Unveiling the AGN in IC 883: discovery of a parsec-scale radio jet

0.47 MB·

by  Cristina Romero-Cañizales

Mon.Not.R.Astron.Soc.000,1–13(2017) Printed26January2017 (MNLATEXstylefilev2.2)
Unveiling the AGN in IC883: discovery of a parsec-scale radio jet
C. Romero-Can˜izales,1,2,3⋆ A. Alberdi,4 C. Ricci,2 P. Are´valo,5 M. A´. Pe´rez-Torres,4,6†
J. E. Conway,7 R. J. Beswick,8 M. Bondi,9 T. W. B. Muxlow,8 M. K. Argo,8,10
F. E. Bauer,2,1,11,12 A. Efstathiou,13 R. Herrero-Illana,4,14 S. Mattila15 and S. D. Ryder16
1MillenniumInstituteofAstrophysics,Chile
7 2InstitutodeAstrof´ısica,FacultaddeF´ısica,PontificiaUniversidadCato´licadeChile,Casilla306,Santiago22,Chile
1 3Nu´cleodeAstronom´ıadelaFacultaddeIngenier´ıayCiencias,UniversidadDiegoPortales,Av.Eje´rcito441,Santiago,Chile
0 4InstitutodeAstrof´ısicadeAndaluc´ıa–CSIC,POBox3004,18080Granada,Spain
2 5InstitutodeF´ısicayAstronom´ıa,FacultaddeCiencias,UniversidaddeValpara´ıso,GranBretan˜aNo.1111,PlayaAncha,Valpara´ıso,Chile
6DepartamentodeF´ısicaTeorica,FacultaddeCiencias,UniversidaddeZaragoza,Spain
n
7DepartmentofEarthandSpaceSciences,ChalmersUniversityofTechnology,OnsalaSpaceObservatory,43992,Onsala,Sweden
a
J 8JodrellBankCentreforAstrophysics,TheUniversityofManchester,OxfordRd,ManchesterM139PL,UK
9OsservatoriodiRadioastronomia-INAF,Bologna,viaP.Gobetti101,40129Bologna,Italy
4
10JeremiahHorrocksInstitute,UniversityofCentralLancashire,Preston,PR12HE,UK
2
11CentrodeAstroingenier´ıa,FacultaddeF´ısica,PontificiaUniversidadCato´licadeChile,Casilla306,Santiago22,Chile

12SpaceScienceInstitute,4750WalnutStreet,Suite205,Boulder,CO80301,USA
]
A 13SchoolofSciences,EuropeanUniversityCyprus,DiogenesStreet,Engomi,1516Nicosia,Cyprus
14EuropeanSouthernObservatory(ESO),AlonsodeCo´rdova3107,Vitacura,Casilla19001,SantiagodeChile,Chile
G 15TuorlaObservatory,DepartmentofPhysicsandAstronomy,UniversityofTurku,Va¨isa¨la¨ntie20,FI-21500Piikkio¨,Finland
. 16AustralianAstronomicalObservatory,POBox915,NorthRyde,NSW1670,Australia
h
p
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o Accepted2017January23.Received2017January23;inoriginalform2016June29
r
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a
ABSTRACT
[
IC883is aluminousinfraredgalaxy(LIRG)classified asa starburst-activegalacticnucleus

1 (AGN) composite. In a previous study we detected a low-luminosity AGN (LLAGN) radio
v candidate.Herewereportonourradiofollow-upatthreefrequencieswhichprovidesdirect
5 andunequivocalevidenceoftheAGNactivityinIC883.OuranalysisofarchivalX-raydata,
2 together with the detection of a transient radio source with luminosity typical of bright su-
0 pernovae,give furtherevidenceof the ongoingstar formationactivity,which dominatesthe
7
energeticsof the system. At sub-parsec scales, the radio nucleushas a core-jetmorphology
0
withthejetbeinganewlyejectedcomponentshowingasubluminalpropermotionof0.6–1c.
.
1 The AGN contributes less than two per cent of the total IR luminosity of the system. The
0 correspondingEddingtonfactoris∼ 10−3,suggestingthisisalow-accretionrateengine,as
7 often found in LLAGNs. However, its high bolometric luminosity (∼ 1044ergs−1) agrees
1
betterwithanormalAGN.Thisapparentdiscrepancymayjustbeanindicationofthetransi-
:
v tionnatureofthenucleusfromasystemdominatedbystar-formation,toanAGN-dominated
i system.Thenucleushasastronglyinvertedspectrumandaturnoverat∼4.4GHz,thusqual-
X ifyingasacandidatefortheleastluminous(L5.0GHz ∼6.3×1028 ergs−1Hz−1)andoneof
r theyoungest(∼3×103yr)gigahertz-peakedspectrum(GPS)sources.IftheGPSoriginfor
a
theIC883nucleusisconfirmed,thenadvancedmergersintheLIRGcategoryarepotentially
keyenvironmentstounveiltheevolutionofGPSsourcesintomorepowerfulradiogalaxies.
Keywords: galaxies:individual:IC883–galaxies:active–galaxies:nuclei–galaxies:jets
–radiocotinuum:galaxies–X-rays:galaxies
1 INTRODUCTION (AGN),isthemergerofgas-richgalaxies(e.g.,Hopkinsetal.2006;
DiMatteoetal. 2007). These systems are a seed for galaxy evo-
One of the mechanisms in the Universe able to trigger bursts of
lution and are thought to produce luminous and ultraluminous
star formation, as well as the onset of an active galactic nucleus
infrared (IR: 8–1000 µm) galaxies (LIRGs: LIR > 1011 L⊙;
ULIRGs: LIR > 1012 L⊙; Sanders&Mirabel 1996). Owing to
(U)LIRGs nature, both the AGN and the active star formation
⋆ E-mail:[email protected]
therein are deeply embedded in dust, and hence obscured. Radio
† VisitingScientist
(cid:13)c 2017RAS
2 C. Romero-Can˜izales,et al.
observationsprovideuswithanextinction-freeviewofthemerger lengths,wherethestarburstrepresentsthemajorcontributiontothe
products,andthusallowustostudytheirinteraction,theirevolu- globalemission.
tionandtheirinfluenceontheirhosts. InPaperIwealsomodelledtheIRspectralenergydistribution
IC883, also known as UGC8387, IZw056 and Arp193, is (SED)andfoundthatthecontributionfromaputativeAGNtothe
one of the closest (D = 100Mpc) LIRGs, with an IR luminos- IRemissionisnomorethan10percentthatofthestarburst,albeit
ity of ∼ 4.7 ×1011 L⊙ (Sandersetal. 2003). The system con- withinherent largeuncertaintiespredictedbyouralmost edge-on
sistsofa∼4kpcrotatingringofmoleculargasanddustobserved viewofthetorus.
edge-on,andtwolargetidaltails(oneprotrudingorthogonallyfrom InthispaperwepresentrecentVLBIradioobservationsmade
thecentreandanotherextendingalongthemainbodytowardsthe withtheEVN,aswell asarchival X-raydata,aimingat confirm-
South),whichindicatetheoccurrenceofpreviousmergerepisodes ing the presence of an AGN in IC883, and at understanding its
(e.g., Smithetal. 1995; Downes&Solomon 1998; Scovilleetal. nature. We also present archival Karl G. Jansky Very Large Ar-
2000). Clemens&Alexander (2004) found kinematic differences ray(VLA)data whichhas aresolution comparable tothat of our
in HI absorption and 12CO emission which they interpret as evi- e-MERLIN observations reported in PaperI. Our study is made
denceofanoutflowofatomicgasperpendiculartothering,though withintheframeworkofthee-MERLINlegacyprojectLuminous
theyalsonoteinflow(orothernon-circularmotion)couldaccount InfraRedGalaxyInventory(LIRGI;PIs:J.Conway&M.A´.Pe´rez-
forthevelocitydifferences.Itispossiblethatreturningtidalmate- Torres)1,whichwillofferaradiocomplementtotheGreatObser-
rialatsomewhatlargerdistances,butseeninprojectionagainstthe vatoriesAll-skyLIRGSurvey(GOALS;Armusetal.2009)NASA
nucleus,couldalsobetheoriginofthekinematicdisparitybetween program. LIRGI pursues the ambitious goal of characterising the
the12COandHI. nuclearandcircumnuclearradioemissionofastatisticallysignifi-
Veilleuxetal.(1995)classifiedthisgalaxyasalow-ionisation cantsampleof42ofthemostluminousnorthernLIRGs.
