Simulating Wide-Field Quasar Surveys from the Optical to Near-Infrared

Abstract

A number of deep, wide-field, near-infrared surveys employing new infrared cameras on 4m-class telescopes are about to commence. These surveys have the potential to determine the fraction of luminous dust-obscured quasars that may have eluded surveys undertaken at optical wavelengths. In order to understand the new observations it is essential to make accurate predictions of surface densities and number-redshift relations for unobscured quasars in the near-infrared based on information from surveys at shorter wavelengths. The accuracy of the predictions depends critically on a number of key components. The commonly used single power-law representation for quasar SEDs is inadequate and the use of an SED incorporating the upturn in continuum flux at lambda~12000 A is essential. The presence of quasar host galaxies is particularly important over the restframe wavelength interval 8000 < lambda < 16000 A and we provide an empirical determination of the magnitude distribution of host galaxies using a low redshift sample of quasars from the SDSS DR3 quasar catalogue. A range of models for the dependence of host galaxy luminosity on quasar luminosity is investigated, along with the implications for the near-infrared surveys. Even adopting a conservative model for the behaviour of host galaxy luminosity the number counts for shallow surveys in the K-band increase by a factor of two. The degree of morphological selection applied to define candidate quasar samples in the near-infrared is found to be an important factor in determining the fraction of the quasar population included in such samples.