Long-slit spectroscopic observations have been made for two starburst galaxies, Henize 2-10 and Markarian 52, in order to discuss physical conditions of the starburst regions in the galactic nuclei. We obtained the spatial variations of physical conditions of the ionized gas, such as Hα surface brightness, electron density, and ionization/excitation conditions. In particular we found an anti-correlation in the spatial variation between the [OIII]/Hβ ratio and the [SII]/Hα and [OI]/Hα ratios in each galaxy. This anti-correlation suggests that the nuclear starburst region consists of a central massive-star cluster and a single envelope of ionized gas.

Which structure do starburst regions have?

One of the most important issues on starburst galaxies has been the typical size of the distribution of ionizing stars. Although it is well known that ionized gas in starburst galaxies extends over a few hundred pc to one kpc, the distribution of ionizing stars themselves is still a controversial issue (e.g. Sugai & Taniguchi 1992; Puxley et al. 1990). There are two models for the structure of starburst regions. In one model, ionizing stars exist in a central cluster with its surrounding ionized nebula (Figure la). This structure is just like that of one giant/supergiant HII region, such as NGC 604 in M33 and 30 Doradus in LMC. In the other model, on the other hand, ionizing stars are distributed over the whole starburst region, with individual ionized regions (Figure 1b).