nuclear emission-line region (LINER) based on optical spec- Weorganisethemanuscriptasfollows.In§2wedescribethe
troscopy. Posterior analysis suggested its classification as a radio and X-ray observations used, as well as the data reduction
starburst–AGN composite (Yuanetal. 2010). The thermal nature process.In§3weprovidemapsandmeasurementsfromourdata.
ofthesoftX-rayemission,theIRcoloursindicatingthermalemis- In§4weinvestigatethenatureoftheIC883nucleusbasedonits
sion from cold dust (Modicaetal. 2012, and references therein), morphology, fluxdensityvariability, itsradioSEDand theX-ray
thesteepglobalradiospectralindex(−0.64)andthefar-IR/radio emission.Werecapin§5withourfinalconclusion.
fluxratio(q=2.28;Condonetal.1991),amongothercharacteris-
tics,providesolidevidenceofactivestarformationinthesystem.
Infact,IC883liesclosetothewellknownstarburstgalaxyM82in
thediagnostic plotof the6.2 µmPolycyclicAromaticHydrocar- 2 OBSERVATIONS:DESCRIPTIONANDDATA
bonvsthe9.7µmsilicatestrength(withvaluesof0.6and−1,re- REDUCTION
spectively; Stierwaltetal.2013)proposedbySpoonetal.(2007).
2.1 VLBIradiofollow-up
SupportforthepresenceofanAGNinIC883comesfromthe[Ne
V] 14.32 µm emission linedetection in theheavily obscured nu- Withtheaimofcharacterisingthenuclear emissioninIC883we
clearregion(Dudiketal.2009),andthedetectionofcompactradio started an observing program using the EVN under project code
sourcesatmilli-arcsecresolution(Lonsdaleetal.1993;Smithetal. ER030(PI:C.Romero-Can˜izales).Theprogramconsistedofthree
1998;Parraetal.2010).TheAGNhashoweverremainedelusivein yearly epochs from 2012 to 2014 (see details in Table 1) at the
X-raysandmid-IRcontinuumobservations,whereAGNsaretypi- frequency bands centred at ν = 8.4GHz (λ = 3.6cm, or X-
callyuncovered,andhenceitspresencehasbeenjudgeduncertain band),ν = 4.9GHz(λ = 6.1cm,orC-band) andν = 1.6GHz
(e.g., Asmusetal. 2015). We note that the starburst-AGN classi- (λ = 18.7cm, or L-band), using the following stations (loca-
fication means solely that thereis both emission from a starburst tion,diameter): Ef-Effelsberg(DE,100m), Wb-Westerborkarray
andmerger-drivenshocks,whichtogethercanmimicstrongAGN (NL, 14×25m), Jb1-Lovell (UK, 76m), Jb2-MKII (UK, 25m),
emission(Richetal.2014). On-Onsala (SE, 20m at λ < 5cm, and 25m at λ & 5cm),
Papadopoulosetal. (2014) studied the spectral line energy Mc-Medicina(IT,32m),Nt-Noto(IT,32m),Tr-Torun(PL,32m),
distribution (SLED) of CO lines in IC883, plus emission lines Ys-Yebes(ES,40m),Sv-Svetloe(RU,32m),Zc-Zelenchukskaya
of dense gas tracers such as high-J HCN transitions, and found (RU,32m),Bd-Badary(RU,32m),Ur-Nanshan(CN,25m),Sh-
that the system basically lacks a dense molecular gascomponent Sheshan (CN, 25m), Hh-Hartebeesthoek (SA, 26m) and Ro-
(n > 104cm−3).Thisisasomewhatunexpectedresultforasys- Robledo(ES,70m).
temwhichispresumablyundergoingastarburst. Eachobservingsegment(seeTable1)lasted4hrintotal,from
In Romero-Can˜izalesetal. (2012b), henceforth PaperI, we which2.5hrwerespentontarget.Anintegrationtimeof2secwas
presentedradioobservationsofthenuclearandcircumnuclear re- usedinalltheepochs,exceptforsegmentsDandG,whereweused
gionsofIC883usingverylongbaselineinterferometry(VLBI)fa- 1sectoobtainasimilarfieldofviewasfortheotherfrequencies
cilitiesandtheelectronicMulti-ElementRemotelyLinkedInterfer- in the same epochs. We used 8×16MHz sub-bands, each with
ometerNetwork(e-MERLIN)array.Ourobservationsrevealedthe dual polarisation. In the 2012 epoch the observations at 4.9 and
presenceofatleastsixnon-thermalcompactcomponents(labelled 1.6GHz were carried out under project EP076 (segments C and
ascomponentsA1toA6)withintheinnermost100pcdiameternu- D),andsubjecttoacorrelationflawwhichmadethoseobservations
clearregionofIC883(namedcomponentA),likelyconstitutinga unusable.
supernova(SN)factoryincoexistencewithalow-luminosityAGN WereducedthedataintheNRAOAstronomicalImagePro-
(LLAGN)candidate(componentA1).WefoundthattheAGNcan- cessingSystem(AIPS).TakingasastartingpointtheEVNpipeline
didate is powering the radio emission at both circumnuclear and
nuclear scales, as seen with the e-MERLIN and European VLBI
Network(EVN)arrays,unlikethescenariodepictedatotherwave- 1 http://lirgi.iaa.es/
(cid:13)c 2017RAS,MNRAS000,1–13
TheAGNactivityinIC883 3
products,weimprovedthecalibrationconsideringionosphericcor- 2.3.2 Chandraobservations
rections,radiointerferenceremoval,andphaseandamplitudeself-
Weretrievedandre-analysedChandra(Weisskopfetal.2000)ob-
calibration on the phase reference source, J1317+3425. The av-
servationsofIC883(ObsID7811,PI:D.Sanders)madeonUT20
erage peak intensities of this calibrator in the different bands
February2007,andoriginallypublishedbyIwasawaetal.(2011).
were 0.24 ±0.01Jy/beam, 0.32 ±0.02Jy/beam and 0.19 ±
TheobservationwascarriedoutwithACIS-S(Garmireetal.2003)
0.01Jy/beam, with a variability in the peak intensity among
andlastedfor14.2ks.WeperformedthedatareductionusingCIAO
epochsofupto11,18and9percentat8.4,4.9and1.6GHz,re-
v.4.6 and following the standard procedure. We reprocessed the
spectively.Inallsegments,weusedJ1159+2914asafringefinder.
data with CHANDRA REPRO and then extracted the spectra using
Imaging using different weighting regimes was performed
theSPECEXTRACTtool.
both in AIPS and the Caltech imaging programme DIFMAP
To be consistent with the spectral extraction of XMM-
(Shepherdetal.1995),toassesstherobustnessofourresults.
Newton/EPIC PN and MOS, which have much lower spatial res-
Throughout this paper, the uncertainties of the peak intensi-
olution than Chandra, the ACIS-S source spectrum used for the
tiesandfluxdensitiesincludea5percentcalibrationuncertainty
spectral fitting was extracted from a circular region of 10arcsec
added in quadrature to the r.m.s. measured in the image. For the
radius.Thebackgroundspectrumwasextractedfromacircularre-
flux densities we also consider a multiplying factor for ther.m.s.
gionofthesamesizeonthesameCCD,wherenoothersourcewas
term,equaltothenumberofbeamscoveringtheemittingregion.
detected.
