Cosmic evolution of radio selected active galactic nuclei in the COSMOS field

1.5 MB·English

by  V. Smolcic

PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
COSMIC EVOLUTION OF RADIO SELECTED ACTIVE GALACTIC NUCLEI IN THE COSMOS FIELD0
V. Smolcˇic´1, G. Zamorani2, E. Schinnerer3, S. Bardelli2, M. Bondi4, L. Bˆirzan5,6, C. L. Carilli7, P. Ciliegi2,
O. Ilbert8, A. M. Koekemoer9, A. Merloni10,11 T. Paglione12,13, M. Salvato1, M. Scodeggio14, N. Scoville1
ABSTRACT
We explorethe cosmicevolutionofradioAGN with low radiopowers(L .5×1025 WHz−1)
1.4GHz
out to z = 1.3 using to-date the largest sample of ∼ 600 low luminosity radio AGN at intermediate
9
redshift drawn from the VLA-COSMOS survey. We derive the radio luminosity function for these
0
AGN,andits evolutionwithcosmictime assumingtwoextreme cases: i)pure luminosityandii)pure
20 densityevolution. TheformerandlatteryieldL∗ ∝(1+z)0.8±0.1,andΦ∗ ∝(1+z)1.1±0.1,respectively,
both implying a fairly modest change in properties of low radio-power AGN since z = 1.3. We show

n that this is in stark contrastwith the evolution of powerful (L >5×1025 W Hz−1) radio AGN
1.4GHz
a over the same cosmic time interval, constrained using the 3CRR, 6CE, and 7CRS radio surveys by
J
Willott et al. (2001). We demonstrate that this can be explained through differences in black hole

1 fueling and triggering mechanisms, and a dichotomy in host galaxy properties of weak and powerful
2 AGN.Ourfindingssuggestthathighandlowradio-powerAGNactivityistriggeredindifferentstages

duringtheformationofmassiveredgalaxies. WeshowthatweakradioAGNoccurinthemostmassive
]
O galaxies already at z ∼ 1, and they may significantly contribute to the heating of their surrounding
medium and thus inhibit gas accretion onto their host galaxies, as recently suggested for the ‘radio
C
mode’ in cosmological models.
.
h Subject headings: galaxies: fundamental parameters – galaxies: active, evolution – cosmology: obser-
p vations – radio continuum: galaxies
-
o
r 1. INTRODUCTION heating through energetic radio outflows – the ‘radio’
t
s 1.1. AGN feedback: The impact of radio sources on mode16 – managed to reproduce well many observed
a
galaxy properties (e.g. the galaxies’ stellar mass func-
[ galaxy formation
tion, color bimodality; Croton et al. 2006, Bower et al.
1 Radio activity from active galactic nuclei (AGN) has 2006). Theparticularchoiceofa‘radiomode’asthe rel-
v recentlybeeninvokedincosmologicalmodelsasasignifi- evant heating mechanism has been motivated by many
2 cantingredientintheprocessofgalaxyformation(‘AGN observationalresults verifying the interplay between the
7 feedback’). Given that, in the past, cosmologicalmodels emissionofradiogalaxiesingalaxyclusters,andtheclus-
3 led to a systematic over-predictionof the high-mass end ter’s hot X-ray emitting gas on large-scales (e.g. Fabian
3 of the galaxy stellar mass function (e.g. White & Rees et al. 2003, Forman et al. 2005).
1. 1978; White & Frenk 1991), the implementation of gas In the centers of galaxy clusters the radiative cooling
0 time of the intra-cluster medium (ICM) is shorter than
0BasedonobservationswiththeNationalRadioAstronomyOb-
9 theageofthecluster. Thus,thecentralICMisexpected
servatory which is a facility of the National Science Foundation
0 to be significantly colder compared to outskirt-regions
operatedundercooperativeagreementbyAssociatedUniversities,
: Inc. (such clusters are referred to as ‘cooling flow clusters’).
v 1 CaliforniaInstituteofTechnology,MC105-24,1200EastCal- However, generally the expected cool X-ray phases are
i
X ifo2rnIiNaABFou-leOvassredr,vPataosraidoeAnas,trConAom91i1c2o5di Bologna, via Ranzani 1, notobserved,andthisisknownasthe‘coolingflowprob-
r 40127,Bologna,Italy lem’. Themostlikelysolutiontothisproblemisthought
a 3 Max Planck Institut fu¨r Astronomie, Ko¨nigstuhl 17, Heidel- toberadiogalaxiesasitisusuallyobservedthatthesyn-
berg,D-69117,Germany chrotron plasma ejected by their radio jets inflates bub-
4INAF - Istituto di Radioastronomia, via Gobetti 101, 40129
bles in the hot X-ray emitting cluster gas and thereby
Bologna,Italy
5Department of Astronomy and Astrophysics, Pennsylvania heats it (see Fabian 1994 for a review). In theoretical
StateUniversity,UniversityPark,PA16802 models, a similar interplay on smaller scales has been
6DepartmentofPhysicsandAstronomy,OhioUniversity,Clip- assumed to be at work between the radio outflows of a
pingerLaboratories,Athens,OH45701
7National RadioAstronomyObservatory, P.O.Box0, Socorro, galaxy and its surrounding hot gas halo.
NM87801-0387 The first observational evidence for the ‘radio mode’
8 Institute for Astronomy, 2680 Woodlawn Dr., University of feedback in the context of galaxy formation has been
Ha9wSapiia,cHeoTneoleluscluo,pHeaSwciaeini,ce96In82st2itute,3700SanMartinDrive,Bal- found by Best et al. (2006). Based on X-ray and ra-
dio observations of a local sample of massive elliptical
timore,MD21218
10Excellence Cluster Universe, Technische Universitt Mnchen, galaxies (Best et al. 2005) they have shown that the ra-
Boltzmannstr. 2,D-85748,Garching,Germany dio source heating may indeed balance the radiative en-
11Max-Planck-Institut fu¨r Extraterrestrische Physik, Giessen-
ergy losses from the hot gas surrounding the galaxy, as
bachstr.,D-85741Garching,Germany
12York College, City University of New York, 94-20 Guy R. postulated in the models. Based on an independent
BrewerBoulevard,Jamaica,NY11451
13AmericanMuseum of Natural History, Central ParkWest at 16 The ‘radio mode’ is defined here as given in Crotonetal.
79thStreet,NewYork,NY10024 (2006).
