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Title:
The optical/near-IR colours of red quasars
Authors:
Francis, P. J.; Whiting, M. T.; Webster, R. L.
Publication:
Publications Astronomical Society of Australia, vol. 17, no. 1, p. 56-71 (PASA Homepage)
Publication Date:
04/2000
Origin:
PASA
Keywords:
QUASARS, OBSERVATIONAL METHODS
Bibliographic Code:
2000PASA...17...56F

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 Fν ∝ ν0 to Fν ∝ ν-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.

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