The Square Kilometre Array (SKA)

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 have conducted observations of 22 pulsars at frequencies of 0.7, 1.4 and 3.1 GHz and present their polarization profiles. The observations were carried out for two main purposes. First we compare the orientation of the spin and velocity vectors to verify the proposed alignment of these vectors by Johnston et al. (2005). We find, for the 14 pulsars for which we were able to determine both vectors, that 7 are plausibly aligned, a fraction which is lower than, but consistent with, earlier measurements. Secondly, we use profiles obtained simultaneously at widely spaced frequencies to compute the radio emission heights. We find, similar to other workers in the field, that radiation from the centre of the profile originates from lower in the magnetosphere than the radiation from the outer parts of the profile.