1999AJ....117.2582B

Quasars that are representative of the populous faint end of the luminosity function are frustratingly dim, with m∼24 at intermediate redshift; moreover, ground-based surveys for such faint QSOs suffer substantial morphological contamination by compact galaxies with similar colors. In order to establish a more reliable ultrafaint QSO sample, we used the Apache Point Observatory 3.5 m telescope to take deep ground-based U-band CCD images in fields previously imaged in V and I with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). Our approach hence combines multicolor photometry with the 0".1 spatial resolution of HST to establish a morphological and multicolor survey for QSOs extending about 2 mag fainter than most extant ground-based surveys. We present results for the ``Groth-Westphal strip'', in which we identify 10 high-likelihood UV-excess candidates having stellar or stellar-nucleus + galaxy morphology in WFPC2. For m₆₀₆<24.0 (roughly B≲24.5), the surface density of such QSO candidates is 420⁺¹⁸⁰_–130 deg^–2, or a surface density of 290⁺¹⁶⁰_–110 deg^–2 with an additional V-I cut that may further exclude compact emission-line galaxies. Even pending confirming spectroscopy, the observed surface density of QSO candidates is already low enough to yield interesting comparisons: our measures agree extremely well with the predictions of several recent luminosity function models.