Table Of ContentProceedingsofthe7thEuropeanVLBINetworkSymposium
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,
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