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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, ..)
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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) ..
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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
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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
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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)
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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
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HST/ACS SPITZER/IRAC MIPS
HST/ACS VLT/ISAAC SPITZER/IRAC
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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
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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)
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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
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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
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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..)