Compact Nuclei in Galaxies at Moderate Redshift. I. Imaging and Spectroscopy

Abstract

This study explores the space density and properties of active galaxies to z~=0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as active galactic nuclei (AGNs) or starbursts. Candidates are selected by fitting imaged galaxies with multicomponent models using maximum likelihood estimate techniques to determine the best model fit. We select those galaxies requiring an unresolved, point-source component in the galaxy nucleus, in addition to a disk and/or bulge component, to adequately model the galaxy light. We have searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing an unresolved nuclear component >=3% of the total galaxy light is 16%+/-3% corrected for incompleteness and 9%+/-1% for nuclei >=5% of the galaxy light. Spectroscopic redshifts have been obtained for 35 of our AGN/starburst candidates, and photometric redshifts are estimated to an accuracy of σ_z~=0.1 for the remaining sample. In this paper, the first of two for this study, we present the selected Hubble Space Telescope imaged galaxies having unresolved nuclei and discuss the selection procedure. We also present the ground-based spectroscopy for these galaxies as well as the photometric redshift predictions for those galaxies without spectra.

Based on observations made with NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with NASA.