Prof. Matt Jarvis

Abstract:

Context: X-ray surveys of Active Galactic Nuclei (AGN) indicate `cosmic downsizing', with the comoving number density of high-luminosity objects peaking at higher redshifts (z about 2) than low-luminosity AGN (z<1). Aims: We test whether downsizing is caused by activity shifting towards low-mass black holes accreting at near-Eddington rates, or by a change in the average rate of accretion onto supermassive black holes. We estimate the black hole masses and Eddington ratios of an X-ray selected sample of AGN in the Chandra Deep Field South at z<1, probing the epoch where AGN cosmic downsizing has been reported. Methods: Black hole masses are estimated both from host galaxy stellar masses, which are estimated from fitting to published optical and near-infrared photometry, and from near-infrared luminosities, applying established correlations between black hole mass and host galaxy properties. Both methods give consistent results. Comparison and calibration of possible redshift-dependent effects is also made using published faint host galaxy velocity dispersion measurements. Results: The Eddington ratios in our sample span the range 10^{-5} to 1, with median log(L_bol/L_Edd)=-2.87, and with typical black hole masses about 10^{8} solar masses. The broad distribution of Eddington ratios is consistent with that expected for AGN samples at low and moderate luminosity. We find no evidence that the CDF-S AGN population is dominated by low-mass black holes accreting at near-Eddington ratios and the results suggest that diminishing accretion rates onto average-sized black holes are responsible for the reported AGN downsizing at redshifts below unity.

Abstract:

We analyse the first publicly released deep field of the UK Infrared Deep Sky Survey (UKIDSS) Deep eXtragalactic Survey to identify candidate galaxy overdensities at z ∼ 1 across ∼1 deg2 in the ELAIS-N1 field. Using I - K, J - K and K - 3.6 μm colours, we identify and spectroscopically follow up five candidate structures with Gemini/Gemini Multi-Object Spectrograph and confirm that they are all true overdensities with between five and 19 members each. Surprisingly, all five structures lie in a narrow redshift range at z = 0.89 ± 0.01, although they are spread across 30 Mpc on the sky. We also find a more distant overdensity at z = 1.09 in one of the spectroscopic survey regions. These five overdense regions lying in a narrow redshift range indicate the presence of a supercluster in this field and by comparing with mock cluster catalogues from N-body simulations we discuss the likely properties of this structure. Overall, we show that the properties of this supercluster are similar to the well-studied Shapley and Hercules superclusters at lower redshift. © 2007 RAS.