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

We present H α and red continuum observations for a sample of late‐type low surface brightness (LSB) dwarf irregular galaxies, consisting of all the ImIV and V galaxies with m B ≤ 17.2 in the Virgo cluster, and compare them with similar data for a representative sample of high surface brightness (HSB) dwarf irregular galaxies, also in the Virgo cluster. Line fluxes and equivalent widths are listed for individual H II regions, and total H α emission is measured for the entire galaxy. Although significant line emission originates in the H II regions that we have identified, it does not make up the entire H α output of all galaxies. For those objects in the LSB sample with Hα emission, we find typical star formation rates (SFRs) from 6.9×10 −3 to as high as 4.3×10 −2 M ⊙ yr −1 . This is, on average, one order of magnitude weaker than for HSB objects, although the SFRs overlap. On average, ∼2 H II regions are detected per LSB galaxy, for a total of 38 H II regions among 17 galaxies with Hα emission. The H II regions are smaller and fainter than in HSB galaxies in the same Virgo cluster environment, have Hα line equivalent widths about 50 per cent of those in HSBs, and cover similar fractions of the galaxies. When more than one H II region is present in a galaxy, we observe a strong intensity difference between the brightest and the second brightest H II regions. The line‐emitting regions of LSB galaxies are preferentially located at the periphery of the galaxy, while in HSBs they tend to be central. The Hα line strength of an H II region is correlated with the red continuum light underneath the region; this holds for both LSBs and 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.