radio-excess signature of hidden AGN activity in distant star-forming galaxies

28 Pages·2012·2.95 MB·English

A&A549,A59(2013) Astronomy
DOI:10.1051/0004-6361/201219880 &
(cid:13)c ESO2012 Astrophysics
GOODS-Herschel: radio-excess signature of hidden AGN activity
in distant star-forming galaxies(cid:63)
A.DelMoro1,D.M.Alexander1,J.R.Mullaney1,2,E.Daddi2,M.Pannella2,F.E.Bauer3,4,A.Pope5,M.Dickinson6,
D.Elbaz2,P.D.Barthel7,M.A.Garrett8,9,10,W.N.Brandt11,V.Charmandaris12,R.R.Chary13,K.Dasyra2,14,
R.Gilli15,R.C.Hickox16,H.S.Hwang17,R.J.Ivison18,S.Juneau19,E.LeFloc’h2,B.Luo11,G.E.Morrison20,
E.Rovilos1,15,M.T.Sargent2,andY.Q.Xue11,21
(Affiliationscanbefoundafterthereferences)
Received25June2012/Accepted28September2012
ABSTRACT
Context. A tight correlation exists between far-infrared and radio emission for star-forming galaxies (SFGs), which seems to hold out to high
redshifts(z ≈ 2).Anyexcessofradioemissionoverthatexpectedfromstarformationprocessesismostlikelyproducedbyanactivegalactic
nucleus(AGN),oftenhiddenbylargeamountsofdustandgas.Identifyingtheseradio-excesssourceswillallowustostudyapopulationofAGN
unbiasedbyobscurationandthusfindsomeofthemostobscured,Compton-thickAGN,whichareinlargepartunidentifiedeveninthedeepest
X-rayandinfrared(IR)surveys.
Aims. We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant
star-forminggalaxiesintheGOODS-Herschel(North)fieldandtorevealthepresenceofhidden,highlyobscuredAGN.
Methods.ThroughextensiveSEDanalysisof458galaxieswithradio1.4GHzandmid-IR24µmdetectionsusingsomeofthedeepestChandra
X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN
(∼1300deg−2)outto redshiftz ≈ 3.Theseradio-excess AGNhaveasignificantlylowerfar-IR/radioratio(q < 1.68,3σ)thanthe typicalre-
lationobservedforstar-forminggalaxies(q≈2.2).
Results.Wefindthat≈45%oftheseradio-excesssourceshaveadominantAGNcomponentinthemid-IRband,whilefortheremaindersthe
excessradioemissionistheonlyindicatorofAGNactivity.ThepresenceofanAGNisalsoconfirmedbythedetectionofacompactradiocorein
deepVLBI1.4GHzobservationsforeightofourradio-excesssources(≈16%;≈66%oftheVLBIdetectedsourcesinthisfield),withtheexcess
radiofluxmeasuredfromourSEDanalysisagreeing,towithinafactoroftwo,withtheradiocoreemissionmeasuredbyVLBI.Wefindthat
thefractionofradio-excessAGNincreaseswithX-rayluminosityreaching∼60%atL ≈1044−1045ergs−1,makingthesesourcesanimportant
X
part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data,
suggestingthatsomeofthesesourcesmightbeheavilyobscured.Amongsttheradio-excessAGNwecandistinguishthreegroupsofobjects:i)
AGNclearlyidentifiedininfrared(andofteninX-rays),afractionofwhicharelikelytobedistantCompton-thickAGN;ii)moderateluminosity
AGN(L (cid:46) 1043 ergs−1)hostedinstrongstar-forminggalaxies;andiii)asmallfractionoflowaccretion-rateAGNhostedinpassive(i.e.weak
X
ornostar-forming)galaxies.Wealsofindthatthespecificstarformationrates(sSFRs)oftheradio-excessAGNareonaveragelowerthatthose
observedforX-rayselectedAGNhosts,indicatingthatoursourcesareformingstarsmoreslowlythantypicalAGNhosts,andpossiblytheirstar
formationisprogressivelyquenching.
Keywords.galaxies:active–quasars:general–infrared:galaxies–galaxies:starformation–X-rays:galaxies
1. Introduction inordertounderstandthenatureofthelinkbetweenSMBHand
galaxies and their cosmic co-evolution. This paper aims to ex-
The discovery of a strong correlation between the properties
pandourknowledgeoftheAGNpopulation,byselectingobjects
of galaxies and those of the supermassive black holes (SMBH)
withradioemissioninexcessofthatexpectedfromstarforma-
hostedintheircentres,suchastheM −M orM −σrela-
BH bulge BH tion. As discussed below, this method selects many AGN that
tions(Magorrianetal.1998;Ferrarese&Merritt2000;Gebhardt
cannot be identified using other established techniques, and so
etal.2000;Marconi&Hunt2003),haspointedoutthatSMBHs
movesusclosertoacompletecensusofgrowingSMBHsinthe
mustplayanimportantroleinthegrowthandevolutionofgalax-
Universe.
ies(seeAlexander&Hickox2012,forageneralreview).Inthe
DeepX-raysurveyshaveprovedtobeaverypowerfultool
past decades many studies have focussed on understanding the
indetectingobscuredandunobscuredAGNdowntofaintfluxes
relation between nuclear activity (AGN) and host galaxies and
and to high redshifts (z ∼ 5; e.g. Alexander et al. 2001; Fiore
haverevealedacommonhistory,wherebothstarformationand
et al. 2003; Hasinger 2008; Brusa et al. 2009; Xue et al. 2011;
blackholeaccretionweremuchmorecommoninthepast,witha
peakatredshiftz≈2(e.g.Fioreetal.2003;Merlonietal.2004; Lehmer et al. 2012). However, it is now evident that even the
deepest X-ray surveys are not complete (e.g. Tozzi et al. 2006;
Marconietal.2004;Hopkinsetal.2006b,2007;Merlonietal.
Hasinger 2008) and miss a significant part of the AGN pop-
2007).CompletingthecensusofAGNactivity,especiallyatred-
ulation, in particular the most obscured, Compton-thick (CT)
shiftswheremostoftheaccretionoccurred,isthereforeessential
AGNs,wheretheX-rayemissionbelow10keVisstronglysup-
(cid:63) Tables1,3andAppendicesareavailableinelectronicform pressed by large column density gas (NH > 1024 cm−2). A
athttp://www.aanda.org largepopulationofheavilyobscuredAGNisindeedpredictedby
ArticlepublishedbyEDPSciences A59,page1of28
A&A549,A59(2013)
synthesismodelsoftheX-raybackground(XRB;Comastrietal. radiation from cosmic ray electrons and positrons, accelerated
1995; Gilli et al. 2001, 2007; Treister et al. 2009; Ballantyne by supernova remnants, which mainly occur in young stellar
et al. 2011) in order to reproduce the high energy peak of the populations in star-forming regions (see Condon 1992, for a
observedX-raybackgroundemission(E ≈ 30keV),whichhas review). The radio emission observed in star-forming galaxies
notyetbeendirectlyresolvedbycurrentX-raysurveys. tightly correlates with the emission in the far-infrared (FIR;
Since large amounts of gas and dust are responsible for the λ ≈ 40−120 µm) band, since they both originate from star for-
suppression of the radiation in the UV, optical and soft X-ray mation processes (e.g. Helou et al. 1985; Condon 1992; Yun
bands, perhaps the most obvious waveband to search for these et al. 2001; Appleton et al. 2004; Ivison et al. 2010). This cor-
heavily obscured objects is where the dust emission peaks, i.e. relation, observed primarily in local star-forming galaxies and
theinfrared(IR)band.Infact,thedustsurroundingtheSMBH starbursts,isfoundtoholdouttohighredshifts(z≈2;e.g.Ibar
is heated by the nuclear radiation, reaching temperatures T ∼ et al. 2008; Sargent et al. 2010b; Ivison et al. 2010; Mao et al.