2.2 VLAarchivalobservations
WereducedarchivalVLAdataat32.5GHz,whichhasaresolution 3 ANALYSISANDRESULTS
comparabletoourpreviouslyreportede-MERLINobservationsat
3.1 Thepc-scalejetinIC883
7.9GHz(PaperI).Comparingtheradioemissionatthesetwofre-
quencies canoffer further understanding on thenature of the nu- The8.4GHzEVNimagesbenefitfromanexquisiteangularreso-
cleus, given that the LLAGN candidate in IC883 dominates the lutionandreveal for thefirsttimeajet-likecomponent emerging
radioemissionatbothcircumnuclearandnuclearscales. fromtheIC883core(seeFig.1).InTable2weshowtheparam-
IC883wasobservedat32.5GHz(centralfrequency)withthe etersobtainedfromthe8.4GHzimagesconvolvedwithabeamof
VLA on 12, 14 and 15 June 2011 under project AL746 (PI: A. 0.98×0.59mas2 atPA=2.95◦.Forthefluxmeasurementsofthe
K.Leroy),whenthearraywasinitsmostextendedconfiguration. core,weconsideredonlytheemissionwithinthe9σcontourtoex-
These observations have been reported by Barcos-Mun˜ozetal. cludemostoftheextendedemissionthatislikelyassociatedwith
(2016). thejet(seeFig.1).
We used the Common Astronomy Software Application In the 2013 and 2014 epochs the core appears slightly ex-
(CASA;McMullinetal.2007)toreducethedata.Thethreedatasets tended toward the direction opposite to the movement of the jet,
wereprocessedindependentlyusingtheVLApipelineandremov- with the largest morphology distortion in the 2013 epoch. This
ing obvious radio frequency interference features. The phase ref- could hint at the existence of a counter-jet, which we cannot di-
erencesourcewasJ1317+3425,asinourEVNobservations.The rectlydetect, probably due tothe effectsof Doppler de-boosting.
peakintensitydidnotvaryamongdatasetsandwethusproceeded However, we acknowledge that the 2013 observations were sub-
to combine them into a single dataset that allowed us to make a jecttotechnicalproblemswhichaffecteddirectlytheuv-coverage
deeperimage. ofthesource, posingdifficultiesfortheimagereconstructionand
loweringitsfidelity.
Only in the 2014 epoch is it possible to disentangle
2.3 X-rayarchivaldata the jet component from the core, and we detect these two
distinct components with peak positions α (J2000) =
2.3.1 XMM-Newtonobservations core
13h20m35s.3184, δ (J2000) = 34◦08′22′.′352 and
core
IC883 was observed by XMM-Newton (Jansenetal. 2001) on 09 α (J2000) = 13h20m35s.3183, δ (J2000) = 34◦08′22′.′351.
jet jet
January2001for21ks(ObsID0093640401,PI:F.Bauer).Theob- The jet component is thus at a projected distance of 0.62pc
servationswerepublishedbyCarreraetal.(2007).ThePNcamera South-Westfromthecore,andatapositionangle(PA)of∼215◦,
(Stru¨deretal. 2001) was operated in Extended Full Frame mode whichisalmostperpendicular totheIC883ringstructureseenat
and the MOS1 and MOS2 cameras (Turneretal. 2001) in Prime largescaleswithe-MERLINatPA=130◦,measuredfromEastto
Full Window, all using the Medium filter. We processed the data North(PaperI,andreferencestherein).
usingthelatestversionofSASavailable(14.0.0).Sourceandback- IC883 was also observed at 8.4GHz on 15 May 2011 with
ground events were extracted for all detectors using an aperture thevery long baseline array (VLBA), asreported in PaperI. The
radius of 40′′ and selecting background regions on a source-free angular resolution of this observation was twice that of the later
area on the same chip as the target. For the PN camera we se- EVNepochs,henceboththecoreandanypossiblejetcomponent
lected single and double events and for both MOS detectors we wouldhavebeencontainedwithinthesamebeam.Hence,theuse
selected single, double, triple and quadruple events. In all cases oftheVLBAobservations asazero-epochisnotstraightforward.
only events with quality flag= 0 were retained. The resulting However,wenotethatin2011,thefluxdensitywas4.38±0.28mJy
net (source minus background) count rates and exposures were (PaperI),andby2012ithadhalved.Weinferthatthe2014jetcom-
0.015counts/s and 19.5ks for MOS1, 0.013counts/s and 19.6ks ponent must havebeen ejected some timebetween 15 May 2011
for MOS2, and 0.043counts/s and 13.8ks for the PN. Wegener- and2November2012,thusprovidingalowerlimittoitsapparent
atedresponsefilesforeachdetectorusingthe RMFGENand ARF- speedof0.6c.
GENtasksandgrouped thespectrawithaminimumof 20counts Usingthe2012epochasareference,wefindthatthejethas
perbinusingthetaskSPECGROUP. movedatapproximately1.0c.Thisisanupperlimittoitsapparent
(cid:13)c 2017RAS,MNRAS000,1–13
4 C. Romero-Can˜izales,et al.
Table 1. Observation setup for EVN project ER030. Stations in italics either did not observe, or were completely lost due to weather/technical
problems.
ER030 Central Observing Participating
segment frequency(GHz) date stations
- 8.4 02-Nov-2012 Ef,Wb,On,Mc,Nt,Ys,Sv,Zc,Bd,Ur,Sh,Hh,Ro
B 4.9 24-Oct-2013 Ef,Wb,Jb2,On,Mc,Nt,Tr,Ys,Sv,Zc,Bd,Ur,Sh,Hh
C 1.6 31-Oct-2013 Ef,Wb,Jb1,On,Mc,Nt,Tr,Sv,Zc,Bd,Ur,Sh,Hh
D 8.4 04-Nov-2013 Ef,Wb,On,Mc,Nt,Ys,Sv,Zc,Bd,Ur,Sh,Hh
E 4.9 22-Oct-2014 Ef,Wb,Jb1,On,Nt,Tr,Ys,Sv,Zc,Bd,Sh,Hh
F 1.6 30-Oct-2014 Ef,Wb,Jb1,On,Nt,Tr,Sv,Zc,Bd,Sh,Hh
G 8.4 04-Nov-2014 Ef,Wb,On,Nt,Ys,Sv,Zc,Bd,Sh,Hh
Table2.Parametersestimatedfromthe8.4GHzEVNimagesat0.98×0.59mas2,PA=2.95◦.
Epoch Component r.m.s. Size Peakintensity Fluxdensity Luminosity Brightnesstemperature
(µJy/beam) (mas2) P8.4(mJy/beam) S8.4(mJy) L8.4(×1027ergs−1Hz−1) TB(×107K)
2012 core 59.2 <0.98×0.59 2.19±0.12 2.19±0.12 26.19±1.49 >6.64
2013 64.2 0.87×0.57 2.30±0.13 3.11±0.25 37.25±2.96 11.04±0.88
2014 83.7 0.80×0.31 1.86±0.13 2.14±0.24 25.66±2.81 15.15±1.66
2014 jet 83.7 1.37×0.57 0.59±0.09 0.80±0.09 9.54±1.11 1.77±0.21
speed, asinthe2012 epoch wealreadyseesomeextended emis- increasing.Weattributethistotheexpansionoftheemittingregion
sionfromthecoretowardsthedirectionwherethejetcomponent within the beam. From the high-resolution 8.4GHz EVN images
isdetectedin2014. (Fig.1),weknowthereisanemergingjet-likecomponentdepart-
ingfromthecore,andweassociateitsappearancetotheincrease
influx(at8.4GHz)andsizeoftheemittingregionmeasuredinthe
3.2 RadioevolutionoftheIC883nuclearregion
lower-resolutionimages.
