The role of environment in low-level active galactic nucleus activity: no evidence for cluster enhancement

0.29 MB·English

by  Brendan Miller

Draftversion January16,2012
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
THE ROLE OF ENVIRONMENT IN LOW-LEVEL ACTIVE GALACTIC NUCLEUS ACTIVITY:
NO EVIDENCE FOR CLUSTER ENHANCEMENT
Brendan Miller,1 Elena Gallo,1 Tommaso Treu,2 and Jong-Hak Woo3
Draft version January 16, 2012
ABSTRACT
WeusetheAMUSE-VirgoandAMUSE-FieldsurveysfornuclearX-rayemissioninearly-typegalaxies
2
to conduct a controlled comparison of low-level supermassive black hole activity within cluster and
1
field spheroids. While both the Virgo and the Field samples feature highly sub-Eddington X-ray
0 luminosities (L /L between ∼ 10−8 −10−4), we find that after accounting for the influence of
2 X Edd
hostgalaxystellarmass,the fieldearly-typegalaxiestendtowardmarginallygreater(0.38±0.14dex)

n nuclear X-ray luminosities, at a given black hole mass, than their cluster counterparts. This trend is
a qualitativelyconsistentwiththe fieldblackholeshavingaccesstoagreaterreservoiroffuel,plausibly
J in the form of cold gas located near the nucleus. We are able to rule out at high confidence the

3 alternative of enhanced X-ray activity within clusters. Presuming nuclear X-ray emission correlates
1 withthetotalenergyandmomentumoutputofthese weaklyaccretingblackholes,this indicatesthat
low-levelactivegalacticnucleusfeedbackisnotgenerallystrongerwithintypicalclustergalaxiesthan

] in the field. These results confirm that for most cluster early-type galaxies (i.e., excluding brightest
E
cluster galaxies) direct environmental effects, such as gas stripping, are more relevant in quenching
H
star formation.
. Keywords: black hole physics — galaxies: nuclei
h
p
-
o 1. INTRODUCTION of outflows (Blandford & Begelman 1999; Pellegrini et
r The gas content and stellar populations of early-type al.2012)orjets(Falckeetal.2004). Theinclusioninsim-
t ulationsof“radiomode”feedbackforAGNsatthecenter
s galaxies are observed to depend upon their large-scale
a surroundings. Field early-typegalaxiestypically contain ofrichgroupsorclusterssuccessfullysuppressesstarfor-
[ mation and reproduces the red colors of large ellipticals
more cold (H I) gas, sometimes alongside young stel-
2 lar populations (Oosterloo et al. 2010). Significant star (Croton et al. 2006; Merloni & Heinz 2007; Khalatyan
etal.2008). Brightestclustergalaxies(BCGs)oftendis-
v formationin highdensity environmentsoccursprimarily
playdirectevidenceofmechanicalfeedbackintheformof
0 between 3 < z < 5, whereas in low density environ-
radio jets and inflated bubbles displacing the hot X-ray-
9 ments it persists to 1 < z < 2 or even lower for low-
emitting intraclustermedium (ICM)(e.g., McNamara&
4 mass galaxies (Thomas et al. 2005; Treu et al. 2005b;
Nulsen 2007). At least in luminous early-type galaxies,
4 Gobat et al. 2008). Proposed mechanisms of inhibiting
the rate of accretiononto the supermassive black hole is
. star formation within clusters include gas removal (e.g.,
2 well correlated with the emerging jet power (e.g., Allen
starvation through ram pressure stripping, tidal strip-
1 et al. 2006; Balmaverde et al. 2008).
ping, thermal evaporation, or other possibilities; Treu
1
The frequency of AGN activity, within both early and
1 et al. 2003; Moran et al. 2005; and references therein)
late-typegalaxies,mayitselfdependonlocalgalaxyden-
: or morphological quenching (i.e., stabilization of a gas
v disk through the build-up of a stellar spheroid; Martig sity, but interpretation is complicated by the sometimes
i non-overlapping nature of various observational activity
X et al. 2009). Such processes would operate only at low
indicators. The fractional rate of emission-line galaxies
efficiency within the field.