200−1000 K, and re-emits the radiation predominantly in the 2011;Bourneetal.2011).
mid-infrared(MIR;λ≈5−40µm)band,peakingat∼20−30µm Joint analyses in the FIR and radio bands, therefore, allow
(Netzer et al. 2007; Mullaney et al. 2011). Moreover, at these us to separate star-forming galaxies from the AGN population.
wavelengths the effects of extinction are small, making it eas- AlthoughtheweakestradioAGN(RQ)havebeenfoundtofol-
ier,intheory,tofindeventhemostobscuredAGN(e.g.Gandhi lowthesameFIR/radiocorrelationofstar-forminggalaxies(e.g.
et al. 2009; Goulding et al. 2012). The downside of using the Moric´ etal.2010;Padovanietal.2011),giventheirwiderange
IR band to search for AGN activity is that dust is present not in radio-loudness, it is possible to identify AGN via their devi-
only in the circumnuclear region of AGN, but also in the host ationfromtheexpectedFIR/radiorelation,theso-called“radio-
galaxy, in particular in star-forming regions. The dust in these excess” sources (e.g. Roy & Norris 1997; Donley et al. 2005).
regions is heated on average to lower temperatures, and there- Sincetheradioemissionisnotaffected(oronlylightlyaffected)
foreitsemissionpeaksatlongerwavelengths,thanthataround by extinction, the radio-excess source selection can potentially
theblackhole(typicallyatλ ≈ 100µm,T ∼ 20−50K;e.g. identify AGN that are often missed in optical or even deep
dust
Calzettietal.2000;Chary&Elbaz2001).Howeverstarforma- X-raysurveys(i.e.themostobscuredCompton-thickAGN;e.g.
tionoftendominatesthespectralenergydistribution(SED)over Donleyetal.2005).
the entire IR band (e.g. Elvis et al. 1994; Richards et al. 2006; Inthiswork,wecombinetwomethodstoidentifythepres-
Netzeretal.2007;Mullaneyetal.2011)anditisoftennottrivial ence of AGN in star-forming galaxies out to high redshift
toseparateitfromtheAGNemission. (z≈3): i) detailed IR SED decomposition, which allows us to
Potentially a very powerful approach to obtaining an unbi- measuretheAGNcontributiontothetotalSED,typicallydom-
ased look at obscured and unobscured AGN is through radio inated by star formation emission; ii) radio-excess signature
observations. In fact, at radio frequencies, where the emission compared to the typical FIR-radio relation observed for SFGs,
is mainly due to non-thermal processes, such as synchrotron which is most likely due to the presence of nuclear activity.
radiation, the effects of extinction are negligible. Historically, We investigate here the radio-excess sources in the GOODS-
AGNdetectedinradiosurveyshavebeendividedintotwomain North field, using deep infrared, radio and X-ray data, which
classes: i) radio-loud (RL) AGN, which are the strongest radio are some of the deepest data available to date. The paper is or-
emitters (typically L (cid:38) 1024−25 W Hz−1; Miller et al. 1990; ganisedasfollows:thedataandcataloguesusedinourinvesti-
rad
Yunetal.2001)andshowstrongextendedradioemission,such gation are presented in Sect. 2. In Sect. 3, our SED fitting ap-
as kpc-scale relativistic jets and lobes, and ii) radio-quiet (RQ) proach is described in details as well as the definition of the
AGN, the weaker radio emitters, whose radio emission is con- FIR-radio flux ratio (q) and the radio-excess sample selection,
fined in a small, unresolved region (≤0.1 pc; “core”); the lat- togetherwithacomparisonwithotherselectioncriteriausedin
tergroupconstitutesthemajorityofthepopulation(∼90%;e.g. previousstudies.InSect.4,weinvestigatetheX-ray,radioand
Miller et al. 1990; Stocke et al. 1992). The separation between IRpropertiesofourradio-excessAGNsampleandtheirSEDs.
radio-loudandradio-quietAGNhastypicallybeensetatR=10, In Sect. 5 we discuss the mixed population found amongst our
whereR,theradio-loudnessparameter,isdefinedastheratiobe- radio-excess AGN, attempting to constrain the fraction of can-
tween the monochromatic flux density in the radio and optical didateCompton-thickAGN;wealsoexaminethestarformation
bands1 R = S /S (e.g.Kellermannetal.1989;Laor2000). propertiesoftheradio-excessAGNhoststhroughtheirspecific
rad opt
However,morerecentstudiesbasedondeepradiosurveyshave star-formationrate(sSFR)incomparisonwiththoseofX-rayse-
shownthatwhiletheradio-loudnessparameterspansaverywide lectedAGNhosts.InSect.6wesummariseourresultsandgive
rangeofvaluesforAGN,thereisnoclearevidenceofbimodal- our conclusions. In Appendix A, the tests performed to refine
ity in the population (e.g. White et al. 2000; Brinkmann et al. ourSEDfittingapproachareexplainedandthebest-fitSEDsfor
2000; Cirasuolo et al. 2003; La Franca et al. 2010). AGN can theentireradio-excesssampleareshowninAppendixB.
thereforebeidentifiedintheradiobandwithawidedistribution Throughoutthepaperweassumeacosmologicalmodelwith
ofradiopower. H0 = 70 km s−1 Mpc−1, ΩM = 0.27 and ΩΛ = 0.73 (Spergel
etal.2003).
While at bright fluxes the radio population is almost en-
tirely composed of AGN, at low radio fluxes (sub-mJy regime)
star-forminggalaxies(SFGs)constituteasignificantfractionof 2. Observationsandcatalogs
theradiosourcepopulationandbecomedominantatµJyfluxes
(e.g. Seymour et al. 2008). The non-thermal radio continuum The Great Observatories Origins Deep Survey-North field
observed in star-forming galaxies is produced by synchrotron (GOODS-N;Giavaliscoetal.2004)isoneofthedeepestmulti-
wavelength surveys currently available and it constitutes an
1 Mostrecently,otherdefinitionsoftheradio-loudnessparameterhave unprecedented resource in terms of its broad-band coverage
beenused,e.g.R = log(νL /L ),whichusesthemonochro- and sensitivity. It covers ≈160 arcmin2 centred on the Hubble
matic radio lumiXnosity and hraadrd2X−1-0rkaeyV luminosity (e.g. Ballantyne Deep Field North (HDF-N, 12h36m, +62◦14(cid:48); Williams et al.
2009;LaFrancaetal.2010). 1996) and it includes very deep X-ray Chandra data (2 Ms;
A59,page2of28
A.DelMoroetal.:Radio-excesssourcesinGOODS-Herschel
Alexanderetal.2003), optical Hubble Space Telescope (HST; The data reduction was performed following the procedure
Giavaliscoetal.2004)andmid-infrared(MIR)Spitzerobserva- described in Berta et al. (2010) and the resulting images have
tions(PI:M.Dickinson);theGOODS-Nfieldhasalsobeenthe pixelscalesof1.2(cid:48)(cid:48) and2.4(cid:48)(cid:48) atPACS100µmand160µm,re-
target of several deep optical imaging and spectroscopic cam- spectively and 3.6(cid:48)(cid:48), 5.0(cid:48)(cid:48) and 7.2(cid:48)(cid:48) at SPIRE 250 µm, 350 µm
paigns from 8–10 m ground-based telescopes. Recently, new and 500 µm. Since the 350 µm and 500 µm data suffer from
deep observations of this field in the far-infrared (FIR) band strongsourceblendingduetothelargepixelscales,wedidnot
withHerschel(Elbazetal.2011)andtheradiobandwithVLA include data at these wavelengths in our SED fitting procedure
(Morrison et al. 2010) have usefully increased the potential of (Sect. 3.1) and we only used the PACS 100 µm and 160 µm
theGOODS-Ndataset. and SPIRE 250 µm data. The 350 µm and 500 µm flux densi-
tiesand/orupperlimits(S ≈ 20.0mJyandS ≈ 30.0mJy,
350 500
5σ) from the catalogue described in Elbaz et al. (2011) were
2.1. SpitzerMIRdata
only used in some of the plots (see e.g. Fig. B.1) to verify the
TheGOODS-NfieldhasbeenobservedatMIRwavelengthsby accuracyofourSEDfittingattheselongerwavelengths.
Spitzerat3.6,4.5,5.8and8.0µmwithIRAC(Fazioetal.2004), TheHerschel100µm,160µmand250µmfluxeswerecal-
withameanexposuretimeperpositionof≈23hperband,and culated using PSF fitting at the positions of the Spitzer-MIPS
at24µmwithMIPS(Riekeetal.2004),aspartoftheGOODS 24µmsources,whichareusedaspriors(Daddietal.,inprep.).
