Abundances in southern Local Group dwarf irregular galaxies⋆

Abstract

We have obtained optical spectrophotometry of H II regions in the southern Local Group dwarf irregular galaxies NGC 6822, WLM and SagDIG. Oxygen abundances are deduced via direct electron temperatures for NGC 6822 and WLM and via an empirical method for all three galaxies. These galaxies conform to the luminosity–abundance relationship for irregular galaxies. Although WLM shows a relatively low abundance, we find it unlikely that this is the cause of the lack of long-period Cepheid variables as suggested by Sandage & Carlson. It is more probable that WLM has experienced an interrupted star formation history, as is often predicted for dwarf galaxies. The relatively low value of N/O in NGC 6822 may be consistent with the 108-yr old burst of star formation noted by Hodge. Observations of very metal-poor galaxies like SagDIG may play a key role in understanding the N/O versus O/H relationship for dwarf irregular galaxies.