Faint quasar candidates from Hubble Space Telescope imaging: number counts from 31 new high-latitude fields

Abstract

Quasars representative of the populous faint end of the luminosity function are dim with m ~ 24 at intermediate redshift, and traditional ground-based surveys for such faint quasars often suffer from severe contamination by compact faint galaxies. In order to limit the latter morphological contamination, we are conducting a combined multicolour and morphological survey for faint quasars, to B < 24.5 and z < 2.1, using archival Hubble Space Telescope (HST) images. Our previous work provided a similar survey of the Groth-Westphal Strip. Here, we extend our survey to an additional 31 high galactic latitude (pure parallel) fields that cover another 0.04 deg² of sky, imaged with the superb 0.1-arcsec spatial resolution of the Wide Field Planetary Camera 2 aboard HST. In our expanded survey, we identify an additional 19 high-likelihood ultraviolet-excess quasar candidates having stellar or unresolved nucleus plus host-galaxy morphology, and yielding a cumulative quasar surface density of 511⁺¹⁴²_-113 deg^-2 to B < 24.5 and z < 2.1. Our results are in reasonable agreement both with our own earlier survey of the Groth-Westphal Strip and with other faint quasar surveys that utilize very different selection approaches.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programme 8743.

E-mail: bbeckwin@depaul.edu