The Optical/Near-IR Colours of Red Quasars

Paul J Francis , Matthew T. Whiting , Rachel L. Webster, PASA, 17 (1), 56.
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Contents Page: Volume 17, Number 1

The Optical/Near-IR Colours of Red Quasars

Paul J Francis 1,2
Matthew T. Whiting 3
Rachel L. Webster 3

1 Research School of Astronomy and Astrophysics, Australian National University, Canberra ACT 0200
pfrancis@mso.anu.edu.au

2 Joint appointment with the Department of Physics and Theoretical Physics, Faculty of Science

3 School of Physics, University of Melbourne, Parkville, VIC 3052
mwhiting,rwebster@physics.unimelb.edu.au
[3mm]

Abstract:

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

$F_{\nu } \propto \nu ^{0}$ to

$F_{\nu } \propto \nu ^{-2}$. 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 $H \sim 19.6$. 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: general - Methods: observational





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