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The Optical/Near-IR Colours of Red Quasars

Published online by Cambridge University Press:  05 March 2013

Paul J. Francis ,
Matthew T. Whiting  and
Rachel L. Webster
Paul J. Francis
Affiliation:
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 0200, Australia; pfrancis@mso.anu.edu.au Joint appointment with the Department of Physics and Theoretical Physics, Faculty of Science, Australian National University, Canberra, ACT 0200, Australia
Matthew T. Whiting
Affiliation:
School of Physics, University of Melbourne, Parkville, Vic. 3052, Australia; mwhiting@physics.unimelb.edu.au
Rachel L. Webster
Affiliation:
School of Physics, University of Melbourne, Parkville, Vic. 3052, Australia; rwebster@physics.unimelb.edu.au
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Abstract

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We present quasi-simultaneous multi-colour optical/near-IR photometry for 157 radio selected quasars, forming an unbiassed sub-sample of the Parkes Flat-Spectrum Sample. Data are also presented for 12 optically selected QSOs, drawn from the Large Bright QSO Survey. The spectral energy distributions of the radio- and optically-selected sources are quite different. The optically selected QSOs are all very similar: they have blue spectral energy distributions curving downwards at shorter wavelengths. Roughly 90% of the radio-selected quasars have roughly power-law spectral energy distributions, with slopes ranging from Fv∝v0 to Fv∝v−2. The remaining 10% have spectral energy distributions showing sharp peaks: these are radio galaxies and highly reddened quasars. Four radio sources were not detected down to magnitude limits of H ∼ 19·6. These are probably high redshift (z > 3) galaxies or quasars. We show that the colours of our red quasars lie close to the stellar locus in the optical: they will be hard to identify in surveys such as the Sloan Digital Sky Survey. If near-IR photometry is added, however, the red power-law sources can be clearly separated from the stellar locus: IR surveys such as 2MASS should be capable of finding these sources on the basis of their excess flux in the K-band.

Keywords

quasars: generalmethods: observational
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 17 , Issue 1 , 2000 , pp. 56 - 71
Copyright
Copyright © Astronomical Society of Australia 2000

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