The Nuclei of Radio Galaxies in the Ultraviolet: The Signature of Different Emission Processes

Abstract

We have studied the nuclei of 28 radio galaxies from the 3CR sample in the UV band. Unresolved nuclei (central compact cores: CCCs) are observed in 10 of the 13 FR I galaxies, and in five of the 15 FR II galaxies. All sources that do not have a CCC in the optical do not have a CCC in the UV. Two FR I galaxies (3C 270 and 3C 296) have a CCC in the optical but do not show the UV counterpart. Both of them show large dusty disks observed almost edge-on, possibly implying that they play a role in obscuring the nuclear emission. We have measured optical-UV spectral indices α_o,UV between ~0.6 and ~7.0 (F_ν~ν^-α). Broad-line radio galaxies have the flattest spectra, and their values of α_o,UV are also confined to a very narrow range. This is consistent with radiation produced in a geometrically thin, optically thick accretion disk. On the other hand, FR I nuclei, which are most plausibly originated by synchrotron emission from the inner relativistic jet, show a wide range of α_o,UV. There is a clear trend with orientation in that sources observed almost edge-on or with clear signs of dust absorption have the steepest spectra. These observations imply that in FR I galaxies, obscuration can be present, but the obscuring material is not in a ``standard'' geometrically thick torus. The most striking difference between these absorbing structures and the classic active galactic nucleus ``tori'' resides in the lower optical depth of the FR I galaxy obscuring material. Based on observations obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.