14IASFMilano-INAF,ViaBassini15,I-20133,Milan,Italy
2 V. Smolˇci´c et al.
sample of radio galaxies in galaxy clusters (studied by (hereafter LERAGN and HERAGN, respectively)
Bˆirzan et al.2004)Best et al.(2006)havecorrelatedthe reflect different modes of black hole (BH) accretion.
mechanicalheatingprovidedbytheradiosourcesontheir Independent studies have shown that i) LERAGN are
surrounding medium with monochromatic 20 cm radio a class of radio luminous AGN that accrete radiatively
power. Combining this with the local radio AGN lumi- inefficiently (Evans et al. 2006; Hardcastle et al. 2006),
nosity function (Best et al. 2005), they have found that and ii) Bondi accretion of the hot X-ray emitting inter-
the volume-averaged mechanical energy heating rate of galactic medium (IGM) is sufficient to power the jets of
localradioluminousAGNisaboutafactorof10-20lower low-powerradiogalaxiesinthecentersofgalaxyclusters
than predicted by the Croton et al. (2006) model. They (based on a sample of nearby galaxies; Allen et al.
assigned this difference to a systematic over-prediction 2006). Based on these findings Hardcastle et al. (2007)
of the heating rate in the model (by a factor of 2-3) and havedevelopedamodelinwhichalllow-excitationradio
the intention of the model to jointly describe both the sources are powered by radiatively inefficient accretion
large- and small- scale heating from radio sources (i.e. (i.e. at sub-Eddington accretion rates) of the hot phase
on both cluster and galaxy scales). More recent calcula- of the IGM, while high-excitation sources are powered
tions of the volume averaged mechanical energy heating by radiatively efficient accretion (at ∼ Eddington rates)
rate due to AGN (K¨ording et al. 2008; Merloni & Heinz of cold gas (that is in generalunrelated to the hot IGM;
2008)yieldhighervaluesatz =0comparedtotheresults see also Merloni & Heinz 2008). This model successfully
ofBest et al.(2006),andthusclosertothe Croton et al. explains a variety of properties of radio luminous AGN,
(2006) prediction. such as their environments, signs of galaxy mergers in
To date no clear picture exists on how the AGN ra- the hosts of powerful (high-excitation) radio sources,
dio mode feedback works. More observations are needed and the break of the radio AGN luminosity function
to provide better constraints on the physics of this pro- (for details see Hardcastle et al. 2007 and references
cess, as well as its evolution with cosmic time. Here therein).
we attempt to shed light on the latter utilizing a large Inthe past two decades it has become clearthat radio
statistical sample of radio AGN, drawn from the VLA- luminous AGN evolve differentially: Low-power sources
COSMOS survey (Schinnerer et al. 2007). evolve less strongly than high-power sources. Numer-
ous studies of high luminosity radio AGN (L &
2.7GHz
1.2. Radio luminous AGN and their evolution 1025 W Hz−1 sr−1 corresponding to L1.4GHz ∼ 2 ×
1026 W Hz−1; Dunlop & Peacock 1990; Willott et al.
Fanaroff & Riley (1974, FR hereafter) were the first
2001) have found a strong positive density evolution
to note that the radio luminous AGN population con-
with redshift of these sources out to a redshift peak of
sists of two apparently distinct types of sources, re-
z ∼2. Beyond this redshift their comoving volume den-
ferred to as FR type I and II, with prominent differ-
sity starts declining (a possible sharpdensity cut-off has
ences in both radio morphology and luminosity. The
been suggested by Dunlop & Peacock 1990, but has not
radio emission from FR I radio galaxies is core dom-
been confirmed by Willott et al. 2001). Such a decline
inated, while FR IIs are edge-brightened with highly
wouldbe consistentwiththe resultsobtainedviaoptical
collimated large-scale jets. FR IIs are typically more
surveys (Schmidt et al. 1995) and, recently, X-ray sur-
powerful in the radio than FR Is, and a dividing lumi-
nosity of L ∼ 2.5 × 1026 W Hz−1 (correspond- veys (Silverman et al. 2008; Brusa et al. 2008).
ing to L 178MH∼z 6 × 1025 W Hz−1)17 has been sug- Analyzing the evolution of lower-power radio AGN
1.4GHz (L > 2×1025 W Hz−1) Waddington et al. (2001)
gested (Fanaroff & Riley 1974). It has been later shown 1.4GHz
have found a significantly slower evolution of this popu-
(Ledlow & Owen 1996) that the FR I/FR II division is
lation, with the comoving volume density turn-over oc-
alsoafunctionofthehostgalaxyopticalluminosity(with
curring at a lower redshift (z ∼ 1 − 1.5), compared
ahigherdividingradioluminosityforhigheropticalhost
to the high-power radio population. Different radio-
luminosity).
optical surveys are still somewhat controversial. While
An alternative way of classifying radio AGN is based
Clewley & Jarvis (2004) and Sadler et al. (2007) find no
ontheexistenceofhighexcitation(HE)emissionlinesin
evidence for particularlystrong evolutionoutto z ∼0.7,
theopticalspectraoftheirhostgalaxies(Hine & Longair
Cowie et al.(2004)havefoundastrongdensityevolution
1979; Laing et al. 1994). In this scheme, objects with-
outto z ∼1.5. Thus, althoughonaveragea weakerevo-
out high-excitation emission lines are referred to as low-
lutionoflow-power(comparedtohigh-power)radioAGN
excitation (LE) radio galaxies, and they are most com-
isimplied,itisstillnotverywellunderstoodhowthelow-
mon at low radio luminosities. Almost all FR I radio
luminosity radio AGN evolve with redshift. In this work
galaxies are LE sources, while optical hosts of the most
we use the VLA-COSMOS AGN sample of low luminos-
powerful radio sources, i.e. FR IIs, usually have strong
ity (96% have L <1025 W Hz−1) radio sources in
emission lines. It is noteworthy, however, that the cor- 1.4GHz
order to comprehensively constrain the evolution of the
respondence between the FR class and the presence of
low-power radio AGN out to z = 1.3. We combine our
emission lines is not one-to-one. Many FR II galaxies
resultswith the new findings andideasonthe cosmolog-
havebeen found to be low-excitationradiogalaxies(e.g.
icalrelevanceofradioAGNinordertostudy the impact
Evans et al. 2006).
of radio luminous AGN on galaxy formation.