Spitzer Legacy program (PI: M. Dickinson). The source cata- In the resulting catalogue 819 sources (∼42%) have S/N > 3
logue was produced using the SExtractor source detection rou- in at least one of the Herschel bands: 633 sources at 100 µm,
tine (Bertin & Arnouts 1996) on a combined 3.6 µm + 4.5 µm 537 sources at 160 µm and 435 at 250 µm, with 5σ (3σ) sen-
image,withmatchedaperturephotometryperformedinthefour sitivity limits of ∼1.7 (∼1.2) mJy, ∼4.5 (∼2.3) mJy and ∼6.5
IRACbandsindividually(Dickinsonetal.,inprep.).Theresult- (∼4.0) mJy, respectively. It is important to note, however, that
ing IRAC catalogue includes 19437 objects detected at 3.6 µm thesensitivityofthe250µmdatastronglyvariesacrossthefield,
witha∼50%completenesslimitof0.5µJy. depending on the local source density of the 24 µm priors. We
The 24 µm observations consist of a final mosaic image of included in our sample all of the 1943 24 µm detected sources
1.2(cid:48)(cid:48)pixelscaleanda5σsensitivitylimitof∼30µJy.Thesource within the GOODS-Herschel field (restricted to the area with
extractionwasperformedwithaPSFfittingtechniqueusingthe deeperMIPSdata),withorwithoutasignificantHerscheldetec-
positionsoftheIRAC3.6µmsourcesdetectedat>5σaspriors tion;thisistoavoidbiasesagainstfaintFIRsources,whichare
(seeMagnellietal.2011,fordetails).TheIRAC3.6µmdatais morelikelytobeAGNdominated.
used to define the source priors because it is ∼30 times deeper
than the 24 µm observations, and therefore all real 24 µm de-
tected sources should also be detected at 3.6 µm. The resulting 2.3. VLAradiodata
24µmcatalogueincludes2552sourcesdetectedwithsignal-to-
Deep,high-resolutionradioobservationsoftheGOODS-Nfield
noiseratio(S/N)>3intheGOODS-Nfield.However,wenote
were taken at 1.4 GHz using the National Radio Astronomy
that in the outer regions of the GOODS-N field the MIPS data
Observatory’s(NRAO)VeryLargeArray(VLA)intheA,B,C
isshallowerandtheuncertaintiesonthesourcefluxesaretypi-
andDconfigurations(165h).Thecombinedradioimagereaches
callylarger;wethereforelimitourcataloguestoasmallerarea
armsnoiselevelof∼3.9µJybeam−1nearthecentrewithabeam
(∼135arcmin2)withintheGOODS-NfieldwheretheMIPSdata
sizeof∼1.7(cid:48)(cid:48).Theseareamongstthedeepestradiodatatakenso
is deeper (1943 sources detected at 24 µm, ∼76%). We require
far. From the VLA image the radio flux density measurements
at least a detection (S/N > 3) at 24 µm for the sources in our
havebeenobtainedthroughPSFfittingateach3.6µmsourcepo-
sampletobeabletoconstrainthesourceSEDsintheMIRband
sition(coincidingwiththe24µmpositions;seeSect.2.1;Daddi
(seeSect.3.1).
et al., in prep.): 1.4 GHz flux measurements were obtained for
An area of ∼150 arcmin2 of the GOODS-N field has also
all1943sourcesdetectedat24µmwithintheGOODS-Herschel
been surveyed at 16 µm using the Infrared Spectrograph (IRS)
area,with489sourceshavingS/N >3andS >13µJy.There-
peak-upimaging(PUI)withpointingsof∼10mineach.Theob- ν
maining1454sourceshaveS/N < 3andtheyareconsideredas
servations and data reduction are described in detail by Teplitz
radioupperlimits.Withinthisfield,weestimatedthat∼20radio
etal.(2011);theresultingmosaicimageischaracterisedby0.9(cid:48)(cid:48)
detected sources (5σ) are undetected at 24 µm down to a flux
pixel scale and has an average 5σ depth of ∼40 µJy (Teplitz
limitofS ≈ 21µJy;thisgivesanestimateofthecompleteness
etal.2011).ThesourcecataloguewasconstructedusingSpitzer- ofourradνioand24µmdetectedsample(hereafterVLA/24µm
MIPS 24 µm priors (>5σ sources) and the 16 µm fluxes were
sample) of ∼93%, as compared to a pure radio-selected sam-
calculated through PSF-fitting, similarly to the procedure used
ple.WenotethatthedetectionlimitadoptedforourVLAradio
for the 24 µm data; the 16 µm catalogue contains 770 sources
catalogue is lower than that used in Morrison et al. (2010) (5σ
(Daddietal.,inprep.).
detection threshold) and therefore the number of radio detec-
tionsfoundhere(489sources)ismuchlargerthanthatfoundby
2.2. GOODS-HerschelFIRdata Morrisonetal.(2010)overthesamearea(256sources).
The GOODS-N field has been observed by the Herschel Space
ObservatoryaspartoftheGOODS-Herschelsurvey(PI:Elbaz), 2.4. ChandraX-raydata
which consists of deep FIR observations of the GOODS-North
and GOODS-South fields for a total exposure of 361.3 h. The Chandra X-ray observations of GOODS-N field cover an
Imaging of the full northern field (GOODS-N; 10(cid:48) ×16(cid:48)) was area of ≈448 arcmin2 in the 0.5–8.0 keV energy band, with
performed using PACS (Poglitsch et al. 2008) at 100 µm and an exposure of ≈2 Ms (Chandra Deep Field North, CDF-N;
160µm(124hofobservations)andSPIRE(Griffinetal.2010) Alexanderetal.2003),reachingasensitivity(on-axis)of≈2.5×
at250µm,350µmand500µm(31.2hintotal);seeElbazetal. 10−17ergcm−2s−1(0.5–2.0keV)and≈1.4×10−16ergcm−2s−1
(2011)fordetailsonthePACSandSPIREobservations. (2–8 keV). The main source catalogue of the CDF-N includes
A59,page3of28
A&A549,A59(2013)
503 X-ray detected sources (Alexander et al. 2003). A supple- field (i.e., Wirth et al. 2004; Cowie et al. 2004; Chapman et al.
mentarycataloguecontaining430X-raysourcesisalsoavailable 2005; Barger et al. 2008; Chapin et al. 2009), as well as some
inthisfield.Thesecatalogueswhereconstructedusingasource unpublished spectroscopic redshift identifications (courtesy of
detection algorithm with false-positive probability threshold Dickinson). The optical positions of the sources in these cata-
of 10−7 for the main catalogue, and a more relaxed threshold logues were matched with the 24 µm positions of our detected
of 10−5 for the supplementary catalogue (see Alexander et al. sourcesusingasearchradiusof1.0(cid:48)(cid:48).Sincetheerrorsontheop-
2003,fordetails).Althoughthissecondcatalogueislikelytoin- ticalpositionsaretypicallyverysmall,weusedasmallersearch
clude many spurious X-ray sources, it can be used to robustly radius than that used for the X-ray catalogues (see Sect. 2.4).
identifyfainterX-raycounterpartsassociatedtoknownsources With this search radius and considering the sky density of the
(seee.g.,Sect.3.4.2.ofAlexanderetal.2003). 24 µm detected sources (Sect. 2.4), we estimated the spurious
These catalogues were used to identify the X-ray counter- detectionstobe∼3%.
parts of the 489 VLA/24 µm sources. The 24 µm positions Spectroscopic redshift measurements were found for
were matched to the X-ray positions using a small search ra- 1225sourcesamongstthe24µmdetectedsample(∼63%),with
dius of 1.5(cid:48)(cid:48); taking into account the high positional resolution thelargemajoritycomingfromtheBargeretal.(2008)spectro-
oftheChandradata(medianpositionaluncertainties≈0.3(cid:48)(cid:48))and scopicredshiftcatalogue(1030/1225).Twomoreredshiftiden-
the small pixel size of the MIPS-24 µm images, the majority tificationswereobtainedfromSpitzerIRSMIRspectra(Murphy
of the true counterparts are expected to lie within this radius. etal.2009),yieldingatotalof1227z measurementsforour
spec
In fact, calculating the probability P to find a random object sources.
within1.5(cid:48)(cid:48) fromtheX-raypositionsfollowingtheprescription In order to increase the redshift identification completeness
of Downes et al. (1986), we obtained a maximum probability ofthesample,wealsoincludedphotometricredshiftsfromacat-
ofrandomassociationP = 0.03(consideringaspacedensityof aloguebuiltfollowingtheproceduredescribedinPannellaetal.
