LATE-TYPE DWARF GALAXIES IN THE VIRGO CLUSTER - II - STAR-FORMATION PROPERTIES

We study star-formation-inducing mechanisms in galaxies through multiw avelength measurements of a sample of dwarf galaxies in the Virgo clus ter described in Paper I. Our main goal is to test how star-formation- inducing mechanisms depend on several parameters of the galaxies, such as morphological type and hydrogen content. We derive the star format ion rate and star formation histories of the galaxies, and check their dependence on other parameters. Comparison of the sample galaxies wit h population synthesis models shows that these objects have significan tly lower metallicity than the solar value. The colours can generally be explained as a combination of two different stellar populations: a young (3-20 Myr) metal-poor population which represents the stars curr ently forming presumably in a starburst, and an older (0.1-1 Gyr) popu lation of previous stellar generations. There is evidence that the old er stellar population was also formed in a starburst. This is consiste nt with the explanation that star formation in this type of objects ta kes place in short bursts followed by long quiescent periods. No signi ficant correlation is found between the star formation properties of t he sample galaxies and their hydrogen content. Apparently, when star f ormation occurs in bursts, other parameters influence the star formati on properties more significantly than the amount of atomic hydrogen. N o correlation is found between the projected Virgocentric distance and the rate of star formation in the galaxies, suggesting that tidal int eractions are not significant in triggering star formation in cluster dwarf galaxies.