r is higher in the field, to a degree exceeding differences
a Another potential mechanism for terminating star
inmorphologicaldistributions (Dressler etal. 1985). On
formation is feedback from an active galactic nucleus
the other hand, the fractionalrate for AGNs selected by
(AGN). Radiation from an efficiently accreting super-
X-ray luminosities is similar between field and cluster
massiveblack hole canlaunch winds or directly heat the
samples (Martini et al. 2007; Haggard et al. 2010). Po-
surrounding gas (e.g., Ciotti & Ostriker 2007; Proga et
sition within the cluster may also play a role (Gavazzi
al. 2010). The duration of such “quasar mode” highly-
luminousAGN activityisonly <108 yr(Yu &Tremaine et al. 2011), although Atlee et al. (2011) find the intra-
∼
cluster radial distribution of X-ray or IR-selected AGNs
2002), outside of which black hole feeding is highly sub-
to be consistent with that of non-AGNs (while noting
Eddington. Numerical and observational work indicates
luminous X-ray AGNs may be more centrally concen-
that mechanical feedback may be of persistent impor-
trated). Within relaxed clusters, Ruderman & Ebeling
tance even in weakly accreting systems, in the form
(2005)findanexcessofX-raypointsourcesbothpeaked
within the central regions and more broadly distributed
1DepartmentofAstronomy,UniversityofMichigan,AnnAr- nearthe virialradius,whichthey attribute to blackhole
bor,MI48109, USA activity triggered by interaction with the BCG and by
2Physics Department, University of California, Santa Bar-
mergers, respectively.
bara,CA93106, USA
3AstronomyProgram,DepartmentofPhysicsandAstronomy, We aim to assess the incidence and magnitude of low-
Seoul NationalUniversity,Seoul,RepublicofKorea level supermassive black hole activity within represen-
2
tative field versus cluster early-type galaxies, and to
determine whether star formation in ordinary cluster
early-type galaxies is likely primarily AGN-quenched.
We characterize activity using nuclear X-ray emission,
whichdirectly measureshigh-energyaccretion-linkedra-
diative output but more importantly serves as a plau-
sible proxy for mechanical feedback (Allen et al. 2006;
Balmaverdeetal.2008). Our samplesarethe AMUSE4-
Virgo and AMUSE-Field surveys. Together, these tar-
get 203 optically-selected local early-type galaxies, with
both surveys centered around Large Chandra Programs
of ACIS-S3 snapshot (3–15 ks) observations (Virgo: ID
08900784,454ks,PITreu;Field: ID11620915,479ks,PI
Gallo),supplementedwithdeeperarchivalChandra cov-
erage. Thesampleselection,datareductionandanalysis,
and nuclear X-ray properties for the AMUSE-Virgo and
AMUSE-FieldsurveysarepresentedinGalloetal.(2008,
2010; hereafter G08, G10) and Miller et al. (2011; here-
after M11), respectively, from which we obtain values
of L , M , and M (the nuclear 0.3–10 keV X-ray
X BH star
luminosity,5 black hole mass, and galaxy stellar mass,
which have units of erg s−1, M⊙, and M⊙, respec-
tively). We emphasize thatthe AMUSE samplesareun-
biased with respect to nuclear X-ray properties, and in
fact almost all of the objects have L < 1041 erg s−1
X
and L /L < 10−5, reaching luminosities well be-
X Edd
low commonly utilized formal AGN classification limits.
Throughout this work, errors are quoted as 1σ.
2. NUCLEARACTIVITYINFIELDANDCLUSTER
SPHEROIDS
Before considering the overall characteristics of the
Virgosample,wearemotivatedbytheradialdependence
of cluster potentials, gas properties, and galactic densi-
tiestoexploretherelativecolorsandX-raypropertiesof
theincludedearly-typegalaxiesasafunctionofdistance
fromM87.6 Distances fromM87 are calculatedfromthe Figure 1. Properties of the M87-associated Virgo galaxies
projectedseparationandthe Meiet al.(2007)catalogof versus distance from M87. The vertical dotted line is r200,
distance moduli. Figure 1 shows the relative color,7 the and open symbols are X-ray upper limits. Top: Relative
color(dashed/dottedlinesaremeanforVirgo/Fieldsamples).
X-ray luminosity, the residual X-ray luminosity, and the
Middle : X-rayluminosity and residual luminosity. Bottom :
detection fraction, as a function of distance from M87.
