Type 2 AGN host galaxies in the Chandra-COSMOS Legacy Survey: No Evidence of AGN-driven ...

12 Pages·2017·2.23 MB·English

Draftversion May12,2017
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
TYPE 2 AGN HOST GALAXIES IN THE CHANDRA-COSMOSLEGACY SURVEY: NO EVIDENCE OF
AGN-DRIVEN QUENCHING
Hyewon Suh1, Francesca Civano2, Gu¨nther Hasinger1, Elisabeta Lusso3,17, Giorgio Lanzuisi4,5, Stefano
Marchesi6, Benny Trakhtenbrot7, Viola Allevato8,9, Nico Cappelluti10, Peter L. Capak11,12, Martin Elvis2,
Richard E. Griffiths13, Clotilde Laigle14, Paulina Lira15, Laurie Riguccini16, David J. Rosario17, Mara
Salvato18, Kevin Schawinski7, Cristian Vignali3,4
7
1 Draft version May 12, 2017
0
2 ABSTRACT
Weinvestigatethestarformationpropertiesofalargesampleof∼2300X-ray-selectedType2Active
y
a GalacticNuclei(AGNs)hostgalaxiesouttoz ∼3intheChandraCOSMOSLegacySurveyinorderto
M understandtheconnectionbetweenthestarformationandnuclearactivity. Makinguseoftheexisting
multi-wavelength photometric data available in the COSMOS field, we perform a multi-component

0 modeling from far-infrared to near-ultraviolet using a nuclear dust torus model, a stellar population
1 modelandastarburstmodelofthespectralenergydistributions(SEDs). Throughdetailedanalysisof
SEDs,we derivethe stellarmassesandthe starformationrates(SFRs) ofType 2 AGNhostgalaxies.
A] The stellar mass of our sample is in the range 9 < logMstellar/M⊙ < 12 with uncertainties of ∼0.19
dex. We find that Type 2 AGN hostgalaxieshave,on average,similar SFRs comparedto the normal
G star-forming galaxies with similar M and redshift ranges, suggesting no significant evidence for
stellar
. enhancement or quenching of star formation. This could be interpreted in a scenario, where the
h relative massive galaxies have already experienced substantial growth at higher redshift (z >3), and
p
grow slowly through secular fueling processes hosting moderate-luminosity AGNs.
-
o Subject headings: galaxies: active — galaxies: nuclei — quasars: general — black hole physics
r
t
s
a 1. INTRODUCTION presence of a supermassive black hole (SMBH) affects
[ its host galaxy. A connection between the growth of
One of the outstanding issues for understanding
SMBHs and their host galaxies has been widely ac-
1 the formation and evolution of galaxies is how the
ceptedby observedcorrelationsbetweenblackhole mass
v
0 1InstituteforAstronomy,UniversityofHawaii,2680Woodlawn and host galaxy properties (e.g. Magorrianet al.
9 Drive,Honolulu,HI96822,USA 1998; Gebhardt et al. 2000; Ferrarese & Merritt
2Harvard-SmithsonianCenterforAstrophysics,Cambridge,MA 2000; Gu¨ltekin et al. 2009; McConnell & Ma 2013;
8
02138,USA
3 3INAF Osservatorio Astrofisico di Arcetri, I-50125 Florence, Kormendy & Ho 2013), andthe remarkableresemblance
0 Italy between the evolutionary behavior of the growth of
. 4Dipartimento di Fisica e Astronomia, Universit`a di Bologna, active galactic nuclei (AGN) and star formation his-
5
vialeBertiPichat6/2,40127Bologna,Italy tory (e.g. Madau et al. 1996; Giacconi et al. 2002;
0 5INAF Osservatorio Astronomico di Bologna, Via Ranzani 1, Cowie et al. 2003; Steffen et al. 2003; Ueda et al. 2003;
7 40127,Bologna,Italy
1 6Department of Physics & Astronomy, Clemson University, Barger et al. 2005; Hasinger et al. 2005; Hopkins et al.
: Clemson,SC29634,USA 2007; Aird et al. 2015; Caplar et al. 2015). The exis-
v 7Institute forAstronomy, DepartmentofPhysics,ETHZurich, tence ofthese correlationsseemsto supportthatnuclear
Xi Wo8lDfgeapnagr-tPmaeunlit-StorfassPeh2y7s,icCsH, -8U0n93iveZrusriticyh,oSfwiHtzeelrsliannkdi, Gustaf activity and star formation might co-exist with the
same gas reservoir fueling black hole accretion and star
H¨allstr¨ominkatu2a,FI-00014Helsinki,Finland
r
a 9Universityof Maryland, BaltimoreCountry, 1000 HilltopCir- formation simultaneously (e.g. Springel et al. 2005;
cle,Baltimore,MD21250, USA Netzer 2009; Mullaney et al. 2012; Rosario et al. 2013;
10Yale Center for Astronomy and Astrophysics, 260 Whitney
Vito et al. 2014). However, our current understanding
Avenue,NewHaven,CT06520,USA
11InfraredProcessingandAnalysisCenter(IPAC),1200E.Cal- of the effects that AGN can have on the star formation
iforniaBlvd.,Pasadena, CA,91125,USA processes is still under debate (see Alexander & Hickox
12California Institute of Technology, 1200 E. California Blvd., 2012; Kormendy & Ho 2013; Heckman & Best 2014 for
Pasadena,CA,91125, USA
recent reviews).
13DepartmentofPhysics&Astronomy,UniversityofHawaiiat
Hilo,200W.KawiliStreet,Hilo,HI96720, USA There has been a general consensus that the majority
14Sorbonne Universit´es, UPMC Universit´e Paris 6 et CNRS, ofstar-forminggalaxiesshowatightcorrelationbetween
UMR 7095, Institut d’Astrophysique de Paris, 98 bis Boulevard the star formation rate (SFR) and their stellar mass,
Arago,75014Paris,France
15DepartmanentodeAstronom´ıa,UniversidaddeChile,Casilla commonly referredto as the main sequence (MS) of star
36-D,Santiago, Chile formation (e.g. Noeske et al. 2007; Daddi et al. 2007;
16Observato´rio do Valongo, Universidade Federal do Rio de Elbaz et al.2007;Rodighiero et al.2011;Whitaker et al.
