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

The Large Scale Structure of the Universe 12.6 billion yr ago z=5.7 From Springel et al billion yr ago z=1.4 Now z=0 (SDSS, Zehavi et al. 2002) Matter distribution is everything but random DARK MATTER GALAXIES (theory) (observations)

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 sources/deg 2, even in moderately deep surveys  high clustering significance with relatively small number of objects e.g. Croom 05, Porciani 05 Martini & Weinberg 01

AGN clustering as a function of.. 1) redshift: measured for optical QSOs r 0 increases with redshift  halo mass ~ constant ~ M sun (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 r 0 on L. Similarly, there is no evidence yet for any dependence of r 0 on obscuration (X-ray samples still small)

X-ray fields with measured 3D clustering Field Area/deg 2 S lim [cgs] N/deg 2 N(zspec)z med logL med Ref. CDFS Gilli 05 CDFN ’’ CLASXS Yang 05 COSMOS in prep. NEP 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

Statistical description of LSS: the 2-point correlation function Spatial Correlation Function: excess probability over random of finding a source in dV 1 and another in dV 2 separated by a distance r 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 Projected correlation function w(r p ): Allows to get rid of distortions in redshift space (i.e. peculiar velocities, redshift errors)

Expected 3D clustering significance in X-ray surveys SNR=(DD-RR)/RR 0.5 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

3D correlation function in the Chandra Msec fields CDFS r 0 =10.3±1.7  1.33±0.14 CDFN r0=5.5±0.6  1.50±0.12 unabsorbed AGN absorbed AGN CDFS vs CDFN AGNCDFN: 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

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

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

Preliminary measurements in COSMOS unobscured obscured Based on Hardness Ratios All sources r 0 = 7.3 ± 0.5 γ = 1.8 ± 0.1

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

AGN Correlation length vs redshift (and luminosity) Late,early type r 0 from Coil et al. 03 late early Halo catalogs from Kauffman et al. 99

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 N sources 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 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).