The stellar populations and evolution of H II galaxies - I. High signal-to-noise optical spectroscopy.

High signal-to-noise spectroscopic data are presented for 32 selected H II galaxies, most of which have not previously been published. All objects are metal-poor, with gaseous oxygen abundances from about 0.05 to 0.5 of the solar neighborhood value. The nitrogen-to-oxygen abundance ratio has a mean value of 0.036, typical of H II and blue compact galaxies, and is uncorrelated with the oxygen abundance, consistent with most of the nitrogen being of primary origin. Relationships amongst emission-line parameters demonstrate the importance of the temperature of the ionizing cluster T(ion), in determining the spectroscopic properties of H II galaxies. T(ion) increases systematically with decreasing oxygen abundance in the youngest objects, indicating that the initial mass function in H II galaxies may be abundance-sensitive. The scatter in emission-line parameters is interpreted as at a given oxygen abundance, as evolution of the ionizing clusters. Several H II galaxies exhibit Wolf-Rayet features indicative of the presence of several 10,000 WN stars. The lowest-abundance objects have narrow He II emission at the few percent of H-beta level. Low abundance H II galaxies may contain less dust.