IC883 shows compact structure at both 4.9 and 1.6GHz as ob- Thespectralindicesremainquiteconstant(withintheuncer-
servedwiththeEVN.Wehavecreatedimagesforallobservations tainties) from 2013 to 2014, being flat between 4.9 and 8.4GHz
using a common convolving beam of 7 × 6mas2, which is the (αC-X) and inverted between 1.6 and 4.9GHz (αL-C), pinpointing
largestsynthesisedbeamobtainedinanyepoch(correspondingto tosomeabsorptionmechanism.Wealsonotethattheluminosities
the1.6GHzobservationsin2014imagedwithuniformweighting). (Lν) andbrightness temperatures(TB)aretypical ofsynchrotron
This allows us to obtain flux densities from similar regions and emittingsources(Table3).
eases the interpretation of the spectral index behaviour, although As wesee in§3.5, the largeconvolving beam in the images
attheexpenseofincreasingther.m.s.Typicallytheuseofnatural weanalysedinthissection,mightalsoincludecontaminationfrom
weightingimprovesther.m.s.whilstitprovidesslightlylowerres- transient sources. However these sources would represent only a
olution.Here,inadditiontotheuseofnaturalweighting,wehave few percent of the total flux, and thus their contribution is taken
alsousedamuchlargerconvolvingbeamthanthatachievedwith intoaccountwithintheuncertainties.
onlynaturalweighting(especiallyat8.4GHz).Thisiseffectively
taperingthedataandconsequently, itresultsinalargerr.m.s.We
3.3 IC883athighfrequencies
useαtodenotespectralindices(definedbyS ∝να).Theresults
ν
areshowninTable3. Further insight into the nature of the nuclear region (component
At 8.4GHzthere isasignificant increasein theflux density A) can be obtained when comparing our previously reported e-
from2012to2013.Wecaninferthisfromboththehigh-resolution MERLINimageat6.9GHz(PaperI)withtheimagefromVLAA-
imagesat0.98×0.59mas2,PA=2.95◦ (Table2),andthelower- configuration observations at 32.5GHz (see§2.2). The combined
resolutionimagesat7×6mas2(Table3).From2013to2014there VLA dataset from June 2011 was observed somewhat close in
isanapparent increaseinthetotalfluxdensity. Wenotehowever timetooure-MERLINobservations(March2011),andtheircom-
that adding the emission from the core and the jet in the high- parison can thus provide us with a rough estimate for the 6.9 to
resolutionimagesresultsinsimilarfluxeswithintheuncertainties 32.5GHzspectralindexofthenuclearregion.
in 2013 and 2014. Most of the increase in flux we see at lower- Weshow thecontour mapof thenatural weighted VLAim-
resolutionislikelyduetoextendedfluxunrelatedtothejet,which age in Fig. 3. The image has a convolving beam of 89×66mas
isrecoveredbythelargerbeam.At4.9and1.6GHzthereisasig- at PA=77◦ and an r.m.s.= 19µJy/beam. We recover a struc-
nificantincreaseinthefluxdensitiesofabout20percent(seeFig. turesimilartothatobtainedwithe-MERLIN(figure1inPaperI).
2).Suchanincrementcanalsobeattributedtothepresenceofex- We note that from the different knots of emission, only compo-
tendedemission.Thepeakintensitiesareconsistentwithnovaria- nent Ahas a32.5GHzcounterpart at thesame position. Thedif-
tionfrom2013to2014, andseemtobedominatedmostlybythe ferenceintherestofthecomponentsmightarisefromthefactthat
evolutionofthecoreitself. at ν & 30GHz we are tracing thermal radiation (e.g., from HII
At8.4andat1.6GHzitisclearthatthedeconvolvedsizesare regions),whereasatlower frequenciesitisthenon-thermal radio
(cid:13)c 2017RAS,MNRAS000,1–13
TheAGNactivityinIC883 5
Table3.Estimatedparametersfromthematched-beamEVNimagesofthenucleusat7×6mas2.
Epoch-Frequency r.m.s. Deconvolved Peakintensity Fluxdensity Luminosity-Lν Brightnesstemp. Spectral
year-ν(GHz) (µJy/beam) sizes(mas2) Pν(mJy/beam) Sν(mJy) (×1027ergs−1Hz−1) TB(×107K) indices
2012–8.4 77.0 <7×6 2.66±0.15 2.66±0.15 31.86±1.84 >0.11
2013–8.4 75.5 1.68×0.80 4.65±0.24 4.78±0.31 57.19±3.76 6.22±0.41
α =0.20±0.16
2013–4.9 42.2 <7×6 4.30±0.22 4.30±0.22 51.46±2.62 >0.51 C-X
α =2.09±0.09
2013–1.6 26.0 3.97×2.63 0.33±0.03 0.41±0.03 4.89±0.40 1.86±0.15 L-C
2014–8.4 96.8 4.18×1.14 4.79±0.26 5.71±0.39 68.29±4.71 2.09±0.14
α =0.15±0.18
2014–4.9 70.0 2.90×1.89 4.62±0.24 5.26±0.34 62.99±4.03 4.82±0.31 C-X
α =2.10±0.09
2014–1.6 15.0 5.38×3.37 0.35±0.02 0.50±0.04 5.93±0.46 1.30±0.10 L-C
emissionfromSNe,SNremnantsandevenanAGN,thatdominate FFA.Owingtothescarcityofdata,wefocusedoureffortsinesti-
theglobal emission(Condon 1992).Infact,weobtainbrightness matingtheturnoverfrequency(ν )intheSED.
t
temperatures< 103Kforthe32.5GHzemissionblobsclosestto
B1a,B1b,B2aandB2b,whichagreeswithathermalorigin.
The e-MERLIN components B2a, B1a, A and B1b we re- 3.4.1 Synchrotronself-absorption
portedinPaperIareatexactlythesamepositionasthe12CO2–1
clumpsreportedbyZaudereretal.(2016),labelledasC1,C2,C3 Thespectralindexbetween1.6and4.9GHz(αL-CinTable3)isvery
andC4,respectively.Intheirstudytheydeterminedthatcomponent closetothespectral indexexpectedfromSSAinahomogeneous
C3(i.e.,componentAinourradioobservations)isthedynamical source(α=2.5).
centreofthemolecularringwhichshapesthemainbodyofIC883. To construct the SED we can use a bending power-law,
Itisinthiscomponentthatthecore-jetstructureisembedded. whichproducesasmoothtransitionbetweentheoptically-thickand
ToallowapropercomparisonoftheVLAandthee-MERLIN optically-thinpartsoftheSED.Anotherpossibilityistouseabro-
observations, we have convolved the VLA image with the e- kenpower-law,butthisapproachdoesnothaveanempiricalbasis.
MERLIN beam: 165×88masat PA= −11.6◦. In theVLA map, For the bending power-law, we consider α1 = 2.5 for the
at the position of component A we measure a peak intensity of optically-thick part. We do not have information at frequencies
4.86±0.26 mJy/beam and a flux density of 5.14±0.31mJy. where the spectrum becomes completely optically thin, but we
These values are very similar to those reported for component A consider α2 = −0.03 todescribeit.Following Callinghametal.
inPaperIbasedonthee-MERLINmap.Hence,weobtainatwo- (2015),thefluxdensityatthefrequencyνisdescribedby
pointspectral indexbetween6.9and32.5GHzof−0.03±0.05.
