ABSTRACT

We used optical spectra to investigate the nuclear regions of the starburst galaxies NGC 2782, NGC 4102 and NGC 6764. In addition to the central starburst, we find evidence for extranuclear shock-ionised gas, with kinematical properties consistent with outflow along the minor axis. The observations are consistent with the presence of dense shockionised shells, formed by the starburst-driven winds. The shells, observed in NGC 2782, NGC 4102 and NGC 6764 respectively, appear to be in a different evolutionary phase, which is explained by differences in age and strength of the central starbursts.

INTRODUCTION

Evolutionary models of starburst nuclei predict the formation of a thin, dense, shock-ionised shell, surrounding a hot cavity (e.g. Tomisaka and Ikeuchi 1988). The shell expands predominantly perpendicular to the galactic plane, gradually elongating until it finally breaks open at the top. The nearby starburst galaxies NGC 253 and M82 are probably examples of the broken-shell phase.

We investigated the starbust galaxies NGC 2782, NGC 4102 and NGC 6764 with the aid of long-slit spectra with high spatial resolution. All three galaxies have been classified previously as starburst galaxies, but the optical line ratios derived from 1-dimensional spectra put the galaxies close to the borderline between starbursts and LINERs in diagnostic diagrams.

RESULTS

From the spatial behaviour of the line ratios in our spectra we find, that the high line ratios are due to the presence of an extranuclear high-ionisation component in addition to the central starburst. The line ratios of the high-ionisation component are in agreement with shock-ionisation; its kinematical properties are consistent with outflow along the minor axis.