Clustering of X-ray selected active galactic nuclei

Abstract

A B S T R A C TA total of 235 active galactic nuclei (AGN) from two different soft X-ray surveys [the ROSAT Deep Survey (DRS) and the ROSAT International X-ray Optical Survey (RIXOS)] with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2j significant detection of clustering of such objects is found on scales < 40-80 h ¹1 Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power-law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1:5 Շ r 0 Շ 3:3 h ¹1 Mpc is implied for comoving evolution, while 1:9 Շ r 0 Շ 4:8 for stable clustering and 2:2 Շ r 0 Շ 5:5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent ultraviolet and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be Շ1:7 on scales Շ10 h ¹1 Mpc, a somewhat smaller value than is inferred from local large-scale dynamical studies.

Tracing the Large‐Scale Structure in theChandraDeep Field South

“…Despite several efforts in the past years, before the deep X-ray surveys by Chandra there was no convincing evidence for three-dimensional clustering of X-ray-selected AGNs. Only a 2 detection was found by Carrera et al (1998) in the ROSAT International X-ray Optical Survey (RIXOS; Mason et al 2000) on scales <60 120 h À1 70 Mpc. Interestingly, the 2 signal detected in the RIXOS refers to the subsample of sources in the redshift range 0.5-1.0, where the biggest structures in the Chandra deep fields are also detected, although the lack of clustering signal at z < 0:5 and z > 1 might be due to the small volume sampled and to the falling sensitivity of the RIXOS, respectively.…”

“…Boyle & Mo (1993) studied the AGNs at z < 0:2 in the Einstein Medium Sensitivity Survey (EMSS; Stocke et al 1991) without finding any positive clustering signal. Carrera et al (1998) considered the AGNs in the ROSAT International X-Ray Optical Survey (RIXOS; Mason et al 2000) and in the Deep ROSAT Survey (Boyle et al 1994), detecting only a weak ($2 ) clustering signal on scales <60 120 h À1 70 Mpc for the RIXOS AGN subsample in the redshift range z ¼ 0:5 1:0.…”

BeppoSAXObservations of the Maser Seyfert 2 Galaxy ESO 103‐G35

“…Few studies have been made in X-rays (see Barcons et al 2000 for a recent review). Studies of complete samples of X-rayÈselected AGNs that probe the D1 Mpc scale of interest have become possible only in the last few years (Boyle & Mo 1993) and have found positive clustering signals on intermediate scales (0.5@È10@ÈVikhlinin & Forman 1995 ; correlation length h~1 Mpc, Carrera et al 1998) & Plionis 2000). The question here is, Is the present overdensity consistent with a random sample of cosmic variance ?…”

Imaging Large-Scale Structure in the X-Ray Sky

“…Structure is visible in both the soft-and hard-band maps, but the hard-band sources are more clustered. To test whether the observed overdensity could arise from Poisson fluctuations, we employed the likelihood test described in Carrera et al (1998). Comparing with 10,000 simulated samples that are Poisson distributed, we found 97.67% of the soft-band simulations and 99.99% of the hard-band simulations had better likelihood than the observations.…”