Awatsil6slu.a9mlGsionHgbzetwhthaietthdateo-m3M2in.E5aRnGLtHrIaNzd,tihoweeemminiitlftleeir-ratfrhrcoasmtetchcecoomcroeproe(ncheoanmstpAaol,snoaesnantvoAete1rdy) Sν =S0(cid:18)ννt(cid:19)α1(cid:18)1−e−(cid:16)ννt(cid:17)α2−α1(cid:19) (1)
flatspectralindexallthewayfrom8.4to32.5GHz. whereS0isaconstant.
Usingaspectralindexof−0.03±0.05forthecoreandthe We performed a non-linear least-squares minimisation to fit
fluxdensityat8.4GHzfromTable3,wecalculatea32.5GHzflux the bending power-law to the data and solved for S0 and νt. We
densityof4.60±0.44mJyin2013,and5.50±0.54mJyin2014. fitted our EVN data which covers the evolution of the core be-
tween1.6and8.4GHz,andincludedtheexpectedfluxdensityat
32.5GHz,whichprovidesuswithanadditionaldegreeoffreedom
3.4 RadioSEDofthenucleus (dof).Thegoodnessofthefitgivenbyχ2resultedinavalueof3.5
for2dof,andaturnoverat4.44+0.72GHz.TheresultingSEDis
−0.62
InTable2wereportedthe8.4GHzfluxdensitiesmeasuredin2014 shownasasolidlineinFig.4.
forboththecoreandthejet.Thecoreisapproximatelythreetimes Wealsoperformedthefitusingα =−0.7,whichisatypical
2
asluminousasthejet,andthusitsemissionwillbethemajorcon- spectralindexfortheoptically-thinpartoftheSED(Orienti2015).
tributortothetotalnuclearfluxmeasuredat7×6mas2. However, this leads to χ2 values > 20, consequently with a p-
In2012and2013thejetcomponenthasnotbeencompletely value ≈ 0. Weacknowledge that the scarcityof data reduces the
ejected, and we expect that its contribution to the total flux is reliabilityofthefitsandallowsthequestioningoftheassumptions
smaller than inepoch 2014. Thecore willbe thedominant emit- wehavemade,althoughtheseseemtobequitefair.
terin2012.Sincewehaveonlymeasurementsatonefrequencyin Asthebehaviourofthecoreissimilarforthefluxesandspec-
2012,weresorttoconsideringthe2013epochasclosesttoquies- tralindicesinboth2013and2014,forcompleteness,wehavere-
cence,anduseittoinvestigatethenatureofthenucleus. peatedtheaboveprocessforthevaluesmeasuredin2014.Thisled
Thehighluminosities(Lν)andbrightnesstemperatures(TB) toaverysimilarturnoverfrequencyof4.42+0.68GHz.
−0.59
estimatedforthecoreandthejet(Tables2and3),areindicativeof
asynchrotronoriginfortheradioemissionatallfrequencies. We
notethatthespectrumishighlyinvertedanditshowsaturnoverat
3.4.2 Free-freeabsorption
lowfrequencies.Free-freeabsorption(FFA)andsynchrotronself-
absorption(SSA)arethemostcommonlyassumedmechanismsre- There is ongoing star formation activity in the nuclear region of
sponsiblefortheturnover.WethusattemptedtofittheradioSED IC883(see§3.5and4.4),henceinternalFFAcouldbethemecha-
withtheavailabledatain2013consideringbothpureSSAandpure nismresponsibleforproducingtheinvertedspectrumweobserve.
(cid:13)c 2017RAS,MNRAS000,1–13
6 C. Romero-Can˜izales,et al.
2.0 Sν (mJy) Pν (mJy/beam)
02-Nov-2012
6 X-band(8.4GHz)
1.5
5
0.25 pc
1.0 4
3
0.5
5.6
ec 5.4 C-band(4.9GHz)
cs 0.0 5.2
milliar-0.5 445...680
4.4
4.2
-1.0
0.55
L-band(1.6GHz)
0.50
-1.5
0.45
0.40
-2.0 59.2 microJy/beam x (-3,3,5,9,15,27)
0.35
2.0 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 0.30
04-Nov-2013
0.25
0 100 200 300 400 500 600 700
1.5 Time(days)
Figure2.Evolutionoftheradiofluxdensityandpeakintensityofthenu-
1.0
cleus(coreplusjet-likestructure)atthreefrequencybands.Theerrorbars
represent±1σvalues.
0.5
c
e
cs 0.0 Weassumethatthefluxdensityfollows:
r
a
milli-0.5 Sν =S0να2(cid:18)ννt(cid:19)2.1(cid:18)1−e−(cid:16)ννt(cid:17)−2.1(cid:19) (2)
-1.0 (asdescribedbyCallinghametal.2015)whereweusedthesame
notationasin§3.4.1.Sincethespectralindexintheoptically-thin
partofthespectrumisnotknown,weproceededwithα =−0.03
-1.5 2
asin§3.4.1.TheSEDobtainedthiswayisshowninFig.4.From
visual inspectionweinferthatthefitusingFFAisslightlyworse
-2.0 64.2 microJy/beam x (-3,3,5,9,15,27)
thanthatobtainedusingSSA.Infact,weobtainalargerχ2value,
2.0 2.0 1.5 1.0 0.5 0.0 -0.5 -1.004-N-1o.v5-20-124.0 13for2dofandassumingα2 =−0.03,and49.5for2dofandas-
sumingα =−0.7.Theturnoverfrequencyinthiscaseis5.4GHz.
2
Similarvaluesareobtainedwhenfittingthe2014measurements.
1.5
Threeorfourdatapointsarenotsufficienttoassessthereli-
abilityofeitherSSAorFFA,andalthoughwecannot neglectthe
1.0
influenceofFFA,ourfitfavoursSSA,andthusthisistheabsorp-
tionmechanismweadopthereafter.
0.5
c
e
cs 0.0 3.5 TransientradiosourcesinIC883
r
a
milli-0.5 EachofourEVNobservationsachievedanr.m.s.>15 µJy/beam
(see Table 3) which is equivalent to a < 5.6σ detection of a ra-
dio supernova with luminosity ∼ 1 × 1027ergs−1Hz−1 (e.g.,
-1.0
SN1993J).
Toeasethedetectionofradiotransientsinour observations,
-1.5 wecreateddeepimagesfollowingtwopathsinAIPS:(i)combining
epochs intheimage plane; and(ii) imaging theconcatenated uv-
-2.0 83.7 microJy/beam x (-3,3,5,9,15) dataateachfrequency.Theformerpathgaveunreliableresults,as
itenhanced artifactspresent inindividual images. Thelatter path
2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
milliarcsec led to a decrease in the r.m.s. We additionally attempted phase-
only self-calibration to further improve image quality. However,
Figure1.ContourmapsofIC883nuclearregionfrom8.4GHzEVNob- thisprocessfailedtoprovide good solutionsgiventhat ourtarget
servations showingtheevolution oftheradiomorphologyataresolution sourcesarefaint.
of 0.98 × 0.59mas2 at PA=2.95◦. The maps are centred at the posi- Using theconcatenated uv-data wemade naturally-weighted
tion of the core: αcore(J2000) = 13h20m35s.3184, δcore(J2000) =
34◦08′22′.′352.Negativecontoursappearindashedstyle. (cid:13)c 2017RAS,MNRAS000,1–13
TheAGNactivityinIC883 7
Table4.Characteristicsofnaturally-weighteddeepimages.
34 08 23.2 June 2011
(32.5 GHz) B2b
Frequency FWHM,PA r.m.s.
23.0 (GHz) (mas2,◦) (µJy/beam)
100 pc
8.4 1.45×0.77,0.1 48.6
22.8
4.9 3.34×2.00,−4.1 42.8
B1b 1.6 10.14×7.12,2.4 19.1
0) 22.6
0
20 A
on (J 22.4 deepimages,whicharebesttotestdetectionfeasibility.Theseim-
nati agesareshowningrey-scalemapsinFig.5(excludingthe8.4GHz
Decli 22.2 B1a frequencyband)andtheircharacteristicsarelistedinTable4.
