Core Radio and Optical Emission in the Nuclei of nearby FR I Radio Galaxies

Abstract

In this paper we analyze the relation between radio, optical continuum and Hα+[N II] emission from the cores of a sample of 21 nearby Fanaroff and Riley type I galaxies as observed with the VLBA and HST. The emission arises inside the inner tens of parsecs of the galaxies. Core radio emission is observed in 19/20 galaxies, optical core continuum emission is detected in 12/21 galaxies and Hα+[N II] core emission is detected in 20/21 galaxies. We confirm the recently detected linear correlation between radio and optical core emission in FR I galaxies and show that both core emissions also correlate with central Hα+[N II] emission. The tight correlations between radio, optical, and Hα+[N II] core emission constrain the bulk Lorentz factor to γ~2-5 and γ<~2 for a continuous jet and a jet consisting of discrete blobs, respectively, assuming jet-viewing angles in the range 30°-90°. Radio and optical core emissions are likely to be synchrotron radiation from the inner jet, possibly with a significant contribution from emission by an accretion disk and/or flow. Elliptical galaxies with LINER nuclei without large-scale radio jets seem to follow the core emission correlations found in FR I galaxies. This suggests that the central engines could be very similar for the two classes of active galactic nuclei. Based on observations with the NASA/ESA Hubble Space Telescope 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.