Janeiro, Ladeirado Pedro Antonio 43, Sau´de, Riode Janeiro, RJ 2012; Rodighiero et al. 2014). Speagle et al. (2014)
20080-090, Brazil
17Centre for Extragalactic Astronomy, Department of Physics, present the calibrated relationship between stellar mass
DurhamUniversity,SouthRoad,Durham,DH13LE,UK and SFR out to z ∼ 6 using a compilation of 25
18Max-Planck-Institutefu¨rPlasmaPhysics,BoltzmannStrasse star-forming MS studies in a variety of fields, report-
2,D-85748Garching,Germany ing that the MS galaxies have a ∼0.2 dex scatter in
2 Suh et al.
the slope of their M -SFR relationand remains con-
stellar
stant over cosmic time. The existence and tightness of
thisstarformationsequencecanbeinterpretedassuming
that the growth of the majority of star-forming galaxies
have been regulated more by internal secular processes
rather than by merger process (e.g. Elbaz et al. 2011;
Rodighiero et al. 2011; Wuyts et al. 2011).
Controversial results were found for AGN host galax-
ies: some studies have indicated equivalent or enhanced
star formation compared to normal star-forming galax-
ies (e.g. Silverman et al. 2009; Mullaney et al. 2012;
Rosario et al. 2012; Santini et al. 2012; Juneau et al.
2013), whereas some others have shown that AGN host
galaxies lie below the MS of star-forming galaxies, sug-
gesting that AGN accretion might suppress and eventu-
allyquenchstarformationviaaprocessoffeedback(e.g.
Barger et al. 2015;Mullaney et al.2015; Riguccini et al.
2015; Shimizu et al. 2015). Furthermore, several studies
have addressed that the majority of AGN host galax-
Fig. 1.— The absorption-corrected X-ray (2-10 keV) luminos-
ies in the local universe are preferentially found in the
ity versus spectroscopic (solid circle) and/or photometric (open
greenvalley onthe color-magnitudediagram,transition- square) redshift for our sample of Type 2 (non-broad-line or ob-
ingfromstar-forminggalaxiesinthebluecloudtopassive scured) AGNs from CCLS. We splitour sample into four redshift
bins (vertical dashed lines). Colored symbols indicate sources in
galaxiesontheredsequence(e.g. Schawinski et al.2009;
Trump et al. 2013). Therefore, the question of whether different L2−10keV bins. For sources which are not detected in
hard band but in other bands (full and/or soft), upper limits of
AGN activity can significantly enhance or quench star L2−10keV areshownwithdownwardtriangles.
formation in galaxies is still unsettled.
Different results can be produced by either physical
Caplar et al. 2015). The alreadyexisting extensive com-
properties of the sources, or observational biases, or
pilation of multi-wavelength data in the COSMOS field
both. The sample selection including completeness and
allowsustoinvestigateAGNhostgalaxiestohaveabet-
biases due to a specific selection method (X-ray ver-
ter understanding of nuclear activity and its connection
sus infrared selected AGNs, for example), as well as the
with star formation.
use of different SFR indicators could introduce system-
In this paper, we investigate the properties of AGN
atics since the contribution of AGN emission may sig-
host galaxies in the Chandra COSMOS-Legacy survey.
nificantly hamper the precise determination of SFRs of
Since the SMBH-powered emission contributes signifi-
AGN host galaxies. The Herschel Space Observatory
cantly to the ultraviolet-to-optical parts of the spectra
(Pilbratt et al. 2010; Poglitsch et al. 2010; Griffin et al.
ofType1AGNs (e.g. Elvis et al.2012;Hao et al.2013),
2010) covers the far-infrared emission from dust in-
itisextremelydifficult todetermine reliablestellarmass
cluding the characteristic far-IR bump typically seen
for Type 1 AGNs (e.g. Maiolino et al. 2010). A detailed
in star-forming galaxies, allowing us for more precise
analysisoftheType1AGNhostgalaxiesintheChandra
measurements of the total IR luminosity, especially for
COSMOS Legacy Survey, including spectroscopic anal-
dusty galaxies and AGN host galaxies, since many of
ysis in the optical and near-infrared wavelength ranges,
the often used SFR indicators (e.g. Hα, UV contin-
willbepresentedinasecondpaper,Suhetal. (inprepa-
uum)canbesubstantiallycontaminatedbyAGN-related
ration). Thus, we focus on the non-broad-line and/or
emission (e.g. Dale et al. 2007; Schweitzer et al. 2007;
obscuredAGNhostgalaxiesbasedeitheronthespectro-
Netzer et al. 2007; Lutz et al. 2016).
scopic or the photometric classification. Hereafter, we
Deep, large-area X-ray observations with Chandra in
refertonon-broad-lineand/orobscuredsourcesas“Type
the COSMOS field (i.e. Chandra-COSMOS, Chandra-
2” AGNs. We utilize multi-wavelength data from near-
COSMOSLegacySurvey;Elvis et al.2009;Civano et al.
ultraviolet to far-infrared of a large sample of AGNs in
2016) open up a new regime for studying a large sample
the Chandra COSMOS Legacy field. We use the Spec-
oftheAGNpopulationoverabroadrangeofluminosities
tralEnergyDistribution(SED)fittingtodisentanglethe
(41<log L0.5−10 keV(erg/s)<45) out to z ∼ 5, provid- galaxyandnuclearcontributionsinordertomeasurethe
ing a unique opportunity of studying AGN evolution.
stellar masses and the SFRs accurately. Finally we dis-
X-ray surveys are the most efficient way for selecting
cussthe effects ofthe nuclearactivity onthe starforma-
AGNsoverawiderangeofluminositiesandredshiftsbe-
tion in Type 2 AGN host galaxies by comparing to the
cause they are less affected by obscuration, and also the
normal star-forming galaxies.
contaminationfromnon-nuclearemission,mainly dueto
ThroughoutthispaperweassumeaΛCDMcosmology
star-formation processes, is far less significant than in with Ω =0.3, Ω =0.7, and H =70 km s−1 Mpc−1.