X-ray coverage (open) and detections (solid). Crosses show
No strong trends are observed with any of these quanti-
typicaluncertainties.
tieswithintheVirgosample(cf.§1;Mart´ınezetal.2010)
and so it is hereafter treated in its entirety. given as the value for the nearest object in the sorted
list. In addition to the nuclear X-ray luminosity L , we
X
2.1. Comparison of AMUSE-Field and AMUSE-Virgo consider Eddington-scaled and residual X-ray luminosi-
samples
ties log(L /L ) and logL /L (M ), respectively,
X Edd X X star
Table 1 contains the properties of the full AMUSE- whereLX(Mstar)=38.36+0.71×(logMstar−9.8)is the
Field and AMUSE-Virgo samples as well as for the sub- best-fit relation determined for the Field sample from
set of objects for which M was calculated from the M11. For quantities in Table 1 derivedfrom X-raydata,
BH
M − σ relation (Gu¨ltekin et al. 2009). For quanti- the Kaplan Meier distribution, incorporating upper lim-
BH
ties derived from optical data (logM , logM , and its, was determined using the survival analysis package
star BH
logM /M ),the 25th,50th,and75thpercentilesare ASURV8 (Lavalley et al. 1992); note, however, that at
BH star
least the 25th percentile values are dominated by cen-
4 AMUSE:AGNMultiwavelengthSurveyofEarly-TypeGalax- soredpointsandsoshouldbetakenasroughlyindicative
ies only. To ensure a uniform comparison, six Field X-ray
5Weassumenointrinsicabsorptionofthenucleusintheseearly- measurements (two detections) with logL < 38.2 are
ctyopluemgnalaoxfi1e0s.21LX(10w2o2u)ldcmb−e2∼;0t.h1e(f0o.5rm)derexvaglrueeatmerayforbeantyipnitcrainlsoicf not used for this analysis; further, VirgoXupper limits
late-typegalaxies(Bogda´n&Gilfanov2011). with logLX < 38.2 are adjusted (by 0.15 dex) to match
6 Here only, Virgo galaxies associated with the subcluster cen- the limiting Field sensitivity.
teredonM49areexcluded. The X-ray detection fraction is higher for the Field
7Wedefinerelativecoloras∆(g−z)=(g−z)−(0.21×logMstar− sample compared to Virgo (50±7% versus 32±6%, with
0.83),basedontheVirgoredsequence. TheVirgoandFieldsam-
ples have mean ∆(g−z)=0.00 and −0.05, respectively (see also
Cassataetal.2007). 8 http://astrostatistics.psu.edu/statcodes/asurv
3
Table 1
SampleProperties
Sample n Mean 25th 50th 75th ndet/n Mean 25th 50th 75th
log(Mstar/M⊙) log(LX/1037)
Field 103 9.66±0.12 8.56 9.75 10.84 50/97 1.71±0.07 0.63 1.37 2.02
Withσ 61 10.54±0.09 10.02 10.70 11.10 46/57 2.01±0.09 1.52 1.92 2.33
Virgo 100 9.90±0.08 9.20 9.80 10.40 32/100 1.43±0.05 0.46 0.91 1.55
Withσ 54 10.48±0.09 10.10 10.40 10.80 28/54 1.60±0.08 0.73 1.44 1.72
log(MBH/M⊙) log(LX/LEdd)
Field 103 6.90±0.12 5.75 6.79 7.96 50/97 −6.96±0.10 −7.50 −7.05 −6.49
Withσ 61 7.66±0.12 7.25 7.89 8.40 46/57 −6.99±0.11 −7.50 −7.07 −6.57
Virgo 100 6.98±0.09 6.28 6.79 7.70 32/100 −7.42±0.12 −7.78 −7.38 −6.87
Withσ 54 7.49±0.14 7.04 7.52 8.20 28/54 −7.45±0.13 −7.79 −7.43 −7.02
log(MBH/Mstar) log(LX/LX(Mstar))
Field 103 −2.76±0.04 −2.90 −2.75 −2.53 50/97 0.05±0.08 −0.36 0.05 0.38
Withσ 61 −2.88±0.06 −3.07 −2.87 −2.64 46/57 0.03±0.08 −0.38 0.04 0.34
Virgo 100 −2.92±0.04 −3.01 −2.86 −2.72 32/100 −0.46±0.10 −1.12 −0.45 −0.03
Withσ 54 −2.99±0.07 −3.21 −2.94 −2.62 28/54 −0.45±0.11 −1.13 −0.45 −0.04
Note. —Quantitiesaredefinedin§1or§2.1. X-raydistributionsfortheFieldsamplearerestrictedtologLX>38.2andVirgo
limitswithlogLX <38.2havebeen adjusted by0.15dex tomatch theFieldsensitivity(§2.1). Kaplan-Meiervalues fortheX-ray
distributionsareslightlybiasedbecausethefirstupperlimitistreatedasadetection.
