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The spatial clustering of X-ray selected AGN

The spatial clustering of X-ray selected AGN. R. Gilli Istituto Nazionale di Astrofisica (INAF) Osservatorio Astronomico di Bologna On behalf of the CDFS and COSMOS teams. AGN clustering… AGN can trace the Large Scale Structure of the Universe

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The spatial clustering of X-ray selected AGN

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  1. The spatial clustering of X-ray selected AGN R. Gilli Istituto Nazionale di Astrofisica (INAF) Osservatorio Astronomico di Bologna On behalf of the CDFS and COSMOS teams

  2. AGN clustering… • AGN can trace the Large Scale Structure of the Universe to cosmologically significant redshifts (z>1-2) • AGN physics from clustering: e.g. host halo mass and lifetimes Also used to estimate the host galaxy type …in the X-ray band • X-rays efficiently sample obscured AGNs (which outnumber unobscured ones by a factor of at least 3-4; see XRB synthesis and the results from the deep surveys) • High surface density, above 1000-2000 sources/deg2, even in moderately deep surveys  high clustering significance with relatively small number of objects e.g. Croom 05, Porciani 05 Martini & Weinberg 01

  3. AGN clustering as a function of.. 1) redshift: measured for optical QSOs r0 increases with redshift  halo mass ~ constant ~ 3 1012 Msun (e.g 2dF – Croom et al. 04,05 ; Porciani et al. 05,06) 2) luminosity: still to be measured 3) obscuration: still to be measured From optical and X-ray studies (Yang 05) there is no clear evidence for any dependence of r0 on L. Similarly, there is no evidence yet for any dependence of r0 on obscuration (X-ray samples still small)

  4. X-ray fields with measured 3D clustering Field Area/deg2 Slim[cgs] N/deg2 N(zspec) zmed logLmed Ref. CDFS 0.1 5 10-17 6000 130 0.8 43.0 Gilli 05 CDFN 0.13 2 10-17 8000 240 0.9 43.2 ’’ CLASXS 0.4 5 10-16 1500 230 1.2 43.9 Yang 05 COSMOS 2 7 10-16 1000 380 1.0 44.1 in prep. NEP 81 2 10-14 10 220 0.4 44.7 Mullis 03 Surveys with different sensitivities sample different z and Lx regimes: one needs to always specify at which average z and Lx clustering is computed

  5. Statistical description of LSS:the 2-point correlation function Spatial Correlation Function: excess probability over random of finding a source in dV1 and another in dV2 separated by a distance r Projected correlation function w(rp): Allows to get rid of distortions in redshift space (i.e. peculiar velocities, redshift errors) ξ(r) increases at small separations, then one needs either deep fields in which the source density is very high or large volumes – or, preferably, both

  6. Expected 3D clustering significance in X-ray surveys SNR=(DD-RR)/RR0.5 Where DD and RR are the numbers of data-data and random-random pairs within a given separation (e.g. 10 Mpc) SNR ≈V-0.5 N

  7. CDFS r0=10.3±1.7 • g=1.33±0.14 unabsorbed AGN absorbed AGN CDFN r0=5.5±0.6 g=1.50±0.12 3D correlation function in the Chandra Msec fields CDFS vs CDFN AGN CDFN: abs vs unabs AGN Large variance between the two Chandra Msec fields No statistically significant difference between clustering of obscured and unobscured AGN within a single CDF

  8. Comparing redshift distributions CDFS CDFN 13% 30% of sources in the 2 most prominent spikes CLASXS COSMOS 6% 5-8% Due to redshift spikes, cannot divide in z–bins in the CDF-S/N Need large area to avoid cosmic variance

  9. XMM-COSMOS CDF-S/N 0.1 deg2 CLASXS 2 deg2 0.4 deg2 First XMM-COSMOS results: Hasinger 06, Cappelluti 06, Brusa 06, Mainieri 06, Miyaji 06, Finoguenov 06

  10. Preliminary measurements in COSMOS All sources unobscured obscured r0 = 7.3 ± 0.5 γ= 1.8 ± 0.1 Based on Hardness Ratios

  11. Clustering of ~ obscured vs unobscured AGN unobscured obscured No statistical evidence for any difference. Based on hardness-ratios: need to be done using NH measurements from spectral fits

  12. early late AGN Correlation length vs redshift(and luminosity) Halo catalogs from Kauffman et al. 99 Late,early type r0 from Coil et al. 03

  13. Summary • Spatial clustering of X-ray selected AGN has been measured so far in only 5 fields. Large cosmic variance is observed between the two pencil beam Chandra Msec fields. Need large areas to get an “unbiased” estimate of AGN clustering and need large area x Nsources product (i.e. large and sensitive surveys) to measure clustering signal at high significance. • Within each field no significant difference is found between the clustering properties of obscured and unobscured AGN (but check using X-ray spectra rather than HR). • X-ray selected AGN appear hosted by halos with mass larger than 1-2 x 1012 Msun • At z~1 the AGN correlation length is in between that of early and late type galaxies at the same redshift (but large scatter: COSMOS ~ early).

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