m Λ 0
optical and infrared surveys (Donley et al. 2008, 2012;
Lehmer et al.2012;Stern et al.2012). Therefore,theX- 2. X-RAY-SELECTED AGN SAMPLE
ray emission is a relatively clean signal from the nuclear
2.1. The Chandra COSMOS Legacy Survey
component. Furthermore, using the AGN sample with
thelarge,uniformX-raydepthandcoherentobservations The Chandra COSMOS-Legacy Survey (CCLS;
intheCOSMOSfield,wecanminimizethesystematicse- Civano et al. 2016) is a large area, medium-depth X-ray
lectioneffects(e.g. Lauer et al.2007;Rosario et al.2013; survey covering ∼2 deg2 of the COSMOS field obtained
Star formation in Type 2 AGN host galaxies 3
by combining the 1.8 Ms Chandra COSMOS survey (C-
COSMOS;Elvis et al.2009)with2.8MsofnewChandra TABLE 1
Detection Fraction foreach Photometry
ACIS-I observations. The CCLS is wide enough to have
Bands
one of the largest samples of X-ray AGNs selected from
a single contiguoussurvey region,containing 4016X-ray Photometry Band Detection fraction
pointsources,andalso deepenoughto find faint sources
down to limiting fluxes of 2.2×10−16 erg cm−2 s−1, GALEXNUV 3%(64/2267)
CFHTU 62%(1395/2267)
1.5×10−15 erg cm−2 s−1, 8.9×10−16 erg cm−2 s−1 in SubaruB 72%(1634/2267)
the soft (0.5-2 keV), hard (2-10 keV), and full (0.5-10 SubaruV 72%(1637/2267)
keV) bands. Moreover,CCLS sources are bright enough Subarur 82%(1866/2267)
Subarui 84%(1896/2267)
so that 97% of these were identified in the optical
Subaruz+ 87%(1964/2267)
and infrared bands and therefore photometric redshifts UltraVistaY 72%(1640/2267)
were computed. Thanks to the intense spectroscopic UltraVistaH 75%(1697/2267)
campaigns in the COSMOS field, ∼54% of the X-ray UltraVistaJ 78%(1761/2267)
UltraVistaKs 79%(1798/2267)
sources have been spectroscopically identified and
Spitzer3.6µm 92%(2079/2267)
classified. The full catalog of CCLS has been presented Spitzer4.5µm 92%(2079/2267)
by Civano et al. (2016) and Marchesi et al. (2016), Spitzer5.8µm 86%(1946/2267)
including X-ray and optical/infrared photometric and Spitzer8.0µm 76%(1721/2267)
spectroscopic properties.
We select a sample of Type 2 AGNs using the spec-
Spitzer/IRAC bands (3.6, 4.5, 5.8 and 8.0µm). The
troscopic type when available (sources classified as non-
detection fraction for each photometry bands is pre-
broad-line AGN, which show only narrow emission line
sented in Table 1. In addition, we use the 24µm and
and/or absorption line features in their spectra), or
70µmMultibandImagingPhotometerforSpitzer(MIPS)
the photometric type (sources which are fitted either
bands (Sanders et al. 2007; Le Floc’h et al. 2009) with
with an obscured AGN template or with a galaxy tem-
∼59% (1329/2267) of the sources detected in the 24µm
plate) from the catalog of X-ray point sources in the
band, which is particularly important for identifying
CCLS (Marchesi et al. 2016). 2716 sources are clas-
AGN dusty obscuring structure. We also constrain
sified as Type 2 AGNs with spectroscopic redshifts
the SEDs in the far-IR wavelength range for ∼25%
(1027)or photometric redshifts (1689). We compute the
(568/2267) of the sources which have been detected by
absorption-corrected X-ray luminosity of Type 2 AGNs
the Herschel Space Observatory (PACS 100µm (∼12%;
using absorption-correction factor from Marchesi et al.
262/2267),160µm(∼10%;222/2267)andSPIRE250µm
(2016) which is obtained assuming an X-ray spectral
(∼20%; 451/2267), 350µm (∼10%; 224/2267), 500µm
index Γ=1.8. In Figure 1, we show the absorption-
(∼3%; 60/2267); Pilbratt et al. 2010; Poglitschet al.
corrected 2-10 KeV X-ray luminosity L2−10 keV of Type 2010; Griffin et al. 2010). We limit the work to only
2AGNsasafunctionofredshift(spectroscopicorphoto-
thoseobjectswithatleastfivedetectedphotometricdata
metric). WeestimateL2−10 keV valuesusingupperlimits points (∼91%; 2056/2267), to guarantee a reliable mea-
for sources which are not detected in the hard band but
surement of the SED fits.
detected in the full band. 1980 sources have been de-
tected in the full band (1618 in the soft and 1532 in the
3. SPECTRAL ENERGY DISTRIBUTION FITTING
hardband). Sourceswithphotometricandspectroscopic
redshiftsareindicatedwithopenandsolidcircles,respec- To derive the physical properties of AGN host galax-
tively. The final sample analyzed in this paper consists ies, we developed a 3-component SED fitting technique
of 2267 out of 2716 Type 2 AGNs in the redshift range which allows to disentangle the nuclear emission from
0.5<z <3.0,inordertoavoideffectsrelatedtotheflux- the stellar light. Over the far-IR to near-UV wave-
or volume-limits of the survey, and because at z > 3.0 lengthcoverage,wedecomposetheentireSEDintoanu-
sources only have partial spectral coveragewhich makes clearAGN dusty obscuring structure (i.e., torus), a host
it challenging to perform a statistically significant SED galaxy with stellar populations, and a starburst com-
fitting. Our Type 2 AGN sample has X-ray luminosities ponent, which is crucial for estimating reliable physical
spanning 3 orders of magnitude from 1042 to 1045 erg/s propertiesofhostgalaxiessuchasgalaxymassandSFR.
in the hard band. Colored symbols indicate sources in The method used here is similar to the one applied by
different X-ray luminosity (L2−10 keV) bins. For sources Lusso et al. (2011) and Bongiorno et al. (2012) on the
which are not detected in the hard band but in the full XMM-COSMOSdataset,withsignificantimprovements,
band,weshowupper limitsofL2−10 keV withdownward includingtheBayesianmethoddescribedinthefollowing
triangles. sections.
