Quasar Population (quasar + population)

Distribution by Scientific Domains


Selected Abstracts


A unified model for the evolution of galaxies and quasars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000
Guinevere Kauffmann
We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few per cent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 107 yr. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z,2 to z=0 is a result of the combination of three effects: (i) a decrease in the merging rate; (ii) a decrease in the amount of cold gas available to fuel black holes, and (iii) an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Ly, systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starburst galaxies is inextricably linked to the hierarchical build-up of galaxies. [source]


Evidence for a large fraction of Compton-thick quasars at high redshift

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2007
Alejo Martínez-Sansigre
ABSTRACT Using mid-infrared and radio selection criteria, we pre-select a sample of candidate high-redshift type 2 quasars in the Subaru XMM,Newton Deep Field (SXDF). To filter out starburst contaminants, we use a Bayesian method to fit the spectral energy distributions (SEDs) between 24-,m and the B -band, obtain photometric redshifts, and identify the best candidates for high- z type 2 quasars. This leaves us with 12 zphot, 1.7 type 2 quasar candidates in an area ,0.8 deg2, of which only two have secure X-ray detections. The two detected sources have estimated column densities NH, 2 & 3 × 1027 m,2, i.e. heavily obscured but Compton-thin quasars. Given the large bolometric luminosities and redshifts of the undetected objects, the lack of X-ray detections suggests extreme absorbing columns NH, 1028 m,2 are typical. We have found evidence for a population of ,Compton-thick' high-redshift type 2 quasars, at least comparable to, and probably larger than, the type 1 quasar population, although spectroscopic confirmation of their active galactic nuclei nature is important. [source]


Most supermassive black hole growth is obscured by dust

ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2006
A. Martínez-Sansigre
Abstract We present an alternative method to X-ray surveys for hunting down the high-redshift type-2 quasar population, using Spitzer and VLA data on the Spitzer First Look Survey. By demanding objects to be bright at 24 µm but faint at 3.6 µm, and combining this with a radio criterion, we find 21 type-2 radio-quiet quasar candidates at the epoch at which the quasar activity peaked. Optical spectroscopy with the WHT confirmed 10 of these objects to be type-2s with 1.4 , z , 4.2 while the rest are blank. There is no evidence for contamination in our sample, and we postulate that our 11 blank-spectrum candidates are obscured by kpc-scale dust as opposed to dust from a torus around the accretion disk. By carefully modelling our selection criteria, we conclude that, at high redshift, 50,80% of the supermassive black hole growth is obscured by dust. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Radio galaxies and type-2 quasars in the Spitzer Extragalactic First Look Survey

ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2006
M. Lacy
Abstract We use mid-infrared colour-colour plots made with data from the Spitzer First Look Survey to explore the emission mechanisms of faint radio sources in that field. Using mid-infrared colours alone, we are able to see the transition from a starburst-dominated radio source population at sub-mJy levels to the radiogalaxy/quasar population at mJy levels. Both FR I and some FR II galaxies show little or no mid-infrared excess, suggesting they either lack dusty tori, or a strong continuum source to heat them, or both. Objects with hot, dusty tori, most likely radio-intermediate and radio-quiet AGN, make up a significant fraction of the faint radio source population. We also discuss the use of mid-infrared colour-colour plots to select radio quiet obscured AGN. Early results from imaging the host galaxies of these type-2 quasars at high spatial resolution with adaptive optics indicates that they, like type-1 quasars and radio galaxies, are often found to be hosted by giant elliptical galaxies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]