Host galaxies, obscuration, and nuclear structure of three nearby compact symmetric objects

We present three-band Hubble Space Telescope imaging of three nearby (z les s than or equal to 0.1) compact symmetric objects : 4C 31.04, 1946 + 708, a nd 1146 + 596. These objects were chosen on the basis of proximity to Earth as well as H I 21 cm line absorption. The inner H-band isophotes of these galaxies are well fitted by Nuker models, typical of nearby elliptical gala xies. Each shows a significant flattening in the isophotal profile at radii similar to0."5 as well as significant variations in ellipticity and P.A. H owever, as previous , authors have noted, neither is uncommon for elliptica l galaxies. All three objects show modest departures from Nuker law models at radii of 1-5 h(60)(-1) kpc. Each galaxy shows large well-distributed dus t features, which are somewhat concentrated in the nuclear regions in featu res that resemble disks or tori. We find that the amount of dust in these g alaxies is about 10 times higher than normal for elliptical galaxies and ra dio galaxy hosts. The major axes of the nuclear dust disks tend to be orien ted roughly perpendicular to the radio axis. One galaxy, 4C 31.04, exhibits bright nuclear regions well aligned with the radio axis, while another, 11 46+596, shows a significant near-IR excess resembling a stellar bar along i ts dust disk. The combination of outwardly normal isophotal profiles with s ignificant variations in P.A. and ellipticity is consistent with the host g alaxies being relatively recent merger remnants and the mergers having occu rred greater than or similar to 10(8) yr ago. Such a merger could have "tri ggered" the onset of the current active phase seen in these objects, but ou r data require a significant time delay between the merger event and the on set of nuclear activity. However, these data are also consistent with the h ypothesis that the onset of nuclear activity in radio galaxies is due to re latively minor "feeding" events and/or the formation of "bars within bars," events which would disturb the internal kinematics only slightly.