An Emission-line Study of Supergiant Ionized Filaments in Irregular Galaxies^*

We present low- and high-dispersion emission-line observations of kiloparsec-sized, low surface brightness filaments of ionized gas in four irregular galaxies. These four galaxies span a range in sizes and star formation rates. In Sextans A, deep Hα images show that the filaments are part of a complicated shell of radius 600 pc around the eastern HII complex. In NGC 2366 there are several filaments emanating from the supershell around the giant HII region NGC 2363, extending up to 1.3 kpc from its center. In NGC 1569 we examine the brightest filament which is 800 pc in length. In NGC 4449 several filaments in the outer part of the galaxy appear to form a coherent structure of radius 1.7 kpc. However, part of the filaments fall along the inner edge of a 2 kpc diameter H I supershell. At the base of this structure near the center of the galaxy ionized gas is expanding at 30 km s^-1.

A comparison with models for the formation of large shells suggests that the filament in NGC 1569 and the shell in Sextans A are fit reasonably with standard stellar-powered supershell models of order 10 Myr old. The filaments in NGC 2366, on the other hand, are sufficiently far from the giant H II region NGC 2363 that they would have to be older than the age of the massive stars in the H II region and so probably are the result of a previous star-forming event. In NGC 4449 we are probably seeing several supershell events, one being the H I supershell with the filaments as the ionized inner edge and the other seen only in Hα. The structures in NGC 4449 may be too large to be entirely in the plane of the galaxy. The FWHM of the velocity profiles of the filaments are surprisingly high and are similar to those seen in normal H II regions in the same galaxies.

Emission-line ratios show that the shells and filaments of all size scales are primarily photoionized. Since the supergiant filaments are located large distances from concentrations of massive stars, this implies that ionizing photons are able to travel distances of order kiloparsecs or that a diffuse source of ionizing photons exists, even in the outer parts of galaxies. Large distance propagation of ionizing radiation can most readily occur in cases such as NGC 1569 where filaments are above the Galactic plane, but long path lengths are still required. The Hα luminosities imply that the percentage of photons that must escape H II regions is about 25%. The large velocity FWHM of the emission-line profiles of supergiant filaments are understood in a photoionization model as a result of the high velocity dispersion in the neutral gas.

These data show that energy input from newly created massive stars plays an important role in modifying the ISM of small galaxies and that these effects include both organized gas flows and photoionization over kiloparsec-sized distances. This process is expected to be important in regulating star formation in small, low-mass irregular galaxies.