24µmsourcesofn=5.2×104deg−2). (2009b) and Strazzullo et al. (2010). The photometric redshifts
FromthemaincataloguewefoundX-raycounterparts(inthe (z ) were estimated using a PSF-matched multi-wavelength
phot
0.5–8keVenergyband)for137ofthe489VLA/24µmsources catalogue including 10 photometric optical/near-IR passbands
(≈28%),withamedianpositionalseparationof≈0.2(cid:48)(cid:48).Amongst (from the U band to 4.5 µm), through a comparison with a li-
the matched sources, we found that in none of the cases was braryofgalaxySEDtemplates,spanningawiderangeofgalaxy
there more than one counterpart within the search radius, with types (from elliptical, to star-forming to QSO-dominated) and
theclosest neighboursbeing atseparations (cid:38)2(cid:48)(cid:48).From thesup- star formation histories (SFHs). The construction of the multi-
plementary catalogue we identified a further 22 X-ray counter- wavelengthcatalogueandthephotometricredshiftestimateswill
parts to the VLA/24 µm sources, yielding a total of 159 X-ray be described in detail in a paper by Pannella et al. (in prep.).
detectedsources(i.e.≈33%oftheVLA/24µmsample).Wenote The photometric redshift catalogue includes 1893 z within
phot
thatsincethesourcecataloguesinalloftheMIRandFIRbands the GOODS-Herschel area considered here. Photometric red-
consideredhere,aswellastheVLAradiocatalogue,arebased shift estimates were available for 671 of the 24 µm detected
on the 3.6 µm positions there was no need to cross-match the sources (∼35%) without z measurements. To verify the re-
spec
X-raysourcepositionswithanyoftheotherbands.FortheX-ray liabilityofthephotometricredshiftswecomparedthez with
phot
undetectedsources,3σupperlimitswerederivedfromaperture- thespectroscopicredshiftsfromBargeretal.(2008);inthepho-
correctedphotometryintheChandraimagesatthe24µmsource tometriccataloguebyPannellaetal.thereare1030sourcesover-
positions, assuming a power-law model with Γ = 1.4 (see e.g., lappingwiththeBargeretal.(2008)spectroscopicsample.The
Sect.3.4.1ofAlexanderetal.2003;Baueretal.2010). relativeaccuracyofz ,definedastheaverageabsolutescatter
phot
TheX-rayluminosities(2–10keV;rest-frame)ofthesources (AAS = mean[|∆z|/(1+z )], where ∆z = (z −z ); e.g.
spec phot spec
wereextrapolatedfromtheobserved2–8keVfluxescalculated Raffertyetal.2011)is≈5%,with≈4%ofoutliers(AAS > 0.2;
from detailed X-ray spectral analysis (Bauer et al., in prep.) see also Mullaney et al. 2012). Two further z were taken
phot
and from Alexander et al. (2003) for the sources in the supple- fromPopeetal.(2006),yieldingatotalof673photometricred-
mentaryChandracatalogue.Weusedtheredshiftsdescribedin shiftestimatesforoursample.Thisgivesafinalredshiftidenti-
Sect. 2.5 and assumed a constant photon index Γ = 1.9 (which fication completeness of ∼98% (1900/1943 sources, including
gives a band conversion factor L2−10keV = 1.08 L2−8keV), in- spectroscopic and photometric redshifts) amongst the 24 µm
cluding appropriate k-correction. We note that the X-ray lu- detected sources in the GOODS-Herschel field with a redshift
minosities have not been corrected for absorption because the rangez=0.02–6.54.
column density estimates (NH, available from Bauer et al., For the purposes of our analysis, we want to investigate
in prep.) often have large uncertainties; moreover, NH values here only the sources with a significant radio detection, in
were not available for the sources in the supplementary cata- order to have reliable measurements of the FIR-radio cor-
logue(Alexanderetal.2003).Therefore,toavoidaddingfurther relation. We therefore only included in our sample sources
uncertainties to the X-ray luminosities and to keep consistency with a redshift identification amongst the VLA/24 µm sample
intheL2−10keVmeasurementsbetweenthemainandthesupple- (484/489 sources). Due to the limitations dictated by our SED
mentary Chandra catalogues, we did not apply any absorption fittingtool(Sect.3.1),wealsoimposedaredshiftlimitofz≤3.0
corrections. to our sources, yielding a sample of 458 VLA/24 µm sources
withspectroscopicorphotometricredshiftsofz≤3.0.Theanal-
ysis of the whole 24 µm detected sample, including radio un-
2.5. Redshifts
detectedsources,willbepresentedinafuturepaper(DelMoro
Thanks to the large spectroscopic follow-up observations per- etal.,inprep.).
formed in the GOODS-N field, ≈3000 redshift identifications
areavailablefortheobjectsinthisfield.Acompilationofspec- 2.6. Stellarmasses
troscopic redshifts (z ) were obtained from the major pub- Themulti-wavelengthoptical/near-IRcatalogueusedtoestimate
spec
licly available spectroscopic redshift surveys of the GOODS-N thephotometricredshifts(Pannellaetal.,inprep.;seeSect.2.5)
A59,page4of28
A.DelMoroetal.:Radio-excesssourcesinGOODS-Herschel
sample is log M ≈ 10.4 M , and for the VLA/24 µm sam-
∗ (cid:12)
ple the median is log M ≈ 10.8 M . We note that the stellar
∗ (cid:12)
mass distribution of the VLA/24 µm sample sources is consis-
tentwiththatoftheX-raydetectedsources(medianstellarmass:
logM ≈10.9M ;seeFig.1).
∗ (cid:12)
3. FIR-radiocorrelation
3.1. SEDfittingapproach
The emission observed in the MIR and FIR bands is produced
by dust heated by the radiation emitted through star formation
and/oraccretionontoaSMBH.Star-formation,whichoccurson
large scales in galaxies, heats the dust to a wide range of tem-
peratures:thehotdustproducesemissionatnear-IR(NIR;λ ≈
2.0−5.0 µm) and MIR wavelengths and gives rise to the char-
acteristic PAH features (e.g. Chary & Elbaz 2001; Smith et al.
Fig.1.GalaxystellarmassesinunitsofM fortheentire24µmdetected 2007), while a large amount of colder dust (T ≈ 20−50 K;
(cid:12) dust
samplewithredshiftidentification(spectroscopicorphotometric).The seealsoSect.1)producesatypicalSEDthatpeaksatFIRwave-
stellarmassesforthesourcesdetectedintheVLAradiobandareshown lengths (λ ∼ 100 µm). AGN activity yields on average hot-
asgreyhistogramandtheX-raydetectedsourcesareshownasshaded ter dust temperatures (T ≈ 200−1000 K) than star forma-
dust
blackhistogram.
tion, so that the bulk of the AGN emission is produced in the
MIRbandwithapeakat shorterwavelengths(λ ∼ 20−30µm;
was also used to calculate the galaxy stellar masses (M ). The e.g.Netzeretal.2007;Mullaneyetal.2011).Thelackofcolder
∗
stellarmasseshavebeenderivedusingtheSEDfittingcodede- dust (T < 200 K) causes a fast decline of the SED at wave-
dust
tailed in Drory et al. (2004, 2009) to fit our multi-wavelength lengthslongerthanλ(cid:38)30µm(e.g.Netzeretal.2007;Mullaney
data.Thestarformationhistorieshavebeenparameterisedwith etal.2011).AGNactivityandstarformationareoftencoupledin
alinearcombinationofamainSFevent,withSFRexponentially agalaxyanditisnottrivialtoseparatetheemissionduetothese
declining with time as ψ(t) ∝ exp(−t/τ) (where the time-scale two processes. To disentangle the two components we there-
τ = 0.1–20 Gyr), and a secondary burst. The main component fore performed a detailed analysis of the IR SEDs of the 458
has solar metallicity and an age between 0.01 Gyr and the age VLA/24 µm detected sources, with spectroscopic or photomet-
oftheUniverseatthesourceredshift,whilethesecondaryburst ricredshiftsouttoz=3(Sect.2.5).
