EXOs: Candidate AGN at z ≥ 6 and intermediate-z evolved populations

30 March 2006 Galaxies and Structures through Cosmic Times, Venice 1

EXOs: Candidate AGN at z ≥ 6

and intermediate-z evolved populations Anton Koekemoer (Space Telescope Science Institute)

+ GOODS (Alexander, Bauer, Brandt, Chary, Conselice, Cristiani, Daddi, Dickinson, Elbaz, Grogin,

Mainieri, Tozzi,..)

+ COSMOS-XMM (Brusa, Comastri, Elvis, Finoguenov, Fiore, Gilli, Hasinger, Impey, Mainieri,

Salvato, ..)


Large X-ray/optical/IR surveys are opening up new regions of parameter space:

Depth probes faint end of AGN LF to high zArea probes high end of AGN LF at high zCombined optical + X-ray depth allows wider

exploration of FX/FOpt and new populations

FOpt (mag)

log

FX (

erg

s-1cm

-2)

AGN Relevant surveys:

GOODS/CDFN+S (15’)E-CDFS (30’)XMM-LH (30’)EGS (10’x 60’)COSMOS (1.4o x 1.4o) ..


FX/FOpt parameter space:AGN typically ~1-2 dexSB are lower Fx/Fopt

Highest FX/FOpt:“EXO’s” - Extreme X-ray /

Optical sources:– Fx/Fopt > ~100– Only revealed by

extending optical depthbelow i or z ~ 24 - 25

– Appear to have no comparable analogs in the local universe

Why do we only start seeing them at faint magnitudes?What produces the high FX/FOpt - these sources likely

consist of two sub-populations:– Balmer break: z ~ 2 - 3 evolved or dusty hosts (DRGs,

EROs)– Lyman break: candidate z > 6-7 agn

FX/Fopt =

0.1

FX/Fopt = 10


Fx/Fopt vs Colour:Sources with low FX/FOpt

are generally blue: low-zstar-forming galaxies

Some blue sources alsohave FX/FOpt typical ofunobscured AGN- quasars

Moderately red galaxies(z - K > 2) all have higherFX/FOpt, obscured AGN

Reddest z - K sources: for z - K > 4, FX/FOpt increases to ~ 10 - 100x the value

for even typical obscured AGNNot a selection effect:

– high FX/FOpt should not necessarily imply bright IR flux

Thus red opt/IR colour is intrinsic characteristic of EXOs


Previous studies of optically faint X-ray sources:

Initial Deep Chandra/XMM fields revealed that ~20-30% ofX-ray sources are “optically faint”, R > 24(Koekemoer et al. 2002, Tozzi et al. 2002)

Most optically faint sources are also X-ray faint, ie have fairly normal FX/FOpt typical of obscured AGN at z ~ 1-3 (Brusa et al. 2003, Mainieri et al. 2004, Fiore et al 2005)

Some optically faint sources are ERO’s (z ~ 1-1.5) - but also have normal FX/FOpt (Stevens et al. 2003, Yan et al. 2003, Rigby et al. 2005)

EXO’s:Optically faint sources with anomalously high FX/FOpt

>100No apparent local analogs at brighter magnitudesTypically have redder z-K than ERO’s (Koekemoer et al.

2004, 2006)


Nature of EXOs:Need further IR constraints on SED to determine which

sub-population a given EXO belongs to based on its red opt/IR:

– Balmer break (z ~ 2 - 3 evolved or dusty populations)– Lyman break (z >6 AGN)

NOTE: expect EXOs to contain both z~2-3 and z>6 sources

Spitzer data:currently based purely on GOODS CDFS + HDFN

(Dickinson)all EXOs are detected in IRAC data:

– red K - IRAC colour– across IRAC, have a mixture of SEDs (red, flat or blue)

MIPS detections/non-detections are consistent with IRAC properties:

– flat/red IRAC -> MIPS detections, usually brighter than IRAC– blue IRAC colours -> generally undetected in MIPS


HST/ACS SPITZER/IRAC MIPS

HST/ACS VLT/ISAAC SPITZER/IRAC



SED fitting:Parameters SSP + constant SFR, reddening, IMF, dust Models:

– initally used CB2003– will also now include Maraston

Results:Most EXOs are

well fit by:– z ~ 2 - 3– evolved– ~1010-11 Mo

Some requirereddening AV ~1

A few EXOs arenot well fit byz ~ 2 - 3 models


Using EXOs to trace high-z AGN evolution:AGN regulate galaxy growth / SFR via feedbackmay trace hierarchical dark matter halospossibly different types of accretion mechanisms:

– luminous AGN may trace major mergers– lower-luminosity AGN may trace more minor interactions and

accretion events (e.g. Merloni et al. 2004)Hasinger etal. luminosity-dependent density evolution LDDE:

– High-lum AGN grow earlier in universe, peak at z ~ 2– Lower-lum AGN peak much later, z~1, decline by 10x to z=0

Questions:how does the faint end of the AGN LF evolve beyond z ~6?does obscured/unobscured AGN ratio increase beyond z ~6? does more rapid evolution of high-lum AGN trace merging

history of spheroid formation? (e.g, Franceschini et al 1999)


Using EXOs to count high-z AGN in GOODS:Use XLF to estimate expected number of optically

unidentified sources as a function of redshiftMost of the optically unidentified AGN are evolved

interlopers at intermediate z > 2Compare with observed number of undetected

sources:– use existing X-ray detection limits– apply optical detection cut-off (z(AB) ~ 27.5 for ACS)

Integrate over X-ray luminosities at each redshift binassume Type 1/2 ratio found in GOODS by Treister et

alUse the difference to calculate cumulative number

N(>6)Compare with N(>6) from XLF


Predict optically unidentified sources in each redshift bin using Hasinger et al. LDDE description, extrapolating upto z ~ 7

Apply to GOODS X-rayselection, including theoptical detection limits, tocalculate EXOs expected.

Number of optically unID’d sources N(z) based onz(AB)=27.5 limit, for currentChandra catalogs:

LDDE predicts 9 - 16 EXOs in GOODS (out of 607 X-ray sources):

– 8-13 should be at z ~ 2 - 5– 1-3 should be at z ~ 6 - 7

Actually detect 13 EXOs:– SED modelling confirms

that 10 are at z ~ 2 - 4– 3 are indeterminate


Conclusions:Number of EXO’s found in GOODS agrees well with that

expected based on LDDE (13 vs 9-16, respectively)Number of intermediate-z interlopers among EXOs also

agrees with expected from LDDE (10/13 vs 8-13/9-16)Remaining number of 3 EXOs in GOODS with possible

high redshifts (ie > 6) is consistent with the 1-3 EXOs expected at z > 6 based on extending LDDE to z ~ 6 – 7

Therefore LDDE appears to extend up to at least z ~ 6 - 7

This suggests that AGN growth/accretion mechanisms continue to track galaxy growth into reionization:

– AGN feedback regulating star formation up to early epochs– black holes tracing dark matter halos since at least z ~ 7

Future:extend to COSMOS to improve LF constraints (whenever

Spitzer catalogs are available for SED fitting..)

Documents

EXOs: Candidate AGN at z ≥ 6 and intermediate-z evolved populations