Wecreatednew images atthe samespatial resolution asthe
deepimages,butrestrictingthecleaningtotheverycentralregion,
22.0
which is dominated by the core and the jet. We then subtracted
B2a
theseimagesfromtheircorrespondingconcatenateddatasets.This
21.8 resultedinuv-datasetsthatarecore-jetemissionfree,whoseimages
areshownascontoursoverlaidontopofthegrey-scalemapsinFig.
21.6 Cont. = 19 x (-3,3,5,9,15,27,45,81,135) microJy/b 5.Thecontoursrepresenttheresiduals,andweidentifytwoputa-
tivetransientsourceswithaS/N> 5(labelledwithaT),andthus
13 20 35.36 35.34 35.32 35.30 35.28
Right ascension (J2000) correspond to robust detections. We carefully checked that these
detectedsourcesdonot lieontopofsidelobes. Thereisnoobvi-
Figure 3. Contour map of the 32.5GHz VLA image at a resolution of ous emission left after the subtraction of the nuclear emission at
89×66masatPA=77◦.Thepositionofthee-MERLINdetected compo-
8.4GHz. We note as well that previously known nuclear compo-
nents(PaperI)areindicatedwithcrossesofthesizeoftheconvolvingbeam
nents (sources A2, A3 and A4 from PaperI) were not detected,
in the e-MERLIN image (165×88mas at PA= −11.6◦), and identified
henceconfirmingtheirtransientnature.
withtheircorrespondinglabels.
To further test the reliability of the T sources, we produced
natural weighted maps of the individual epochs at the resolution
10.00 indicatedinTable4accordingtothecorrespondingfrequency.The
2013 image fidelity is slightly worse than in the deep images as there
arefewervisibilities,howeverwefindthattheTsourceat4.9GHz
isdetectedonlyinepoch2013,andthe1.6GHzTsourceappears
onlyinepoch 2014. The4.9GHzTsource isstillmarginallyde-
tectedinauniformly-weightedmap,whilstthe1.6GHziswashed
out when using that weighting scheme. We therefore report only
onereliabletransientsourceinepoch2013at4.9GHz,withalu-
minosityof(4.9±1.2)×1027ergs−1Hz−1,basedonthe4.9GHz
y) deepradioimage.Suchaluminosityiscomparabletothatofbright
mJ 1.00 radioSNeattheirmaximumlight.
(
Sν
3.6 XMM-NewtonandChandraresults
ThetoppanelofFig.6showsthethreeXMM-Newtonspectrainthe
0.2–10keVrange:theEPIC/PNspectrumisshowninblueandthe
MOS1andMOS2spectrainblackandredrespectively.Forallfits
SSA(eq. 1) weusedtheXSPECpackageandfitthethreespectrasimultaneously
InternalFFA(eq. 2) withallparameterstied.
0.10 We first tried a simple fit with a power-law model (power-
1 2 3 4 5 6 7 8910 20 30 40
ν (GHz) law) obscured by neutral material using the Tuebingen-Boulder
absorption model for X-rayabsorption bythegas, molecular and
Figure4.Radio spectral energydistribution ofthe corefromcontempo- dustphasesoftheinterstellarmedium(ISM)(tbabs;Wilmsetal.
raneousmeasurementsatthreedifferentfrequencies in2013(circles)and 2000),withahydrogencolumndensityfixedattheGalacticvalue
theestimatedfluxdensityat32.5GHz(square).Twodifferentfitsassuming in the direction of the source, N = 1020cm−2. This model
H
SSA(solidline)andFFA(dashed-line)havebeenappliedtothedataasde- gives a poor fit with χ2 = 130 for 57dof and obvious residu-
scribedinthelegendinsidetheplotandinthetext.Theerrorbarsareata
als peaking sharply at 0.8keV, and overestimates the flux at low
1σlevel.
and high energies. Freeing the column density and maintaining
the powerlaw slope and normalisation free improves the fit sig-
nificantly to χ2 = 74.5 for 56dof. This model is plotted as a
solid line in the top panel of Fig. 6 and the ratio between the
data and the model is plotted in the middle panel. The best fit-
tingvaluesoftheparametersareN =(1.8±0.3)×1021cm−2,
H
(cid:13)c 2017RAS,MNRAS000,1–13
8 C. Romero-Can˜izales,et al.
1 2 3 PN
mJy/beam
V−1 0.01 MOS1
4300 Contour levels = 59.7 microJ4y.9/b GeaHmz x(C (--3b,a3n,d5)) nts s ke−1 10−3 MOS2
u
o
c
20 10−4
2.5
10 o 2
ec rati 1.15
milliarcs-100 T o 02..255
ati 1.5
r 1
-20
0.5
0.5 1 2 5
-30 Energy (keV)
Figure 6. XMM-Newton spectrum of IC883. Middle and lower pan-
-40 els correspond to the data to model residuals, tbabs*powerlaw and
tbabs*(powerlaw+apec),respectively.
40 30 20 10 0 -10 -20 -30 -40
milliarcsec
100 200 300
microJy/beam Chandra (3-8 keV)
1.6 GHz (L-band)
40
Contour levels = 22.0 microJy/beam x (-3,3,5)
30
20
10
ec 10’’
arcs 0 10.2203"
milli-10 0.071 0.08 0.17 1.1
Figure 7. HardX-rays Chandra image centred on IC883. Theunits are
-20
T countssmoothedwithaGaussiankernelofradius3pixels.Thecrossrep-
resentsthepositionoftheradiocore.
-30
-40
theredshiftatthesystemicvaluez = 0.023, givesaslightlybet-
40 30 20 10 0 -10 -20 -30 -40 ter fit with χ2 = 63.6 for 54dof. The hot gas component is al-
milliarcsec
wayssub-dominantbutmakesitslargestcontributionbetween0.7
Figure5.Grey-scalemapsofthe4.9(toppanel)and1.6GHzdeepimages, and 1keV removing a small peak in the residuals at these ener-
withoverlaidcontoursoftheimageswherethenuclearemissionhasbeen gies. The ratio of the data to this model is shown in the bottom
subtracted.PutativetransientsourcesarelabelledwithaT. panel inFig.6,where thepeak at 0.85 keV, most clearlyseen in
the PN (blue points) is better modelled. The best fitting parame-
ters are N = (1.57±0.35) ×1021cm−2, Γ = 1.85 ±0.14,
H
photon index Γ = 2.1 ±0.1, and normalisation (3.7±0.4) × powerlawnormalisation=(2.8±0.4)×10−5photonss−1cm−2
10−5 photonss−1cm−2 at 1keV. These values result in an un- at 1 keV, apec temperature = 0.86 ± 0.14keV and apec nor-
absorbed 2–10keV fluxof8.11×10−14ergs−1cm−2 anda2– malisation(4.6±1.4)×10−6photonss−1cm−2at1keV.These
10keVluminosityL2−10keV =9.75×1040ergs−1. values result in an unabsorbed 2–10 keV flux of the powerlaw
ThehigherangularresolutionChandraimagesofIC883show onlyof9.0+−15..02 ×10−14 ergs−1cm−2 andL2−10keV = 1.07×
aspatiallyextendedemissionregionaroundacentralpointsource 1041ergs−1,similartothevaluereportedbyModicaetal.(2012).
