Evidence for Widespread AGN Activity among Massive Quiescent Galaxies at z ~ 2

0.47 MB·English

by  Karen Pardos Olsen

Draftversion January17,2013
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
EVIDENCE FOR WIDESPREAD ACTIVE GALACTIC NUCLEUS ACTIVITY AMONG MASSIVE QUIESCENT
GALAXIES AT Z ∼2
Karen P. Olsen, Jesper Rasmussen, Sune Toft, and Andrew W. Zirm
DarkCosmologyCentre,NielsBohrInstitute, UniversityofCopenhagen, JulianeMariesVej30,DK-2100Copenhagen, Denmark;
[email protected]
Draft version January 17, 2013
ABSTRACT
3
1 We quantify the presence of active galactic nuclei (AGNs) in a mass-complete (M∗ >5×1010M⊙)
0 sample of 123 star-forming and quiescent galaxies at 1.5 ≤ z ≤ 2.5, using X-ray data from the 4
2 Ms Chandra Deep Field-South (CDF-S) survey. 41%±7% of the galaxies are detected directly in
X-rays, 22%± 5% with rest-frame 0.5-8keV luminosities consistent with hosting luminous AGNs
n
(L > 3×1042ergs−1). The latter fraction is similar for star-forming and quiescent galaxies,
a 0.5-8keV
J anddoesnotdependongalaxystellarmass,suggestingthatperhapsluminousAGNsaretriggeredby
external effects such as mergers. We detect significant mean X-ray signals in stacked images for both
6
the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star
1
formationonly and/ora low-luminosity AGN in both cases. Comparing star formation rates inferred

from the 2-10keV luminosities to those from rest-frame IR+UV emission, we find evidence for an
]
O X-ray excess indicative of low-luminosity AGNs. Among the quiescent galaxies, the excess suggests
that as many as 70%−100%ofthese containlow- or high-luminosity AGNs, while the corresponding
C
fraction is lower among star-forming galaxies (43%−65%). Our discovery of the ubiquity of AGNs
.
h in massive, quiescent z ∼ 2 galaxies provides observational support for the importance of AGNs in
p impeding star formation during galaxy evolution.
- Subject headings: galaxies: active – galaxies: evolution – galaxies: high-redshift – galaxies: star-
o
formation – infrared: galaxies – X-rays: galaxies
r
t
s
a
[ 1. INTRODUCTION and otherwise provide the dominant source of star for-
Massive galaxies at z ∼ 2 are likely progenitors of to- mation at z ∼ 2–3 (Dekel et al. 2009). To help test this
2 picture for massive galaxies at z ∼2 we need better ob-
day’s massive ellipticals (e.g., van Dokkum et al. 2010).
v servationalconstraints on the incidence of AGN and the
Combining broadband UV–MIR photometry with high-
8 connection to star formation in these galaxies.
resolution imaging has shown that a relation akin to
5 the Hubble sequence is in place at z ∼ 2, dividing A general concern when estimating the star forma-
1 tion rate (SFR) by fitting the broadband spectral en-
massive galaxies into two main categories: star-forming
1
ergy distribution (SED) of a galaxy with stellar popula-
. galaxies with disk-like or irregular structures and qui-
2 escent galaxies with little or no star formation and ex- tion synthesis models, is the relative dominance of AGN
1 versus star formation. The dominant contributor to the
tremelycompactmorphologies(e.g.,Cimatti et al.2008;
2 mid-infrared(MIR)lightisfromUV-lightreprocessedby
Toft et al.2009; Williams et al.2010;Wuyts et al.2011;
1 dust, but the UV light can be emitted by either young
Szomoru et al. 2012). The quiescent galaxiesform a sig-
v: nificantfraction(30%–50%)ofallmassivez∼2galaxies starsoranAGN.Atz ∼2itisnotclear,withouttheaid
of high spatial and spectral resolution, what is causing
i (e.g., Kriek et al. 2006; Williams et al. 2009; Toft et al.
X theobservedMIRemissionfrommassivegalaxies;isdust
2009), but how they got to this stage remains an open
in the central regions being heated by AGN activity, is
r question.
a dustacrossalargerregionofthe galaxybeingheatedby
Active galactic nuclei (AGNs) are one of the preferred
star formation, or is a combination of the two scenarios
hypothesized mechanisms for arresting star formation,
taking place?
as radiation, winds and jets from the central accreting
X-rayemission,ontheotherhand,doesnotsufferfrom
black hole can remove the gas or heat it so that con-
strongdustobscuration. ObservinginX-raycanthuslift
traction cannot take place (see Fabian 2012 for a re-
the apparentdegeneracyininterpretingthe originofthe
view of observational evidence). Semi-analytical simula-
MIR light, and thereby help to give the true AGN frac-
tions by, e.g., Di Matteo et al. (2005) and Croton et al.
tionofasampleofgalaxies. AgalaxydominatedbyAGN
(2006), show how reasonable descriptions of the AGN
activitycanbedistinguishedfromagalaxydominatedby
feedback on star formation in elliptical galaxies can re-
star formationby having a harder X-ray spectrum if the
sult in galaxies which resemble those observed in terms
AGN is obscured by dust, or by simply having a very
of, e.g., their mass function. This could be the main
mechanism leading the massive galaxies at z ∼ 2 from high X-ray luminosity, L0.5-8keV. The most heavily ob-
scured “Compton-thick” AGNs, with column densities
the blue star-forming cloud through the “green valley”
N > 1024 cm−2, might be missed by X-ray selection,
and onto the red and dead sequence (Schawinski et al. H
but can instead be identified via an excess of MIR emis-
2007; Hopkins et al. 2008). An AGN would be capable
sionoverthatexpectedfrompurelystar-forminggalaxies
of heating or expelling the gas residing in the galaxy as
(e.g., Daddi et al. 2007; Treister et al. 2009).
