Journal Article

The relative growth of optical and radio quasars in SDSS

Francesco Shankar, Gregory R. Sivakoff, Marianne Vestergaard and Xinyu Dai

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 401, issue 3, pages 1869-1881
Published in print January 2010 | ISSN: 0035-8711
Published online January 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.15764.x
The relative growth of optical and radio quasars in SDSS

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We cross-correlate the Sloan Digital Sky Survey Data Release 3 quasar sample with Faint Images of the Radio Sky at Twenty centimetres (FIRST) and the Vestergaard et al. black hole (BH) mass sample to compare the mean accretion histories of optical and radio quasars. We find significant statistical evidence that radio quasars have a higher mean Eddington ratio λ at z > 2 with respect to optical quasars, while the situation is clearly reverse at z < 1. At z > 2, radio quasars happen to be less massive than optical quasars; however, as redshift decreases radio quasars appear in increasingly more massive BHs with respect to optical quasars. These two trends imply that radio sources are not a mere random subsample of optical quasars. No clear correlation between radio activity and BH mass and/or accretion rate is evident from our data, pointing to other BH properties, possibly the spin, as the driver of radio activity. We have checked that our main results do not depend on any evident bias. We perform detailed modelling of reasonable accretion histories for optical and radio quasars, finding that radio quasars grows by a factor of a few, at the most, since z∼ 4. The comparison between the predicted mass function of active radio quasars and the observed optical luminosity function of radio quasars, implies a significantly lower probability for lower mass BHs to be radio loud at all epochs, in agreement with what is observed in the local universe.

Keywords: galaxies: active; galaxies: jets; quasars: general

Journal Article.  10422 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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