is limited to <10% of the galaxy total stellar mass and is mod- To represent the galaxy emission we used five star-forming
elled as a 100 Myr old constant SFR episode with solar metal- galaxy(SFG)templatesdefinedbyMullaneyetal.(2011),cov-
licity. We adopted a Salpeter (1955) IMF for both components eringthewavelengthrange6–1000µm.Thesefivetemplatesare
and an extinction law (Calzetti et al. 2000), allowing ranges of defined as composites of a sample of local star-forming galax-
A = 0–1.5 mag and A = 0−2.0 mag to extinguish the main ieswith L (cid:46) 1012 L (Brandletal.2006)andaredesignedto
V V IR (cid:12)
componentandtheburst,respectively(Pannellaetal.,inprep.; samplethefullrangeofIRAScoloursobservedforthesegalaxies
see also Mullaney et al. 2012). The uncertainties on the stellar (seeMullaneyetal.2011,fordetails).Wehaveextendedthefive
masses are estimated from the dispersion on the mass-to-light SFGtemplatestoshorterwavelengths(3µm)usingtheaverage
ratio(M/L)distributionoftheentirelibraryofmodelsadopted, starburst SED derived by Dale et al. (2001). To verify whether
aswellasfromthesystematicuncertainties(duetotheadopted the Dale et al. (2001) template was suitable for extending the
models, IMF, SFH, metallicity, etc.; see e.g. Marchesini et al. Mullaney et al. (2011) templates, we obtained publicly avail-
2009).Theuncertaintiesaretypicallylargeratlowstellarmasses able NIR and MIR data (from the NASA/IPAC Extragalactic
and range from ≈0.4 dex at log M = 9.0 M to ≈0.2 dex at Database, NED3) for the Brandl et al. (2006) sample of local
∗ (cid:12)
log M = 11.0 M (Pannellaetal.,inprep.).Wenotethatalso star-forminggalaxiesandweplottedthesedatapointsoverour
∗ (cid:12)
AGNemissionintheUV/opticalbandcancauseuncertaintieson extended templates as a check; we thus verified that data and
thestellarmassestimates.However,thecontaminationfromthe modelsagreedwithreasonablescatter.
AGNaffectsthestellarmassonlywhentheAGNisverylumi- We also extended the five SFG templates to the radio band
nous(L >1044 ergs−1;e.g.Rovilos&Georgantopoulos2007; withapower-lawslopeS ∝ ν−α,withα = 0.7(e.g.,Ibaretal.
X ν
Xueetal.2010;Mullaneyetal.2012). 2009, 2010; see Fig. 2). The normalisation of the radio power-
Stellar masses were measured for 1894 of the 1900 24 µm lawcomponentwasfixedaccordingtothetypicalFIR-radiore-
detected sources with a redshift identification within the lation found for local star-forming galaxies (Helou et al. 1985;
GOODS-Herschel field (Sect. 2.5), which include 456 of the Condon1992).WestressthateventhoughwelimitedourSFG
458VLA/24µmsourceswithspectroscopicorphotometricred- templatelibrarytoonly5templates,theyrepresentawiderange
shiftz ≤ 3.0thatconstituteourfinalsample.Thegalaxystellar ofIRcolor–colorpropertiesofstar-forminggalaxies(Mullaney
mass values obtained range between log M = 7.0−12.5 M et al. 2011), even broader than those reproduced by, e.g., the
∗ (cid:12)
(log M = 8.7−11.9 M for the VLA/24 µm sample), with Chary & Elbaz (2001) galaxy template library (105 templates;
∗ (cid:12)
the large majority of the sources (≈97%) having log M = seeFig.2).
∗
9.0−11.5 M (Fig. 1)2. The median stellar mass for the 24 µm To reproduce the emission from the AGN we used the em-
(cid:12)
piricallydefinedAGNtemplatebyMullaneyetal.(2011),com-
2 Wenotethatthefractionofsourceswithverylow(logM∗<8.0M(cid:12)) posed by a broken power-law, declining at wavelengths long-
orveryhigh(log M∗ > 12.0 M(cid:12))massvaluesamongstthe24µmde- ward of λ (cid:38) 30 µm as a modified black-body. We note that
tectedsample(3%)isconsistentwiththeestimatedfractionofphoto-
metricredshiftoutliers(seeSect.2.5). 3 http://ned.ipac.caltech.edu/
A59,page5of28
A&A549,A59(2013)
SFGtemplates(Fig.2)usingχ2minimisationtoevaluatethe
goodnessofthefit;
2. as a second step, we performed new fits, again using χ2
minimisation, by adding the AGN component, also includ-
ing extinction of the AGN component as a free parame-
ter (varying between A ≈ 0−30 mag, corresponding to
V
N ≈ 0−5×1022 cm−2 assumingtheaveragegalacticdust-
H
to-gas ratio A = N /(1.8×1021); e.g. Predehl & Schmitt
V H
1995),toeachSFGtemplate(SFG+AGN);
3. finally,an f-testwasperformedusingtheχ2 valuesandthe
degrees of freedom (d.o.f.) for all of the five pairs of solu-
tionstoevaluatetheimprovementofthefitduetotheaddi-
tion of the AGN component. We accepted the SFG + AGN
modelasthebest-fitiftheAGNcomponentsignificantly(i.e.
>90% confidence level, according to the f-test probability)
improved the resulting χ2 for the majority of the solutions,
Fig.2. Comparison of the five star-forming galaxy templates used in
i.e.inatleastthreeoutoffivefittingsolutions.
ourapproach(blacklines)withthe105Chary&Elbaz(2001)galaxy
templates(greylines);allofthetemplateshavebeennormalisedto1
at 30 µm. With our 5 SFG templates we cover a broad range of star- Thecriteriaweadoptedtodefinethebest-fittingmodelwerees-
forming galaxy properties, even broader than those of the Chary & tablished after performing several tests on our SED fitting ap-
Elbaz(2001)templates. proach; in particular, we tested these criteria on a sub-sample
of our sources for which detailed Spitzer-IRS MIR spectra are
available(seeAppendixA,fordetails).
thistemplateisinagreementwiththetypicalSEDsproducedby
clumpytorusmodels(e.g.Nenkovaetal.2008a,b).Wefixedthe Oncethebest-fitmodelwasdefined,thefinalmeasurements
power-lawindicesattheaveragevaluesΓ = 1.7andΓ = 0.7, ofthepropertiesofthesources(i.e.,FIRflux,relativeAGN/SFG
1 2
with a break pointat λ = 19 µm (see Mullaney etal. 2011). contribution,etc.)andtheirerrorswerederivedasweightedav-
Brk
We have allowed the AGN component to be modified due to erages of the values obtained from the five best-fit model solu-
dustextinction,usingtheextinctionlawofDraine(2003),which tions(seeAppendixAfordetails).FortheAGN+SFGmodels
mainlyaffectsthetemplateatλ (cid:46) 30µmandalsoproducesthe we included in the average only the solutions where the AGN
typical silicate absorption feature at 9.7 µm, often observed in componentwassignificant(>90%confidencelevel).Thisisbe-
AGN(e.g.Rocheetal.1991;Shietal.2006;Rocheetal.2007; cause,duetothesparsedatausedtoconstraintheSEDs,insome
Martínez-Sansigreetal.2008). cases the five solutions obtained from the different SFG tem-
Theflux densitiesat8 µm,16 µm,24 µmfrom Spitzer and plates were similar (small difference in χ2 values) and did not
100 µm, 160 µm and 250 µm from Herschel have been used allowustounambiguouslydetermineauniquesolutionthatbest
intheSEDfittingprocesstoconstraintheSEDsofoursources characterisedthedata(seeAppendixA,fordetails).
(Fig.3).Inthecaseofnon-detections(S/N < 3),themeasured
fluxes at each source positions, with the large associated un-
3.2. Radio-excesssampleselection
certainties, were used in the SED fits. The fluxes in the shorter
Spitzer-IRACbands(3.6,4.5,5.8µm)werenotincludedinthe
The FIR-radio correlation is typically defined as the ratio be-
SED fitting process as they fall out of the wavelength range
tween the flux in the FIR band (∼40−120 µm; rest-frame) and
covered by our templates at relatively low redshifts z > 0.2.
thefluxdensityintherest-frameradioband(1.4GHz;e.g.Helou
Moreover,atthesewavelengthstheobservedSEDisoftendom-
etal.1985;Sargentetal.2010a).Manystudiesperformedsofar
inated by the old stellar population emission, which is not ac-
athighredshiftreliedoneitherMIRfluxesasa“proxy”forthe
counted for in our SED templates. Data at longer wavelengths
FIR(orbolometricIR)flux(e.g.fromtheS ;Appletonetal.