wellasthegasthatisbelievedtoflowinviacoldstreams
2 Olsen et al.
TheshapeoftheMIRspectrumcanalsorevealAGNs, Ω = 0.3, Ω = 0.7 and h = 0.72, and magnitudes are
m Λ
as the intense nuclear emission re-emitted by dust leads in the AB system.
to a power-law spectrum in the MIR. For this purpose,
colorcutshavebeendevisedusingthe Spitzerdata(e.g., 2. GALAXYSAMPLE
Stern et al. 2005; Donley et al. 2012). While this tech- WeselectourgalaxiesfromtheFIREWORKS1catalog
niquehasthecapabilityofdetectingevenCompton-thick (Wuyts et al. 2008), which covers a field situated within
AGNs, otherwise missed in X-ray, it has a low efficiency the CDF-S and contains photometry in 17 bands from
forlow-tomoderate-luminosityAGNsanditsrobustness the U band to the MIR. For this study we extract a
has yet to be verified at z &2 (Cardamone et al. 2008). mass-complete sample of M∗ > 5×1010M⊙ galaxies at
In a galaxy where star formation is dominant, the to- 1.5 ≤ z ≤ 2.5 in a way similar to that of Franx et al.
talhardbandX-rayluminosity, L ,canbeusedto (2008) and Toft et al. (2009). We will use spectroscopic
2-10keV
estimatetheSFR(e.g.,Grimm et al.2003;Ranalli et al. redshifts when available(Vanzella et al.2008; Xue et al.
2003; Lehmer et al. 2010). An AGN will reveal itself if 2011),andphotometricredshiftsfromtheFIREWORKS
the X-ray SFR inferred this way is much largerthan the catalog otherwise. In order to maximize the signal-to-
SFRderivedfromothertracerssuchasHα, UV,orMIR noise (S/N) on results from X-raystacking (Zheng et al.
luminosity. Also, if star formation dominates the radia- 2012) and ensure a relatively homogeneous PSF across
tion output, the L →SFR conversion will give an up- the Chandra field employed, we consider only galaxies
x
per limit on the SFR as has been obtained for sub-mm that lie within 6′ of the average Chandra aimpoint.
galaxies(Laird et al.2010)andformassive,star-forming We adopt galaxy stellar masses from the SED fitting
galaxies at 1.2<z <3.2 (Kurczynski et al. 2012). results of Franx et al. (2008) and Toft et al. (2009). In
Atredshiftsaround2,high-ionizationlinesinthe rest- short, the SED fits were carried out with models by
frame UV can be used as AGN indicators, but X-ray Bruzual & Charlot (2003), choosing the best fit from
observations remain a more efficient way of identify- adopting three different star formation histories (a sin-
ing AGNs (Lamareille 2010). Several studies of mas- gle stellar population with no dust, an exponentially de-
sive z ∼ 2 galaxies have therefore been made with clining star formation history of timescale 300Myr with
the aim of uncovering AGN fractions, using the Chan- dust, and a constant star formation history with dust).
dra X-ray observatory. Rubin et al. (2004) performed Incaseswherethespectroscopicredshiftdifferedbymore
a study of 40 massive (M∗ = (1-5) × 1011M⊙) red than 0.1 from the original FIREWORKS photometric
(J −K ≥ 2.3) galaxies at z & 2 by analyzing a 91 ks redshift, we redid the SED fits in FAST2 using an expo-
s s
Chandra exposure. Roughly 5% of these were found to nentiallydecliningstarformationhistorywitharangeof
hostanAGNwithintrinsicL >1.2×1043ergs−1. possibletimescalesfrom10Myrto∼1Gyr. Asaquality
2-10keV
Assuming that the stacked X-ray signal from the re- parameter of the SED modeling, we demand an upper
maining X-ray undetected galaxies in X-ray comes from limit onthe reduced χ2 of10 onthe best-fit model. The
ν
star formation alone, they derived a mean SFR broadly SED fits provide SFR estimates, but we will be using
consistent with the typical mean SFRs estimated from SFRs derived from rest-frame UV+IR luminosities (see
SED fits. Alexander et al. (2011) analyzed the 4Ms Section4.2)astheseincludeobscuredstarformationand
Chandra observations of 222 star-forming BzK galaxies aresubject to less assumptions. Fromthe observedpho-
(M∗ ∼ 1010-1011M⊙, Daddi et al. 2007) in the Chandra tometry,rest-framefluxesinU,V,J bandandat2800˚A
DeepField-South(CDF-S).10%(23)showedX-rayemis- have been derived using InterRest3 (Taylor et al. 2009).
sion consistent with AGN dominance, of which 5%(11) We divide the resulting sample of 123 galaxies into
were found to contain heavily obscured AGNs, 4%(9) quiescent and star-forming galaxies using the rest-frame
to have luminous, unobscured AGNs, and 3 out of 27 U, V and J (falling roughly into the observed J, K and
low-luminosity systems showed excess variability over IRAC4.5µmbandsatz ∼2)colors. Dust-freebutquies-
that expected from star formation processes, indicating centgalaxiesdifferfromdust-obscuredstarburstgalaxies
that atleastsome low-luminosity(rest-frame L < inthattheyobeythefollowingcriteriabyWilliams et al.
