2009MNRAS.394..533G

We use photometric data from the Spitzer mission to explore the mid- and far-infrared properties of 10 red QSOs (J - K > 2, R - K > 5mag) selected by combining the Two Micron All Sky Survey (2MASS) in the near-infrared with the Sloan Digital Sky Survey (SDSS) at optical wavelengths. Optical and/or near-infrared spectra are available for 8/10 sources. Modelling the spectral energy distribution (SED) from ultraviolet to far-infrared shows that moderate dust reddening (A_V= 1.3-3.2) can explain the red optical and near-IR colours of the sources in the sample. There is also evidence that red QSOs have 60/12µm luminosity ratio higher than PG QSOs (97 per cent significance). This can be interpreted as a higher level of star formation in these systems (measured by the 60-µm luminosity) for a given active galactic nucleus (AGN) power (approximated by the 12-µm luminosity). This is consistent with a picture where red QSOs represent an early phase of AGN evolution, when the supermassive black hole is enshrouded in dust and gas clouds, which will eventually be blown out (possibly by AGN driven outflows) and the system will appear as typical optically luminous QSO. There is also tentative evidence significant at the 96 per cent level that red 2MASS QSOs are more often associated with radio emission than optically selected SDSS QSOs. This may indicate outflows, also consistent with the young AGN interpretation. We also estimate the space density of red QSOs relative to optically selected SDSS QSOs, taking into account the effect of dust extinction and the intrinsic luminosity of the sources. We estimate that the fraction of red QSOs in the overall population increases from 3 per cent at M_K= -27.5mag to 12 per cent at M_K = -29.5mag. This suggests that reddened QSOs become more important at the bright end of the luminosity function. If red QSOs are transition objects on the way to becoming typical optically luminous QSOs, the low fractions above suggest that these systems spent less than 12 per cent of their lifetime at the `reddened' stage.