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Abstract:
Aims. We study the spatial clustering of 632 (1130) XMM-COSMOS active galactic nuclei (AGNs) with known spectroscopic or photometric redshifts in the range z = [0.1–2.5] in order to measure the AGN bias and estimate the typical mass of the hosting dark matter (DM) halo as a function of AGN host galaxy properties.
Methods. We created AGN subsamples in terms of stellar mass, M*, and specific black hole accretion rate, LX/M*, to study how AGN environment depends on these quantities. Further, we derived the M*−Mhalo relation for our sample of XMM-COSMOS AGNs and compared it to results in literature for normal non-active galaxies. We measured the projected two-point correlation function wp(rp) using both the classic and the generalized clustering estimator, based on photometric redshifts, as probability distribution functions in addition to any available spectroscopic redshifts. We measured the large-scale (rp ≳ 1 h−1 Mpc) linear bias b by comparing the clustering signal to that expected of the underlying DM distribution. The bias was then related to the typical mass of the hosting halo Mhalo of our AGN subsamples. Since M* and LX/M* are correlated, we matched the distribution in terms of one quantity and we split the distribution in the other.
Results. For the full spectroscopic AGN sample, we measured a typical DM halo mass of log (Mhalo/h−1 M⊙) = 12.79−0.43+0.26, similar to galaxy group environments and in line with previous studies for moderate-luminosity X-ray selected AGN. We find no significant dependence on specific accretion rate LX/M*, with log (Mhalo/h−1 M⊙) = 13.06−0.38+0.23 and log (Mhalo/h−1 M⊙) = 12.97−1.26+0.39 for low and high LX/M* subsamples, respectively. We also find no difference in the hosting halos in terms of M* with log (Mhalo/h−1 M⊙) = 12.93−0.62+0.31 (low) and log (Mhalo/h−1 M⊙) = 12.90−0.62+0.30 (high). By comparing the M*−Mhalo relation derived for XMM-COSMOS AGN subsamples with what is expected for normal non-active galaxies by abundance matching and clustering results, we find that the typical DM halo mass of our high M* AGN subsample is similar to that of non-active galaxies. However, AGNs in our low M* subsample are found in more massive halos than non-active galaxies. By excluding AGNs in galaxy groups from the clustering analysis, we find evidence that the result for low M* may be due to larger fraction of AGNs as satellites in massive halos.
