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Session 65 - ISM in Nearby Galaxies.
Oral session, Tuesday, January 16
Salon del Rey South, Hilton

[65.04] The Dynamical and Physical State of the ISM in Dwarf Galaxies

C. L. Martin (Steward Obs., U. Arizona)

Supernovae and stellar winds drive a gamut of dynamical activity in the interstellar medium (ISM) ranging from isolated supernova remnants to galactic-scale outflows. This feedback between star formation and the interstellar medium (ISM) plays an important but poorly understood role in the formation, chemical evolution, and star formation history of galaxies. In particular, the fate of a dwarf galaxy may be determined by the global response of the ISM to the first generation of stars. The low escape velocity, low metallicity, and lack of differential rotation in dwarfs facilitate the development of a galactic wind; and it has been suggested on theoretical grounds that the ensuing mass loss may regulate the bursting period in dwarfs or even halt star formation entirely.

We present preliminary results from an observational thesis designed to quantify this feedback in 15 nearby ``dwarf'' galaxies (-18.5 < M_B < -13.5). Deep emission-line images reveal a wealth of structure in the ionized gas which often extends beyond the continuum emission. We use high-resolution spectroscopy to demonstrate that many of these arcs and filaments are associated with expanding supergiant shells of warm, ionized gas. A hot component of interstellar gas inside these shells is believed to drive their expansion, and we have used ROSAT observations to detect the X-ray emission from the most energetic bubbles. We discuss the star formation history and chemical evolution of dwarf galaxies using simple dynamical models for the observed gas kinematics.

The violent gas dynamics may also play an important role in shaping the physical state of the ISM. Much of the emission-line flux from these galaxies is emitted by diffuse ionized gas (DIG) with an emission-line spectrum distinctly different from that of the HII regions. We have obtained longslit moderate-resolution spectra for the sample and will describe the variations in the DIG's signature within these galaxies. Our objectives are to determine the dominant excitation mechanism of the DIG and the DIG's contribution to the integrated spectra of the galaxies.

Program listing for Tuesday