SOFT-X-RAY EMISSION FROM NGC-5253 AND THE IONIZED INTERSTELLAR-MEDIUM

A new ROSAT PSPC observation of the gas-rich dwarf galaxy NGC 5253 reveals an extended, soft X-ray source near the center of the galaxy. The pulse-height spectrum is well fit with a Raymond-Smith model for a thermal plasma at a temperature of (4.0 +/- 1.3) x 10(6) K for a foreground absorbing column of log N-H = 21.4 cm(-2). The X-ray luminosity between 0.1 and 2.4 keV is L(x) = (6.5(-1.6)(+5.8)) x 10(38) ergs s(-1). Higher resolution X-ray imaging is needed to determine the contribution to L(x) from point sources. However, we argue that hot interstellar gas produces most of the X-ray emission and investigate whether stellar winds and supernovae in the central starburst can heat the required mass of X-ray-emitting gas. We find the simplest superbubble model underpredicts L(x) by a factor of 16 but show that a superbubble evolving in a clumpy ambient medium can easily reproduce the observed X-ray properties. Our model suggests the most recent burst is young (similar to 5 x 10(6) yr) and requires evaporating clouds or filaments to fill a small fraction of the nebula. To provide additional insight into the dynamics of the interstellar medium (ISM) in NGC 5253, we present results from H alpha imaging and follow-up high-resolution spectroscopy of the H alpha emission line. The spectacular filaments extending from the galaxy in the H alpha image do not show unusually high velocities; in the outer part of the nebula, the velocity of the gas producing the H alpha emission shifts randomly by +/- 10-30 km s(-1) over scales of similar to 100-200 pc. The complex velocity field in the core can be interpreted as a slow, global expansion of the warm ionized medium coincident with the region of X-ray emission. We discuss the relation of the hot and warm phases of the ISM in NGC 5253 and draw analogies to giant H II regions and superwind galaxies.