The Morphology of 9 Radio-selected Faint Galaxies from deep HST Imaging

Using the HST WFPC2 we perform deep I-band imaging of 9 radio-selected (F(8.5 GHz) ≥ 14μJy) faint galaxies from the Roche, Lowenthal and 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 7 are disk galaxies and 2 are bulge-dominated. Four of the six with redshifts have a high optical surface brightness in comparison with previous studies of disk galaxies at similar redshifts (e.g. Lilly et al. 1998). The HST imaging reveals that 2 of the 9 galaxies are in close interacting pairs and another 5 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 50 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 e.g. Windhorst et al. (1995) and Serjeant. et al. (2000). 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 radioluminous 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 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.