Long slit spectroscopy of diffuse ionized gas in NGC 55

Spectroscopic measurements of emission line ratios and velocities are prese nted for ionized gas across the central plane of NGC 55, a late-type galaxy in the Sculptor group. The low metallicity in NGC 55 leads to relatively l ow line ratios of [S II]/H alpha and [N II]/H alpha for HII regions as well as the diffuse ionized gas. These are the first spectroscopical measuremen ts of line ratios in ionized gas exterior to the stellar disc of NGC 55. An alysis of the line ratios and the relative velocities of different features suggests that photoionization is a plausible explanation for the ionizatio n of this diffuse gas. The observed shell structures and the corresponding velocities support the idea of diffuse gas being pushed into the halo by su pernova explosions and stellar winds.