Holes and Shells in the Interstellar Medium of the Nearby Dwarf Galaxy IC 2574

Abstract

We present HI synthesis observations of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) made with the NRAO Very Large Array at high spatial and velocity resolution (95 pc x 2.6 km/s). The VLA HI observations show a stunning amount of detail in the form of 48 mostly expanding HI shells and holes in its neutral interstellar medium. These features range in size from about 100 (a limit set by the size of the beam) to about 1000 pc dominating the appearance of the HI surface brightness map. Their dynamics clearly influence the velocity field of IC 2574. Current star formation, as traced by H-alpha emission, is predominantly found along the rims of the larger HI holes, suggesting propagating star formation. On linear scales of about 95 pc, star formation occurs if the HI surface density reaches values higher than 10^21 cm^-2. The scaleheight of the HI layer is found to be about 350 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed onedimensional velocity dispersion of about 7 km/s). A comparison with other galaxies shows that the energies needed to create the expanding HI structures are the same for all types of galaxies, at least to first order. The overall statistical properties of the HI holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities and ages.