from Herschel-SPIRE 350 µm and 500 µm (Elbaz et al. 2011) 24µm
2004; Donley et al. 2005), 70 µm flux density (e.g. Appleton
werealsonotincludedinthefits,becauseofthelargeruncertain-
et al. 2004; Seymour et al. 2009; Sargent et al. 2010a; Bourne
tiesonthemeasuredfluxesduetostrongblending(seeSect.2.2)
et al. 2011), or on SED fitting spanning only the rest-frame
andthelownumberofsignificantlydetectedsources.However,
MIRband(e.g.Sargentetal.2010a;Padovanietal.2011).These
thesefluxes(orupperlimits)havebeenincludedwhenplotting
methods are often inaccurate because they require assumptions
theSEDs,asavisualcheckofthebest-fittingSEDsolutions(see aboutthesourceSEDoverthewholeIRbandand/oronthebolo-
Figs.3andB.1).
metric corrections. Through our detailed SED analysis of the
Wenotethatonlyusingphotometryatλ<250µm(SPIRE- 458 VLA/24 µm sources with z ≤ 3.0, using the approach de-
250µmband)meansthatathighredshifts(z (cid:38) 1.3)wearenot
scribedintheprevioussection(Sect.3.1)andtheHerscheldata
abletofullyconstraintheFIRSEDpeakatwavelengthslonger
toconstraintheFIRSEDpeak,wecanovercometheseissuesby
thanλ ≈ 100µm(rest-frame)andthereforewecannotexclude
directlymeasuringthegalaxyemissionoverthewholeIRband.
the presence of a colder dust component, which would yield
From the best-fit models, we calculated the FIR flux (f )
higherFIRfluxesthanthatpredictedfromourmodel.However FIR
bydirectlyintegratingthetotalSEDsovertherest-framewave-
we can anticipate (see Sect. 3.2) that we find very good agree-
length range λ = 42.5−122.5 µm (e.g. Helou et al. 1985). We
ment between our average FIR-radio relation and that found in
usedthetotalSED,whichinmanycasesincludescontributions
severalpreviousworks.
frombothSFGandAGNcomponents,tocalculatetheFIRflux
TheSEDfittingprocesswasdevelopedasfollows:
becauseweaimataconservativeselectionofradio-excessAGN,
1. initially,onlythestar-forminggalaxytemplateswereusedin sinceradioquietAGNoftenfollowthetypicalFIR-radiorelation
thefit;wefittedthedataofeachsourcewitheachofthefive ofstar-forminggalaxies.
A59,page6of28
A.DelMoroetal.:Radio-excesssourcesinGOODS-Herschel
Fig.3.Examplesofspectralenergydistributions(SEDs)todemonstratethevarietyofSEDsfoundfortheradio-excesssources(Sect.3.2).The
SEDsontheleftareconsistentwithasimplegalaxytemplate:SFGtemplate(“IRSFG”,top)orellipticalgalaxytemplate(“passive”,bottom;see
Sect.4.3);theSFGtemplateupperlimit(greyline)isalsoshown.Wenotethattheellipticaltemplate(longdashedline)isnotfittedtothedata,
butitisonlyshowntodemonstratethatitcanwellrepresentthedata.TheSEDsontherightarefittedwithastar-forminggalaxy(dashedline)+
AGN(dottedline)model(“IRAGN”):onthetopthereisanX-raydetectedAGN,onthebottomanX-rayundetectedAGN.ThetotalSEDsare
shownasblacksolidlines.ThefilledcirclesrepresenttheSpitzer8,16,24µmandtheHerschel100,160,250µmfluxdensities(inmJy),usedto
constraintheSEDs.TheopensymbolsindicatesthedatathatwerenotincludedintheSEDfittingprocess:redtrianglesareSpitzer-IRAC3.6,4.5,
5.8µmfluxdensities,blackcirclesareSPIRE350and500µmandblacksquaresareVLA1.4GHzfluxdensities;theradiodatadonotmatchthe
SEDsinthesecases,becausethesourceshaveexcessradioemissioncomparedtothatexpectedfrompurestarformation(Sect.3.2).Thebluestar
representsthe6µmluminosityoftheAGNpredictedfromtheX-rayluminosityintherest-frame2–10keV,usingtheLutzetal.(2004)relation
forlocalunobscuredAGN;wenotethatthispointdoesnotmatchtheIRAGNcomponentbecausetheX-rayluminositytendstounderestimatethe
intrinsicAGNpoweriftheAGNemissionisheavilyabsorbed(seeSect.5.1).Onthetopleftcornerofeachplotthesourceredshiftsareindicated
aswellasthecorrespondingsourcenumberinTable1(Col.1).
In only ≈3% of the 458 analysed sources (15 sources) was estimatedbynormalisingtheSFGtemplatestothe24µmdata-
theSEDfittinganalysisunabletoprovideagoodrepresentation point(Figs.3andB.1),whichisthususedasaproxyofstarfor-
ofthemid-andfar-IRdata4.Inmostofthecases(6/15),thiswas mation,andintegratingtheSEDbetween42.5−122.5µm.Ofthe
duetolargeuncertaintiesonthephotometricredshift,orpossibly five measurements obtained from the different SFG templates,
to spurious counterpart associations between the different cata- themaximumhasbeentakenasthe f upperlimit.
FIR
logues (see Sect. 2.5). In a smaller number of cases (4/15), the UsingthedefinitiongivenbyHelouetal.(1985),wecalcu-
poor fitting results seemed to be due to large uncertainties on latedtheratiobetweenthefar-infraredandradioemission(q)as:
thefluxdensitymeasurements,especiallyatwavelengthswhere
thesourcesarenotsignificantlydetected(S/N <3).Weflagged (cid:104) (cid:16) (cid:17)(cid:105)
q=log f / 3.75×1012Hz −log[S (1.4GHz)] (1)
these“problematic”casesinTable1(column“Fit”),beingaware FIR ν
thatthemeasurementsobtainedfromtheirSEDsarenotfullyre-
liable.Theremainderofthesesources(5/15)havestrongemis- where fFIRisinunitsofWm−2,3.75×1012Hzisthefrequency
sionintheSpitzer IRACbands,evenstrongerthanthefluxde- at the centre of the FIR band (λ = 80 µm) and Sν(1.4 GHz)
tected at 24 µm (S /S > 1), and are not detected at longer istheradiofluxdensity(inunitsofWm−2 Hz−1)atrest-frame
8 24
wavelengths(FIR)byHerschel.Thissuggeststhattheemission 1.4 GHz, extrapolated from the VLA data using the power-law
slopeS ∝ ν−α,withα = 0.7,typicalforstar-forminggalaxies
from star formation (or AGN) in these sources is weak, while ν
(e.g.,Ibaretal.2009,2010).
theemissionfromoldstellarpopulation,usuallydominatingthe
rest-frame NIR band (λ < 5 µm, rest-frame), is strong; their Theqdistributionobtainedfortheentiresampleisshownin
Fig.4(left);thesourceswhere f (andthereforealsoq)isan
infraredSEDsarethereforemoreconsistentwiththatofgalax- FIR
upper limit are not included in the histogram. The peak of the
ies dominated by passive stellar populations rather than by ac-
distribution is at q ≈ 2.2, in excellent agreement with the val-
tivestarformation.ForthesesourcesFIRfluxupperlimitswere
ues typically obtained for star-forming/starburst galaxies (e.g.
4 ThepoorfitswereflaggedbyvisualinspectionoftheresultingSEDs; Helou et al. 1985; Condon 1992). However, the distribution is
these are the cases where even the best-fitting SEDs deviate signifi- not symmetrical around the peak and shows a broad tail at low
cantlyfromthedata. q values, indicating a relatively large number of sources with
A59,page7of28
A&A549,A59(2013)
Fig.4.Left:distributionofFIR-radiofluxratio(q)forallofthesourcesintheVLA/24µmsample(458sources,excludingtheupperlimits;see
Sect.3.2);theblueGaussianprofilerepresentsthebestfittothepeak(q = 2.21±0.18),whilethereddashedGaussianistheqdistribution
mean
foundbyHelouetal.(1985)forlocalstarburstgalaxies.Theverticallineindicatestheseparationweassumedtoselectourradio-excesssources,
q = 1.68,correspondingto3σfromthepeak(2σwhenincluderadioupperlimits).Right:FIR-radiofluxratio(q)asafunctionofredshiftfor
alloftheVLA/24µmsources;theradio-excesssourceshaveq < 1.68(dot-dashedline);thesolidhorizontallinerepresentstheaverageqvalue
forthe“radio-normal”sources(q = 2.21)andtheshadedregionindicatesthe±1σerrorfromthisaverage.Theradio-excesssourcesidentified
byDonleyetal.(2005)areplottedascrosses(seeSect.3.3).Thebinnedqaverageforthe“radio-normal”sources,inthreeredshiftbins,isalso
shownasblackcircles;wefoundnosignificantevidenceofevolutionoftheFIR-radiocorrelationouttoz=3.
