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 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 outnumberunobscured ones by a factor of at least 3-4; see XRB synthesisand 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

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 measured3) obscuration: still to be measured

From optical and X-ray studies (Yang 05) there is no clearevidence for any dependence of r0 on L. Similarly, there is noevidence yet for any dependence of r0 on obscuration (X-raysamples still small)

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

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

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(rp):

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)/RR0.5

Where DD and RR are thenumbers of data-data andrandom-random pairs within agiven separation (e.g. 10 Mpc)

SNR ≈ V-0.5 N

3D correlation function in the Chandra Msec fields

CDFS r0=10.3±1.71.33±0.14

CDFN r0=5.5±0.6

1.50±0.12

unabsorbed AGN

absorbed AGN

CDFS vs CDFN AGN CDFN: abs vs unabs AGN

Large variance between the twoChandra Msec fields

No statistically significant differencebetween clustering of obscured andunobscured AGN within a single CDF

Comparing redshift distributions

CDFS

30% of sources in the 2 most prominent spikes

13%

6%

CDFN

CLASXS COSMOS5-8%

Due to redshift spikes, cannot divide in z–bins in the CDF-S/NNeed large area to avoid cosmic variance

XMM-COSMOS

CDF-S/N

CLASXS

0.1 deg2

0.4 deg2 2 deg2

First XMM-COSMOS results: Hasinger 06, Cappelluti 06, Brusa 06, Mainieri 06, Miyaji 06, Finoguenov 06

Preliminary measurements in COSMOS

unobscuredobscured

Based on Hardness Ratios

All sources

r0 = 7.3 ± 0.5γ = 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

unobscuredobscured

AGN Correlation length vs redshift(and luminosity)

Late,early type r0 from Coil et al. 03

late

early

Halo catalogs fromKauffman 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 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).