageofNGC2623restoredwithabeamof27x19mas.Thepeakis10.8 mJy/bmandthecontoursaredrawnat1,2,4,8,16x3mJy/bm. emission,whileLonsdaleetal(1993)foundevidenceforahigh UGC05101whosecomponentsaremuchmoreluminousthan brightnesscompactcomponentembeddedincomplicatedradio 1989JorthesupernovaeinArp220.Thisplausibilityneedsto emission. beexploredwithfurthermulti-frequencyobservationsaswell OurMERLINimage(Fig.3)showsa slightlyasymmetric asX-rayobservations. structure,elongatedinaroughlyeast-westdirection,extending The picture that seems to be emerging is that in most for ∼ 350 pc. The EVN image shows at least two (∼ 6x6 pc) ULIRGs,starburstsprovidemostoftheenergy,howeverade- components,separatedby70pc,withthebrightesthavinglu- tectable AGN is present in a fraction of those and in those minosityof1.031039erg/sandabrightnesstemperatureof3.9 casesasignificantminorityoftheenergy(10%–40%)cancome 106K while the weakerhasa luminosityof 8.11037 erg/sand from AGN accretion. Recent X-ray observations(e.g. Ptak et abrightnesstemperatureof3.9105K.Atleastoneotherweak al(2003)addfurthersupporttothispicturesincestrongAGN- andextendedcomponent,isdetected∼96pctothesouthofthe likeX-raysourcesarefoundwithCHANDRAandBeppo-SAX brightestcomponentwith T = 9.6104K and a luminosity of inasignificantfractionofULIRGs. B 5.11037erg/s. Given that the shape of the hard X-rayspectrum can only Acknowledgements. The European VLBI Network isa joint facility ofEuropean,Chinese,SouthAfricanandotherradioastronomyinsti- be explained by a Compton-thick, cold, reflection dominated tutesfundedbytheirnationalresearchcouncils. AGN (Maiolino et al, 2003) it is plausible that the brightest componentintheEVNimageisthelocationofaweakAGN, whilefortheothertwocomponentsastarburstoriginisplausi- References ble. Condon,J.J.,Helou,G.,Sanders,D.B.,&Soifer,B.T.,1990,ApJS, 73,359 4. Conclusions CondonJ.J.,HuangZ.-P.,YinQ.F.&ThuanT.X.,1991,ApJ378,65 ImanishiM,TerashimaY.,AnabukiN.,NakagawaT.,2003,ApJ,596, InthefourULIRGsdiscussedinthispaper,50%to70%ofthe L167 totalfluxdensityisresolvedoutatparsecscaleresolution.The LipariS.,MediavillaE.,,DiazR.J.,Garcia-LorenzoB.,Acosta-Pulido parsec scale structure consists of multiple components with J.,AguerM.P.,TerlevichR.,2004,MNRAS,348,369 sizesofafewtotensofparsecs,brightnesstemperaturesrang- LonsdaleC.J.,LonsdaleC.J.,&Smith,H.E.1992,ApJ391,629 ingfrom105 to107Kandluminositiesfrom1037 to1040 erg/s LonsdaleC.J.,SmithH.&LonsdaleC.J.,1993,ApJ405,L9 (UGC05101).Allcomponentsaretoolargetobesingleradio LonsdaleC.J.,Lonsdale,C.J.,SmithH.E.,DiamondP.D.,2003,ApJ, 592,804 supernovaeandapartfromUGC05101,theirluminositiesare Mazzarella J.M.,Gaume R.A.,AllerH.D.,Hughes P.A.,1988, ApJ, a few thousand times brighter than Cas A but similar to the 333,168 supernova1989J.It is thereforepossible that at least some of MaiolinoR.,ComastriA.,GilliR.,NagarN.M.,BianchiS.,BkerT., thesecomponentsareclusteredyoungradiosupernovae(RSN) Colbert E., Krabbe A., Marconi A., Matt G., Salvati M.,2003, orsupernovaeremnants. MNRAS,344,L59 Itappearsthatatleastsomeofthehighbrightnesstemper- Ptak A.,Heckman T., Levinson T., Weaver K., Strickland D., 2003, atureparsecscale emission isdueto an AGN,in particularin ApJ,592,782 A.G.PolatidisandJ.E.Conway:ContinuumEVNandMERLINobservationsofUltraLuminousInfraredGalaxies 235 SmithH.E.,LonsdaleC.J.,LonsdaleC.J.&DiamondP.J.,1998a,ApJ 493,L17 SmithH.E.,LonsdaleC.J.&LonsdaleC.J.,1998b,ApJ492,137 TheanA.H.C,GillibrandT.I.,PedlarA.,Kukula,M.J.,2001,327,369