Late-type dwarf irregular galaxies in the Virgo cluster - I. H alpha and red continuum data

We present H alpha and red continuum observations for a sample of late-type low surface brightness (LSB) dwarf irregular galaxies, consisting of all t he ImIV and V galaxies with m(B) less than or equal to 17.2 in the Virgo cl uster, and compare them with similar data for a representative sample of hi gh surface brightness (HSB) dwarf irregular galaxies, also in the Virgo clu ster. Line fluxes and equivalent widths are listed for individual H II regi ons, and total H alpha emission is measured for the entire galaxy. Although significant line emission originates in the H II regions that me have iden tified, it does not make up the entire H alpha output of all galaxies. For those objects in the LSB sample with H alpha emission, we find typical star formation rates (SFRs) from 6.9 x 10(-3) to as high as 4.3 x 10(-2) M. yr(-1). This is, on average, one order of magnitude weaker than for HSB ob jects, although the SFRs overlap. On average, similar to 2 H II regions are detected per LSB galaxy, for a total of 38 H II regions among 17 galaxies with H alpha emission. The H II regions are smaller and fainter than in HSB galaxies in the same Virgo cluster environment, have H alpha line equivale nt widths about 50 per cent of those in HSBs, and cover similar fractions o f the galaxies. When more than one H II region is present in a galaxy, we o bserve a strong intensity difference between the brightest and the second b rightest H II regions. The line-emitting regions of LSB galaxies are prefer entially located at the periphery of the galaxy, while in HSBs they tend to be central. The H alpha line strength of an H II region is correlated with the red continuum light underneath the region; this holds for both LSBs an d HSBs. We do not identify fundamental differences in the star formation properties of the LSB and HSB dwarf galaxies that we have studied, and we infer that these galaxies must be similar, with the difference being the intensity of the present star formation burst.