2-10keV
1043ergs−1) systems may contain AGNs. (2009):
The aim of this paper is to determine the AGN frac-
U −V >1.3 (1)
tioninmassivez ∼2galaxies,andrevealanydifferences
betweengalaxiescharacterizedasquiescentorstarform- V −J <1.6 (2)
ing. WeaddressthematterbyanalyzingtheX-rayemis- (U −V)>0.88×(V −J)+0.49 (3)
sion from a mass-complete (M∗ > 5×1010M⊙) sample
of 1.5 ≤ z ≤ 2.5 galaxies residing in the CDF-S. The The fraction of quiescent galaxies identified within our
CDF-S,observedfor4Ms,iscurrentlythedeepestX-ray sample using this method is 22%±5%(27/123). This is
view of the universe and so provides the best opportu- rather low compared to the 30–50%found by Toft et al.
nitytostudyhigh-zgalaxiesacrossarelativelylargearea (2009) for the same redshift and mass limit, but under
(464.5arcmin2). the requirement that the sSFR (=SFR/M∗) from SED
Following a description of the sample selection (Sec- fitting is less than 0.03Gyr−1. If applying this crite-
tions 2) and of the X-ray data and analysis (Section 3), rion,we wouldarriveata fractionof42%±7%(51/123).
wedetermine AGNfractionsfromthe X-rayspectraand A possible reason for the discrepancy between the two
compare the X-ray inferred SFRs to UV+IR estimates methods may be that we are using the UVJ criterion in
(Section 4.1). Results are presented and discussed in
Section 5, and in Section 6, we sum up the conclusions. 1 http://www.strw.leidenuniv.nl/fireworks/
Throughout the paper we assume a flat cosmology with 2 http://astro.berkeley.edu/~mariska/FAST.html
3 http://www.strw.leidenuniv.nl/~ent/InterRest
AGN in massive galaxies at z ∼2 3
dividually, we stack the X-ray non-detected galaxies in
order to constrain the typical X-ray flux from these.
Stacked X-ray images of the 4 subsamples are shown
in Figure 1 with all galaxies aligned to their K band
s
galaxycenterpositionsfromFIREWORKS.Representa-
tivecountratesandassociatederrorsforthesestacksare
calculatedusing the optimally weightedmeanprocedure
of Cowie et al. (2012) and tabulated in Table 1 together
with S/N values.
CountRate±1σ (10−6s−1)in
FullBand SoftBand HardBand
Q 0.71±0.96 2.01±0.41 −1.67±0.86
(0.7) (4.9) (−1.9)
SF 2.51±0.55 1.08±0.24 1.29±0.49
(4.5) (4.5) (2.7)
Table 1
StackingresultsforQuiescent(Q)andStar-forming(SF)galaxies
notdetected individuallyinX-Rays(inParentheses the
correspondingS/N)
The reliability of our chosen method for X-ray stack-
ing and source count extraction was tested with Monte
Carlo(MC)simulations. With500MCsimulationsof53
Figure 1. Stacked, background-subtracted and exposure-
corrected20′′×20′′imagesinthethreeenergybands(columns)for (corresponding to the number of star-forming galaxies
theindividuallynon-detected(toprow)anddetected(bottomrow) notdetectedinX-rays)randomlyselectedpositionshav-
quiescent and star-forminggalaxies. Images of the non-detections ing no X-ray detections nearby, we obtain a histogram
havebeensmoothedwithaGaussianofwidth2pixels.
of stacked S/N values that is well fitted by a normal
thelimitsoftheredshiftrangeinwhichithassofarbeen distribution with a center at −0.02±0.8, that is, con-
established. As the redshift approaches 2, the quiescent sistent with no detection. Similar results were obtained
populationmovestobluerU−V colors,possiblycrossing using a sample size of 19, the number of quiescent non-
the boundaries of Equation (3). However, we prefer the detections.
UVJ selection technique in contrast to a cut in sSFR, We quantify the hardness of the X-ray spectra using
because rest-frame colors are more robustly determined the hardness ratio, HR= (H −S)/(H +S),5 where H
than star formation rates from SED fits. and S are the net counts in the hard and soft band,
respectively. Assuming a power-law spectrum, we also
3. X-RAYANALYSISANDSTACKINGMETHOD derivea photonindex, Γ,6 with the online missioncount
The raw X-ray data from the CDF-S survey consist ratesimulatorWebPIMMS7 usingaGalacticHIcolumn
of 54 individual observations taken between 1999 Octo- densityof8.8×1019cm−2 (Stark et al.1992)andnotin-
ber and 2010July. We build our analysis onthe workof cludingintrinsicabsorption. WhenevertheS/Nineither
Xue et al.(2011),whocombinedtheobservationsandre- soft or hard band is below 2, we use the corresponding
projected all images. They did so in observed full band 2σ upper limit on the count rate to calculate a limit on
(0.5–8keV), soft band (0.5–2keV) and hard band (2– both HR and Γ, leading to a limit on the luminosity as
8keV), and the resulting images and exposure maps are well. When neither a hard- nor a soft-band flux is de-
publicly available.4 tected with an S/N above 2, a typical faint source value
We extractsource andbackgroundcount rates in the X- ofΓ=1.4(Xue et al.2011)isassumed,correspondingto
raymapsforallgalaxiesusingthemethodofCowie et al. HR∼−0.3foranintrinsicpowerlawspectrumofΓ=1.9
(2012). Source counts are determined within a circu- (Wang et al. 2004).
lar aperture of fixed radii: 0′′.75 and 1′′.25 at off-axis We derive the unabsorbed flux from the count rate by
angles of θ ≤ 3′ and θ > 3′ respectively. Background again using the WebPIMMS tool, now with the Γ tied
counts are estimated within an annulus 8′′–22′′ from the to the observed value of either the individually detected
source, excluding nearby X-ray sources from the catalog galaxy or the stack in question. With this method, a
of Xue et al. (2011). count rate of ∼ 10−5countss−1 corresponds to a flux of
Eachgalaxyisclassifiedas“X-raydetected”ifdetected nearly 10−16ergcm−2s−1 in full band at a typical value
inatleastonebandat≥3σsignificance,therebycreating of Γ = 0.8. Finally, the flux is converted into luminos-
4 subsamples containing 8 quiescent and detected, 19
quiescent and undetected, 43 star-formingand detected, 5Hardnessratiohastheadvantage,incomparisontoΓ,ofavoid-
inganyassumptionsregardingtheshapeofthespectrum.
and 53 star-forming and undetected galaxies.