Recently, strong evidence (Evans et al. 2006;
The paper is outlined as follows. In Sec. 2 we define
Hardcastle et al. 2006, 2007) has emerged support-
theVLA-COSMOSAGNsample. InSec.3wederivethe
ing the idea that low- and high- excitation radio AGN
radio luminosity function for the VLA-COSMOS AGN,
17 Aspectralindexofα=0.7(Fν ∝ν−α;whereFν istheradio andextendittohighradiopowersusingtheresultsfrom
fluxdensity)isassumedthroughout thepaper. Willott et al. (2001). In Sec. 4 we constrain the evolu-
Evolution of radio selected AGN 3
tion of radio AGN out to z = 1.3. In Sec. 5 we analyze 3.1. Derivation of the luminosity function (LF)
the properties of local and intermediate redshift weak
We derive the radio LF (Φ) for our AGN galaxies in
andpowerfulradioAGN;inSec.6wecomputetheradio
four redshift bins using the standard 1/V method
max
AGNmassfunction,andderiveandcomparethestarfor-
(Schmidt 1968). We limit the accessible volumes a) on
mation quenching and radio-AGN triggering rates. We
the bright end by the minimum redshift cut-off of the
discussourresultsinthecontextofgalaxyformationand
redshift range in consideration or the minimum redshift
evolution in Sec. 7, and summarize them in Sec. 8.
out to which an object could be observed due to the
We report magnitudes in the AB system, adopt H0 = optical saturation limit of i∗ = 16 (AB mag; see also
70, Ω = 0.3,Ω = 0.7, and define the radio syn-
M Λ Capak et al. 2007), and b) on the faint end by the max-
chrotron spectrum as F ∝ν−α, assuming α=0.7.
ν imum redshift out to which a galaxy could be observed
given the flux limits on both the radio and optical data.
2. THEVLA-COSMOSRADIOAGNSAMPLE
Inpractice,thelatterisdominatedbytheradiodetection
The sample of AGN galaxies used here is presented limits, rather than the optical, as the major fraction of
in Smolˇci´c et al. (2008a, S08a hereafter), and we briefly the sources used here has i band magnitudes brighter
AB
summarize it below. than ∼24 (see Fig. 21 in S08a).
Using radio and optical data for the COSMOS field, We further take into account the non-uniform rms
S08a have constructed a sample of 601 AGN galaxies noise level in the VLA-COSMOS mosaic via the differ-
with z ≤ 1.3 from the entire VLA-COSMOS Large entialvisibility area(i.e. arealcoverage,A ,vs. rms;see
k
Project catalog (Schinnerer et al. 2007). The selection Fig. 13 in Schinnerer et al. 2007). Hence, for a source
required optical counterparts with iAB ≤ 26, accurate with a 1.4 GHz luminosity Lj its maximum volume is
photometry, and a S/N ≥ 5 (i.e. & 50 µJy) detection V (L )= n A ·V (zAk ,L ).
at 20 cm, and is based on a rest-frame optical color mIanxordjer to rko=b1ustlky demraivxe tmhaexLFjwe take several ad-
classification (see also Smolˇci´c et al. 2006). The clas- ditional corrPections into account: a) the VLA-COSMOS
sification method was well-calibrated using a large lo- detection completeness (Bondi et al. 2008), and b) the
cal sample of galaxies (∼ 7,000 SDSS “main” spec- AGN galaxy selection bias due to the rest-frame color
troscopic galaxy sample, NVSSand IRAS surveys) rep- uncertainties. We correct for these in the same way
resentative of the VLA-COSMOS population. It was as described in Smolˇci´c et al. (2008b, S08b hereafter).
shown that the method agrees well with other indepen- The median of the first correction (as a function of flux
dent classification schemes based on mid-infrared colors density) is ∼ 10% (reaching a maximum value of 60%
(Lacy et al. 2004; Stern et al. 2005) and optical spec- in only one of the lowest flux density bins; see Tab. 1
troscopy(Baldwin, Phillips & Terlevich1981;Best et al. in Bondi et al. 2008). The second correction, based on
2005). The selected sample of AGN is estimated to be Monte Carlo simulations, yields a reduction of ∼ 5% of
∼90% complete. the average number of radio AGN (see below; see also
Therest-framecolorclassificationprocedureefficiently S08b for a more detailed description).
selects mostly type 2 AGN (such as LINER, Seyferts), Hence, in the ith luminosity bin the comoving space
and absorption-line AGN (with no emission lines in the
density (Φ ), and its corresponding error (σ ), are
i i
opticalspectrum),whiletype1AGN(i.e.quasars,.20%
computed by weighting the contribution of each (jth)
ofthetotalAGNsample)arenotincludedinthecurrent
galaxy by the completeness correction factor, f (see
sample(see S08afordetaileddefinitions ofthe samples). det
Bondi et al. 2008):
Although based on a color identification (as opposed
to an optical spectroscopic classification), the output N fj N fj 2
of the selection of our intermediate redshift (z ≤ 1.3) Φ = det ; σ = det (1)
AGN sample is comparable to those of numerous local i j=1 Vmj ax i vuj=1 Vmj ax!
(z ≤ 0.3) radio AGN samples extensively studied in X uX
t
The selection bias due to the rest-frame color uncer-
the literature (e.g. Sadler et al. 2002; Best et al. 2005;
taintiesisaccountedforviaMonteCarlosimulations. As
Mauch & Sadler 2007, drawn from the SDSS, 2dF, and
described in S08a and S08b, in each iteration the rest-
6dFopticalsurveyscombinedwiththeNVSSandFIRST
frame color error distribution is generated (see Fig. 5 in
20 cm radio surveys). This is an important feature as it
S08a)andadded to the rest-frame colorderivedby SED
enablesastraight-forwardandfaircomparisonofourre-
fitting. AGN galaxies are then re-selected, and the LF
sults with those based on these local studies (e.g. the
is derivedas described above. In this way we obtain100
local AGN radio luminosity function).
realizations of (Φ ,σ ) for each luminosity bin, and we
Out of the 601 selected AGN galaxies 262 have spec- i i
take the median values as representative.
troscopic redshifts, while the remaining sources have
very reliable photometric redshifts available (σ ∆z = 3.2. The radio AGN luminosity function
1+z
0.027;see S08a andreferencestherein). Basedo(cid:16)nMo(cid:17)nte The 1.4 GHz radio LFs for our AGN galaxies for
Carlo simulations, S08a have shown that the photomet- the 4 chosen redshift bins are shown in Fig. 1, and
ricerrorsintherest-framecolorintroduceasmall,∼5%, tabulated in Tab. 1. In each panel in Fig. 1 we
uncertainty in the number of the selected galaxies. Here also show the local 20 cm LFs for AGN derived by
we use the S08a sample of AGN galaxies, statistically Condon et al. (2002), Sadler et al. (2002), Best et al.
