Small-Scale Systems of Galaxies. I. Photometric and Spectroscopic Properties of Members

This paper is the first of a series addressed to the investigation of galaxyformation/evolution in small scale systems of galaxies (SSSGs) which arelocated in low density cosmic environments. Our algorithm for SSSG selectionincludes galaxy systems of 2 or more galaxies lying within 1000 km/s and a 200h_{100}^{-1} kpc radius volume. We present the analysis of the photometric andspectroscopic properties of 19 member galaxies belonging to a sample of 11SSSGs. In the $\mu_e - r_e$ plane, early-type members may be considered "ordinary",not "bright" galaxies in the definition given by Capaccioli et al.(1992) with asignificant fraction of galaxies having a disk or disky isophotes. We do notdetect fine structure and signatures of recent interaction events in theearly-type galaxy population, a picture also confirmed by the spectroscopy. At odd, there are several spiral members with open arm configurations asexpected in interacting systems. At the same time, emission lines in thespectra of spiral members fall in the HII regions regime defined withdiagnostic diagrams (Veilleux & Osterbrock 1987). None of the objects displaysunambiguous indication of nuclear activity, although fourspiral nuclei could beascribed to the class of Seyferts. The star formation rate seems enhanced overthe average expected in spiral galaxies only for poorer SSSGs in particularpairs (<50 solar masses per year) but without being in the range of starburstsystems.Comment: 24 pages, including 6 figures and 6 tables. Accepted for publication in A