Host Galaxies, Obscuration and Nuclear Structure of Three Nearby Compact Symmetric Objects

Abstract

We present 3-band HST imaging of three z</=0.1 compact symmetric objects (CSOs): 4C 31.04, 1946+708 and 1146+596. These objects were chosen on the basis of proximity to Earth as well as HI 21 cm absorption. The inner H-band isophotes of these galaxies are well fit by "nuker" models, typical of nearby ellipticals. Each shows a significant flattening in the isophotal profile at radii ~0.5", as well as variations in ellipticity and PA. However, neither is uncommon for elliptical galaxies. All three show modest departures from nuker models at radii of 1-5 $h_{60}^{-1}$ kpc. Each shows large, well distributed dust features, which are somewhat concentrated in the nuclear regions in features which resemble disks or tori. The amount of dust in these galaxies is about 10 times higher than normal for ellipticals and radio galaxy hosts. The nuclear dust disks tend to be oriented roughly perpendicular to the radio axis. One galaxy, 4C31.04, exhibits bright nuclear regions well-aligned with the radio axis, while another, 1146+596, shows a significant near-IR excess resembling a stellar bar along its dust disk. The combination of outwardly normal isophotal profiles with significant variations in PA and ellipticity is consistent with the host galaxies being relatively recent merger remnants, with the mergers having occurred >/= 10^8 years ago. Such a merger could have "triggered" the current activity in these objects, but our data require a significant time delay between the merger and the onset of nuclear activity. However, these data are also consistent with the hypothesis that the onset of nuclear activity in radio galaxies is due to relatively minor "feeding" events and/or the formation of "bars within bars", which would disturb the internal kinematics only slightly.