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

Wed, 31 Dec 2008 23:00:00 GMT

The spatial distribution of X-ray selected AGN in the Chandra deepfields: a theoretical perspective Marulli, Federico; Bonoli, Silvia; Branchini, Enzo; Gilli, Roberto; Moscardini, Lauro; Springel, Volker 2009 MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 396 1404-1414 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.

Modelling the cosmological co-evolution of supermassive black holes and galaxies - II. The clustering of quasars and their dark environment

Wed, 31 Dec 2008 23:00:00 GMT

Modelling the cosmological co-evolution of supermassive black holes and galaxies - II. The clustering of quasars and their dark environment Bonoli, Silvia; Marulli, Federico; Springel, Volker; White, Simon D. M.; Branchini, Enzo; Moscardini, Lauro 2009 Monthly notices of the Royal Astronomical Society 396 423-438 We use semi-analytic modelling on top of the Millennium simulation to study the joint formation of galaxies and their embedded supermassive black holes. Our goal is to test scenarios in which black hole accretion and quasar activity are triggered by galaxy mergers, and to constrain different models for the light curves associated with individual quasar events. In the present work, we focus on studying the spatial distribution of simulated quasars. At all luminosities, we find that the simulated quasar two-point correlation function is fit well by a single power law in the range 0.5 less than or similar to r less than or similar to 20 h(-1) Mpc, but its normalization is a strong function of redshift. When we select only quasars with luminosities within the range typically accessible by today's quasar surveys, their clustering strength depends only weakly on luminosity, in agreement with observations. This holds independently of the assumed light-curve model, since bright quasars are black holes accreting close to the Eddington limit, and are hosted by dark matter haloes with a narrow mass range of a few 10(12) h(-1) M-circle dot. Therefore, the clustering of bright quasars cannot be used to disentangle light-curve models, but such a discrimination would become possible if the observational samples can be pushed to significantly fainter limits.Overall, our clustering results for the simulated quasar population agree rather well with observations, lending support to the conjecture that galaxy mergers could be the main physical process responsible for triggering black hole accretion and quasar activity.

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

Modelling the cosmological co-evolution of supermassive black holes and galaxies - II. The clustering of quasars and their dark environment