excess radio emission over that expected from star formation withoursample5 (seeSect.3.3).InFig.4(right)wealsoshow
processes. thattheaverageqforalloftheradio-normalsources,calculated
in three different redshift bins (z = 0.0−1.0, z = 1.0−2.0,
In order to identify the radio-excess sources, we fitted the z = 2.0−3.0,blackcircles),rema1insfairlyconsta2nt,withinthe
peakoftheqdistribution,consideringonlysourceswithq>2.0, 3
errors,overthewholeredshiftrange,andtherefore,theapparent
withaGaussianprofileandestimatedthespreadoftheFIR-radio
decrease of the q values at high redshift is not significant (e.g.
correlationexpectedforstar-forminggalaxies:fromthebest-fit
Elbazetal.2002;Sargentetal.2010b;Maoetal.2011;Bourne
weobtainedq = 2.21±0.18(seeFig.4).Ifwealsoincludethe
etal.2011).
radioundetectedsources(S/N <3)intheqdistribution,weob-
tainaGaussianprofilewithaverysimilarpeak,butlargerscatter
(q=2.24±0.29),duetothelargeruncertaintiesontheqvalues.
3.3. IR-radiorelation:q andq
24 100
Theqvaluesfortheradioundetectedsourceswerecalculatedus-
ingtheradio1.4GHzfluxmeasurementsatthe24µmsourcepo- SeveralpreviousworksthathaveinvestigatedtheFIR-radiocor-
sitions(Sect.2.3).Wesettheseparationbetween“radio-normal” relation and radio-excess sources have used different methods
and “radio-excess” sources at q = 1.68, corresponding to a 3σ todefinetheFIR-radiofluxratio.Inparticular,inmanystudies
deviation from the peak of the distribution for the VLA/24 µm theFIRfluxhasbeenreplacedbythemonochromaticfluxden-
sample (∼2σ from the peak for the whole 24 µm sample). We sity at 24 µm (e.g. Appleton et al. 2004; Donley et al. 2005;
defined“radio-normal”asthesourceswithq>1.68,thatfollow Ibar et al. 2008; Sargent et al. 2010a), 60 µm (e.g. Vlahakis
thetypicalFIR-radiorelation(q ≈ 2.2);wenotethatthispopu- et al. 2007), or 70 µm (Seymour et al. 2009; Sargent et al.
lationincludesstar-forminggalaxies,butalsotypicalradio-quiet 2010a;Maoetal.2011),asaproxyfortheFIRemissionofthe
AGN(e.g.Moric´etal.2010;Sargentetal.2010a;Padovanietal. galaxy.Itisthereforeinterestingtoseehowthesedefinitionsof
2011), while the large majority of the radio-excess sources are the FIR-radio ratios, calculated from monochromatic flux den-
mostlikelytohostAGNactivity. sities, compare to q estimated by us (Eq. (1)) across the full
FIRwaveband.Inparticularweperformedadirectcomparison
Withourselectionweobtainedasampleof51radio-excess
AGNcandidates,∼11%ofthewholeVLA/24µmdetectedsam- of our sample selection with that used by Donley et al. (2005),
wheretheradio-excesssourceswereselectedusingq <0,with
pleatz ≤ 3.0.InFig.4(right)weshowtheFIR-radioratio(q) 24
for the entire sample as a function of redshift. The horizontal
dashed line indicates the q value at the peak of the distribution
5 Wenotethatfor6sourcestheredshiftslistedinDonleyetal.(2005)
(q=2.21±0.18)with1σuncertainty(shadedregion),whilethe
areindisagreementwithours(seeTable1):infivecaseswehavenew
dot-dashed line at q = 1.68 represents our threshold for radio-
spectroscopicredshiftswheretherewereonlyphotometricredshiftes-
excesssources(3σdeviationfromthepeak).Intheplotwealso timates(ornoestimatesatall)inDonleyetal.(2005);intheremaining
marked12oftheDonleyetal.(2005)radio-excesssourcesthat case,wehaveaphotometricredshiftwheretherewasnoredshiftmea-
arefoundwithintheGOODS-Herschel(North)fieldandoverlap surementforthissourceinDonleyetal.(2005).
A59,page8of28
A.DelMoroetal.:Radio-excesssourcesinGOODS-Herschel
q =log(S /S ),calculatedusingtheobserved24µm
24 24µm 1.4GHz
and1.4GHzfluxdensities.
AsshowninFig.4(right),themajorityoftheDonleyetal.
(2005) sources (11/12 that overlap with our sample) are also
radio-excesssourcesbasedonourdefinition.Howeverq does
24
notprovideacompleteselection,sincethemajorityofourradio-
excesssources(≈78%,40objects)aremissingfromtheDonley
et al. (2005) sample. To verify that this is not only due to the
improvedsensitivityofourradiodata(Sect.2.3),wecompared
thenumberofradio-excesssourceswewouldidentifywithour
qdefinitionandselection(q<1.68)usingtheradiocatalogueby
Richards(2000),thesameasthatusedbyDonleyetal.(2005):
we obtained 24 radio-excess sources in the GOODS-Herschel
field,twicethenumberofthoseidentifiedbyDonleyetal.(2005)
using q within the same sky area (12 sources). Moreover, in
24
one case the q selection by Donley et al. (2005) disagrees
24
withourFIR-radioratio,underestimatingtheFIRpowerofthe
sources(Fig.4,right).ThiseffectisshownmoreclearlyinFig.5,
where we calculated the observed q and q for all of the
24 100
VLA/24µmdetectedsources6.Theq andq ratiospredicted
24 100
for our range of star-forming galaxies are shown as shaded re-
gions in Fig. 5, where our radio-excess sources (q < 1.68) are
marked with open circles; the q = 0 line adopted by Donley
24
etal.(2005)toselectradio-excesssourcesisalsoshown(Fig.5,
top panel). Although the q < 0 criterion would select some
24
of our radio-excess sources, more than 60% of them would be
missed,testifyingthelargeincompletenessofthismethodwhen
comparedtotheFIR-radiorelationq(Eq.(1)).Moreover,using
q < 0 to define the radio-excess sample would include some
24
“radio-normal” galaxies (according to our definition; Sect 3.2),
introducingsomecontaminationtothesample(∼10%).
Ontheotherhand,theq ratio(Fig.5,bottom)seemstobe
100
inbetteragreementwithourqselectionthantheq ratio,asfew
24 Fig.5.Infrared-to-radiofluxratios,usingtheobserved24µm(q ;top)
ofourradio-excesssourcesfallintotheshadedarea.Thisisex- 24
and100µmfluxdensities(q ;bottom);theradio-excesssourcesinour
pectedsincethefluxobservedat100µmisclosertotheFIRSED 100
sample(q<1.68)arehighlightedbyopencircles.Theshadedregions
peak than that observed at 24 µm which is also likely to be af-
inthetwoplotsrepresenttheq andq ratiospredictedforourrange
fected by the PAH emission features at z (cid:38) 1 (as illustrated by of star-forming galaxies as a f2u4nction1o00f redshift. The horizontal line
the shaded region in Fig. 5, top) and/or by AGN contribution. in the top plot is the q = 0 selection used by Donley et al. (2005)
24
By defining a separation line below the shaded area to identify todefinetheirradio-excesssample.Inthebottompanelweidentified
radio-excesssources(e.g.q = 1.5;Fig.5)wewouldrecover q = 1.5asaneasyselectionofradio-excesssources(dashedline),
100 100
the majority of our radio-excess sources (∼60%; ∼80% includ- withreasonablecompleteness(≈60–80%)atleastouttoz≈2.
ing the upper limits that lie right above the separation line), a
much larger fraction than those identified using q . We stress,
24 InFig.6,weshowtherest-frameFIRandtheradio1.4GHz
however, that the q (observed) selection is still not complete
100 luminosities (L − L ; rest-frame) for all of the “radio-
andmuchlessaccuratethanourselectionmethodusingtherest- FIR 1.4GHz
normal” (black crosses) and the radio-excess sources (red cir-
frameFIRfluxtocalculateq.
