BLUE LOW SURFACE-BRIGHTNESS GALAXIES .2. SPECTROSCOPY AND CHEMICAL ABUNDANCES

In our continued search for young/unevolved galaxies at low redshift we have obtained spectroscopic observations of 16 blue low surface-brightness galaxies. These galaxies are among the bluest and dimmest objects found in the ESO-Uppsala Catalogue. Oxygen abundances are derived in 24 HII regions. They are at the lower boundary found for galaxies, 517% of the solar value. The nitrogen abundance relative to oxy gen, N/O, derived in 11 HII regions, varies between 0.025 and 0.048. The constancy of N/O below 12+log(O/H) similar to 8 strongly supports a primary origin for nitrogen. Special attention is devoted to the correction for underlying absorption in the Balmer lines as deduced from spectral evolutionary models. We discuss the discrepancy between the abundances derived with the standard and bright line methods, and show that part of it could be accounted for if the photoionized gas is mixed with a shock heated component.