corrected for this effect. (2005), and Mauch & Sadler (2007). Our derived LF in
the local redshift bin (top left panel) agrees well with
3. THE1.4GHZLUMINOSITYFUNCTIONFORRADIO the local LFs. This is quite remarkable as a) our identi-
AGNINVLA-COSMOS ficationmethodisbasedonlyonphotometrycontraryto
4 V. Smolˇci´c et al.
1025 W Hz−1; Dunlop & Peacock 1990; Willott et al.
TABLE 1 2001; Waddington et al. 2001), while constraining the
Luminosity functionsforVLA-COSMOS
low-luminosity end (sampled here) has been hampered
AGN
by low number statistics due to small field sizes.
redshift L1.4GHz Φ Cowie et al. (2004) combined two deep 1.4 GHz radio
range [WHz−1] [Mpc−3dex−1] surveys of the HDF-N (40′ in diameter; 5σ ∼ 40 µJy in
the central region) and SSA13 (34′ in diameter; 5σ ∼
4.47·1021 5.08±2.25·10−4
1.41·1022 2.54±0.66·10−4 25 µJy in the center) fields, and derived the AGN radio
4.47·1022 2.22±0.40·10−4 LFintwoseparateredshiftrangessimilartoours,andat
1.58·1023 4.54±1.57·10−5 comparable luminosities to our work. Our results are in
0.1<z≤0.35 5.62·1023 1.21±0.70·10−5 qualitativeagreementwiththosederivedbyCowie et al.
1.78·1024 1.66±0.88·10−5 (2004,c.f.theirFig.3). However,theirsampleisafactor
5.62·1024 7.90±5.59·10−6 of ∼7 smaller than the one used here.
1.78·1025 1.46±0.84·10−5
37..1964··11002222 11..9700±±00..5259··1100−−44 4. THEEVOLUTIONOFRADIOAGN
2.00·1023 4.48±1.18·10−5 In this section we constrain the evolution of our low-
0.35<z≤0.6 5.62·1023 1.40±0.57·10−5 power radio AGN using the VLA-COSMOS AGN data
1.41·1024 8.96±3.96·10−6 (Sec. 4.1). We further extend this to high-power ra-
3.16·1024 4.49±2.81·10−6
7.94·1024 4.53±2.84·10−6 dio AGN using the model obtained by Willott et al.
1.00·1023 8.76±2.32·10−5 (2001) based on high-power AGN samples drawn from
2.51·1023 8.75±1.36·10−5 the 3CRR, 6CE, and 7CRS surveys (Sec. 4.2).
6.31·1023 2.86±0.61·10−5
0.6<z≤0.9 1.78·1024 2.45±0.50·10−5 4.1. The evolution of low-power radio AGN galaxies in
4.47·1024 4.11±1.82·10−6 the COSMOS field
1.00·1025 3.43±1.66·10−6
2.24·1025 6.88±7.45·10−7 The evolution of an astrophysical population is usu-
3.16·1023 3.72±0.93·10−5 ally parameterizedby monotonic density andluminosity
7.08·1023 2.28±0.47·10−5 evolution of its local luminosity function:
1.78·1024 1.82±0.34·10−5
0.9<z≤1.3 5.01·1024 3.97±1.38·10−6 L
13..2166··11002255 41..2452±±10..3737··1100−−66 Φz(L)=(1+z)αD ×Φz=0 (1+z)αL (2)
(cid:20) (cid:21)
7.08·1025 7.16±5.48·10−7
1.78·1026 1.46±0.79·10−6 where αD and αL are the characteristic density and
. H0=70,ΩM =0.3,ΩΛ=0.7 luminosity evolution parameters, respectively, L is the
luminosity, Φ (L) is the luminosity function at redshift
z
the spectroscopic selection performed by the other stud- z, and Φ is the local luminosity function.
z=0
ies (Sadler et al. 2002; Best et al. 2005; Mauch & Sadler It is well knownthat strong degeneracybetween lumi-
2007), and b) the 22◦ COSMOS field samples a signif- nosity and density evolution exists, even if the observa-
icantly smaller comoving volume at these low redshifts tionaldatasampletheturn-over(‘knee’)oftheLFatdif-
comparedtothealmostall-skysurveysusedbytheother ferentcosmologicaltimes(seee.g.Le Floc’h et al.2005).
studies. This verifies that both our selection method, TheVLA-COSMOSAGNsampleinparticularlackshigh
as well as the derivation of the LF are correct. Within luminosity objects that could constrain the turn-over of
the errors our lowest-redshift LF seems to be best rep- the LF in all our redshift bins (see Fig. 1). Hence, we
resented by the local LF derived by Sadler et al. (2002, do not attempt to try to break the luminosity – density
Sad02 hereafter), and we adopt this local LF for further evolution degeneracy, but we separately constrain both
analysis of the evolution of our AGN population. pure density (PDE; α =0) and pure luminosity (PLE;
L
In the top right panel in Fig. 1 we compare our de- α = 0) evolution based on our data. We adopt the
D
rived AGN LF (0.35 < z ≤ 0.6) with the volume densi- Sad02 local AGN LF (Φ ) as the representative LF in
z=0
tiescomputedbySadler et al.(2007)foraredshiftrange the local universe, which is given by the following ana-
0.4 < z < 0.7 based on 2SLAQ Luminous Red Galaxy lytical form:
Survey and NVSS, FIRST data. We convert their data
to a base of dlogL, and scale them to match the mean
1−α 2
redshift of our redshift bin (z =0.475 compared to their Φ(L)=Φ∗ L exp − 1 log(1+ L ) (3)
z =0.55)usingpureluminosityevolutionasobtainedby (cid:20)L∗(cid:21) ( 2σ2 (cid:20) L∗ (cid:21) )
Sadler et al.(2007,notethatthisonlydecreasestheirra-
dioluminositiesby10%). TheLFsareinexcellentagree- where α = 1.58, σ = 1.0, Φ∗ = 7.6× 10−6 Mpc−3,
ment. Itisnoteworthythatwewellconstrainthevolume and L∗ = 2.1×1024 W Hz−1 for their AGN population
densities at the low luminosity end (. 1025 W Hz−1), (scaled to the cosmology used here, and to the base of
while the sample of Sadler et al. (2007) extends further dlogL).