2.2. Multi-wavelength dataset 3.1. Model templates
We compile the SEDs of our sample of Type 2 AGNs In order to examine the SEDs for Type 2 AGN host
from near-ultraviolet (near-UV; 2300˚A) to far-infrared galaxies, we use model SEDs which are made by com-
(far-IR;500µm)wavelengthsusing the multi-wavelength biningastellarpopulation,hotdustemissionfromAGN
photometricdataavailableintheCOSMOSfield. Specif- (torus) and IR starburst templates to match the broad-
ically, we use the most recent photometric catalog from bandphotometrySEDsofoursample. Thenuclearemis-
Laigle et al.(2016)includingtheGALEXnear-UVband, sioninobscuredAGNdominatestheSEDonlyintheX-
CFHT U band, five Subaru Suprime-Cam bands (B, V, rayband, while atotherwavelengths,the lightis mainly
r, i, z+), four UltraVista bands (Y, H, J, Ks), and four due to the galaxy emission combined with reprocessed
4 Suh et al.
structed from the study of a large sample of Seyfert
galaxies for which clear signatures of non-stellar nu-
clear emission were detected in the near-IR and mid-
IR, and also using the radiative transfer code GRASIL
(Silva et al. 1998). There are three different templates
dependingontheamountofnuclearobscurationinterms
of hydrogen column density, 1022 <N <1023 cm−2,
H
1023 <N <1024 cm−2, N >1024 cm−2 for Seyfert 2.
H H
The threetemplates ofType2 AGN dusttorusareplot-
tedinFigure2withyellowcurves. Thelargerthecolumn
density, the higher is the nuclear contribution to the IR
emission. Although the X-ray data for our AGNs con-
tains some information on the N towards each source
H
(see Marchesi et al. 2016), we chose to allow N to be a
H
free parameter in our SED fitting.
For the starburst component in the far/mid-IR re-
Fig. 2.— Examples of model templates used in the multi- gion, we adopted 169 starburst templates (105 from
component SED fitting. Green curves indicate some examples of
Chary & Elbaz 2001 and 64 from Dale & Helou 2002)
host galaxy templates with various combinations of τ=[0.1, 1, 3],
and tage=[50 Myr, 2 Gyr] with E(B-V)=[0.0, 0.3]. Yellow curves for fitting the cold dust emission (i.e. far-IR emis-
correspond to three AGN dust torus templates depending on the sion). It has been shown that measuring the far-IR
hydrogencolumndensity,NH. Redcurvescorrespondtothesubset luminosity from fitting the far-IR region to libraries of
ofstarbursttemplates.
SED (Chary & Elbaz 2001; Dale & Helou 2002) gives
roughlythe same results as the modified blackbody plus
nuclear emission in the near-IR and mid-IR. While nu- power-law model (Casey 2012; U et al. 2012; Lee et al.
clear emission, reprocessed by dust, could significantly 2013). The Chary & Elbaz (2001) templates are gen-
contribute to the mid-IR luminosity, the far-IRluminos- erated based on the SEDs of four prototypical star-
ity is known to be dominated by galaxy emission pro- burst galaxies (Arp220, ULIRG; NGC6090, LIRG; M82,
duced by star formation activity (e.g. Kirkpatrick et al. starburst; and M51, normal star-forming galaxy). The
2012). Although a recent study by Symeonidis (2017) Dale & Helou (2002) templates are based on 69 normal
pointed out that the most powerful unobscured quasars star-forming galaxies, representing a wide range of SED
could dominate the far-IR luminosity, we only consider shapes and IR luminosities, complementing each other.
the far-IR luminosity produced by starburst activity for A small subset of starburst templates are shown in Fig-
our sample of moderate-luminosity AGNs. ure 2 as red curves.
The optical SED of a galaxy represents the integrated
light of the stellar populations. We have generateda set 3.2. Multi-component SED fitting
of synthetic spectra from the stellar population synthe- Wedevelopeda3-componentSEDfittingprocedurein
sis models of Bruzual & Charlot (2003). We have used which the observed photometric data is fitted at a fixed
solar metallicity and the Chabrier initial mass function redshiftofthesourcewithalargegridofmodelsobtained
(Chabrier 2003). We have built 10 exponentially decay- by combining the templates described above. The ob-
ingstarformationhistories(SFH),wheretheopticalstar servedfluxcanbe expressedasthe sumof3components
formation rate is defined as SFR ∝ et/τ, with charac- as
teristic times ranging from τ = 0.1 to 30 Gyr, and a
f =C f +C f +C f (1)
model with constant star formation. For each SFH, the obs 1 stellarpopulation 2 AGN 3 starburst
SEDsaregeneratedbymodelswith15gridsofages(t ) wherethe C , C ,andC arecoefficients thatreproduce
age 1 2 3
rangingfrom0.1Gyrto10Gyr,withtheadditionalcon- theobserveddatabyχ2 minimization. Thebest-fitSED
straintoneachcomponentthattheageshouldbesmaller solution could be a stellar population with a negligible
thantheageoftheUniverseattheredshiftofthesource. contributionfromAGN/starburstcomponents,orastel-
The library of stellar population models is composed by lar population with the central AGN component, or a
165 templates. Since the stellar light can be affected by stellarpopulationwithstarburstcomponent, ora stellar
dustextinction,wetakeintoaccountthereddeningeffect population with both AGN and starburst components.
