1. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice1 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,..)

2. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice2 Large X-ray/optical/IR surveys are opening up new regions of parameter space: Depth probes faint end of AGN LF to high z Area probes high end of AGN LF at high z Combined optical + X-ray depth allows wider exploration of F X /F Opt and new populations F Opt (mag) log F X (erg s -1 cm -2 ) AGN Relevant surveys: GOODS/CDFN+S (15’) E-CDFS (30’) XMM-LH (30’) EGS (10’x 60’) COSMOS (1.4 o x 1.4 o )..

3. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice3 F X /F Opt parameter space: AGN typically ~1-2 dex SB are lower F x /F opt Highest F X /F Opt : “EXO’s” - Extreme X-ray / Optical sources: –Fx/Fopt > ~100 –Only revealed by extending optical depth below 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 F X /F Opt - 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 u u u u u u u FX/Fopt = 0.1FX/Fopt = 10

4. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice4 F x /F opt vs Colour: Sources with low F X /F Opt are generally blue: low-z star-forming galaxies Some blue sources also have F X /F Opt typical of unobscured AGN- quasars Moderately red galaxies (z - K > 2) all have higher F X /F Opt, obscured AGN Reddest z - K sources: for z - K > 4, F X /F Opt increases to ~ 10 - 100x the value for even typical obscured AGN Not a selection effect: –high F X /F Opt should not necessarily imply bright IR flux Thus red opt/IR colour is intrinsic characteristic of EXOs

5. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice5 Previous studies of optically faint X-ray sources: Initial Deep Chandra/XMM fields revealed that ~20-30% of X-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 F X /F Opt 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 F X /F Opt (Stevens et al. 2003, Yan et al. 2003, Rigby et al. 2005) EXO’s: Optically faint sources with anomalously high F X /F Opt >100 No apparent local analogs at brighter magnitudes Typically have redder z-K than ERO’s (Koekemoer et al. 2004, 2006)

6. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice6 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

7. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice7 HST/ACS SPITZER/IRAC MIPS HST/ACS VLT/ISAAC SPITZER/IRAC

8. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice8

9. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice9 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 –~10 10-11 M o Some require reddening A V ~1 A few EXOs are not well fit by z ~ 2 - 3 models

10. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice10 Using EXOs to trace high-z AGN evolution: AGN regulate galaxy growth / SFR via feedback may trace hierarchical dark matter halos possibly 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 )

11. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice11 Using EXOs to count high-z AGN in GOODS: Use XLF to estimate expected number of optically unidentified sources as a function of redshift Most of the optically unidentified AGN are evolved interlopers at intermediate z > 2 Compare 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 bin assume Type 1/2 ratio found in GOODS by Treister et al Use the difference to calculate cumulative number N(>6) Compare with N(>6) from XLF

12. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice12 Predict optically unidentified sources in each redshift bin using Hasinger et al. LDDE description, extrapolating up to z ~ 7 Apply to GOODS X-ray selection, including the optical detection limits, to calculate EXOs expected. Number of optically unID’d sources N(z) based on z(AB)=27.5 limit, for current Chandra 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

13. 30 March 2006 Galaxies and Structures through Cosmic Times, Venice13 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..)