Properties of Ionized Interstellar Froth in Irregular Galaxies
Abstract
Results are reported from new observations of large-scale Hα filamentary structures lying outside of classical HII regions in Magellanic irregular galaxies. These structures, which we refer to as froth, are seen as arcs, loops, and filaments, some of which are centered on OB associations and some of which extend away from giant H II regions into regions of lower H I column density. The data consist of measurements of Hα intensities from narrow-band CCD images, long-slit digital spectra from which relative emission ratios have been determined, and high-velocity resolution echelle spectra that yield information on the internal kinematics of these structures. The data show the following: (1) the level of ionization of the froth is lower than in H II regions in the same galaxies, with [S II], [N II], and [O II] relative to Hα being enhanced while [O III]/Hβ is reduced in the froth. (2) Frothy features have large line velocity widths that often exceed those found in classical giant H II regions in the same galaxies. (3) The froth has Hα surface brightnesses of ~100 rayleighs; 10-100 times brighter than typical diffuse Galactic emission. (4) Dimensions are large; a typical filament is 700 x 50 pc. The ionization mechanism for the froth is a puzzle. The absence of strong [O I] emission rules out pure shock models. However, pure photoionization seems implausible in many cases given the large distances (0.2-0.8 kpc) between the nearest concentration of OB stars and the ends of the filaments. A combination of shock and photoionization may best explain these structures, but serious problems remain. The origins of ionized froth are discussed in the context of the evolution of supershells produced by luminous OB associations. However, it is not clear that this standard mechanism can explain the full range of observed frothy structures, and alternative models must be considered. Connections between these types of features in the interstellar medium and feedback processes which could effect star formation rates on large scales are briefly discussed.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1990
- DOI:
- 10.1086/169286
- Bibcode:
- 1990ApJ...362..480H
- Keywords:
-
- Galactic Structure;
- H Ii Regions;
- Interstellar Matter;
- Magellanic Clouds;
- Astronomical Spectroscopy;
- Charge Coupled Devices;
- H Alpha Line;
- H I Regions;
- Photoionization;
- Star Formation;
- Stellar Luminosity;
- Astrophysics;
- GALAXIES: INTERSTELLAR MATTER;
- NEBULAE: H II REGIONS;
- RADIATION MECHANISMS