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| Paper: |
The nuclei of radio galaxies as seen by the HST |
| Volume: |
290, Active Galactic Nuclei: from Central Engine to Host Galaxy |
| Page: |
331 |
| Authors: |
Chiaberge, M.; Macchetto, F. D.; Sparks, W. B.; Capetti, A.; Celotti, A. |
| Abstract: |
We have studied the nuclei of 3CR radio galaxies with z lower than 0.3 through HST images, both in the optical and in the UV. The great majority of them show unresolved nuclear sources (central compact cores, CCC). We find a tight linear correlation between FRI CCC and the radio core emission, both in flux and luminosity, arguing for a non--thermal synchrotron origin of both components. FR II nuclei show a more complex behavior, which is however related to their optical spectral properties. Broad--lined objects (BLO) have the brightest nuclei both in the optical and in the UV. In the radio-optical plane BLO show an optical excess, when compared to the radio-optical correlation of FRI. About 40% of the high and low excitation galaxies (HEG and LEG) show CCC which resemble those detected in FRI galaxies, in apparent contrast to the unification model. The equivalent width of the [OIII] emission line reveals the nature of these nuclei. Most of HEGs nuclei are obscured to our line of sight and only scattered radiation is observed. LEGs (and a minority of HEGs) are instead truly similar to FRI. This implies that the population of FR II is composed by objects with different nuclear properties, and only a fraction of them can be unified with quasars. The different optical-UV spectral indices confirm the different nature of FRI and FRII nuclei: while the synchrotron scenario can account for the spectral properties of FRI CCC, the nuclei of BLO FRII are consistent with thermal radiation from the accretion disk. The UV data suggest that a moderate amount of obscuration (AV ∼1-3) can be present in FRI, but the obscuring material is not in a ``standard'' geometrically thick torus. |
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