The Ultraviolet Continuum Emission of Radio Galaxies. I. Description of Sources from the Hubble Space Telescope Archives

This paper describes the UV continuum radiation in the range from 1400 to 3700 Å of radio galaxies. The analysis is deferred to the next paper in this series. The sample of radio galaxies was compiled by searching the Hubble Space Telescope (HST) archives for images taken with the Faint Object Camera (FOC) prior to 1993. Altogether the sample consists of 30 3C and Parkes radio galaxies that have redshifts below 0.2 (the majority have redshifts of ~0.03) and radio powers of ~10^25-27 WHz^-1 (using H₀ = 50 km s^-1 Mpc^-1 and q₀ = 0.0).

We show the FOC contour plots of the radio galaxies and calculate the UV fluxes and magnitudes within the standard HST medium and wide filter bands (F130M, F140M, F152M, F165W, F170M, F190M, F231M, F320W, F342M, and F372M). We detect UV emission at wavelengths ≤2300 Å only for FR II galaxies that are broad emission line galaxies BLRGs and for FR I galaxies that have optical jets, that are BL Lac objects or that are BLRGs. The UV magnitudes range from 15.0 to 18.0, and the fluxes range from 10^-28.5 to 10^-30.5 W m^-2 Hz^-1. For sources without UV emission, upper limits are typically ~19.0 mag or ~10^-31 W m^-2 Hz^-1.

We model the UV emission by assuming a combination of a point source (the nuclear component excluding the jet) and an extended galaxy component. We find that the nuclear contribution at wavelengths ≤2300 Å is close to 100% for BL Lacs and BLRGs and ranges from 20% to 70% for FR I galaxies with optical jets. At longer wavelengths ≥3100 Å, the nuclear contribution tends to be less (not for BL Lacs which are still dominated by the nuclear component) and extended UV components are detected for all FR I and FR II galaxies in the sample.

We compare the structure of the extended UV emission to optical (mostly R-band HST-WFPC2) images. At wavelengths ≤2300 Å, the extended UV emission looks roughly spherical and shows some differences to the optical structure. Thus, we speculate that scattered light is an important contributor to the extended UV light wavelengths ≤2300 Å. At wavelengths ≥3100 Å, the extended UV and the optical images are roughly comparable although somewhat "blobbier." Thus, the extended flux at wavelengths ≥3100 Å is probably due to hot stars. However, since some of the polarization images show differences in the UV emission, scattered light from the active galactic nuclei may also contribute.