1σ Poisson errors), but within the shorter snapshot ex-
posures, the rates are closer (31±7% versus 24±5%).
However, the distribution of X-ray luminosities also
tends towardhigher values within the Field sample (Ta-
ble 1). At face value, the percentage of objects with
L ≥ 1039 erg s−1 is significantly greater in the Field
X
sample (25±5% versus 10±3%). This holds also for
residual X-ray luminosity: expressed as a percentage of
the full samples, the number of detected galaxies with
logL /L (M ) > 0.4 is 18±4% for the Field ver-
X X star
sus 5±2% for Virgo (similar results apply for 0.0 <
logL /L (M ) < 0.5; Figure 2a). Incorporating up-
X X star
per limits (with the caveats noted above), not only the
75th percentiles, but also the 25th and 50th percentiles
forX-rayluminosity(alsoEddington-scaledorresidual),
are modestly enhanced by ∼0.2–0.5dex in the Field rel-
ative to the Virgo samples.
We desire to compare the functional dependence of
L (M ) in the Field versus Virgo samples. However,
X star
the distribution of logM for the full Field sample
star
is inconsistent [Kolmogorov-Smirnov (KS) test proba-
bility p < 0.001] with that of the Virgo sample. As
bothstarformationandnuclearactivityarestrongfunc-
tionsofstellarmassaswellasenvironment(e.g.,Treuet
al. 2005a; Yee et al. 2005), we control the Field-versus-
Virgo comparison for M as described next.
star
2.2. Controlling for host stellar mass
Figure 2. Top: Residual X-ray luminosities (see §2.1) for
the Field (blue circles: snapshot; purple circles: archival) Itis unlikely that the modest tendency towardgreater
and Virgo (red diamonds) samples. Open symbols are upper X-ray luminosities within the Field sample is due to the
limits. Middle: MBHversusMstar fortheField(greencircles) differing Mstar distributions: (i) the tail to low stellar
and Virgo (gray diamonds) samples. The line indicates the massesthatprimarilydistinguishestheFieldsamplecon-
median log(MBH/Mstar) = −2.8. Filled symbols have MBH tains objects with generally lower, not higher, X-ray lu-
calculated from σ. The tint of the Field points indicates in minosities; (ii) the modest enhancement persists across
whatfractionofweightedsubsamplestheyareincluded. Bot-
the full range of considered stellar masses (Figure 2a);
tom: Histogram of Mstar distribution for the Field (cyan) (iii)thesubsetsoftheFieldandVirgosamplesforwhich
and Virgo (orange) surveys, with overplotted representation
σwasusedtocalculateM dohaveconsistentdistribu-
by four and three Gaussians, respectively. The green line is BH
tionsofM (Figure2b;theseareprimarilythebrighter
the ratio of the Virgo to Field summed Gaussians (arbitrary star
normalization), which istheweighting functionusedtodraw galaxies), and here too the Field subset has modestly
Field subsamples. greaterX-rayluminosity(alsoEddington-scaledorresid-
ual).
4
Table 2
CorrelationswithX-rayluminosity
(logLX−38)=A+B×(logMBH−8)
Samplea n ndet A B σ0
Field(full) 97 50 1.05+−00..1100 0.61+−00..1163 0.61+−00..0087
Virgo(full) 100 32 0.67+−00..0190 0.62+−00..1164 0.49+−00..0077
Field(withσ) 57 46 1.07+−00..1112 0.44+−00..1153 0.62+−00..0098
Virgo(withσ) 54 28 0.67+−00..1113 0.54+−00..1175 0.54+−00..1019
Field(LMXBcor) 97 45 1.04+−00..1101 0.67+−00..1184 0.60+−00..0087
Virgo(LMXBcor) 100 28 0.63+−00..1100 0.64+−00..1184 0.50+−00..0086
Field(lowlum) 90 43 0.90+−00..0088 0.42+−00..1110 0.35+−00..0075
Virgo(lowlum) 99 31 0.64+−00..0099 0.47+−00..1131 0.38+−00..0087
Field(X-raydet) 50 50 1.24+−00..1111 0.30+−00..1133 0.57+−00..0086
Virgo(X-raydet) 32 32 0.99+−00..1121 0.45+−00..1155 0.42+−00..0088
Field(weighted) 45 24+−21 0.99+−00..0087 0.51+−00..0058 0.53+−00..0093
Note. — Fitting of LX(MBH) is described in §2.3. The re-
ported parameters are medians of the posterior distributions, and
thequoted errorscorrespondto1σ foroneparameter ofinterest.
