The HST survey of the B2 sample of radio-galaxies: Optical nuclei and the FR I/BL Lac unified scheme^*

¹ Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
² SISSA/ISAS, Via Beirut 2-4, 34014 Trieste, Italy
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Osservatorio Astronomico di Bologna, Via Ranzani, 1, 40127 Bologna, Italy
⁵ Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁶ Dipartimento di Fisica dell'Università di Bologna, Via Irnerio 46, 40126 Bologna, Italy
⁷ Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands

Corresponding author: A. Capetti, capetti@to.astro.it

Received: 4 October 2001
Accepted: 3 December 2001

Abstract

We examine the optical properties of the nuclei of low luminosity radio-galaxies using snapshot HST images of the B2 sample. In agreement with the results obtained from the analysis of the brighter 3C/FR I sample, we find a correlation between fluxes (and luminosities) of the optical and radio cores. This provides further support for the interpretation that the optical nuclear emission in FR I is dominated by synchrotron emission and that accretion in these sources takes place in a low efficiency radiative regime. In the framework of the FR I/BL Lacs unified scheme, we find that the luminosity difference between FR I and BL Lac nuclei can be reproduced with a common beaming factor in both the radio and the optical band, independent of the extended radio luminosity, thus supporting such a scenario. The corresponding bulk Lorentz factor is significantly smaller than is expected from observational and theoretical considerations in BL Lacs: this can be interpreted as due to a velocity structure in the jet, with a fast spine surrounded by a slower layer.