Spectroscopic study of starburst galaxies

We present spectroscopic results in optical regions on the fourteen supergiant HII regions of the galaxies with UV-excess continua. Emission-line properties (relative intensity ratios and equivalent widths) deduced from these data are compared with the nuclear starburst regions (NSBRs) and the disk starburst regions (DSBRs). The DSBRs show both higher excitation conditions and poorer metal abundances than the NSBRs. It is also shown that the NSBRs mainly contain less powerful ionizing stars such as late O- and/or early B-type stars, while the DSBRs contain more powerful ionizing stars such as early O-type ones. The observed steep Balmer decrements in the DSBRs may be mainly due to the internal reddening by dust grains in the burst regions rather than to the effect of stellar Balmer absorption. From the observational differences between the NSBRs and the DSBRs, it is suggested that there are different modes of starburst activity according to the difference in the physical conditions of the pre-starburst gas. Accordingly, the observational characteristics of the starburst activity have more significant relation to the physical conditions of the regions where the burst occurs than to the global parameters of the underlying galaxies. (author)