Late-type dwarf galaxies in the Virgo cluster - II. Star formation properties

We study star-formation-inducing mechanisms in galaxies through multiwavelength measurements of a sample of dwarf galaxies in the Virgo cluster 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 formation rate and star formation histories of the galaxies, and check their dependence on other parameters. Comparison of the sample galaxies with population synthesis models shows that these objects have significantly 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 currently forming presumably in a starburst, and an older (0.1-1 Gyr) population of previous stellar generations. There is evidence that the older stellar population was also formed in a starburst. This is consistent with the explanation that star formation in this type of objects takes place in short bursts followed by long quiescent periods. No significant correlation is found between the star formation properties of the sample galaxies and their hydrogen content. Apparently, when star formation occurs in bursts, other parameters influence the star formation properties more significantly than the amount of atomic hydrogen. No correlation is found between the projected Virgocentric distance and the rate of star formation in the galaxies, suggesting that tidal interactions are not significant in triggering star formation in cluster dwarf galaxies.