The morphology of nine radio-selected faint galaxies from deep Hubble Space Telescope imaging

Abstract

Using the Hubble Space Telescope (HST) WFPC2 we perform deep I-band imaging of nine radio-selected [F(8.5 GHz) ≥14 μJy] faint galaxies from the Roche, Lowenthal & Koo (2002) sample. Two are also observed in V using HST STIS. Six of the galaxies have known redshifts, in the range 0.4 < z < 1.0. Radial intensity profiles indicate that seven are disc galaxies and two are bulge-dominated. Four of the six with redshifts have a high optical surface brightness in comparison with previous studies of disc galaxies at similar redshifts (e.g. Lilly et al. 1998). The HST imaging reveals that two of the nine galaxies are in close interacting pairs and another five show morphological evidence of recent interactions — two are very asymmetric (Aasym∼ 0.4) and three have large, luminous rings resembling the collisional starburst rings in the Cartwheel galaxy. For the two ring galaxies with redshifts, we measure ring radii of 7.05 and 10.0 h−150 kpc, which suggest post-collision ages 0.1–0.2 Gyr. One has a fainter inner ring, like the original Cartwheel. The remaining two appear to be late-type barred galaxies and relatively undisturbed. Our HST imaging confirms the high incidence of interactions and dynamical disturbance in faint radio-selected galaxies, as reported by Windhorst et al. and Serjeant et al., for example. In the great majority of these galaxies the high radio luminosities are probably the result of interaction-triggered starbursts. However, one interacting galaxy is a very radio-luminous giant elliptical, with red V−I colours, a normal surface brightness and no evidence of star-forming regions, so its radio source is probably an obscured active galactic nucleus (AGN). The mixture of observed morphologies suggests that enhanced radio luminosities often persist to a late stage of interaction, i.e. at least ∼0.2 Gyr after the perigalactic encounter.