The spectacular ionized interstellar medium of NGC 55

We present deep H alpha+[N II], [S II] (lambda lambda 6716, 6731 Angstrom) and [O II] (lambda lambda 3726, 3729 Angstrom) images of the highly inclined, actively star-forming SBm galaxy NGC 55, located in the nearby Sculptor Group. Due to its proximity, NGC 55 provides a unique opportunity to study the disk-halo interface in a late-type galaxy with unprecedented spatial resolution. Our images reveal a spectacular variety of ionized gas features, ranging from giant H II region complexes, to supergiant filamentary and shell features, to patches of very faint diffuse emission. Many of these features protrude well above the plane of the galaxy, including a very faint fragmented shell of emission projected at 2.6 kpc above the disk. We identify candidate 'chimneys' extending out of the disk, which could be the conduits into the halo for hot gas around disk star-forming regions, and could also provide low-density paths for the passage of UV photons from the disk to the halo. Several of the identified chimneys are 'capped' with clumps of ionized gas, one of which, located at 1.5 kpc above the disk plane, appears to be the site of recent star formation. Emission-line ratios ([O II]/H alpha+[N II], [S II]/H alpha+[N II]) constrain the ionization mechanism of the gas, and our images allow the first measurement of [O II]/H alpha+[N II] in extra-planar diffuse ionized gas. The diffuse gas is characterized by emission-line ratios which are enhanced on average by a factor of 2 compared to those of bright H II regions. Each line ratio increases in value smoothly from the cores of H II regions, through the haloes of H II regions, into the diffuse ionized gas. Such a continuous trend is predicted by models in which the diffuse gas is ionized by photons produced by distant OB stars. We discuss the factors which may determine the existence of extra-planar ionized gas in galaxies. (C) 1996 American Astronomical Society.