6 The photon index is defined as the exponent in
While the X-ray detected galaxies can be analyzed in- the relation between photon flux density and energy:
dN/dE[photons cm−2 s−1 keV−1]∝E−Γ
4http://www2.astro.psu.edu/users/niel/cdfs/cdfs-chandra.html 7 http://heasarc.nasa.gov/Tools/w3pimms.html
4 Olsen et al.
L(e0r.5g–s8−k1e)V HR Classification
>3×1042 <−0.2 UnobscuredAGNs(NH<1022cm−2)
>3×1042 >−0.2and <0.8 ModeratelyobscuredAGNs(1022 <NH<1024cm−2)
>3×1042 >0.8 Compton-thickAGNs(NH>1024cm−2)
<3×1042 <−0.2 Star-forminggalaxy
<3×1042 >−0.2 Low-luminosityobscuredAGNsorstar-forminggalaxy
Table 2
Classificationschemeusedinthiswork,withlimitsfromSzokolyetal.(2004),Wangetal.(2004),andTreisteretal.(2009)
ityinthedesiredrest-framebandsusingXSPECversion
12.0.7 (Arnaud 1996).
4. INTERPRETATIONOFX-RAYPROPERTIES
4.1. Luminous AGN Identification
In the top panel of Figure 2 are shown the rest-frame
X-rayfullbandluminosities, L ,notcorrectedfor
0.5–8keV
intrinsicabsorption,versusobservedhardnessratio,HR,
for all X-ray detected galaxies as well as stacked non-
detections. A typical detection limit on L has
0.5–8keV
been indicated with a dotted line in Figure 2, calcu-
lated as two times the background noise averaged over
all source positions. A few detected galaxies are found
below this limit due to these residing at relatively low
z and/or in regions of lower-than-average background.
Galaxies with L > 3×1042ergs−1 are selected
0.5–8keV
as luminous AGN, since star-forming galaxies are rarely
found at these luminosities (Bauer et al. 2004). In ad-
dition, we adopt the HR criteria in Table 2 in order to
identify obscured and unobscured AGNs.
About 53%(27/51) of the X-ray detected galaxies are
identified as luminous AGNs, with 22 moderately to
heavily obscured (−0.2 < HR < 0.8) and 5 unobscured
(HR< −0.2) AGNs. The rest have X-ray emission con-
sistent with either low-luminosity AGNs or star forma-
tion. We do not identify any Compton-thick AGNs di-
rectly,butthreegalaxieshavelowerlimitsontheirhard-
ness ratios just below HR=0.8, thus potentially consis-
tentwithCompton-thickemission(seeTable2). Intotal,
wedetectaluminousAGNfractionof22%±5%(27/123),
afractionwhichis23%±5%(22/96)forthestar-forming
and 19%±9%(5/27) for the quiescent galaxies.
Stacking the non-detections results in average X-ray
source properties that exclude high luminosity AGNs.
However, the inferred limits on their HR are consistent
with a contribution from low-luminosity AGNs, the im-
portance of which cannot be determined from this plot
alone.
4.2. X-ray Inferred SFR
Fromthe rest-framehardbandluminosity, L it
ispossibletoderiveestimatesoftheSFR,asth2e–n10ukmeVber Figure 2. Top: L0.5–8keV vs. HR for all galaxies. Red: quies-
centgalaxies. Blue: star-forminggalaxies. Squares: stacksofnon-
of high-mass X-ray binaries (HMXBs) is proportionalto detectedsamples. Filledcircles: luminousAGNs(selectedwiththe
the SFR (Ranalli et al. 2003). Kurczynski et al. (2012) X-raycriteriaindicated bya shaded area). Open circles: galaxies
dominated by low-luminosity AGNs or star formation processes
made a comparison of different SFR indicators, by ap-
alone (selected with X-ray criteria indicated by a hatched area).
plyingthree differentLx →SFRconversionsto 510star- Dashed line: separating obscured (NH & 1022 cm−2) from un-
formingBzKgalaxiesat1.2<z <3.2,selectedashaving obscured AGNs. Dotted line: typical detection limit. Errorbars
L < 1043ergs−1. While relations by Persic et al. display 2σ errors and limits are indicated with arrows. Bottom:
2–10keV
(2004) and Lehmer et al. (2010) overestimated the true SFRUV+IR vs. SFR2–10keV (see the text). Dashed line: equality.
Typical errors on X-ray detected galaxies are shown in the lower
SFR (from rest-frame UV and IR light) by a factor of
rightcorner. Symbolsareasabove.
∼5,therelationbyRanalli et al.(2003)providedagood
AGN in massive galaxies at z ∼2 5
agreement at 1.5 < z < 3.2. But, as pointed out by 5. RESULTSANDDISCUSSION
Kurczynski et al. (2012), all relations might lead to an 5.1. Luminous AGN Fraction
overestimationdue to contaminationby obscuredAGNs
in the sample of SF galaxies. As we do not know the In total, 22 X-ray detected galaxies have emis-
exact amount of obscured AGN contamination in the sion consistent with containing a luminous (rest-frame
stacks, we have chosen to use the following relation by L0.5–8keV >3×1042ergs−1)andobscured(1022 <NH <
Ranalli et al. (2003) as a conservative estimate of the 1024cm−2) AGN, and a further five have emission con-
SFR: sistent with a luminous unobscured (N < 1022 cm−2)
H
AGN. This leads to a luminous AGN fraction of the full
sample of 22%pm5%(27/123)and of the detected galax-
SFR2–10keV =2.0×10−40L2–10keV (4) ies only, 53%±13%(27/51). The AGN fraction among
both quiescent and star-forming galaxies, according to
withSFRmeasuredinM⊙yr−1 andL2–10keV inergs−1. tThaebirleX3-rsahyowsps,ecmtreaa,niisngmtehaasutrAedGtNosbien amroaussnidve2z0%∼a2s
Anotherreasonfornotusingthe morerecentrelationby
galaxies, even quiescent ones, are common, as proposed
Lehmer et al.(2010)isthatthisrelationwasconstructed
byKriek et al.(2009)whostudiedthenear-IRspectrum
forgalaxieswithSFR>9M⊙yr−1 only,whereasalarge of one quiescent galaxy.
part of our sample has very low SFRs as inferred from
SED fitting (< 1M⊙yr−1). Following Kurczynski et al.
