Hubble Space Telescope observations of X-ray-selected active galactic nuclei

Abstract

We report on the initial results of a comprehensive Hubble Space Telescope (HST) snapshot imaging survey of 76 low-redshift (z<0.15) X-ray-selected active galactic nuclei (AGN) in the Einstein Extended Medium Sensitivity Survey. This survey is expected to show no bias with respect to host galaxy types, and so is arguably one of the best available samples with HST imaging for the study of the host galaxies. The HST observations in the F814W band are complemented by deeper ground-based observations in the B and R bands for most AGN. The absolute magnitudes for AGN in this sample lie in the range −24<MB(AB)<−18, bracketing the extrapolated break in the QSO luminosity function MB(AB)=−22.3] at these low redshifts. We find a weak correlation between the luminosity of the host galaxy and the central AGN. We find no convincing cases of an AGN with no detectable host galaxy, although the faintest host galaxies of moderately luminous AGN do extend as faint as MB(AB)=−18. We find no evidence for strong interaction/merger activity in any of the AGN in this sample. The median ratio of AGN to host galaxy luminosity (LAGNLHost=0.2) is lower than previously observed, although the observed scatter is large. Approximately 55 per cent of these radio-quiet AGN have host galaxies that are fitted best by a ‘bulge-only’ model (or alternatively are classified visually as ellipticals/S0 galaxies), confirming the result by McLure et al. that radio-quiet AGN are not exclusively found in spiral galaxies. A comparison with the Autofib field galaxy survey shows that the morphological type distribution is skewed toward earlier types than a field galaxy sample drawn at random with the same distribution of luminosities. This is consistent with the observation that the luminosity of the host galaxies is higher by 0.75±0.25 mag than a matching sample drawn from the Autofib survey. Given the bias toward early-type galaxies, the AGN host galaxies are consistent with a luminosity and size distribution identical to normal galaxies. In every respect these galaxies are intermediate in their properties between the large, luminous host galaxies found around high-luminosity AGN in the local Universe and the fainter host galaxies identified around lower luminosity Seyfert galaxies. These results suggest that, with the exception of a bias toward early spectral types, host galaxies of AGN are drawn at random from the overall galaxy population, with the nuclear properties governed (weakly) by spheroid mass.