using the Calzetti et al. (2000) law. We have considered The fit is performed differently for sources detected in
E(B-V)values inthe rangebetween0and0.5withsteps the far-IR and those that are not. Specifically, for the
of 0.05, and the range between 0.5 and 1 with a step of sources detected at 24µm but not in any far-IR Her-
0.1. We show some examples of stellar population tem- schel wavelength, there are large uncertainties in the es-
plates with various combinations of τ=[0.1, 1, 3], and timate of C and C , because both could substantially
2 3
t =[50Myr,2 Gyr]with E(B-V)=[0.0,0.3]in Figure 2 contributeinthe observed24µmband,introducingade-
age
(green curves). generacy in the SED fitting. This implies that the fit-
In general, the SED of an obscured AGN is character- ting can produce two different probable solutions with
ized by the near-infrared bump that is a result of the a similar χ2. One is a prominent AGN dominating in
absorption of intrinsic nuclear radiation by dust clouds the IR range with no contribution from the dust emis-
in the proximity of the central region (so-called torus) sion heated by stars, and the other is a negligible AGN
on parsec scales, which subsequently re-radiate at in- contribution in the 24µm band with the infrared emis-
fraredfrequencies(Barvainis1987). ThedusttorusSED sion dominated by star-forming regions. Therefore, we
templates are taken from Silva et al. (2004), as con- perform two different fits for the sources which are not
Star formation in Type 2 AGN host galaxies 5
Fig. 3.—ExamplesoftheSEDfits(leftpanels)forsourceswhicharedetectedinHerschelfar-IRphotometry(LID-1688,CID-360),and
sourceswhicharedetected in24µm MIPSphotometry butfaintinthefar-IR(CID-1771, LID-617). Therest-frameobservedphotometric
data (black points) and the detection limits (arrows) are shown with the best-fit model (black solid curve). For the far-IR faint sources
(CID-1771, LID-617), we show two different best fit models (solid and dashed curves). The galaxy template (green), AGN dust torus
template (yellow), and starburst component (red) arealsoindicated. The residuals areshown inthe lower plot of each spectrum. In the
right panels, we show the PDFs for the stellar mass of each source. The best-fitting values are shown in red solid line. The expectation
values(bluedashed)andthe16and84percentileintervals(grayshades)arealsoindicated.
6 Suh et al.
detectedatanyfar-IRwavelength. (1)thebest-fitmodel
with a possible star-forming component using Herschel
upper limits, adopting the same approach as described
by Calistro et al. (2016). Specifically, we consider Her-
scheldetectionlimits ineachHerschelband(flux )to
limit
make mock data points in the far-IR wavelength range,
assuming the flux to be flux /2 with an uncertainty
limit
±flux /2,tofitthepossiblestar-formingcomponent.
limit
(2) We assume a negligible contribution from star for-
mation in the IR range, L =0, and a significant con-
FIR
tributionfromtheAGNat24µm. Thus,wehavearange
of possible L values for Herschel-undetected sources
FIR
(i.e., minimum to maximum).
WeshowexamplesoftheSEDfitsforthesourceswhich
are detected in far-IRphotometry (top two panels; LID-
1688, CID-360), and the sources which are undetected
in the far-IR (bottom two panels; CID-1771, LID-617)
in the left panels of Figure 3. The rest-frame photomet-
ric data (black points) and the detection limits (arrows)
are shown with the best-fit model (black solid curve).
For the far-IR faint sources (CID-1771, LID-617), we
show two different best fit models in the IR wavelength Fig. 4.— Comparison between stellar masses derived from our
SED fitting and that from Lusso et al. (2011; blue circles), Bon-
range: possiblestar-formingcomponentusingupperlim-
giornoetal. (2012;redsquares),andLaigleetal. (2016,LePhare;
its (solid curve) and negligible star formation contribu-
blackopencircles). Theblacklinedenotes aone-to-onerelation.
tions (dashed curve). The galaxy template (green), the
AGN dust torus template (yellow), and the starburst
ample sources. In each case, the best-fitting values are
component (red) are also indicated. The residuals are
shown as red solid line. We also show the expectation
also shown in the lower panel of each SED fit.
values (blue dashed) and the 16 and 84 percentile in-
The χ2 minimization is used to determine the best fit
tervals (gray shades) derived from the cumulated PDFs.
amongallthe possibletemplate combinations. However,
We note that the expectation and the best-fitting val-
itsabsolutevalueisnotareliableindicator,becausesys-
ues are usually very close to each other. In Figure 4, we
tematicuncertaintiesmaydominatethestatisticalerrors.
showthe comparisonofthe stellarmassesobtainedfrom
Therefore,we compute acomplementarystatistic onthe
ourSEDfitting withthe results fromLusso et al.(2011)
quality of fit, which is the variation of the residual from
(blue circles) and Bongiorno et al. (2012) (red squares)
the fit. We remove ∼1% (27/2056) sources which show
based on their SED fitting, and Le Phare pipeline prod-
largevariationsintheirresiduals(>0.5),sincethisindi-
ucts (Laigle et al. 2016; black open circles). While our
cates that their high χ2 is not due to an underestima-
red sample explores a broader range of redshifts and lumi-
tion of the photometric errors but either caused by the
nosities, we find good agreements on the stellar masses
lack of suitable templates or by the bad photometry.
ofmatchedsources,mainlybrightAGNs. The1σdisper-
sions between the stellar mass derived in this work and
3.3. Estimation of physical parameters
otherworksare0.27dex(Lusso et al.2011)and0.30dex
While the use of the χ2 minimization technique can
(Bongiorno et al. 2012), respectively.