a Thevarioussamplesaredefinedasfollows: full: allobjects;with
σ: onlyobjects forwhichMBH was calculated fromahigh-quality
measurement of σ; LMXB cor: LX changed to an upper limit for
those objects forwhichthe probabilityof LMXBcontamination is
non-negligible;lowlum: onlyobjectssatisfyingLX<1040ergs−1;
X-ray det: onlyobjects withX-raydetections. Figure 3. LX versus MBH for the Field and Virgo sam-
ples, with symbols coded as in Figure 2a. The blue/red
We verify that LX, MBH, and Mstar are all mutu- lines are the Field/Virgo fits to the model (logLX −38) =
allysignificantlycorrelatedusingthemethod9 ofAkritas A+B×(logMBH−8);thesolid linesareforthefullsamples,
& Siebert (1996; cf. Kelly et al. 2007), which incorpo- whilethedashedlinesarefortheobjectswithMBHcalculated
rates censoring into the calculation of Kendall’s partial from σ. The inset shows joint 68% (solid) and 90% (dotted)
τ . For the Field sample, with (1) logM and (2) confidenceellipsesforthefullsamples,andjoint68%(dashed)
12,3 BH
confidenceellipses for theσ subsets. Thebest-fitparameters
logL controlling for (3) logM , the individual co-
efficieXnts are τ = 0.49, τ =st0ar.77, and τ = 0.50 for the21 Mstar–weighted Field subsamples arealso marked.
12 13 23
(all variables are correlated), with τ = 0.19±0.04; B×(logM −8),followingtheBayesianmethodologyof
12,3 BH
the nullhypothesisofzeropartialcorrelationisrejected. G10 for fitting. Uncertainties on L and M are 0.11
X BH
The M −M correlation would introduce degener- and0.44dex,respectively,withGaussianlikelihoodfunc-
star BH
acy were both variables included, while the L −M tions on the log quantities (except uniform probability
X star
correlation suggests L (M ) is best compared across below upper limits on L ). Rotational invariance is en-
X BH X
groups possessing consistent M distributions. forcedonthe power-lawindex B, andthe priordistribu-
star
In additionto analyzingthe L (M ) relationfor the tionofM istakentobelog-uniform. AMarkovChain
X BH BH
full Field and Virgo samples, we conduct two weighted MonteCarlosamplerisusedtoexplorethe{A,B,σ }pa-
0
comparisons. As mentioned, the subsets of early-type rameterspace. Weusethemedianof9000randomdraws
galaxiesforwhichM iscalculatedfromσalreadyhave fromtheposteriordistributionasthe mostlikelyparam-
BH
consistent distributions of M . We also draw 21 ran- etervalue,withestimated1σ errorsreportedasthe16th
star
dom subsamples from the Field survey weighted to cor- and 84th percentiles. We have improved the treatment
respond to the Virgo M distribution (using the ratio of upper limits over that in G10 and here use a more
star
of the Virgo to Field M histograms;Figure 2c). This recent M −σ relation to recalculate their black hole
star BH
importance sampling is conducted without replacement, masses,andsowe giveupdated fits forthe Virgosample
placing an effective limit on the subsample size; we use as well as new results for the Field sample in Table 2,
n = 45 as only ∼5% of such subsamples have logM illustrated in Figure 3.
star
distributions inconsistent (KS p < 0.05) with Virgo. Of The best-fit relation for the full Field sample is
the 97 Field galaxies with logL > 38.2, 16 (all with (logL −38)=(1.05±0.10)+(0.61±0.15)×(logM −8),
X X BH
logM < 8.1) are not included in any of the 21 sub- with intrinsic scatter of 0.61±0.08 dex.10 Fitting simu-
star
samples, while 48 [33] are included in (1 to 14)/21 [(15 lated X-ray luminosities for each Field M point (ran-
BH
to 21)/21] of the subsamples. domlydistributedwithσ =0.61aboutthebest-fitrela-
0
tionandwithvaluesoflogL <38.4translatedintolim-
X
2.3. Field versus cluster LX(MBH) correlations itscloselyscatteredneartheLXsensitivitythreshold)re-
We parameterize the dependence of nuclear X-ray lu-
10 For reference, the coefficients for this fit performed with the
minosity upon black hole mass as (logL −38) = A+
X IDL Bayesian code of Kelly (2007) are A = 1.10±0.10, B =
0.75±0.11, and σ0 = 0.70±0.09; the differing methodologies
9 http://www.astrostatistics.psu.edu/statcodes/cens tau provideresultsconsistentwithintheerrors.