out to 1.6×1027 W Hz−1 (see also Fig. 3, and Sec. 7). For both PDE and PLE we derive the evolution by
The VLA-COSMOS AGN volume densities in the summing the χ2 distributions obtained for a large range
two highest redshift bins (0.6 < z ≤ 0.9 and 0.9 < of evolution parameters α and α [(α ,0) for PLE;
L D L
z ≤ 1.3) are shown in the bottom panels in Fig. 1. (0,α ) for PDE] in each particular redshift bin (exclud-
D
The radio AGN LF at these redshifts has been stud- ing our first – local – redshift bin). The uncertainty in
ied in full detail only for higher power AGN (L > α is then taken to be the 1σ error obtained from the
1.4GHz
Evolution of radio selected AGN 5
Fig. 1.— 1.4GHzluminosityfunctions (LFs)forAGN inthe VLA-COSMOSsurvey(filledredcircles),shownforfourredshiftranges
indicated in each panel. The local AGN volume densities derived by Condonetal. (2002, dashed black line), Sadleretal. (2002, solid
orangeline),Bestetal.(2005,emptysquares),andMauch&Sadler(2007,dottedline)arealsoplottedineachpanel. Theemptysymbols
inthetoprightpanelaretheradioAGNvolumedensitiesderivedfortheredshiftrange0.4<z<0.7bySadleretal.(2007)forthe2SLAQ
LuminousRedGalaxysurveysample,andscaledheretomatchourmeanredshiftvalueof0.475(seetextfordetails).
χ2 statistics. Our results yield a pure density evolution ity evolution with L∗ ∝ (1+z)0.8±0.1 or a pure density
with αD = 1.1±0.1, or alternatively a pure luminosity evolutionwhereΦ∗ ∝(1+z)1.1±0.1. Regardlessofwhich
evolution with α =0.8±0.1. modelisphysicallymoreappropriatetodescribethereal
L
In Fig. 2 we show the luminosity density for our AGN cosmicevolutionoftheVLA-COSMOSAGNpopulation,
in the four redshift ranges defined in Fig. 1. We also both imply a modest evolution of radio luminous AGN
plottheluminositydensitygiventhederivedPDandPL in the luminosity range of ∼1022−25 W Hz−1.
evolutions (lines). From the figure it becomes obvious Our results are consistent with previous findings.
that we cannot distinguish between these two types of Based on a V/V analysis out to a redshift of ∼
max
evolutiongivenourdata,asoursample,tracingonlythe 1, Clewley & Jarvis (2004) have found no strong evo-
lowerluminosity endofthe luminosity function, appears lution of low radio luminosity sources (L .
325MHz
to be described equally well by both. Thus, in the fur- 1025 W Hz−1 sr−1, corresponding to L . 4.5 ×
1.4GHz
ther analysis we will take the range of the two best fit 1025 W Hz−1) at least up to z ∼ 0.6. Further,
evolution models (taking also their errors into account) Sadler et al.(2007)haveconstrainedtheevolutionofthe
as representative of the range of uncertainties. luminosity function for radio luminous AGN (L ∼
1.4GHz
In summary, our results imply either a pure luminos- 1024−27 W Hz−1) out to z ∼ 0.7. Using the local AGN
6 V. Smolˇci´c et al.
AGN LF using two radio populations – a less powerful
(L ∼ 2.5×1025−27 W Hz−1) population compris-
1.4GHz
ing both FR I and FR II sources, and a powerful popu-
lation(L &2.5×1026 WHz−1)comprisingmostly
1.4GHz
FR II sources. They have modeled the evolution of the
firstpopulationasapuredensityevolutionuptoamaxi-
mumredshift(∼0.7)beyondwhichanyevolutionceases.
The evolution of the powerful population has been as-
sumed to change in density following a Gaussian distri-
bution in redshift, which was allowed to have a different
shape beyond its redshift peak at z ∼ 2 (see Tab. 1 in
Willott et al.2001). ItisworthnotingthattheWillottet
al. modelagreeswellwiththe Dunlop & Peacock(1990)
steep spectrum model.
The VLA-COSMOS AGN sample constrains the faint
end of the radio AGN luminosity function, and here we
use the Willott et al. (2001) model to extend our radio
LF to high powers. In Fig. 3 we compare our LFs with
the Willott et al. (2001) model, after the latter hasbeen
convertedtothecurrentcosmologyandthe151MHzra-
dio luminosities scaled to 1.4 GHz. The VLA-COSMOS
AGNdataandtheSad02LFconstraintheradioLFmore
robustly at the faint end compared to the Willott et al.
(2001)modelfortheirless-powerful radio AGN.Thus,in
the further analysis we will constrain the low-power ra-
Fig. 2.— LuminositydensityfortheVLA-COSMOSAGNinfour dio AGN LF and its evolutionusing the VLA-COSMOS
redshiftranges(filledcircles). Thelinesrepresentthebestfitpure sample as described in the previous section, and we will
density(dashed line)and pureluminosity(solidline)evolution of useonlythe powerfulradioAGNmodelbyWillott et al.
thelocal20cmAGNluminosityfunction(seetextfordetails).
(2001)todescribetheevolutionofhighradio-powerAGN
LF given by Mauch & Sadler (2007) they have found (see dash-dotted curves in Fig. 3).
a pure luminosity evolution with L∗ ∝ (1 + z)2.0±0.3.
The VLA-COSMOS AGN LF in the redshift range 4.3. The evolution of the comoving radio luminosity
0.35 < z ≤ 0.6, and the Sadler et al. (2007) AGN LF density for AGN galaxies
(0.4 < z < 0.7) agree remarkably well (see top right
Ataspecificcosmictimetheintegratedcomovinglumi-
panel in Fig. 1 & Fig. 3). If we parameterize the evo- nositydensityrepresentsthetotalpowerperunitcomov-
lution of the VLA-COSMOS AGN in the same way as
ing volume of a given astronomical population. Thus,
described in Sadler et al. (2007, using the same local if divided into distinct populations of objects it traces
LF), we derive a pure luminosity evolution of 1.2±0.4
their relative contribution to the overall power output
for the 0.35 < z ≤ 0.6 redshift bin. It has to be
at a given redshift. To estimate the contribution of low
noted,however,thattheVLA-COSMOSandthe2SLAQ
and high radio power AGN to the AGN radio luminos-
samples constrain different radio luminosity ranges (see
ity output at different cosmic times, we investigate in
Fig. 3). Our results agree within the uncertainties with the following the evolution of the comoving 1.4 GHz lu-
those of Mauch & Sadler (2007), however they on aver-
minosity density, Ω , for our VLA-COSMOS AGN
1.4GHz
age imply a slower evolution. This is consistent with (L . 1025 W Hz−1) and the high-power AGN
1.4GHz
a more modest evolution of low radio-power, compared (L & 1026 W Hz−1), adopting the Willott et al.
