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E. S. Perlman (University of Maryland, Baltimore County), J. T. Stocke (University of Colorado, Boulder), J. E. Conway (Onsala Space Observatory), C. S. Reynolds (University of Colorado, Boulder)
One of the most enduring and difficult mysteries concerning radio galaxies is the conditions which trigger the onset of the active phase. We have investigated these mysteries by observing with HST three nearby (z < 0.1) compact symmetric objects, radio sources whose physical sizes (~100-1000 pc) and lobe advance speeds indicate that their active phase and ejection of radio lobes began 103-104 years ago. Our target objects, 4C 31.04, 1946+708 and 1146+596 (=NGC 3894), were chosen for HST observations both on the basis of proximity as well as detected HI line emission. All three objects are well fit by ``nuker" models typical of nearby ellipticals. They also appear to be in moderately rich environments, with each having a companion within 20-70 kpc projected distance. However, the galaxies are much dustier (by a full order of magnitude) than typical bright, L* ellipticals. The dust features we see include nuclear disks or tori in all three, with apparent geometries consistent with a perpendicular orientation to the radio axis. One galaxy, 4C 31.04, exhibits bright nuclear regions well aligned with the radio axis, while another, 1146+596, shows a significant near-IR excess along its dust disk. We discuss the physical conditions in the dust disk and nucleus, and the possible association of the nuclear obscuration with the radio lobes and the onset of activity in these objects. The lack of observed isophotal anomalies in these objects has important implications for the scenario proposed by Wilson & Colbert, whereby radio-loud AGN are triggered by major mergers of two (possibly active) spiral galaxies: in particular, any such mergers must have happened at least ~108 years ago (the crossing time for the product galaxies).