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

Abstract

We present H-alpha and red continuum observations for a sample of late-type low surface brightness (LSB) dwarf irregular galaxies(DIGs), consisting of all the m_B<17.2 ImIV and V galaxies in the Virgo cluster (VC), and compare them with a representative sample of VC high surface brightness (HSB) DIGs. Line fluxes and equivalent widths are listed for individual HII regions and total H-alpha emission is measured for the entire DIG. The HII regions we identified do not produce all the line emission. For LSBs with line emission we find star formation rates (SFR) of 6.9-43.0 10^{-3} M_sol/yr, ~10x weaker than for HSB objects. About 2 HII regions are detected per LSB galaxy; they are smaller and fainter than in HSB galaxies in the VC, have line equivalent widths ~50% of those in HSBs, and cover similar fractions of the galaxies. The line-emitting regions of LSB galaxies are preferentially located at the periphery of a galaxy, while in HSBs they tend to be central. The line strength of an HII region is correlated with the red continuum light underneath the region. No fundamental differences were identified in the star formation properties of the LSB and HSB dwarf galaxies we studied and infer that these galaxies must be similar, with the difference being the intensity of the present star-formation burst.