Multiwavelength observations of quasars and their environments

Abstract

The relationship between the radio and optical properties of quasars and the connection between these properties and the quasar host galaxies are investigated. Radio data have been analyzed for 359 of the 1055 quasars in the Large Bright Quasar Survey (LBQS). A major result of this work is that the radio-loud fraction is mostly insensitive to redshift and quasar optical luminosity, remaining at ≈ 10% over the absolute magnitude range -28 ≤ M(B) ≤ -23 and from redshifts z = 0.2 to z ∼ 5. Two deviations from these flat distributions occur at z ∼ 1, where there is a modest increase in the radio-loud fraction, and for absolute magnitudes brighter than M(B) = -28, where the fraction climbs to 20-30%. The rise in radio-loud fraction at z ∼ 1 is reproduced by a model based on the optical and radio quasar luminosity functions. The increase at high optical luminosities is consistent with the existence of two radio emission mechanisms, one correlated with optical luminosity, the other independent. The nearly flat distributions in the LBQS differ markedly from those of the optically selected Palomar-Green Bright Quasar Survey and the X-ray selected Extended Medium Sensitivity Survey. A subset of 16 LBQS quasars was imaged with the Hubble Space Telescope to study the dependence of radio and optical luminosity on the absolute magnitudes and morphologies of the host galaxies. There is no distinction in host galaxy magnitude between radio-loud and radio-quiet quasars, assuming they are all of the same galaxy type. The magnitudes of the hosts are ≳ L*, and the optical luminosities of the hosts and nuclear components are positively correlated. Many of the host galaxies have small axial ratios, which may indicate that they are inclined disk systems; or else they have bright elongated features which are visible while the bulk of the underlying lower surface brightness components of the host galaxy are not.