(Fig.7). Thisextended emission isinevitably included in theex- Chandra data were binned to1 count per bin, and we used Cash
traction region of the PN and MOS source spectra. This emis- statistics to fit the ACIS-S spectrum, resulting in C = 72.2 for
sion probably corresponds to hot gas in the galaxy and we ac- 147dof.UsingamodelsimilartotheoneappliedtoXMM-Newton
count for it by adding an optically-thin thermal emission com- dataweobtainacolumndensityofN = 1.8+1.9 ×1021cm−2,
H −1.4
ponent, using the Astrophysical Plasma Emission Code (apec; a photon index of Γ = 1.80+0.78, and a temperature of the
−0.75
Smithetal. 2001). For the XMM-Newton spectrum, fitting the apec of kT = 1.00+0.28keV. All these parameters are consis-
−0.30
modeltbabs*(powerlaw+apec)withfreecolumndensity,pow- tentwiththevaluesobtainedwithXMM-Newton/EPIC.TheChan-
erlawslopeandnormalisation,andapectemperatureandnormal- dra spectrum is shown in Fig.8. The observed 2–10keV flux is
isation,whilekeeping theabundance fixedtotheSolarvalueand (6±3)×10−14 ergs−1cm−2.
(cid:13)c 2017RAS,MNRAS000,1–13
TheAGNactivityinIC883 9
4.2 Anewjetcomponent
0.01
Our EVN observations on 4 November 2014 clearly show a re-
V
e solvedcore-jetstructureinIC883.Thejetwaslikelyejectedsome
k
s/s/ 10−3 timebetween15May2011 and2November 2012 hencemoving
nt awayfromthecorewithanapparentspeedof0.6–1.0c(seeSection
u
o
c 3.1). Such subluminal proper motion iscomparable to the appar-
entvelocitiesofjetsdetectedinLLAGNs(e.g.,Nagaretal.2002a;
10−4
Mezcua&Prieto2014;Argoetal.2015).
4 Wenotethattheappearanceofthejetcomponentwasnotas-
atio sociatedwithastrongradioflareinIC883atleastsince2004,time
r 2 ofthefirstVLBIobservationsatfrequencieshigherthan5GHzre-
portedinPaperIwherethecoreisclearlydetected,thoughevenbe-
forethat,Lonsdaleetal.(1993)reportedanupperlimitat1.49GHz
0.5 1 2 5
Energy (keV) fromglobalVLBIobservationsmadein1991. Thejetisdetected
thankstothehigh-resolutionandhigh-sensitivityobservationsthat
Figure8.Chandra/ACISspectrumofIC883fitwithamodelthatincludes
were made toward IC883 for the first time. Similar observations
anabsorbedpower-lawandcollisionally ionisedthermalcomponent.The
areneededtounveilparsec-scalejetcomponentsinotherAGNsas
bottompanelshowstheratiobetweenthemodelandthedata.
suggestedbyMezcua&Prieto(2014).Weareprobablywitnessing
recurrentAGNactivityinIC883ratherthanitsonset,unlikeinthe
caseofNGC660wheretheradiobrightnessincreasedbyafactor
4 DISCUSSION of>1200,accompanyingtheappearanceofanewcompactsource
andtheproductionofajet(Argoetal.2015).
Thehigh-resolutionradiodatawehavepresentedinthispaperpro-
videunambiguousevidenceoftheexistenceofajetinIC883,and Giventhenuclearfluxdensitylevelin2011(4.38±0.28mJy,
thus,forthepresenceofanAGNinthenucleus.Ontheotherhand, PaperI)anditsdecreaseby2012(seeTable2),wecaninferthere
ouranalysisoftheavailableX-raydataagreesbothwithstarfor- wasprobablyapreviouscomponent similartotheoneweclearly
mationandAGNactivityinIC883.Inthissectionwediscussthe detect in 2014. In PaperI we also reported variability in the flux
observationalevidenceinordertounderstandtheongoingphysical densityofthenucleusat5GHz(labelledascomponentA1inthat
processesinthecoreofIC883. paper)intheunevenlysampledperiodfrom2004to2011.IC883
couldbeproducingjetcomponentscontinuously.However,thejet
componentsdonotseemtoliveverylong,aswedonotseelarger-
scalejets,notevenata2masresolution.
4.1 OriginoftheX-rayemission
ItisthoughtthataverydenseISM(asexpectedinthenuclear
TheoriginoftheX-rayemissionisdifficulttodetermine.The0.2–
region of LIRGs) can halt jets moving through it (Bicknelletal.
10keVspectrumcontainsanoptically-thinthermalcomponentand
2003). The interaction of the jet with the ISM can be probed by
apowerlawwithslopeΓ=1.8.Theformercanbeproducedbya
meansofHIabsorptionobservations.However,thepossibilityex-
collisionallyionisedplasmainastarformingregionwhilethesec-
ists that HI absorption also probes foreground molecular clouds
ondcanbeproducedbyarelatedpopulationofhigh-massX-raybi-
whichhappentolieinthelineofsight(Conway1999),asseemsto
naries.Inthiscase,theentireX-rayemissioncanbeascribedtostar
bethecasehere(see§1).InIC883,theHIabsorptionreportedby
formation.Alternatively,alowluminositySeyfertspectrumcould
Clemens&Alexander(2004)isdisplacedby0.2arcsec(≈100pc)
producethepowerlawemissionandphoto-ioniseitssurroundings
fromcomponentA(Zaudereretal.2016),andhenceitisnotprob-
toproducemostoftheemissionbelow2keV,whichisalsoconsis-
ingdirectlythecore-jetregion.
tentwiththeopticalLINERclassificationofthissource.
Taking into account the background-corrected Chandra Papadopoulosetal. (2014) report a deficit of dense gas to-
countsinthe2–8keV(H)and0.5–2keV(S)bands,thehardness ward the dynamical centre region (which corresponds to the nu-
ratio (HR = (H −S)/(H +S)) for IC883 isHR = −0.56. clear region as we have seen at radio frequencies). Furthermore,
This value is below the threshold set by Iwasawaetal. for AGN Modicaetal.(2012)foundevidenceofanuclearstellarwindbased
(HR > −0.3),andisconsistentwiththeaverageoftheGOALS onthesoftX-rayemissionandmorphology.Infact,thesoftX-ray
sample(HR = −0.56,Iwasawaetal.2011).However,aslopeof spectrumcanbewellreproducedbyacollisionallyionisedplasma,
Γ=1.8,asmeasuredhere,isalsoconsistentwithalow-accretion whichisatypicalsignatureofstarformation.Wecanexcludethe
rateAGN(e.g.,Fanalietal.2013). presence of a strong X-ray jet, since its radio counterpart is not
The X-ray to star formation rate (SFR) correlation of strong. Is the jet clearing its way through the surrounding gas?
Ranallietal.(2003),consideringtheSFRofIC883(185 M⊙yr−1, Probablynot,asitismorelikelythatthestarformationfeedbackis
PaperI),showsthattheexpected2–10keVluminositysolelydueto responsibleforthegasdispersal.
starformationwouldbe∼9.45×1041 ergs−1,whichis10times
higherthanthemeasuredX-rayluminosityofthesource.Thisdis- Whatisthenhinderingtheproductionoflargescaleradiojets
crepancy can berelatedtoahigh obscuration of thestar forming andkeepingthecentralradiosourceconfinedtoverysmallregions?
regionitself.Furthermore,thehighX-rayfluxexpectedwouldsug- The reason could simply be that the AGN is producing weak jet
gestthatmostoftheobservedX-rayemissionarisesfromthestar components which arenot powerful enough tobreak through the
formation. denseISM.
