Star Formation and the Interstellar Medium in Four Dwarf Irregular Galaxies

We present new, high sensitivity VLA observations of HI in four dwarfgalaxies (UGCA 292, GR8, DDO 210, and DDO 216) and we use these data to studyinteractions between star formation and the interstellar medium. HI velocitydispersions and line shapes in UGCA 292, GR8, and DDO 210 show that these threegalaxies contain both warm and cool or cold HI phases. The presence of the coldneutral medium is indicated by a low-dispersion (3--6 km/s) HI component or bythe Gauss-Hermite shape parameter h_4 > 0. Contrary to expectations, we find notrend between the incidence of the low-dispersion (colder) phase and the starformation rate in five dwarf galaxies. The colder HI phase may be a necessaryingredient for star formation, but it is clearly not sufficient. However, thereis a global trend between the star formation rate of a galaxy and the incidenceof asymmetric HI profiles. This trend probably reflects kinetic energy inputfrom young massive stars. Numerical simulations show that the effects ofrotational broadening (finite angular resolution) are minimal for thesegalaxies. Simulations are also used to estimate the errors in the columndensities of the high-dispersion and the low-dispersion HI phases.Comment: Accepted by Ap