Quiescent(27) Star-forming(96)
(2012), we use the observed soft band flux to probe the
LuminousAGNs 5 22
rest-frame hard band luminosity. The uncertainty on
2(det) 19(det)
theSFRisestimatedfromtheerrorontheobservedsoft- Low-luminosityAGNs
12–19(non-det) 0–21(non-det)
bandfluxtogetherwithasystematicerrorintherelation LuminousAGNfraction 19%±9% 23%±5%
itself of 0.29dex, as given by Ranalli et al. (2003).
TotalAGNfraction 70%–100% 43%–65%
For comparison, the “true” SFR is inferred from rest-
frameUVandIRlight,SFRUV+IR,followingthemethod Table 3
of Papovich et al. (2006):
X-RayderivedAGNNumbersandFractionsforQuiescentand
Star-formingGalaxies,DividedintoLuminousAGNsand
SFRUV+IR =1.8×10−10(LIR+3.3L2800)/L⊙ (5) Detected andNon-detected Low-luminosityAGNs
whereLIRisthetotalinfraredluminosityandL2800isthe This luminous AGN fraction is high when compared
monochromatic luminosity at rest-frame 2800˚A. L2800 to the 5% found by Rubin et al. (2004) (see the intro-
comes from the rest-frame UV flux, f2800˚A (see Section duction), but this is likely a consequence of their 4σ de-
2),andinthiscontext,theerrorsonf2800˚Aarenegligible. tection limit of 1.2×1043ergs−1 in rest-frame 2–10keV
WederiveLIRfromtheobserved24µmflux,f24µm,using beingabouttwiceashighasourlimitof5.5×1042ergs−1
redshift-dependent multiplicative factors, a(z), from the (at Γ = 1.4), as calculated from the average back-
work of Wuyts et al. (2008), Section 8.2 in that paper): ground noise in the observed soft band. Adopting the
detection limit of Rubin et al. (2004) and requiring an
S/N of at least 4, we reduce our fraction of luminous
LIR[L⊙,8–1000µm]=10a(z)·f24µm (6) AGN to 8%±3%(10/123),consistentwith the results of
Rubin et al. (2004). Alexander et al. (2011), using also
the 4Ms CDF-S data, found a much lower X-ray detec-
The errorbars on L derive from the errors on f
IR 24µm tionfractionof21%±3%ascomparedtoours(53%±7%),
and we further assume an uncertainty of 0.3 dex in the
and a luminous AGN fraction of only 9%±2%(20/222).
relationofPapovich et al.(2006)asfoundbyBell(2003)
We believe that the discrepancies have several reasons,
whencomparingtoHα-andradio-derivedSFRs. TheX-
the main ones being: (1) our use of a mass-complete
ray undetected quiescent galaxies are not detected with
sample, whereas the BzK selection technique used by
S/N>3in24µmeither(exceptinonecase)leadingtoa
meanflux of1.2±1.7µJy, ofwhichwe adopta 2σ upper Alexander et al. (2011) includes galaxies down to M∗ ∼
limit in the following. 1010M⊙ for which the total AGN fraction, assuming a
In the bottom panel of Figure 2, SFR is com- fixed Eddington ratio, is expected to be lower above our
2–10keV
pared to SFR with the dashed line indicating detection limit,8 (2) our updated source count extrac-
UV+IR
equality. Itisnosurprisethatnearlyalloftheindividual tion and stacking method leading to higher S/N, and
X-raydetectionshaveveryhighSFR ascompared (3) the use of Γ instead of HR in the AGN identifica-
2–10keV
to SFR , as we are largely insensitive to individual tion conducted by Alexander et al. (2011). For compar-
UV+IR
purely star-forming galaxies, given the detection limits ison, Tanaka et al. (2012) recently discovered a group
in the top panel of Figure 2 and the criteria in Table 2. of quiescent galaxies at z = 1.6 with only one “main-
For the star-forming stack, the X-ray inferred SFR of sequence” star-forming galaxy. This group differed from
71±51M⊙yr−1 is consistent with the IR+UV inferred local groups in having a remarkably high AGN fraction
sShFoRwsoafn4S6F±R23–310Mke⊙Vyorf−912,±w6h5eMre⊙asyrt−h1e(q8u9i±es1c7eMnt⊙sytra−ck1 ma8ssFoinrcareafisxeesdwEitdhditnogttaolnstrealltaior,manasds,atshsuemAiGngNtXha-tragyalluaxmyinbousligtye
whenbootstrappingthis sample 200times), wellexceed- isexpectedtoscalewithgalaxystellarmassaccordingtotheMbh–
ing SFRUV+IR ≤3M⊙yr−1. Mbulge relation(H¨aring&Rix2004)
6 Olsen et al.
of38+23%,consistentwithourresult,andwhichtheyin- It should be mentioned that the 24µm flux, espe-
−20
terpret as possible evidence for AGN activity quenching cially at these high redshifts, is an uncertain estimator
star formation. ofthetotalrest-frameinfraredluminosity,i.e.,theentire
dust peak, used in the conversion to SFR. As shown by
5.2. Importance of Low-luminosity AGNs
Bell (2003), one should ideally use the entire 8–1000µm
As seen in Figure 2, the X-ray-identified luminous range, e.g., by taking advantage of Herschel data, which
AGNsingeneralshowanexcessinSFRcomparedtothat canleadtosystematicdownwardcorrectionfactorsupto
inferredfromIR+UVemission. Amongthegalaxiesclas- ∼2.5 for galaxies with LIR ≈1011L⊙ (similar to the in-
sified as being dominated by either low-luminosity AGN ferredIR luminosities of our sample galaxies detected in
orstarformation,about∼90%(21/24)haveSFR2–10keV 24µm, showing a median of LIR =1011.5L⊙) as demon-
more than 1σ above SFRUV+IR. strated by Elbaz et al. (2010). However, using the same
Surprisingly,thequiescentstackalsohasamuchlarger conversionfromf to L as the one implemented in
24µm IR
SFR2–10keV than SFRIR+UV. Even when removing the thisstudy,Wuyts et al.(2011)showedthatthe resulting
marginally undetected galaxies with 2<S/N<3, the re- L for galaxies out to z ∼ 3 are consistent with those
IR
sulting SFR2–10keV =62±19M⊙yr−1 is still more than derived from PACS photometry with a scatter of 0.25
3σ above SFR . This discrepancy is only further dex. Hence, we do not expect the inclusion of Herschel
IR+UV
aggravated if instead assuming the SFR−L relation of photometry in the present study to significantly impact
X
Lehmer et al. (2010). If caused by obscured star forma- any of our results, and we leave any such analysis for
tion, we would have expected an average 24µm flux of future work.