5
turns output coefficients in close agreement (∆A<10%, a range in richness, as well as triples, pairs, or isolated
∼
∆B<7%) with those input. Better data (e.g., dynami- galaxies. Groupsareintermediatebetweenfieldandclus-
∼
cal black hole masses) or alternative priors or methods ters in terms of strength of galaxy-medium interactions,
might yield somewhat different coefficients, but we em- but facilitate strong galaxy-galaxy interactions because
phasize it is the comparisonbetween the (identically fit) thebulkspeedsofthegalaxiesarelower(somewhatsim-
Field and Virgo relations that is of interest here. ilar to cluster outskirts). An apparent smooth decrease
The best-fit slopes for the full Field and Virgo sur- in scaled X-ray luminosities from isolated to group to
veys are consistent, B ≃ 0.6. Consequently, the aver- cluster environments, albeit with large scatter, was in-
ageEddington-scaledX-rayluminosity scaleswith black terpretedbyM11as tentativeevidence ofenvironmental
hole mass as ∝M−0.4 (a “downsizing” effect as noted modulation of supermassive black hole fueling. The di-
BH
by G10). However, the best-fit Field intercept exceeds rect comparison between field and cluster galaxies con-
thatforVirgoby0.38±0.14,consistentwiththemodest ducted in this work supports that possibility.
X-ray enhancement discussed in §2.1. The complemen- The robust result that these Virgo early-type galaxies
taryquestionofwhethertheVirgoearly-typegalaxiesare are not systematically more X-ray luminous than their
systematicallymoreX-rayactiveisrobustlyansweredin Field counterparts indicates that low-level black hole
the negative; the probability that the Field intercept is activity is not generally stronger within typical cluster
>2.5σ lower than that for Virgo is <10−6. galaxies. This result is supported by the insensitivity
Fitting to the measured-σ subsets produces similarre- to environment of the fractional rate of X-ray-identified
sults,withadifferenceininterceptof0.40±0.17. (Some- AGNs (Martini et al. 2007; Haggard et al. 2010). Pre-
what flatter slopes may derive from the incompleteness suming nuclear X-ray emission correlates with the total
of σ measurements in fainter galaxies, which tend to energy and momentum output of these weakly accret-
be X-ray limits.) Fitting most of the M –weighted ingblackholes(§1),thisimplieslow-levelAGNfeedback
star
Field subsamples gives results consistent with those for is not generally stronger within typical cluster galaxies
the full Field sample; 20th, 50th, and 80th percentiles than in the field. This supports that the older stellar
for each parameter from these fits are given in Table 2. populations in cluster early-type galaxies, and the asso-
Converting the handful of X-ray detections with a non- ciatedpaucityofongoingstarformation,arenotdirectly
negligible possibility of LMXB contamination to upper due to recent black hole activity.
