Astrophys. J., 571, 247-255 (2002/May-3)
The nuclei of radio galaxies in the ultraviolet: the signature of different emission processes.
CHIABERGE M., MACCHETTO F.D., SPARKS W.B., CAPETTI A., ALLEN M.G. and MARTEL A.R.
Abstract (from CDS):
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 (Fν∝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.
Galaxies: Active - Galaxies: Nuclei - Radiation Mechanisms: General - Ultraviolet: Galaxies
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