90µJy for the individual galaxies, in order to match the Table3givesanoverviewofthederivedAGNfractions,
lower limit on SFR . This is far above the upper bothathighandlowX-rayluminosity. Addingthenum-
2–10keV
limit of 3.4µJy for the stack, suggesting that the X-ray bers of luminous AGN, X-ray-detected low-luminosity
flux ofthis stackis insteaddominatedby low-luminosity AGNs as well as the estimated lower limit on the low-
AGNs, but that their contribution to the 24µm flux re- luminosity AGN fraction among non-detections, we ar-
mains undetectable in the Spitzer–MIPS data. rive at a lower limit on the total AGN fraction of
We derive a lower limit on the low-luminosity AGN 27+21+0.6·19+0·53
contribution for this stack of 19 objects by constructing fAGN ≥ =0.48 (7)
123
a mock sample of the same size and with the same red-
shifts,butcontainingnlow-luminosityAGNswithlumi- for all massive galaxies at z ∼ 2. While for the star-
nositiesL =1042ergs−1 justbelowthedetection forming galaxies this fraction lies in the range from
0.5–8keV
limit and 19−n galaxies with L = 1041ergs−1 43%–65%, it must be 70%, and potentially 100%, for
0.5–8keV
(all of them with Γ = 1.4). From random realizations the quiescent galaxies. Using the upper limits on these
of this mock sample we find that at least n = 12 low- numbers, a tentative upper limit on the total AGN
luminosity AGNs are required to match the observed fraction is 0.72.
rest-frame 2–10 keV luminosity of the stack. Similarly,
we find that by removing at least 12 randomly selected
5.3. Contribution from Hot Gas Halos
galaxies it is possible to match the low SFR predicted
by IR+UV emission, though only with a small probabil- We have so far considered star formation and
ity (∼ 1% of 200 bootstrappings). In the following, we AGN activity as causes of the X-ray emission ob-
willadopt63%(=12/19)asaconservativelowerlimiton served, but a third possibility is an extended hot gas
the low-luminosityAGNfractionamongquiescentX-ray halo as seen around many nearby early-type galax-
non-detections, but we note that the data are consistent ies in the same mass-range as our sample galax-
with all the quiescent galaxies hosting low-luminosity ies (Mulchaey & Jeltema 2010) and predicted/observed
AGNs if the luminosity of these is assumed to be only aroundsimilarspirals(Toft et al.2002;Rasmussen et al.
L =7×1041ergs−1. 2009;Anderson & Bregman2011;Dai et al.2012). AGN
0.5–8keV
Interestingly, the quiescent stack is only detected in X-ray emission is expected to come from a very small,
the soft band, cf. Table 1, whereas one might expect a R < 1pc, accretion disk surrounding the central
significantcontributiontothehardbandfluxfromalow- black hole of the host galaxy (Lobanov & Zensus 2007),
luminosity AGN population as the one proposed above. whereas very extended star formation in the galaxy or a
The lack of a hard band detection can be explained by hotgashalosurroundingitwouldleadtomoreextended
the fact that the sensitivity of the CDF-S observations emission. We investigate the possibilities for these lat-
dropsaboutafactorofsixfromthesofttothehardband ter cases by comparing radial surface brightness profiles
(Xue et al.2011),meaningthatthelow-luminosityAGN of the 51 individually X-ray detected galaxies out to a
population must be relatively unobscured (Γ > 1, con- radius of 8′′ in both the stacked observed image and a
sistent with the value of 1.4 assumed here), as it would correspondingly stacked PSF image.
otherwise have been detected in the hard band with an The profiles are calculated in full band only, because
S/N>2. of the high S/N here as compared to the other bands
TheSFR ofthe star-formingstackisconsistent (cf. Figure 1). For each galaxy, we extract the back-
2–10keV
with its SFR , meaning that a strict lower limit to groundsubtracted source count per pixel within 10 con-
UV+IR
the low-luminosity AGN fraction here, is zero. However, centric rings of width 0′′.8 around the galaxy center po-
performing the test above with the same model param- sitions from the FIREWORKS catalog, and each profile
eters, a maximum of 40%(21/53) low-luminosity AGNs is normalized to the mean count rate in all rings. The
is possible, before the X-ray inferred SFR exceeds the same procedure is applied to the correspondingPSF im-
upper limit on SFR . ages,extractedwiththelibraryandtoolsthatcomewith
UV+IR
AGN in massive galaxies at z ∼2 7
O’Sullivan et al. (2003), the stacked profile of the halos,
convolved with the corresponding PSFs, is well outside
the errorbars of the measured profile. Only if assuming
anextremelycompactcaseofβ =1andR =1kpc,does
c
the halo emission become sufficiently compact to mimic
thatofthePSFprofile(seedash-dottedlineinFigure3),
but the halo model then overpredicts the observed sur-
face brightness on scales smaller than 1′′. In conclusion,
ahotgashaloalone,describedby aβ-model, cannotex-
plainthe emission, unless one choosesmodel parameters
thatrendertheprofileindistinguishablefromapoint-like
source.