giveanindicationoftheoverallqualityofthefitting,the The SFR is estimated by combining the contributions
best-fitvaluecouldnotbeagoodestimateofrepresenta-
from UV and IR luminosity, which can estimate reli-
tive of physical parameter values in a multi-dimensional able total SFR since dust in the galaxy is heated by UV
parameter space with degeneracies. We, therefore, use
emission produced by young massive stars, and then re-
Bayesian statistics to derive the most representative emittedinthemid-to-farinfraredregime(seee.g. Draine
value for each parameter of galaxy physical properties,
2003). We derive the total SFR conversion using the
andtoevaluatetherobustuncertaintiessinceitaccounts
relation from Arnouts et al. (2013), which is similar to
for the degeneracies inherent in our SED templates. that proposed by Bell et al. (2005) and adjusted for a
We explore any possible combination of SED parame-
Chabrier (2003) Initial Mass Function (IMF),
ters, which includes the age since the onset of star for-
mation, the e-folding time τ for exponential SFH mod- SFRtotal (M⊙/yr)=(8.6×10−11)×(LIR/L⊙+2.3×νLν(2300˚A))
els, and the dust reddening. We take into account the (2)
possible range for each parameter (i.e. for galaxy mass, where L is the total rest-frame star-forming IR lu-
IR
7 < log(Mstellar/M⊙) < 13), and find all the models minosity, which is integrated between 8-1000µm from
that produce a value for the parameter. We then build the starburst template, and L (2300˚A) represents the
ν
a probability distribution function (PDF) for the stel- rest-frame intrinsic absorption-corrected near-UV lumi-
lar mass with the likelihood, exp(−0.5 χ2), associated nosityat2300˚Ainunits ofL⊙. ToaccountforHerschel-
with that model for a given source. We estimate expec- undetectedsources,wederiveupperlimitsontheirSFRs
tation values and uncertainties as the width of the pa- byassumingpossiblestar-formingIRluminosityfromthe
rametervaluescorrespondingtothe16and84percentile best-fit using Herschel detection limits. In addition, we
of the cumulative PDF. In the right panels of Figure 3, also derive the minimum SFRs using only UV luminos-
we show PDFs for the stellar mass for each of the ex- ity, assuming L = 0. Therefore, we have a range of
IR
Star formation in Type 2 AGN host galaxies 7
possiblevaluesforSFRsforHerschel-undetected sources Herschel far-IR photometry and the unobscured SFRs
(i.e. from minimum to the maximum SFRs). In Table from UV observations, using a large sample of ∼62,000
2, we present Type 2 AGN host galaxy properties which star-forming galaxies in the COSMOS field. The SFR
include stellar masses, SFRs, and luminosities, derived indicator used in the Lee et al. (2015) work is consis-
from the SED fitting. tent with the one used for the CCLS sample. They find
InFigure5,weshowthe stellarmassdistribution(top that the slope of the MS is dependent on stellar mass,
left) and the total SFR distribution (top right) for our such that it is steeper at low stellar masses and appears
sample of Type 2 AGN host galaxies, normalized to the to flatten at stellar masses above Mstellar ∼ 1010.3M⊙,
total area. For comparison, the stellar mass distribu- suggesting a curvature of the star-forming MS with a
tions of all galaxies in the COSMOS field (Laigle et al. flat slope at the high mass end (see also Whitaker et al.
2016) are shown in gray shaded histogram. The dis- 2014). Furthermore, Tomczak et al. (2016) present sim-
tributions of Type 2 AGNs in the COSMOS field from ilar measurements of the star-forming MS up to z ∼ 4
Bongiorno et al. (2012) and Lusso et al. (2011) are also using far-IR photometry from the Spitzer and Herschel
indicated with blue and yellow histograms, respectively. observatories. They also suggest that the slope of star-
We also show the redshift evolution of stellar masses forming MS becomes shallower above a turnover mass
and SFRs in the bottom panels of Figure 5. Individ- that is in the range from 109.5−1010.8M⊙.
ualsourcesareindicatedwithgrayfilledstars(Herschel- We show SFRs and stellar masses of our sample of
detected; which are detected at least in one Herschel Type 2 AGN host galaxies, split into four redshift bins
band) and circles (Herschel-undetected), and the range in the upper panels of Figure 6. The individual sources
of SFRs for Herschel-undetected sources are also indi- are indicated with filled gray stars when the sources are
catedwithgraylines. Blackstarsrepresentthemeanand detectedinHerschelfar-IRphotometry,whilethe circles
thestandarddeviationofSFRsfortheHerschel-detected representthe possiblemaximumSFRforthe sourcesde-
sources combined with maximum SFRs of Herschel- tected only up to 24 µm. The range of SFRs (i.e. from
undetected sources, indicating maximum mean SFRs. minimum to maximum) for Herschel-undetected sources
The minimum mean SFRs, of which the combination isindicatedwithgraybars. Weindicatethestar-forming
of SFR of the Herschel-detected source and minimum MSrelationshipsfromTomczaketal. (2016;solidcurve)
SFRs of Herschel-undetected source, are indicated with and Speagle et al. (2014; dashed line) for comparison.
black circles. The typical uncertainties for the stellar The relation reported in the Lee et al. (2015) study is
masses (∼0.19 dex) and the SFRs (for the Herschel- alsoindicatedwithdottedcurvesatthelowredshiftbins.
detected sources; ∼0.20 dex) are shown in the bottom We show mean values of the combination of the SFR of
right corner. The stellar mass of our sample ranges Herscheldetectedsources(filledgraystars)andthemax-
from ∼ 109 to ∼ 1012 M⊙, peaking at higher masses imum SFRs of the Herschel-undetected sources (open
(∼ 5 × 1010 M⊙) compared to normal galaxies, con- graycircles)in the stellar mass bins (black stars). Black
sistent with results from Bongiorno et al. (2012) and circles mark the mean values of the combination of SFR
Lusso et al. (2011). There is a lack of significant evo- of Herschel detected sources (filled gray stars) and the
lution of stellar masses of Type 2 AGN host galaxies minimumSFRsofthe Herschel-undetected sources. The
with redshift, which are relatively massive since z ∼ 3, black thick errorbars representthe range of mean SFRs
indicatingthattheymighthavealreadyexperiencedsub- whichaccountfor the maximumandminimum SFRs for
stantialgrowthathigherredshift(z >3). Oursampleof the Herschel-undetected sources. We also display the
Type 2 AGN host galaxies spans a wide range of SFRs, meanSFRsforthesourcesateachX-rayluminositybins
peaking at higher values toward higher redshifts. We in the stellar mass bins (colored stars).
note that the measurement of the SFR has considerably In the lower panels of Figure 6, we show the SFR
larger uncertainties than that of stellar mass, because it offset (∆SFR) for the AGN host galaxies relative to
depends on the Herschel detections, SFRs could be in- the star-forming MS of Tomczak et al. (2016). The
herently biased againsthigher values, while a significant gray shades mark the intrinsic scatter (∼0.2 dex) of
fraction (∼75%) of our sample are faint in the far-IR the star-forming MS. Most previous studies have found
photometry, which could have lower SFRs. no clear evidence for a correlation between the X-
ray luminosity and the SFR of the AGN host galaxy
4. THE SFR-M RELATION
STELLAR (Lutz et al.2010;Shao et al.2010;Mullaney et al.2012;
To investigate the effects of AGNs on the star forma- Rosario et al. 2012; Rovilos et al. 2012; Harrison et al.