1.4GHz
to high radio-power, radio AGN (Willott et al. 2001;
(2001) model.
Waddington et al. 2001; Clewley & Jarvis 2004). Our
For a given redshift Ω has been computed by
1.4GHz
findings are also in very goodagreementwith the recent
integrating the luminosity density over the entire range
resultsbasedonNVSS/FIRSTandtheMegaZ-Luminous of radio luminosities. For the VLA-COSMOS AGN the
RedGalaxycatalogdrawnfromtheSDSS(Donoso et al.
luminosity density has been constrained using our best
2008).
fit PLE and PDE models (see curves in Fig. 2 and
Sec. 4.1), and for the high-power population using the
4.2. The evolution of high-power radio AGN
Willott et al. (2001) model ‘C’, with the corresponding
Inordertoquantifythecontributionofdifferentpopu- errors (see tab. 1 in Willott et al. 2001 and Sec. 4.2).
lations to the comoving radio energy density at different InFig.4weshowthe evolutionofthecomoving20cm
cosmictimes, all radioAGNpopulationsneedtobe con- integrated luminosity density for all radio AGN as well
sidered. As low- and high- power radio AGN seem to as the low and high radio power AGN separately. The
evolve in a different manner (see Sec. 1) and our VLA- evolutionofthesetwopopulationsisverydifferent;high-
COSMOS AGN sample only the low power radio AGN, powerAGNevolvesignificantlystrongerthanlow-power
we need to make the following assumptions about the AGN. The total Ω for radio AGN is dominated by
1.4GHz
evolution of the high power radio sources. low-power AGN at low redshifts (z . 0.7) where the
Based on the 3CRR, 6CE, and 7CRS radio sur- contribution of high-power radio sources is negligible.
veys combined with complete optical spectroscopy, However, at z & 0.7 high-power sources begin to con-
Willott et al.(2001)havesuccessfully modeledthe radio tribute significantly to the overall integrated luminosity
Evolution of radio selected AGN 7
Fig. 3.— VLA-COSMOSAGNvolumedensitiesat20cminfourredshiftranges(redfilledcircles;analogous toFig.1). Alsoshownin
eachpanel isourbestfitevolution correspondingtotherangegivenbypuredensityandpureluminosityevolution oftheSad02localLF
and their corresponding errors (orange shaded curve), as well as the radio AGN luminosity function model given by Willottetal. (2001,
model ‘C’) for their less luminous population (dashed blue curves), and high-luminosity population (dash-dotted blue curves; scaled to
currentcosmology and 1.4GHzradiofrequency; seetext fordetails). In the toprightpanel the volumedensities derived bySadleretal.
(2007,scaledtomatchourmeanredshiftvalueof0.475;seealsoFig.1)areshown.
density, and at a redshift of ∼ 1.3 their contribution to theywillnolongerbeabletohaveaphaseofhighaccre-
the total AGN Ω is comparable to the one of low- tion(i.e.vigorousBHgrowth),consistentwithnumerous
1.4GHz
power AGN. The implications of these populations for previousstudiesofsimilarsamples(e.g.Allen et al.2006;
galaxy formation and evolution, also out to higher red- Evans et al. 2006; Hardcastle et al. 2006, 2007). For the
shifts (z =2.5), are discussed in Sec. 7. high radio power AGN on the other hand, extensive ev-
idence exists in the literature that they accrete at high
5. PROPERTIESOFRADIOAGN rates – representing a mode of substantial BH growth.
In this section we outline and compare the properties
5.1. Local universe
of low and high radio-powerAGN in the local (Sec. 5.1)
and intermediate-redshift (Sec. 5.2) universe. We find Variouscorrelationsarefoundintheliteraturebetween
that already by z ∼ 1 the host galaxies of low-power, the presence of emission lines in AGN and their e.g. ra-
VLA-COSMOSAGNhavebuilt-upstellarandblackhole dio power, black hole and stellar mass, as well as envi-
massescomparabletothehighestmassgalaxiesobserved ronmentand the galaxies’gas content. We outline these
locally. As their black hole masses are already signif- below.
icant at these intermediate redshifts, this implies that First, almost all FR I – low power – radio galaxies are
8 V. Smolˇci´c et al.
masses, as well as lower black hole masses compared to
LERAGN. Even further, the latter constitute the most
massive galaxies in the universe (M⊙ > 1011 M⊙) that
preferentially occupy the centers of high galaxy density
regions (Baum et al. 1992, Best et al. 2005).
Furthermore, various studies in the literature have
shown that HERAGN, contrary to LERAGN, tend
to show unusually blue off-nuclear continuum colors
(Baum et al. 1992), evidence of recent star formation
(Baldi & Capetti 2008), and often have distorted opti-
cal morphologies suggesting that they have undergone a
recent major merger (Heckman et al. 1986; Baum et al.
1992; Baldi & Capetti 2008). CO and HI observations
of radio galaxies suggest larger amounts of cold gas in
powerful,comparedto low-power,radio sources(see e.g.
Fig. 8 in Evans et al. 2005,see also Emonts et al. 2008),
and there is evidence supporting quantitatively larger
amounts of dust, and therefore gas (Leon et al. 2001;
Solomon & Vanden Bout 2005) in HERAGN compared
to LERAGN (de Koff et al. 2000,Mu¨ller et al. 2004). A
summary of the properties of radio AGN in the nearby
universe is given in Tab. 2.
5.2. Intermediate redshift
Fig. 4.— Evolutionofthecomoving20cmintegratedluminos-
itydensity forVLA-COSMOSAGN (orange curve) galaxies since Inthis sectionweinvestigatethe propertiesoflowand
z = 1.3. Shown is also the evolution of the high-luminosity ra-
high radio-power AGN at intermediate redshifts, and
dio AGN, adopted from Willottetal. (2001, hatched region; the
thick and dashed lines correspond to the mean, maximum and compare these to the properties of their local counter-
minimum results, respectively). The evolution for the total AGN parts. In Sec. 3 and Sec. 4 we have derived the 1.4 GHz
population, obtained by co-adding the VLA-COSMOS and high- radio luminosity function, and its evolution, for a sam-
luminosityAGNenergydensities,isshownasthered-shadedcurve
ple of ∼ 600 radio luminous AGN (0.1 < z < 1.3)
(seetextfordetails).
