The highly inclined very late type galaxy NGC 55 is a perfect target to study the effects of massive star formation on the interstellar medium, especially in the disk-halo interface.

We present deep broad- and narrow-band images of a field centered on the two largest HII regions of NGC 55. The color-magnitude diagrams produced from broad band V, R, i, and z images show the effects of a strong burst of recent star formation and imply the presence of a population of blue clusters which show up as clearly exdended sources on our best seeing images.

In the Hα image a spectacular arrangement of filaments, shells and supergiant shells is visible. This ionized gas at distances of up to 1 kpc from the nearest star-forming region makes NGC 55 a new example of a galaxy in ‘chimney mode’.

Supergiant shells in irregular galaxies

HII supergiant shells (SGSs) in irregular galaxies were first detected in the Magellanic Clouds on deep Hα plates taken with the UK Schmidt telescope (Goudis & Meaburn 1978). Thereafter, more and more such structures were detected in nearby irregular galaxies using modern detectors (e.g. Hunter & Gallagher 1990; Bomans & Hopp 1992; Hunter, Hawley & Gallagher 1993). The objects seem to be related to active star formation and HI holes, but the pattern is far from being understood.