1999AJ....118..273W

We present H I synthesis observations of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) made with the NRAO(2) Very Large Array (VLA) in its B, C, and D configurations at high spatial and velocity resolution (95 pc x 2.6 km.s^–1). In addition, we present optical broad- and narrowband images obtained with the 2.2 m telescope of the Calar Alto Observatory. The VLA H I observations show a stunning amount of detail in the form of 48 mostly expanding H I shells and holes in its neutral interstellar medium. These features range in size from about 100 pc (a limit set by the size of the beam) to about 1000 pc, dominating the appearance of the H I surface brightness map. Their dynamics clearly influence the velocity field of IC 2574. In addition to well defined holes, some large scale (>1000 pc) coherent features are visible in the channel maps. They are probably the remainder of an older shell population. Current star formation, as traced by Hα emission, is predominantly found along the rims of the larger H I holes, suggesting propagating star formation. On linear scales of ~95 pc, star formation occurs if the H I surface density reaches values higher than 10²¹ cm^–2. The radial expansion of the H I holes (8-12 km.s^–1), the indicative ages (10-60 Myr) and the energy requirements for their formation (10⁵⁰-10⁵³ ergs) can be understood in terms of the combined effects of stellar winds and multiple supernova explosions of the most massive stars formed during a recent phase of active star formation.

The scale height of the H I layer is found to be ~350 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed one-dimensional velocity dispersion of ~7 km.s^–1). This puffed-up disk implies a lower H I volume density (0.15 cm^–3). This fact, combined with the reduced gravitational pull and solid-body rotation throughout the disk, explains why the diameter distribution in dwarf galaxies extends to substantially larger values than in spiral galaxies. A comparison with other galaxies shows that the energies needed to create these structures are the same for all types of galaxies, at least to first order. The overall statistical properties of the H I holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities, and ages.