tioningalaxies,weexplorethedistributionofoursample 2012; Stanley et al. 2015). Our results indicate that
of Type 2 AGN host galaxies on the SFR-M dia- thereisnosignificantdifferenceintheSFRswithrespect
stellar
gram compared to normal star-forming galaxies. Orig- to X-ray luminosity. Interestingly, it seems that there is
inally the star-forming MS studies concluded that the a tendency for luminous (1043.5 < L2−10keV(erg/s) <
SFR increases with stellar mass as a single power law, 1044.0) AGN host galaxies to deviate from the star-
while the log SFR–log M slope and the normaliza- forming MS relation in the range 0.5 < z < 0.9. In this
stellar
tionvarybasedonthe redshifts,sampleselection,choice redshift range, AGN host galaxies with Mstellar/M⊙ <
of stellar initial mass function, and SFR indicators (for 1010.5 show higher SFRs than star-forming MS galaxies,
asummaryseeSpeagle et al.2014). Recentstudieshave while massive AGN host galaxies (Mstellar/M⊙ > 1011)
suggestedthattheSFR-M relationflattenstowards seem to have SFRs that lie below the star-forming MS
stellar
thehigh-massend(Whitaker et al.2014;Lee et al.2015; relation.
Tomczak et al.2016). Forexample,Lee et al.(2015) ex- Type 2 AGN host galaxies, on average, seem
amine the star-forming MS, of which the total SFRs are to have SFRs that lie on the star-forming MS at
determined by combination of the obscured SFRs using all redshifts, consistent with previous studies (e.g.
8 Suh et al.
Fig. 5.—StellarmassandSFRdistributionofoursampleofType2AGNhostgalaxies,normalizedtothetotalarea. Thestellarmass
distribution of our sample is shown inthick red histograms in top left panel. The distribution of all galaxies from the COSMOS catalog
(Laigleetal.2016)isalsoshowningrayshadedhistogramforcomparison. WealsoshowthedistributionofType2AGNsintheCOSMOS
fieldfromBongiornoetal. (2012; blue)andLussoetal. (2011; yellow). Intoprightpanel,weshowtheSFRdistributions,splitintofour
redshiftbins. In the bottom panels, the individual sources are indicated with filled stars (Herschel-detected) and open circles (Herschel-
undetected) as a function of redshift. The range of SFRs forHerschel-undetected sources are indicated with grayerror bars. Blackstars
indicate mean values of SFRs of Herschel-detected sources combined with possible maximum SFR of Herschel-undetected sources, while
black circles represent that of SFRs of Herschel-detected sources combined with minimum SFR of Herschel-undetected sources. We also
showthetypical uncertainties inbottom rightcorner.
Xue et al. 2010; Mainieri et al. 2011; Mullaney et al. We discuss the star formation in Type 2 AGN host
2012; Rosario et al. 2013), but with much broader dis- galaxies and the implications of the growth of black
persions. Mullaney et al. (2015) found that AGN host holes and galaxies over cosmic time. We show that
galaxies with log Mstellar/M⊙ &10.3 show significantly the majority of Type 2 AGN host galaxies seem to re-
broaderSFR distributions comparedto the star-forming side along the star-forming MS, consistent with previ-
MS galaxies, compared to normal galaxies (see also ous studies (e.g. Mainieri et al. 2011; Mullaney et al.
Shimizu et al. 2015). We note, however, that Type 2 2012; Rosario et al. 2013). While the “flattening” in the
AGN host galaxies at high mass bins remain on the star-forming MS at high masses could be interpreted as
star-forming MS, when taking into account the depen- a consequence of quenching the star formation in mas-
denceoftheslopeofthestar-formingMSonstellarmass sive galaxies (e.g. Whitaker et al. 2014; Lee et al. 2015;
(Whitaker et al. 2014; Lee et al. 2015; Tomczak et al. Schreiber et al. 2015), the SFRs of Type 2 AGN host
2016). The selection effects and observational biases galaxies are consistent with those expected from nor-
can be important since a significant fraction (∼75%) of malstar-forminggalaxiesinmoststellarmassbinsupto
oursamplearenotdetectedinfar-IRphotometry,which z ∼3,indicating no clearsignaturefor enhanced orsup-
is crucial for precise measurements of the SFRs. The pressed SFRs compared to normal star-forming galax-
SFRdistribution,therefore,ismuchbroaderthanthatof ies. Thiscanbeinterpretedbyinternalsecularprocesses,
star-forming MS galaxies, when taking into account the which might be responsible for driving both star forma-
fact that the SFRs of the Herschel-undetected sources tion and nuclear activity in Type 2 AGN host galaxies.
could ultimately be much lower (i.e. minimum SFRs). These results are consistent with the weak link between
Overall, Type 2 AGN host galaxies remain on the star- mergerfeaturesandthemodestAGNactivity. Frompre-
forming MS over a broad redshift range, indicating no vious works in the literature (e.g. Cisternas et al. 2011;
sign of strong SFR enhancements in the redshift range Mainieri et al. 2011; Schawinski et al. 2012; Fan et al.
0.5<z <3.0. 2014; Villforth et al. 2014), the majority of AGN host
galaxies do not show significant merger features, indi-
5. DISCUSSION
Star formation in Type 2 AGN host galaxies 9
Fig. 6.—Top: SFRversusstellarmassofoursampleofType2AGNhostgalaxiesinthefourredshiftbins. Grayfilledstarsindicatethe
individual sources which aredetected in the far-IR Herschel photometry, and gray open circles represent the possible maximum SFR for
thesources whicharenotdetected inanyHerschelbands. TherangeofSFRs(i.e. fromminimumtomaximum)forHerschel-undetected
sourcesareindicatedwithgrayerrorbars. Weindicatethestar-formingMSrelationshipsfromSpeagleetal. (2014;dashedline),Leeetal.