drawn from the VLA-COSMOS survey. One of the
main characteristics of this radio AGN sample is that
LERAGN, while optical hosts of FR IIs, which are typi-
it consistsof predominantly (96%)low-powerAGN with
cally more powerful than FR Is (Fanaroff & Riley 1974;
L . 1025 W Hz−1. Ledlow & Owen (1996) have
Owen 1993; Ledlow & Owen 1996), usually have strong 1.4GHz
emission lines18. Recently, based on a large statistically shown that the threshold between weak FR I types of
galaxies and powerful FR IIs is a function of the host
significantsample of localradio- opticalsources(SDSS-
galaxy’soptical(Rband)magnitude,M . Hence,tobet-
NVSS-FIRST) Kauffmann et al. (2008) have found that R
ter constrain the properties of the VLA-COSMOS AGN
thefractionofradioAGNwhoseopticalhostshaveemis-
in Fig. 5 we plot them in the L vs. M plane. Al-
sion lines in their spectra (predominantly HERAGN) is 1.4GHz R
mostall(98%)ofourAGNoccupythe weakradioAGN,
a strong function of radio luminosity. This emission-
FR I region in this plane.
line galaxy fraction is roughly constant (∼ 40%) up to
L ∼ 1025 W Hz−1, beyond which it steeply rises In order to get a further insight into the composition
ap1.p4rGoHazching ∼ 80% at ∼ 4×1025 W Hz−1 (see Fig. 3 of the VLA-COSMOS AGN, we make use of the sources
with available optical spectroscopy. Trump et al. (2007)
inKauffmann et al.2008). This ‘critical’luminosity, ob-
have carried out a spectroscopic survey of X-ray and ra-
served by Kauffmann et al., is remarkably close to the
dioselectedAGNcandidatesintheCOSMOSfield(using
FR I – II break luminosity, as well as to the power
the Magellan/IMACS instrument). They have classified
which roughly separates the radio sources which show
their sources into a) broad emission line AGN (‘bl’), b)
strong cosmological evolution from those which do not
narrowemissionlineAGN(‘nl’),c)redgalaxieswithonly
(see e.g. Fig. 4 and Sec. 4). Furthermore, based on the
absorptionfeaturesintheir spectrum(‘e’) andd)hybrid
results of Kauffmann et al., the luminosity of L ∼
1025 W Hz−1 can be thought of as a rough thres1h.4oGlHdzbe- objects showing a mix of narrow emission lines and a
red galaxy continuum shape (‘nle’). Combining a radio
tween high- and low- excitation radio AGN. Thus, most
of the low radio power AGN (L . 1025 W Hz−1) AGN detection with this classification scheme, we can
1.4GHz very roughly consider the red and hybrid galaxy spec-
areLERAGN,whilethemajorityofpowerfulradioAGN
(L &1025 W Hz−1) are HERAGN. troscopic classification (‘e’+’nle’) as a proxy for LER-
1.4GHz AGN, and the narrow emission line AGN classification
On the other hand, the fraction of emission line (i.e.
(‘nl’) as a proxy for HERAGN. About 35% of our VLA-
high-excitation)radioAGNinthelocaluniversestrongly
COSMOS AGN have an available classification based
decreases as a function of both stellar mass and velocity
on IMACS spectroscopy, and in Fig. 6 the fractions of
dispersion (see Fig. 3 in Kauffmann et al. 2008). This
‘e+nle’ and ‘nl’ galaxies are shown as a function of both
implies that, at least locally, high-excitation (or alterna-
redshift (left panel) and radio luminosity (right panel).
tively high radio power) AGN tend to have lower stellar
The absorption line and hybrid galaxies dominate the
18Note,however,thatthecorrespondencebetweentheFRclass VLA-COSMOS sample at a constant ∼ 80% level at all
andthepresenceofemissionlinesisnotexactlyone-to-one. redshifts (z ≤1.3), while the narrow emission line AGN
Evolution of radio selected AGN 9
TABLE 2
Propertiesof LERAGN andHERAGNin thelocal universe
LERAGN HERAGN
Property Ref. Ref.
Objectclass mainlyFRI (1),(2) mainlyFRII (1),(2)
radioluminosity L1.4GHz .1025 WHz−1 (3) L1.4GHz &1025 WHz−1 (3)
densityenvironment moderate-to-high (4),(5) moderate-to-low (4)
opticalmorphology regular (8),(9) oftendistorted (8),(9),(10)
opticalcolor red (9),(11) bluer(comparedtoLERAGN) (10),(9)
stellarmass highest(&5×1010 M⊙) (3),(11) lower(comparedtoLERAGN) (3),(12)
BHmass highest(∼109 M⊙) (9),(13) lower(comparedtoLERAGN) (3),(14)
ISMcontent low (15),(16),(17) higher(comparedtoLERAGN) (15),(16),(17),(18),(19)
accretionmode radiativelyinefficient (1),(2),(14),(20) radiativelyefficient (1),(2),(20),(21),(22)
Note. — References: (1) Evansetal. 2006, (2) Hardcastleetal. 2006, (3) Kauffmannetal. 2008, (4) Baumetal. 1992,
(5) Bestetal. 2005, (8) Heckmanetal. 1986, (9) Baumetal. 1992, (10) Baldi&Capetti 2008, (11) Smolˇci´cetal. 2008a, (12)
Tasseetal. 2008, (13) this work, (14) Chiabergeetal. 2005, (15) Leonetal. 2001, (16) deKoffetal. 1996, (17) Mu¨lleretal.
2004,(18)Evansetal.2006,(19)Emontsetal.2008,(20)Hardcastleetal.2007,(21)Barthel1989,(22)Haas2004
radio AGN sample. Thus, similar to the findings in the
localuniverse,ourlowradiopowerAGNatintermediate
redshift are preferentially LERAGN.
Stellar masses, using a Chabrier (2003) initial mass
function(IMF),havebeencomputedfortheentireVLA-
COSMOS galaxy sample (z ≤ 1.3) by S08a. In Fig. 7
(top panels) we show the stellar masses for our VLA-
COSMOS AGN as a function of redshift and 1.4 GHz
luminosity. The median stellar mass of our radio AGN
is 1.6×1011 M⊙, i.e. < logM∗ >= 11.2 (in any given
redshift range) with a 1σ scatter of 0.4 dex. This is
consistent with the stellar masses of the most massive,
local galaxies (e.g. Baldry et al. 2004; Best et al. 2005).
In addition, it is worth noting that their average rest-
frame colors are consistent with red galaxy colors (see
e.g. Fig. 9 in S08a).