5.4. Quenching of Star Formation by AGNs?
It remains debated whether the presence of AGN
is connected with internal galaxy properties associated
withsecularevolutionor,to a higherdegree,with exter-
nal processes (Darg et al. 2010). For example, the cos-
Figure 3. Comparisonoftheradialsurfacebrightnessprofilesin
PSFimagesvs. observedimagesinfullbandforalldetectedgalax- mic star formation rate density and the number density
ies. Black: stacked observed image centered on K-band positions of AGN share a common growth history with a peak in
fromFIREWORKSforalldetected galaxies (solidline)andifex- activity around z ∼ 2–3 (Treister & Urry 2012; Dunlop
cludingthethreeX-raybrightestsources(dashedline). Greyline:
2011),hintingataco-evolutionbetweenSFRandsuper-
stackedPSFimage. Red: stackofhalomodelimages,withβ=0.5,
Rc=2.5kpc(dashed) andβ=1,Rc=1kpc(dot-dashed). massiveblackhole(SMBH)accretion(Schawinski2011).
Inthis section,we willinvestigatethe correlation,if any,
CIAO,9 allowing for extraction at the exact source po- betweenthepresenceofAGNsandinternal/externalpro-
sitions on the detector. We verified the robustness of cesses, treating luminous and low-luminosity AGNs sep-
using PSF models from the CIAO calibration database arately. As typical internal properties governing secular
forthispurpose,byrepeatingtheaboveprocedurefor51 evolution we will focus on stellar mass, M∗, and SFR
known X-ray bright point-like sources from the catalog (Peng et al. 2010), while major mergers are taken as a
of Xue et al. (2011), and confirming that the resulting likely case of external processes and as a phenomenon
mean profile was fully consistent with that of the corre- often associated with luminous AGNs (Treister et al.
sponding model PSFs. 2012).
As can be observed in Figure 3, the combined radial Startingwith our X-rayidentified luminous AGNs, we
profile of our X-ray detected galaxies in the full band is already found that the fraction of these does not corre-
consistent with the PSF of point sources stacked at the late with star formation in our sample (cf. Table 3). In
same detector locations. A Kolmogorov-Smirnov (K-S) addition, the distribution of luminous AGNs and that
test yields a statistic of 0.2 and a probability of 99.96% of the rest with regards to their SFRIR+UV are similar,
that the two profiles are drawn from the same parent with a K-S test yielding a statistic of 0.21 with a proba-
sample. bility of 59%. Similarly, Harrison et al. (2012) found no
Omitting the three sources with L > correlation between AGN luminosity and quenching of
0.5–8keV
1044ergs−1 (cf. Figure 2) leads to a more extended starformation,albeitata higherluminosity(L2−8keV >
profile (dashed black line in Figure 3), but the result 1044ergs−1) than probed here. Dividing the sample ac-
still shows a high correspondingK-S probability of 70%. cording to M∗ instead, by constructing two bins around
We also compare the profiles while recentering the im- themedianmassof1.1×1011M⊙,wearriveatsimilarlu-
agesonthe centerofthe X-rayemissionasderivedusing minous AGN fractions above and below this mass limit,
WAVDETECT10 insteadof the K-bandcenter positions namely 21%±7%(13/61)and 23%±7%(14/62),respec-
listed in the FIREWORKS catalog, in order to test for tively. We thus find no clear evidence for the luminous
the impact of any off-set between the X-ray and opti- AGN fraction of our sample to correlate with internal
cal centroids. However, the recentering only resulted in properties,suggestingthatanexternalfactoris oflarger
smallvariationswithintheerrorbarsontheoriginalpro- importance.
file. The same conclusions apply to the subsample of X- An alternative is that luminous AGNs are primarily
raydetectedstar-forminggalaxiesonly,whereastheS/N triggered by non-secular processes such as major merg-
was too low to performa similar study onthe quiescent, ers. Treister et al. (2012) found that major mergers
X-ray detected sample alone. are the only processes capable of triggering luminous
For comparison, we also simulated the stacked pro- (Lbol & 1045ergs−1) AGN at 0 < z < 3. Our luminous
file of extended hot gas halos, using for the sur- AGN fraction is consistent with the major merger frac-
face brightness profile the β-model first introduced by tion of massive (M∗ > 1011M⊙) galaxies at 1.7 < z < 3
Cavaliere & Fusco-Femiano(1976). Withdefaultparam- found by Man et al. (2012) to be 15%±8% (compared
eters of β = 0.5 and core radius R = 2.5kpc, both to a luminous AGN fraction of 11±3%(13/123) when
c
taken from the study of z . 0.05 early-type galaxies by excluding galaxies with M∗ < 1011M⊙). This is consis-
tentwiththeideathatourluminousAGNsaretriggered
9 http://cxc.cfa.harvard.edu/ciao4.3/ahelp/mkpsf.html bymajormergers,buta moredirecttestofthis scenario
10 WAVDETECTispartoftheCIAOsoftware. would be to search for major mergers using the imaging
8 Olsen et al.
data available in CDF-S. Indeed, Newman et al. (2012) amongmassive(M∗ >5×1010M⊙)galaxiesatred-
usedHSTCANDELSimagingintheGOODS-Southfield shifts 1.5 ≤ z ≤ 2.5, using their X-ray properties
residing within CDF-S (along with the UKIRT Ultra extractedfromthe4MsChandraDeepFieldSouth
Deep Survey) to arrive at roughly equal pair fractions observations. Among the X-ray detected galaxies,
when comparing massive (M∗ > 1010.7M⊙) quiescent to 53% ± 13% harbor high-luminosity AGNs, while
massive star-forming galaxies at 0.4 < z < 2. Again, stacking the galaxies not detected in X-ray, leads
this is in agreement with our result that the luminous to mean detections consistent with low-luminosity
AGN fractiondoes not varybetween quiescent andstar- AGNs or pure star formation processes. The lu-
forming galaxies. A detailed morphological study of our minous AGN fraction among quiescent and star-
X-ray identified luminous AGN using the HST/WFC3 forminggalaxiesissimilar(19%±9%and23%±5%,
data to search for evidence of merging is an interesting respectively) and does not depend on galaxy M∗.
