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 galaxy formation/evolution in small-scale systems of galaxies (SSSGs), which are located in low-density cosmic environments. Our algorithm for SSSG selection includes galaxy systems of two or more galaxies lying within Δcz ≤ 1000 km s^-1 and a 200 h kpc radius volume. We present the analysis of the photometric and spectroscopic properties of 19 member galaxies belonging to a sample of 11 SSSGs. In the μ_e-r_e plane early-type members may be considered "ordinary," rather than "bright," galaxies, with a significant fraction of galaxies having a disk or disky isophotes. We do not detect fine structure or signatures of recent interaction events in the early-type galaxy population, a picture also confirmed by the spectroscopy. At odds with these findings are several spiral members with open arm configurations, as expected in interacting systems. At the same time, emission lines in the spectra of spiral members fall in the H II regions regime. None of the objects displays unambiguous indications of nuclear activity, although four spiral nuclei could be ascribed to the class of Seyferts. The star formation rate seems enhanced over the average expected in spiral galaxies only for poorer SSSGs in particular pairs (≤50 M_⊙ yr^-1) but without being in the range of starburst systems.