We further compute the BH masses for the full VLA-
COSMOS AGN sample using the local correlation given
by Marconi & Hunt (2003) which relates the K-band lu-
minosity to the BH mass:
log M =8.21+1.13×(log L −10.9) (4)
10 BH 10 K
L the rest-frame K-band luminosity (also in so-
Fig. 5.— Monochromatic 1.4 GHz radio power for VLA- K
lar units). The scatter in the relation is 0.5 dex
COSMOSAGNasafunctionoftheirhostgalaxyabsoluteRband
magnitude. Thedashedlinecorrespondstotheseparationbetween (Marconi & Hunt 2003). The K-band rest-frame lu-
FRIandFRIItypesofgalaxiesgivenbyLedlow&Owen(1996). minosity for our VLA-COSMOS AGN was computed
Note that almost allof the VLA-COSMOSAGN occupy the low- via SED fitting as described in detail in S08a. In or-
powerFRIregionofthisplane.
der to take into account passive luminosity evolution
contribute about ∼ 20% at all redshifts. There is an in- (see Hopkins et al. 2006b and references therein) we
dication that in the highest redshift bin (1 < z < 1.3), change the normalization constant of the above rela-
where our most luminous (L > 1025 W Hz−1) ra- tion as a function of redshift following the results from
1.4GHz
dioAGNareobserved(seeFig.1),thefractionofnarrow Hopkins et al. (2006b, see short-dashed line in the bot-
emissionline AGN rises. However,giventhe largeerror- tomleftpanelin their Fig.2). InFig.7(bottom panels)
bars in this redshift range (due to the low number of we plot the estimated BH masses as a function of both
spectroscopically observed radio AGN) we cannot draw redshift and radio 1.4 GHz luminosity. The median BH
anyrobustconclusionsregardingthis. Inthe rightpanel mass of our radio AGN is log M = 8.8 with a stan-
10 BH
ofFig.6weshowthe fractionof‘e+nle’and‘nl’galaxies dard deviation of 0.4. Note that the apparent trend of
asafunctionof1.4GHz power. Althoughthenumberof BH mass with radio luminosity (bottom right panel in
sources with L &1025 W Hz−1 is low in the VLA- Fig. 7) reflects the dependence of radio luminosity on
1.4GHz
COSMOS AGN sample, there is an indication that the redshift in our radio flux limited sample (see Fig. 16 in
fraction of narrow-line objects (thus roughly HERAGN) S08a) rather than a real trend.
increases beyond ∼ 1025 W Hz−1, consistent with the The black hole masses of our intermediate redshift ra-
findings in the local universe (Kauffmann et al. 2008). dioAGNarecomparabletothehighestblackholemasses
Assumingthatthespectroscopicsub-samplerepresents known in the local universe (see e.g. McLure & Dunlop
wellthe full sample (note that this is a rather robustas- 2002,2004). Thisimplies thatthe low-powerradioAGN
sumptionasshowninS08a;seetheir Figs.3 and21),we have assembled their BH masses already by these in-
conclude that LERAGN dominate the VLA-COSMOS termediate redshifts, and that their BHs cannot grow
10 V. Smolˇci´c et al.
Fig. 6.— The fraction of VLA-COSMOS AGN with available Magellan/IMACS spectroscopy (Trumpetal. 2007) as a function of
redshift(leftpanel)and1.4GHzradioluminosity(rightpanel). Thegalaxies havebeendividedintotwosub-samples,i)narrowemission
line (nl) galaxies, and ii) elliptical galaxies (e) plus composite objects showing a red galaxy continuum and narrow emission lines (nle;
see Trumpetal.2007 for details). These two subsamples roughlycorrespond to high- and low-excitation AGN, respectively (see text for
details). Thequantityandsymbolsforobjectsofeachtypeisindicatedintheleftpanel. Notethatthereisaslight(althoughnoisy)trend
showing that the fraction of narrow emissionline objects (i.e. roughly high-excitation AGN) increases as a function of both redshift and
radioluminosity.
at sub-Eddington rates (Allen et al. 2006; Evans et al.
2006; Hardcastle et al. 2006, 2007).
Given all of the above, the composition of our VLA-
COSMOS AGN, the majority of which are shown to be
LERAGN,is consistentwith the propertiesoflowradio-
power AGN in the local universe: Our intermediate red-
shift AGN have already by z ∼ 1 assembled both their
stellar and black hole masses, comparable to the high-
mass end of the galaxies known today.
Ontheotherhand,powerfulradiogalaxiesathighred-
shifts tend to show strong emission lines in their optical
spectra (Rawlings et al. 1989, Baum & Heckman 1989,
Rawlings & Saunders 1991; Willott et al. 1999, 2000),
and are often associated with ongoing star-formation
(Archibald et al. 2001, Greve et al. 2006, Seymour et al.
2008)assuggestedbytheirbluerrest-framecolors(com-
paredtored-sequencegalaxies). Again,thisisconsistent
with local observations of such sources. In numerous
studies in the literature (e.g. Barthel 1989; Haas et al.
2004) these objects have been shown to accrete radia-
tively efficiently at high (∼ Eddington) accretion rates.
Fig. 7.— Logarithm of stellar (top panels) and black hole Thus these HERAGN present a mode of significant BH
(bottom panels) masses for the VLA-COSMOS AGN as a func-
growth – unlike the low radio power LERAGN.
tion of redshift (left panels) and 1.4 GHz radio luminosity (right
panels). The stellar masses were computed via SED fitting using To summarize, the properties of local and
the Bruzual&Charlot (2003) stellar population synthesis models intermediate-redshift radio AGN, as shown above,
(Chabrier 2003 IMF; see S08a for full details). The BH masses suggestthatradioactivityistriggeredinsimilarpopula-
werederivedusingeq.4,andcorrectedforpassiveluminosityevo-
tionsofobjects independent oftheir redshift,i.e. cosmic
lution (see text for details). The dashed line in the right column
corresponds to the lowest 1.4 GHz luminosity observable out to time. This is in agreement with the finding that the
z=1.3giventhe VLA-COSMOSLargeProjectdetection limitof rest-frame optical colors of radio source host galaxies
∼50µJybeam−1. do not change with redshift (Barger et al. 2007, S08a,
Huynh et al. 2008). In addition, there is converging
significantly since z ∼ 1.3. Therefore, they must be in evidence that low radio power AGN reflect a modest –
a mode of modest growth in BH mass. This result is radiatively inefficient – mode of BH growth, while high
in agreement with numerous findings in the literature radio power AGN are undergoing a phase of significant
implying that LERAGN accrete radiatively inefficiently,