extensionto the workpresentedhere, whichwe leave for We confirmed that extended X-ray emission from
a future study. a hot gaseous halo is not a viable explanation for
Turning toward our low-luminosity AGNs, we do see the observed X-ray emission of the X-ray detected
X-ray evidence for an enhanced population among the galaxies.
quiescent galaxies when compared to their star-forming
2. Limits on total AGN fraction
equivalents. The mean masses of our non-detected sam-
We convert the rest-frame hard band X-ray lumi-
ples of quiescent and star-forming galaxies are similar
nosity into an upper limit on the star formation
(14.6±1.6 and 12.7±1.0×1010M⊙ respectively), sug-
rate,SFR andcomparetothatderivedfrom
gesting that the relevant factor here is SFR. At 0.01 < 2–10keV
the rest-frame IR+UV emission, SFR . All
z < 0.07, Schawinski et al. (2009) found that massive IR+UV
luminous AGNs show an excess in SFR as
(M∗ & 1010M⊙) host galaxies of low-luminosity AGNs expected, and so does a large fraction (∼2–1900k%eV) of
all lie in the green valley, that is, at some intermedi-
the remaining detected galaxies. While the star-
ate state after star formation quenching has taken place
forminggalaxiesnotdetectedinX-rayhaveamean
apnodpublaetfioornes.thTehSeEDobsiesrvtarutiloyndtohmatintahteedsebhyosotldgasltaexlliaers X-ray inferred SFR of 71 ± 51M⊙yr−1, consis-
tent with their SFR , the stack of quiescent
had been quiescent for ∼ 100Myr, ruled out the pos- IR+UV
galaxies shows an excess in SFR of a fac-
sibility that one short-lived, luminous AGN suppressed 2–10keV
tor >10 above the upper limit on SFR . For
star formation and, at the same time, made it unlikely IR+UV
these galaxies,we find that a minimum fraction of
that the same AGN quenching star formation was still
∼ 60% must contain low-luminosity (L ≈
active, given current AGN lifetime estimates of ∼ 107– 0.5–8keV
1042ergs−1) AGNs if the SFR estimates from X-
108yr (Di Matteo et al. 2005). Rather, the authors fa-
ray are to be explained, and that low-luminosity
voreda scenarioin whicha low-luminosityAGN already
AGNs might be present in all of them. On the
shut down star formation, followed by a rise in lumi-
other hand, for the star-forming stack, we derive a
nosity, making the AGN detectable. At z ∼ 2, SED
low-luminosity AGN fraction of 0–40%.
fits of massive (M∗ > 1011M⊙) quiescent galaxies show
Gathering all low- and high-luminosity AGNs, we
that the quenching typically took place ∼ 1 Gyr before
derive a lower limit to the total AGN fraction of
the time of observation (Toft et al. 2012; Krogager, J.-
48%, with a tentative upper limit of 72%.
K. et al., in preparation), demanding an even longer de-
lay or an episodic AGN activity as frequently applied Ourstudyisthefirsttopresentobservationalevidence
in models (Croton et al. 2006). Episodic AGN activ- that, at z ∼ 2, the majority of quiescent galaxies host a
ity could explain why we see evidence for a higher low- low- to a high-luminosity AGN, while the AGN fraction
luminosity AGN fraction among quiescent as compared issignificantlylowerinstar-forminggalaxies. Thesefind-
to star-forming galaxies, but it would also require the ingsareconsistentwithanevolutionaryscenarioinwhich
low-luminosity AGN phase in quiescent galaxies to last low-luminosity AGN quench star formation via the en-
at least as long as the dormant phase. Future modeling ergetic output from SMBH accretion, which, if believed
and observations will show whether this is in fact possi- to continue in an episodic fashion as often invoked by
ble. models, would need to have “dormant” phases at least
We conclude that our data are consistent with a sce- as long as “active” phases.
nario in which luminous AGNs in massive galaxies at We find that the high-luminosity AGNs are likely re-
z ∼ 2 are connected with major mergers or other non- latedto non-secularprocessessuchasmajormergers. In
secular processes, while the presence of low-luminosity the future, examining the X-ray properties of galaxies
AGNsinthemajorityofquiescentgalaxiessuggeststhat in a larger sample, cross-correlated with signs of major
theseAGNspresentanimportantmechanismforquench- mergers, may shed further light on the co-evolution of
ing star formation and keeping it at a low level. Ulti- AGNs and host galaxy.
mately what happens at z ∼ 2 has to agree with the
subsequent evolution that changes size and morphology
ACKNOWLEDGMENTS
of quiescent galaxies (Barro et al. 2012)
We thank the anonymousrefereefor carefulreadingof
6. SUMMARY the manuscript and suggestions that improved the pa-
Our main conclusions are on the following two topics: per. We also enjoyed useful discussions with Ezequiel
Treister, Marianne Vestergaard, Tomotsugu Goto and
1. Luminous AGN fraction Harald Ebeling. We thank Stijn Wuyts and Natascha
We find a luminous AGN fraction of 22% ± 5% M. F¨orster Schreiber for photometric redshifts and SED
AGN in massive galaxies at z ∼2 9
modelingresults. Wegratefullyacknowledgethesupport Mulchaey,J.S.,&Jeltema, T.E.2010,ApJ,715,L1
from the Lundbeck foundation. J.R. acknowledges sup- Newman,A.B.,Ellis,R.S.,Bundy,K.,&Treu,T.2012,ApJ,
port provided by the Carlsberg Foundation. The Dark 746,162
O’Sullivan,E.,Ponman,T.J.,&Collins,R.S.2003,MNRAS,
CosmologyCentre is funded by the Danish NationalRe-
340,1375
search Foundation. Papovich,C.,Moustakas,L.A.,Dickinson,M.,etal.2006,ApJ,
640,92
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