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 Angstrom of radio galaxies. The analysis is deferred to the n ext 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 sampl e consists of 30 3C and Parkes radio galaxies that have redshifts belo w 0.2 (the majority have redshifts of similar to 0.03) and radio power s of similar to 10(25-27) WHZ(-1) (using H-0 = 50 km s(-1) Mpc(-1) and q(0) = 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 less than or equal to 2300 Angstrom only for FR II galaxies that are broad emission line galaxies BLRGs and for FR I galaxies that have optical j ets, that are BL Lac objects or that are BLRGs. The UV magnitudes rang e 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 typi cally similar to 19.0 mag or similar to 10(-31) W m(-2) Hz(-1). We mod el the UV emission by assuming a combination of a point source (the nu clear component excluding the jet) and an extended galaxy component. W e find that the nuclear contribution at wavelengths less than or equal to 2300 Angstrom is close to 100% for BL Lacs and BLRGs and ranges fr om 20% to 70% for FR I galaxies with optical jets. At longer wavelengt hs greater than or equal to 3100 Angstrom, the nuclear contribution te nds to be less (not for BL Lacs which are still dominated by the nucle ar component) and extended UV components are detected for all FR I and FR II galaxies in the sample. We compare the structure of the extende d UV emission to optical (mostly R-band HST-WFPC2) images. At waveleng ths less than or equal to Angstrom, the extended UV emission looks rou ghly spherical and shows some differences to the optical structure. Th us, we speculate that scattered light is an important contributor to t he extended UV light wavelengths less than or equal to Angstrom. At wa velengths greater than or equal to 3100 Angstrom, the extended UV and the optical images are roughly comparable although somewhat ''blobbier .'' Thus, the extended flux at wavelengths greater than or equal to 31 00 Angstrom is probably due to hog stars. However, since some of the p olarization images show differences in the UV emission, scattered ligh t from the active galactic nuclei may also contribute.