cles) in the GOODS-Herschel field. The diagonal lines repre-
sent the mean q value obtained for the “radio-normal” sources
4. Results (q=2.21)andtheseparationbetween“radio-normal”andradio-
excess sources (q = 1.68). For comparison we also indicate
4.1. Sampleproperties in the plot the average q obtained for a sample of low-redshift
(z (cid:46) 0.15) radio-loud AGN (RL; (cid:104)q (cid:105) = −0.38) from Evans
We established in Sect. 3 that the direct measurement of the RL
et al. (2005). For these sources the FIR flux was calculated us-
FIR flux from the best-fit SED provides a reliable and unam-
ingthephotometryat60µmand100µm,followingHelouetal.
biguouswaytoidentifysourcesthathaveradioemissioninex-
(1985).SomewellstudiedlocalCompton-thickAGNtakenfrom
cesstothatexpectedpurelyfromstarformation.Asdescribedin
literature7 (Della Ceca et al. 2008) are also plotted, as a guide;
theprevioussections,fromouranalysisweidentified51radio-
althoughthemajorityofthesesourceshaveFIR-radioratioscon-
excesssources,whicharemostlikelyhostinganAGN.
sistent with that of star-forming galaxies and most radio-quiet
6 q =log(S /S ),whereS andS areobserved AGN (q ≈ 2.2), three out of the eight AGN are radio-excess
100 100µm 1.4GHz 100µm 1.4GHz
monochromatic fluxes. Since we do not have photometric measure- sources (i.e., Mkn 3, NGC 7674 and NGC 1068), suggesting
ments at 60 µm or 70 µm to directly compare our results with those that some Compton-thick AGN might also be included in our
obtained in previous works (e.g. Vlahakis et al. 2007; Seymour et al. radio-excesssample.
2009; Sargent et al. 2010a; Mao et al. 2011), we used the observed
100µmfluxdensity,whichistheclosestavailabledatapointtoallow 7 The FIR and radio data for these sources are taken from the
anycomparisonwithq orq . NASA/IPACExtragalacticDatabase(NED).
60 70
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A&A549,A59(2013)
sources. Amongst our radio-excess AGN sample, only ∼53%
(27/51) are detected in the X-ray band (Table 1), hence about
half of the sample is X-ray undetected. This suggests that the
radio-excessselectionisapowerfulmethodtoidentifyAGNthat
wouldotherwisebemissedbyeventhedeepestX-rayAGNsur-
veys,suchastheChandra2MsX-raysurvey(Alexanderetal.
2003).
In Fig. 7, we show the fraction of radio-excess sources as
a function of X-ray luminosity. The sources have been divided
intofiveX-rayluminositybinsfrom L = 1040 ergs−1 to
2−10keV
L = 1045 ergs−1. We found that the fraction of radio-
2−10keV
excess AGN increases from ≈7% at low X-ray luminosities
(L < 1042 erg s−1) to ≈60% in the highest luminosity
2−10keV
bin(L =1044−1045ergs−1).Thislargeincreaseofradio-
2−10keV
excessAGNathighluminositiessuggeststhatthebrightestAGN
alsoproducethemostpowerfulradioemission(e.g.Brinkmann
et al. 2000; La Franca et al. 2010), presumably the majority of
itcomingfromaradiocore.Wenotethatthefactthatmorelu-
minousAGNarealsomoreradiobrightdoesnotmeanthathigh
luminosity AGN are typically radio loud. On the contrary, the
radio-loudness, measured as R = log(νL /L ), de-
X 1.4GHz 2−10keV
creases with increasing X-ray luminosity (e.g. La Franca et al.
2010).
To investigate in detail the radio properties of our radio-
excess sources we compared the excess radio emission (i.e.
Fig.6.Toppanel:fractionofradio-excesssourcesasafunctionofthe the excess above the radio emission expected from star for-
rest-frame FIR luminosity (42.5–122.5 µm, in units of L(cid:12)); the frac- mation), with the AGN radio core emission detected in deep
tion decreases with increasing LFIR, as expected since sources with VLBI 1.4 GHz observations of the Hubble Deep Field North
highLFIRaremorelikelytobeluminousstar-forminggalaxies.Bottom (HDF-N) and the Hubble Flanking Fields (HFF). The details
panel: rest-frame FIR vs. radio 1.4 GHz luminosity (rest-frame); the
of the VLBI observations are given in Chi et al. (2009) and
radio-excess sources are plotted as red circles and some local well
Chi et al. (in prep.). Briefly, the sensitivity of the VLBI data
known Compton-thick AGNs (open squares) are shown for compari-
variessignificantlyfromthecentreofthefield(rmsnoiselevel
son (data taken from literature). The black dotted line correspond to
(cid:104)q (cid:105) = −0.38 (average for a sample of RL AGN from Evans et al. of 7.3 µJy/beam within 2(cid:48) from the phase centre) to the outer
20R0L5);theblacksolidlinecorrespondtoq=2.21(averagefor“radio- parts of the field (2(cid:48)−8(cid:48); rms noise level of 14–37 µJy/beam),
normal”sources)andthereddashedlinecorrespondtoq = 1.68,our which means that only sources with a radio core brighter than
selectionlimitforradio-excesssources. S (cid:38)100µJyarelikelytobedetected.Amongstourradio-
1.4GHz
excesssample, only13sourcesare brightenough(onthe basis
of the VLA 1.4 GHz flux density; Table 1) to be detected in
It is important to note that, although some of our radio- the VLBI images and for eight of them a compact radio core
excess sources have radio luminosities typical of radio-loud was indeed detected by VLBI, positively confirming that these
AGN(Lrad > 1024 WHz−1),themajorityliesinaregioninbe- sources host an AGN and that the excess radio emission is in
tween those occupied by RL AGN and “radio-normal” sources fact due to a radio core. The remaining five sources have on
and therefore can also be referred to as “radio-intermediate” average lower radio VLA fluxes compared to the sources de-
sources(e.g.Drakeetal.2003).Wethereforestressthatthedefi- tected by VLBI (the only exception is #47, in Table 1, which
nitionof“radio-excess”doesnotnecessarilymean“radio-loud” is an extended radio-loud, wide-angle-tailed source) and lie in
(seeSect.1). theouterregionsoftheVLBIfield;thenon-detectiontherefore
InthetoppanelofFig.6,thefractionofradio-excesssources mightbeduetosensitivityissues,or,asinthecaseofsource#47,
as a function of the rest-frame FIR luminosity is shown (with to the radio flux being dominated by extended emission, more
1σuncertainties;Gehrels1986).Thesourcesinoursamplespan than a compact core. The rest of our radio-excess sources lie
awiderangeofFIRluminosities,LFIR ≈109−1012 L(cid:12)(Table1), outside the VLBI region (13 sources) or are typically too faint
typical of normal star-forming galaxies, luminous IR galax- (S <100µJy)tobesignificantlydetectedabovetheVLBI
1.4GHz
ies (LIRG; LFIR ≈ 1011 L(cid:12)) and ultra-luminous IR galaxies sensitivitylimit(25sources).
(ULIRGs; LFIR ≈ 1012 L(cid:12)). However, the fraction of radio- In Fig. 8 we show the comparison between the AGN radio
excess sources decreases with increasing FIR luminosity, and coreemissionmeasuredfromtheVLBIdataandtheexcessra-
only one source out of 51 reaches the high luminosities typical dio emission obtained for our sources. The excess radio emis-
ofULIRGs.Thisisnotunexpected,sinceatthehighFIRlumi- sionwasestimatedbysubtractingthe1.4GHzfluxpredictedfor
nosities of ULIRGs, the radio core must be very bright (Lrad > starformationassumingtheaveragevalueq = 2.21(Sect.3.2),
2 × 1024 W Hz−1; RL AGN regime) to be identified as radio- fromthetotalradiofluxdensity(fromtheVLAdataat1.4GHz;
excessabovetheverystrongstar-formationactivity. Sect.2.3).Theobtainedradioexcessfluxdensitiesagreewiththe
VLBIAGNcoreemissionwithinafactorof≈2.Thisisafurther
validationthatourSEDanalysisisreliableandthatwithitweare
4.2. X-rayandradioemissionfromtheAGN
abletopredicttheAGNradio-coreluminositieswithreasonable
Sincetheexcessradioemissionisattributedtothepresenceofan accuracy.Wenotethatourestimatestendtoslightlyoverpredict
AGN,itisinterestingtoinvestigatetheX-raypropertiesofthese the AGN radio-core flux densities because they might include
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