A new model for the infrared emission of quasars

Abstract

The optical-UV continua of 75 PG quasars are modelled to test two extreme hypotheses: (a) the range of continuum shapes observed is the result of different amounts of reddening of a single underlying continuum, and (b) the range of continuum slopes is entirely intrinsic, due to accretion discs being viewed at different inclinations. Although (a) is consistent with the range of continua observed, and an underlying average de-reddened spectrum can be derived, the extinctions implied would require a far greater bolometric power in the infrared than is observed. On the other hand, at least part of the spread in continuum shapes is almost certainly due to extinction, because the redder continua tend to show the 2200-A dust feature in absorption. A compromise is adopted, in which bluer than average continua are assumed to be due to viewing an accretion disc at non-zero inclination from the normal, while redder than average continua are assumed to be due to dust extinction. The infrared emission of the 24 PG quasars with IRAS detections is then modelled in terms of (i) emission from dust clouds in the narrow-line region at lambda=3-30 mum, and (ii) a starburst contributing to the emission at 30-100 mum. Correlations between the luminosities in these components and the optical luminosity support this resolution into distinct infrared components, and suggest that the covering factor of the narrow-line-region dust clouds is 0.2-0.7,with a mean of 0.32. At bolometric luminosities greater than 10^12 L_ (for H_0=50 km s^-1 Mpc^-1, Omega_0=1), the space density of starburst galaxies is 10 times that of quasars, but these space densities may become comparable for L_bol>=1^13 L_. For galaxies containing some kind of AGN (QSO, Seyfert 1 or 2), i.e. about 10-20 per cent of galaxies, the broad correlation of luminosities in starburst and QSO components suggests a common fuelling mechanism, for example via galaxy interactions and mergers. A QSO or Seyfert event appears to be inevitably accompanied by a starburst, but the reverse is not true.