(2015;dottedcurve),andTomczaketal. (2016;solidcurve)forcomparison. BlackstarsindicatemeanvaluesofSFRsofHerschel-detected
sources combined with possible maximum SFR of Herschel-undetected sources, while black circles represent that of SFRs of Herschel-
detected sources combined withminimum SFR of Herschel-undetected sources. Blackthick errorbars represent the range of meanSFRs
whichaccount forthemaximum andminimumSFRsof theHerschel-undetected sources. We alsodisplaythe meanSFRsforthe sources
ateach X-rayluminositybin(coloredstars). Bottom: SFRoffsets (∆SFR)relativetothestar-formingMSofTomczaketal.(2016). The
grayshadesmarkthe∆SFR∼±0.2dex.
cating that mergers do not dominate the triggering of ies. Xue et al. (2010) also found that for mass-matched
AGN activity, at least for moderate-luminosity AGNs. samples, the SFRs of AGN host galaxies are similar to
Allevato et al. (2011, 2012, 2016) further point out those of non-AGN galaxies at z ∼1−3, consistent with
that moderate-luminosity AGNs inhabit group-sizedha- ourresults. WefurtherconsiderdifferentX-rayluminosi-
los (1013−13.5 M⊙), almostindependent ofredshift upto ties to minimize potential luminosity-dependent effects.
z ∼ 5. This also implies that major mergers cannot be Within each stellar mass bin, we subdivide our sam-
the main driver of the evolution of AGNs. ple into bins of the X-ray luminosity. With luminosity-
However, this result could be also interpreted by the selection effects taken into account, we find that there
differenttimescalesandthespatialscalesassociatedwith is no clear signature for a correlation between the AGN
the star formation and nuclear activity (Hickox et al. luminosity and the SFRs of AGN host galaxies. The
2014) in the sense that most AGN vary on a timescale Eddington ratio could be a factor that creates a bias
much shorter (∼ 105yr) than that of star formation againstlow-luminosityAGNsaccretingatthelowestEd-
(∼ 100Myr) (e.g., Hickox et al. 2009; Aird et al. 2012; dington ratios at high redshift. Our AGN sample may
Bongiorno et al. 2012). According to this scenario, all also bias against the heavily obscured, Compton-thick
episodesofstarformationandAGNactivitycouldbein- sources, which might be missed by X-ray selection (e.g.
timatelyconnectedatanytime. Furthermore,weshould Treister et al. 2004; Kocevski et al. 2015). However, at
point out that these could be driven by the selection least for our sample of moderate-luminosity X-ray se-
biases, mainly due to the interplay between the lim- lected AGNs, we find that there is no significant differ-
ited X-ray luminosity, Eddington ratio, SFRs and stel- encebetweenAGNhostsandnormalstar-forminggalax-
lar masses of AGN host galaxies (e.g. Lauer et al. 2007; ies.
Xue et al. 2010). While AGNs preferentially reside in From the perspective of our investigation on the star-
massive galaxies, when considering in the same stellar formationinType2AGNhostgalaxies,weproposethat
massbins,SFRsofAGNhostgalaxiesindicatenosignif- the relatively massive galaxies have already experienced
icant difference compared to normal star-forming galax- substantial growthby major mergers,which are capable
10 Suh et al.
of triggeringboth a significant starburstand high accre- use multi-band photometry (from near-UV through the
tion AGN activity at higher redshift (z > 3), and grow far-IR) to decompose the entire SED into separate com-
slowly through secular fueling processes hosting moder- ponents with nuclear AGN emission, the host galaxy’s
ate luminosity AGNs. Aird et al. (2012) present that stellar populations, and a starburst contribution in the
AGN Eddingtonratiosareindependent ofstellarmasses far-IR. We derive stellar masses of our sample in the
of their hosts at z < 1, suggesting that the same physi- range 9 < logMstellar/M⊙ < 12 with uncertainties of
calprocessesregulateAGN activityin galaxiesatstellar ∼0.19dex. The SFRis estimatedby combiningthe con-
masses 9.5.log Mstellar/M⊙ .12.0. Suh et al. (2015) tributions from UV and IR luminosity. Our sample of
further point out that a substantial fraction of massive Type 2 AGN host galaxies span a wide range of SFRs
black holes accreting significantly below the Eddington (−1<logSFR(M⊙/yr)<3)withuncertaintiesof∼0.20
limitatz <2,suggestingthatmodestAGNactivity can dex.
be triggered via internal, secular processes in massive We explore the distribution of AGN host galaxies on
galaxies. This is also compatible with the lack of sig- the SFR-stellar mass diagram compared to the normal
nificant evolution of stellar masses of Type 2 AGN host star-forming galaxies. Overall, Type 2 AGN host galax-
galaxies. Our results suggest that the majority of Type ies seem to have SFRs that lie on the star-forming MS
2 AGN host galaxies at z < 3 might be driven more by up to z ∼3, independent of X-ray luminosities. Our re-
internal secular processes, implying that they have sub- sults indicate that AGN host galaxies do not show clear
stantially grown at much earlier epoch. signature for enhanced or suppressedSFRs comparedto
normal star-forming galaxies.
6. SUMMARY
We present the host galaxy properties of a large sam-
ple of ∼2300 X-ray-selected Type 2 AGNs out to z ∼ 3 This work was supported in part by NASA Chan-
in the CCLS in order to examine whether AGN activ- dra grant number GO3-14150C, GO3-14150B, and also
ity cansignificantlyenhanceor quenchstar formationin GO5-16150A.K.S.acknowledgessupportfromSwissNa-
galaxies. To derive the physical properties of AGN host tional Science Foundation Grants PP00P2 138979 and
galaxies,wedevelopamulti-componentSEDfittingtech- PP00P2 166159. E.L.issupportedbyaEuropeanUnion
niquetodisentanglethenuclearemissionfromthestellar COFUND/Durham Junior Research Fellowship (under
light, and derive host galaxy properties. Specifically, we EU grant agreement no. 609412).
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