FUSEObservations of Outflowing Oviin the Dwarf Starburst Galaxy NGC 1705

We report FUSE far-UV spectroscopy of the prototypical dwarf starburst galaxyNGC 1705. These data allow us for the first time to probe the coronal-phase gas(T = 10E5 to 10E6 K) that may dominate the radiative cooling of thesupernova-heated ISM and thereby determine the dynamical evolution ofstarburst-driven outflows. We detect a broad (100 km/s) and blueshifted (by 80km/s) OVI absorption-line arising in the previously-known galactic outflow. Theproperties of the OVI absorption are inconsistent with the standard superbubblemodel in which this gas arises in a conductive interface inside the outershell. We show that the superbubble in NGC 1705 is blowing out of the galaxyISM. During blow-out, coronal-phase gas can be created by hydrodynamical mixingas hot gas rushes out through fissures in the fragmenting shell of cool gas. Asthe coronal gas cools radiatively, it can naturally produce the observed OVIcolumn density and outflow speed. The OVI data show that the cooling rate inthe coronal-phase gas is less than about 10% of the supernova heating rate.Since the X-ray luminosity from hotter gas is even smaller, we conclude thatradiative losses are insignificant. The outflow should be able to vent itsmetals and kinetic energy out of the galaxy. This process has potentiallyimportant implications for the evolution of dwarf galaxies and the IGM.Comment: ApJ (in press