Hubble Space Telescope observations of nearby radio-loud early-type galaxies

We present and analyze Hubble Space Telescope (HST) WFPC2 broad- and narrowband observations of the central regions of 19 nearby radio-loud early-type galaxies. Together with two more galaxies, they form a complete sample of Fanaroff and Riley Type I galaxies. We obtained V- and I-band images and narrowband images centered on the H alpha + [N II] emission lines. We use archival data for six galaxies. We describe the data reduction, give isophotal fits, and analyze the central emission-line gas and dust distributions. Our main conclusions are the following. Although obscuration by dust inhibits a direct determination of central cusp slopes, the data suggest that most but not all galaxies have shallow cores. Dust is detected in all but two galaxies. There are a wide variety of central dust morphologies, ranging from central disks to lanes and irregular distributions. The analysis suggests that the difference between disks and lanes is intrinsic and not due to different viewing angles. Central emission-line gas is detected in all galaxies. Extended low surface brightness emission is always associated with the dust features. In a subsequent paper we will present a detailed analysis of the relation between these central properties and the nuclear activity.