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

In this paper we analyze the relation between radio, optical continuum andHalpha+[NII] emission from the cores of a sample of 21 nearby Fanaroff & Rileytype I galaxies as observed with the VLBA and HST. The emission arises insidethe inner tens of parsec of the galaxies. Core radio emission is observed in19/20 galaxies, optical core continuum emission is detected in 12/21 galaxiesand Halpha+[NII] core emission is detected in 20/21 galaxies. We confirm therecently detected linear correlation between radio and optical core emission inFR I galaxies and show that both core emissions also correlate with centralHalpha+[NII] emission. The tight correlations between radio, optical andHalpha+[NII] core emission constrain the bulk Lorentz factor to gamma ~ 2-5 andgamma =< 2 for a continuous jet and a jet consisting of discrete blobs,respectively, assuming jet viewing angles in the range [30deg,90deg]. Radio andoptical core emissions are likely to be synchrotron radiation from the innerjet, possibly with a significant contribution from emission by an accretiondisk and/or flow. Elliptical galaxies with LINER nuclei without large-scaleradio jets seem to follow the core emission correlations found in FR Igalaxies. This suggests that the central engines could be very similar for thetwo classes of AGNs.Comment: 46 pages, 10 figures and 8 tables. Accepted for publication in AJ. Corrected few typos in tex