Quasar Candidates in the Hubble Deep Field

The Hubble Deep Field (HDF) gives us an unprecedented view of our universe and an opportunity to study a wide range of questions in galaxy evolution and cosmology. Here we focus on the search for unresolved faint quasars and active galactic nuclei (AGNs) in the crude combine images using a multicolor imaging analysis that has proved very successful in recent years. Quasar selection was carried out both in multicolor space and in ``profile space.'' The latter is the multiparameter space formed by the radial profiles of the objects in the different images. By combining the dither frames available for each filter, we were able to obtain well-sampled radial profiles of the objects and measure their deviation from that of a stellar source. This procedure efficiently helps to overcome the problems caused by the WPFC2 undersampling. Furthermore, to identify areas of multicolor space where quasars might be expected, we generated synthetic quasar spectra in the range 1.0<z<5.5, including effects of intrinsic emission lines and absorption by Lyalpha forest and Lyman limit systems, and computed expected quasar colors. We also developed routines to determine the completeness of our data to point sources in the observed filters. The data are 90% complete at 26.2, 28.0, 27.8, 26.8 mag in the F300W, F450W, F606W, and F814W filters, respectively. We find 41 compact objects in the HDF: one pointlike object with colors consistent with quasars but inconsistent with that of ordinary stars; seven pointlike objects with colors consistent with quasars or stars; 18 stars; and 15 slightly resolved objects, 12 of which have colors consistent with quasars or stars. The latter objects could be resolved AGNs or regions of intense star formation. We estimate the upper limit of unresolved and slightly resolved quasars/AGNs with V_606<27.0 and z<3.5 to be 20 objects (16,200 deg^-2). We independently recovered a resolved, star-forming galaxy with a spectroscopically confirmed redshift of 3.368 and five spectroscopically confirmed stars. All provide confirmation for the validity of our color and morphological modeling. We make a detailed comparison with previous studies of the HDF. We find good agreement among different authors on the number of stars in the HDF and the lack of quasar candidates with z>3.5. We find more quasar candidates than previous work because of our more extensive modeling and use of the color information. Spectroscopic observations of the candidates are needed to determine which, if any, are quasars or AGNs.