limits does not significantly alter the results. If ob- Possibly past AGN outbursts could have already sup-
jects with logL > 40 erg s−1 are arbitrarily excluded, pressed star formation and expelled gas from cluster
X
thereby removing even weak Seyfert-level activity, the galaxies, leaving no obvious current link. Both star for-
slopes flatten somewhat and the difference between in- mation(Moranetal.2005)andAGNactivity(Ruderman
terceptsis0.26±0.12. RestrictingconsiderationtoX-ray &Ebeling2005)maybe triggeredatinfallneartheclus-
detectionsalsoflattenstheslopes;herethedifferencebe- ter virialradius, either throughenvironmentalor galaxy
tween intercepts is 0.25±0.16. Joint confidence regions interactions. The AGN fraction increases with redshift
forthefullandσ subsetfitsareplottedinsettoFigure3, (e.g., Martini et al. 2009; Haggard et al. 2010; Aird et
alongwiththebest-fitparametersfortheM –weighted al. 2011), and at z ∼ 3 may be significantly higher in
star
Field subsamples. clusters (Lehmer et al. 2009). However, given that su-
permassive black hole activity is not generally greater
3. DISCUSSION within clusters in the local universe, this scenario re-
WebrieflyconsidertheimplicationsofenhancedL in quires AGN activity to be terminated or reduced more
X
field early-types. As both the Field and Virgo galax- rapidly within clusters. While various environmental ef-
ies have highly sub-Eddington luminosities (10−8 < fectsmayremovegasotherwiseavailableforAGNfueling
L /L < 10−4), their accretion structures should be in clusters, as described above, this depletion also acts
X Edd
physically similar (see, e.g., Soria et al. 2006 for discus- to quench star formation and so it is unclear that past
sion of inefficient flow models). One obvious parameter AGN activity is required for this task. Our findings are
that could link nuclear activity to the large-scale envi- consistent with other arguments that AGN activity is
ronment is the fuel supply. Hot gas is subject to off- notthe proximatecauseofstarformationquenching,in-
setting effects in clusters, with ram pressure stripping cludingtheobservedgradualdeclineinremnantstarfor-
counteredby accretionfromthe ICM and potential con- mation with decreasing cluster radius which led Moran
finement of winds (Brown & Bregman 2000), but radio et al. (2005) to associate the cessation of star formation
and optical observations establish cold gas and younger with a slow-actingeffect such as starvation(see also von
stellar populations as more prevalent in field early-type derLindenetal.2010),andthesimilarityinthedistribu-
galaxies (§1). Major tidal interactions, which are less tionofX-rayluminosities for9.5<log(Mstar/M⊙)<12
frequent within clusters due to the large galaxy velocity which led Aird et al. (2011)11 to state no evidence for
dispersions,couldhelpbringgastothenucleus,although AGN-quenching.
this is not required as stochastic events may suffice for We note BCGs occupy a privileged position centered
low-level fueling (as conjectured for Seyferts; Hopkins withintheclusterpotentialandshowdirectevidence(§1)
& Hernquist 2006). While accretion in local early-type of mechanical AGN feedback. An additional complica-
galaxiesdoesnotappeartobelimitedbygassupply(So- tionisthatBCGsincoolcoreclustersdoshowrecentstar
ria et al. 2006; Pellegrini 2010), a ∼2× larger infall rate formation related to the cooling gas (Hicks et al. 2010),
(small in absolute terms; e.g., Allen et al. 2006) could again distinguishing them from typical cluster galaxies.
plausibly produce the modest observedenhancements in
LX assuming uniform efficiency and outflow fraction. 11 Pellegrini (2010) also mention the large scatter in X-ray lu-
TheFieldsamplecontainsgalaxiesingroupsspanning minositiesaboveMstar≃6×109M⊙.
6
OurclustersampleisexclusivetoVirgo,sowedonotin- Gobat,R.,Rosati,P.,Strazzullo,V.,Rettura,A.,Demarco,R.,&
vestigateBCGsingeneral,butourconclusionsaboutthe Nonino,M.2008, A&A,488,853
importance of environmental effects versus AGN feed- Gu¨ltekin,K.,etal.2009,ApJ,698,198
Haggard,D.,Green,P.J.,Anderson,S.F.,Constantin,A.,
back likely do not apply to BCGs.
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Hicks,A.K.,Mushotzky,R.,&Donahue,M.2010,ApJ,719,1844
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assistance with fitting the L (M ) relations, and an Kelly,B.C.,Bechtold, J.,Siemiginowska,A.,Aldcroft,T.,&
X BH
anonymous referee for comments that improved this Sobolewska, M.2007, ApJ,657,116
Khalatyan,A.,Cattaneo, A.,Schramm,M.,Gottl¨ober, S.,
work. T. T. acknowledges support from the NSF
Steinmetz, M.,&Wisotzki,L.2008,MNRAS,387,13
through CAREER award NSF-0642621, and from the
Lavalley,M.,Isobe,T.,&Feigelson,E.1992, AstronomicalData
PackardFoundationthroughaPackardResearchFellow- AnalysisSoftwareandSystemsI,25,245
ship. J. H. W. acknowledges support by the Basic Sci- Lehmer,B.D.,Alexander,D.M.,Geach, J.E.,etal.2009,ApJ,
ence Research Program through the National Research 691,687
Martig,M.,Bournaud,F.,Teyssier,R.,&Dekel,A.2009,ApJ,
Foundation of Korea funded by the Ministry of Educa-
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tion, Science and Technology (2010-0021558). Support
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