Star Formation in Sculptor Group Dwarf Irregular Galaxies and the Nature of “Transition” Galaxies

We present new Hα narrowband imaging of the H II regions in eight Sculptor group dwarf irregular (dI) galaxies. The Hα luminosities of the detected H II regions range from some of the faintest detected in extragalactic H II regions (∼1035 ergs s-1 in SC 24) to some of the most luminous (∼1040 ergs s-1 in NGC 625). The total Hα luminosities are converted into current star formation rates (SFRs). Comparing the Sculptor group dI's to the Local Group dI's, we find that the Sculptor group dI's have, on average, lower values of SFR when normalized to either galaxy luminosity or gas mass (although there is considerable overlap between the two samples). The range for both the Sculptor group and Local Group samples is large when compared with that seen for the sample of gas-rich, quiescent, low surface brightness (LSB) dI's from van Zee et al. (published in 1997) and the sample of isolated dI's from van Zee (from 2000 and 2001). This is probably best understood as a selection effect since the nearby group samples have a much larger fraction of extremely low luminosity galaxies and the smaller galaxies are much more liable to large relative variations in current SFRs. The Sculptor group and LSB samples are very similar with regard to mean values of both τgas and τform, and the Local Group and isolated dI samples are also similar to each other in these two quantities. Currently, the Sculptor group lacks dI galaxies with elevated normalized current SFRs as high as the Local Group dI's IC 10 and GR 8. The properties of "transition" (dSph/dIrr) galaxies in Sculptor and the Local Group are also compared and found to be similar. The transition galaxies are typically among the lowest luminosities of the gas-rich dwarf galaxies. Relative to the dwarf irregular galaxies, the transition galaxies are found preferentially nearer to spiral galaxies and are found nearer to the center of the mass distribution in the local cloud. While most of these systems are consistent with normal dI galaxies, exhibiting temporarily interrupted star formation, the observed density-morphology relationship (which is weaker than that observed for the dwarf spheroidal galaxies) indicates that environmental processes such as "tidal stirring" may play a role in causing their lower SFRs.