(cid:13)c 2017RAS,MNRAS000,1–13
10 C. Romero-Can˜izales,et al.
4.3 AGigahertzPeaked-Spectrumgalaxycandidate 0.15in2014.ThisistheturnoverregionoftheradioSED.Wehave
also determined that at higher frequencies, the core has a much
TheIC883nucleushasaspectralindexbetween1.6and4.9GHz
flatterspectralindex(−0.03±0.05),whichholdsatleastfrom8.4
(in2013and2014)veryclosetothespectralindexexpectedfrom
to32.5GHz(see§3.3).Usingtheseaslowerandupperfrequencies,
synchrotron self-absorption inahomogeneous source (α = 2.5), wecalculatearadioluminosityof (1.3±0.2)×1039ergs−1 in
with a turnover frequency of 4.44+−00..7622GHz. This characteris- 2013and(1.6±0.2)×1039ergs−1 in2014.Wethusdetermine
tic, together with the compactness of the nucleus (at a few mas-
amagnetic fieldstrength of 2.6±0.2mG in2013, anda similar
scale),aretypicalfeaturesoftheso-calledGigahertz-PeakedSpec-
valuein2014thatdiffersbyonly5percent.
trum(GPS)sources(e.g.,O’Deaetal.1991;deVriesetal.1997;
Tyul’Bashev(2001)obtainedmeanmagneticfieldstrengthsof
Snellen2009),whichalsosharecommonpropertieswithLLAGNs
theorderof1–10mGforasampleofGPSsources,ingoodagree-
(Nagaretal. 2002b). GPS sources are important as they repre-
mentwiththeB valuescompiledbyO’Dea(1998)fromthelit-
sentthefirststagesofpowerfulradiogalaxies(e.g.,O’Dea1998; eq
erature. The value we estimated for IC883 is consistent with the
Snellenetal.2000,andreferencestherein).
valuesreportedbythoseauthors.Itisremarkablethateventhough
GPS galaxies have typical luminosities at 5GHz above
IC883isalsoanadvanced merger,itsmagneticfieldstrengthex-
1031 ergs−1Hz−1 (1024Ws−1). This is about three orders of
ceedsbyatleastanorderofmagnitudethatofothersourcesbelong-
magnitude higher luminosity than that displayed by IC883 (see
ing to the same class (Momjianetal. 2003; Drzazgaetal. 2011;
Table3).However,itisknownthatcurrentGPScataloguesarebi-
Romero-Can˜izalesetal.2012a).
ased toward bright sources at redshifts > 0.1. The discovery by
Wecannowcalculatethelifetimeoftheelectronswithmini-
Tingayetal. (1997) of the nearest GPS galaxy at the time posed
mumenergymovinginsuchamagneticfieldandatafrequencyν
thequestionwhetherthelow-luminosity,low-redshiftGPSsources c
(consideredhereas8.4GHz)as:
arehidingamongorwithinLINERsources(O’Dea1998).Infact,
Tingayetal.(2003)presentedthreenearbyGPSsourceshostedby τ ν B 3 −1/2
elliptical galaxies which can be thought of as IC883 “cousins” syn ≈1.06×103 c eq (4)
basedonthecommonpropertiessharedbytheirhosts:(i)showing (cid:18)Myr(cid:19) "(cid:16)GHz(cid:17)(cid:18)µG(cid:19) #
strong evidence of interacting/merging activity; (ii) showing op- (Pacholczyk1970),resultinginanageof(2.8±0.3)×103yr,which
tical spectra identifying themas LINERs;(iii)being low-redshift
isatthelowerendofthetypicalagerangeforGPSgalaxies.Anes-
galaxies(<100Mpc)withluminositiesbelowtheaverageforGPS timateoftheelectronlifetimeaccordingtovanderLaan&Perola
galaxies(see alsoHancocketal.2009; Tingay&Edwards2015), (1969)leadstobasicallythesameresult(t =3.1×103yr).Inthe
e
althoughIC883remainstheleastluminousone. following,weadoptanageof∼3×103yrfortheIC883core.
Ithasbeensuggestedthatthecompactness(onsub-kpcscales)
of GPS galaxies is due to interaction with very dense surround-
ing gas, which is responsible for hindering the jet growth (e.g., 4.4 Ongoingstarformationinthesystem
Bicknelletal.1997),althoughthereisnowmountingevidencethat
In PaperI we estimated a core-collapse SN (CCSN) rate of
their sizes can be explained in terms of their age (O’Dea 1998;
Snellen2009).Therearedifferentmethodstodeterminetheageof 1.1+−10..36yr−1andaSFR=185M⊙yr−1forIC883.Suchhighrates
makeIC883aprimetargetforCCSNsearchesusinghigh-spatial
GPSsources.Forinstance,usingthecurvatureofthesynchrotron
resolutionradioandnear-IRobservations.
spectrum inasample of compact lobe-dominated objectsand as-
WehaveconcatenatedalltheEVNepochstoproducedeepim-
suming equipartition magnetic fields, Murgia (2003) found spec-
tralagesbetween102–105yrforsuchobjects.Polatidis&Conway agesofIC883atdifferentfrequenciestofacilitatetheSNsearch.
Wedetectedatransientsourceat4.9GHzin2013withaluminos-
(2003)measuredthesubluminalpropermotionofhotspots,andde-
itycomparabletothatofluminousradioSNeandgivefurtherevi-
terminedkinematicageswithinthesamerange.However,Murgia
denceoftheongoingstarformationactivityinthenuclearregions
(2003)notedthatwhenthemorphologyoftheGPSsourceisdom-
ofIC883.
inatedbystrongjetsorhotspots,thespectralagewillbealower
IfthetransientisindeedaSN,itseemstoevolvefast,aswe
limittothesourceage.
donotdetectitinconsecutiveyearlyepochsanditappearsatonly
InthecaseofIC883thereisnoavailableinformationabout
onefrequencyatatime.Anotherpossibilityisthatthisisaslowly
thefrequencyatwhichthespectrumbreaksandbecomessteep(i.e.,
evolvingSNanditappearstobecaughtclosetoitsmaximumlight,
whereradiationlossesdominate)andthuswecannotdeterminethe
andhadalreadyfallenbelowthedetectionlimitinatimespanofa
spectralage.InanattempttoestimatetheageoftheAGNinIC883,
year.
weconsidertheformulationforthemagneticfieldandsynchrotron
On account of IC883’s distance, the discovery of transients
timegivenbyPacholczyk(1970),
intheradio isbiased toonly thebrightest SNe. SNe2010cu and
2011hiexplodedinthecircumnuclearregionsofIC883andwere
(cid:18)BµeGq(cid:19)≈8.1"(1+φk)(cid:18)1c0127(cid:19)(cid:18)1kRpc(cid:19)−3(cid:18)1039LeRrgs−1(cid:19)#2/7 (3) dIIePteecvteendtisnbtahseendeoanr-IthRe(irKnaenakra-IrReeltigahl.t-2c0u1r2ve).sTahnedseraadrieoluikpepleyrTlyimpe-
whereφisthefillingfactoroffieldsandparticles,kistheratioof its(Kankareetal.2012,PaperI).TypeIIPSNecaneasilygounno-
heavyparticleenergytoelectronenergy,adoptingforsimplicitythe ticedatradiowavelengthsastheytypicallyhavelowpeakluminosi-
valuesof0.5and100,respectively(seee.g.,Beck&Krause2005). ties(. 1026 ergs−1Hz−1).Thiswouldimplythatthetransients
R is the linear size of the radio emitting region, with a value of wedetect(reportedinPaperI,andinthispaper),iftheyareSNe,
7.3pc(15mas),asmeasuredfromtheEVNimagesat7×6mas2. areeitherTypeIb/c(whicharebrightandevolvefast),orarerela-
c isafunctionoftheupperandlowerfrequenciesconsideredand tivelybrightTypeII’scaughtclosetotheirmaximumlight,falling
12
theircorrespondingspectralindex,whichinthiscaseis∼ 7.76× below our detection limits rather soon. In both cases, we do not
106. expecttodetecttheminconsecutiveyearlyepochs.
From Table 3, we see that α varied from 0.20 in 2013 to We do not detect non-thermal milliarcsec radio sources off
C-X
(cid:13)c 2017RAS,MNRAS000,1–13