The optical/near-IR colours of red quasars

We present quasi-simultaneous multi-colour optical/near-IR photometry for 1 57 radio selected quasars, forming an unbiassed sub-sample of the Parkes Fl at-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 optic ally selected QSOs are all very similar: they have blue spectral energy dis tributions curving downwards at shorter wavelengths. Roughly 90% of the rad io-selected quasars have roughly power-law spectral energy distributions, w ith slopes ranging from F-nu proportional to nu(0) to F-nu proportional to nu(-2). The remaining 10% have spectral energy distributions showing sharp peaks: these are radio galaxies and highly reddened quasars. Four radio sou rces were not detected down to magnitude limits of H similar to 19.6. These are probably high redshift (z > 3) galaxies or quasars. We show that the c olours of our red quasars lie close to the stellar locus in the optical: th ey 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 th e K-band.