THE ENVIRONMENTAL INFLUENCE ON THE EVOLUTION OF LOCAL GALAXIES

The results of an Ha photometric survey of 30 dwarf galaxies of various morphologies in the Centaurus A and Sculptor groups are presented. Of these 30, emission was detected in 13: eight are of late-type, two are early-type, and three are of mixed morphologies. The typical flux detection limit of similar to 2 x 10(-16) erg s(-1) cm(-2) translates into a star-formation rate (SFR) detection limit of 4 x 10(-6)M(circle dot) yr(-1). In light of these results, the morphology -density relation is re-examined, and it is shown that, despite a number of unaccounted parameters, there are significant correlations between the factors determining the morphological type of a galaxy and its environment. Dwarf galaxies in high-density regions have a lower current SFR and lower neutral gas content than their low-density counterparts, confirming earlier results from the Local Group and other denser environments. The effect of environment is also seen in the timescale formed from the ratio of blue luminosity to current SFR -dwarfs in higher-density environments have larger values, indicating relatively higher past average SFR. The influence of environment extends very far, and no dwarfs from our sample can be identified as field objects.