Agn Host Galaxies: HST at Z ~ 0.1 and Gemini Adaptive Optics at Z ~ 2

Abstract

A study of 76 host galaxies of Active Galactic Nuclei (AGN) in the local universe (z ~ 0. 1) carried out with Hubble Space Telescope shows that these hosts are remarkably similar to typical field galaxies. They have normal distributions of disk and bulge size and surface brightness and do not show an obvious preponderance of peculiar structures or morphologies which might suggest recent strong interactions. The outstanding characteristic is that early-type galaxies constitute 55% of the AGN host sample compared to 10-20% of the field population suggesting a spheroid-black hole linkage. This AGN sample was selected on the basis of X-ray flux from the Einstein Medium Sensitivity Survey and is expected to show no bias with respect to the host galaxy properties.

There are some challenges associated with comparing this local sample with one at higher redshift. The first requirement is that the two samples be observed at equivalent restframe wavelength and the second is that similar physical resolution in the galaxies frame should be achieved. These two requirements can be met in a satisfactory (but not perfect) manner using adaptive optics on a large-aperture telescope and observing in the K-band. Although the resolution is lower by a factor of a few at z ~ 2 relative to HST observations at z ~ 0.1, it is possible to derive the global properties of the host galaxies in an equivalent manner given sufficient signal-to-noise ratio.

A third and very difficult problem is selecting comparable and unbiased samples at high and low redshift. This problem requires a careful analysis of selection effects. We present Gemini Quick Start observations of two AGNs at z ~ 1.7 with the University of Hawaii Hokupa’a adaptive optics system which delivered resolution of 0.12 arcseconds (FWHM) in the K-band. The guide-star requirement has driven most earlier observers to guide upon the AGN itself which restricts one to highly luminous quasars. This constraint was removed by selecting candidates for observation from the 2dF QSO Redshift Survey (Boyle et al. 2000) allowing us to image typical L ~ L* galaxies at high redshifts and thus greatly strengthen the validity of the comparison between the low and high redshift AGN samples.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555

Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina)