The spatial distribution of X-ray selected AGN in the Chandra deepfields: a theoretical perspective

Abstract

We study the spatial distribution of X-ray selected active galacticnuclei (AGN) in the framework of hierarchical coevolution ofsupermassive black holes and their host galaxies and dark matterhaloes. To this end, we have applied the theoretical model developed byCroton et al., De Lucia & Blaizot and Marulli et al. to the output ofthe Millennium Run and obtained hundreds of realizations of past lightcones from which we have extracted realistic mock AGN catalogues thatmimic the Chandra deep fields. We find that the model AGN number countsare in fair agreement with observations both in the soft and in thehard X-ray bands, except at fluxes <= 10(-15) erg cm(-2) s(-1), wherethe model systematically overestimates the observations. However, alarge fraction of these faint objects are typically excluded from thespectroscopic AGN samples of the Chandra fields. We find that thespatial two-point correlation function predicted by the model is welldescribed by a power-law relation out to 20 h(-1) Mpc, in closeagreement with observations. Our model matches the correlation lengthr(0) of AGN in the Chandra Deep Field-North but underestimates it inthe Chandra Deep Field-South. When fixing the slope to gamma = 1.4, asin Gilli et al., the statistical significance of the mismatch is 2sigma-2.5 sigma, suggesting that the predicted cosmic variance, whichdominates the error budget, may not account for the differentcorrelation length of the AGN in the two fields. However, the overallmismatch between the model and the observed correlation functiondecreases when both r(0) and gamma are allowed to vary, suggesting thatmore realistic AGN models and a full account of all observationalerrors may significantly reduce the tension between AGN clustering inthe two fields. While our results are robust to changes in the modelprescriptions for the AGN light curves, the luminosity dependence ofthe clustering is sensitive to the different light-curve modelsadopted. However, irrespective of the model considered, the luminositydependence of the AGN clustering in our mock fields seems to be weakerthan in the real Chandra fields. The significance of this mismatchneeds to be confirmed using larger data sets.