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Using COSMOS to Probe the High-Redshift AGN Population

Using COSMOS to Probe the High-Redshift AGN Population. Anton Koekemoer (Space Telescope Science Institute)

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Using COSMOS to Probe the High-Redshift AGN Population

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  1. AAS 207, Washington DC, 10 January 2006 Using COSMOS to Probe theHigh-Redshift AGN Population Anton Koekemoer (Space Telescope Science Institute) + COSMOS XMM / AGN Team: M. Brusa, A. Comastri, N. Cappelluti, F. Civano, M. Elvis, A. Finoguenov, F. Fiore, R. Gilli, G. Hasinger, C. Impey, V. Mainieri, M. Salvato, C. M. Urry, C. Vignali, G. Zamorani

  2. AAS 207, Washington DC, 10 January 2006 • Supermassive BH’s - questions: • How & when do they form? • How do they grow & evolve? • What is their impact on galaxy growth (eg feedback) • What sets SBH mass host bulge mass ? • Context: • Already have SBH ~ 109 Moat z~6 (Fan et al. 02, 03, 05) • Quasar LF changes with redshift: • simple luminosity evolution(PLE) is ruled out • instead, seem to havedensity evolution at highend of the LF (Fan et al 2003)

  3. AAS 207, Washington DC, 10 January 2006 • However: • LF density evolution is not the same for all luminosities • LF shape changes with redshift: Lum.-depdendent density evolution (Hasinger et al 2005) • Higher-lum objects: • grow early in universe • peak at z ~ 2 • decline by 100x from z ~ 2 to present • Lower-lum objects: • growth peaks much later, z ~ 1 • decline only by <10x from z ~ 1 to present

  4. AAS 207, Washington DC, 10 January 2006 (from Hasinger et al. 2005)

  5. AAS 207, Washington DC, 10 January 2006 • Below LX ~ 1044 erg s-1, density evolution is drastically different from higher-lum sources, peaking at lower z • higher-lum sources show essentially pure density evolution • suggests possible difference in accretion / galaxy evolution as a function of luminosity (Hasinger et al. 2005)

  6. AAS 207, Washington DC, 10 January 2006 • Physical picture to date: • rapid evolution of high-lum AGN appears to trace merging history of spheroid formation(e.g, Franceschini et al 1999) • much later peak and slower decline of lower-lum AGN more closely resembles star formation history which peaks later at z ~ 1 • thus potentially two different modes of accretion and black hole growth with radically different accretion efficiency(eg Merloni et al. 2004) - corresponds essentially to galaxy mergers vs interactions • Next steps: • need to extend picture for z < 2 - 4 to higher-z & low-lum: • do these modes of BH growth / accretion apply at < 1 Gyr? • what is the role of AGN feedback in early universe in determining the eventual bulge / BH mass relation?

  7. AAS 207, Washington DC, 10 January 2006 • How can COSMOS help? • unique combination of wide area and depth, opt/Xray • 2 sq deg large enough to probe rare high end of AGN LF at LX > 1045 - 46 erg s-1X-ray coverage deep enough to probe fainter end of AGN LF (LX ~ 1044 erg s-1) up to z ~ 6 - 7 • At least ~ 1000 AGN from XMM (654 to date; Brusa et al) • Extensive optical spectroscopic coverage • deep multi-band optical/NIR coverage • Spitzer IRAC observations will trace host stellar mass forz > 1-2; MIPS will help constrain thermal dust emision

  8. AAS 207, Washington DC, 10 January 2006 • XMM Observations: • Initial dataset covers 12 pointings (Brusa et al) • Area covered ~1.3 sq deg • Total of 715 X-ray sources detected; ~20 extended • Limiting fluxes: • F(0.5-2 keV) ~ 1 x 10-15 erg cm-2 s-1 • F(2-10 keV) ~ 5 x 10-15 erg cm-2 s-1 • Final survey: • total of 23 fields, covering 2 sq deg • aim for ~1500 X-ray sources

  9. Searching for High-z AGN AAS 207, Washington DC, 10 January 2006 • First, ensure most X-ray sources have ID, z: • ~80% identified (Brusa et al.) • spectroscopy as complete as possible (Impey, Trump et al.) • Next, examine ambiguous IDs: • mostly expected from limited XMM spatial resolution • corresponds to multiple optical IDs inside formal positional error circles • Finally, produce sample of EXOs: • some of these are red/evolved obscured AGN at z ~ 2 - 5 • remaining fraction are candidates for z > 6 AGN • Really need combined optical/NIR/Spitzer to help disentangle these possibilities, for any given source

  10. EXOs to date: AAS 207, Washington DC, 10 January 2006 • 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) • 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) • EXO’s: • Optically faint sources with anomalously high FX/FOpt >100 • Typically have much redder z-K colour than even the ERO’s (Koekemoer et al. 2004) • SED models: single-burst / continuou SFR + dust reddening (see also Mainieri et al 2005)

  11. AAS 207, Washington DC, 10 January 2006 • Using EXOs to count High-z AGN in COSMOS: • Use XLF to estimate expected number of optically unidentified sources as a function of redshift • expect some X-ray AGN to be optically undetected starting at z > 2 • Compare with observed number of undetected sources: • use existing X-ray detection limits • apply optical detection cut-off (I(AB) ~ 26 for Subaru,I(AB) ~ 27 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. AAS 207, Washington DC, 10 January 2006 • predict optically unidentified sources in each bin using Hasinger et al. LDDE description • apply to COSMOS X-rayselection, including theoptical detection limits • Number of optically unID’d sources N(z) based onI(AB)=26 limit, for current(12-pointing) XMM catalog • LDDE predicts ~70 EXO’s • Compare with ~40 sources(Brusa et al)

  13. Summary AAS 207, Washington DC, 10 January 2006 • Preliminary results: • Based on luminosity-dependent density evolution, expect a total of ~10% optically unidentified sources to AB~26 in the current XMM catalog, with ~2% expected at z~6 • current total of unidentified sources (Brusa et al) is ~5% • Once lower-z EXOs are accounted for, this suggests ~ 2x less AGN than expected at z~5-6 • Marginally inconsistent with extension of LDDE to z~6 • suggests AGN accretion / growth mechanisms at z~6 may be starting to differ from those seen at z < 2 - 4, eg more dominated by extreme accretion events • Future: • Spectroscopy (Impey, Trump); Spitzer imaging (Sanders) • SED modelling to better constrain redshifts

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