FUSE observations of outflowing OV1 in the dwarf starburst galaxy NGC 1705

We report FUSE far-UV spectroscopy of the prototypical dwarf starburst gala xy NGC 1705. These data allow us for the first time to directly probe the c oronal-phase (T = a few times 10(5) K) gas that may dominate the radiative cooling of the supernova-heated interstellar medium (ISM) and thereby deter mine the dynamical evolution of the starburst-driven outflows in dwarf gala xies. We detect a broad (similar to 100 km s(-1) FWHM) and blueshifted (Del ta upsilon = 77 km s(-1)) O VI lambda 1032 absorption line arising in the p reviously known galactic outflow. The mass and kinetic energy in the outflo w we detect is dominated by the warm (T similar to 10(4) K) photoionized ga s which is also seen through its optical line emission. The kinematics of t his warm gas are compatible with a simple model of the adiabatic expansion of a superbubble driven by the collective effect of the kinetic energy supp lied by supernovae in the starburst. However, the observed properties of th e O VI absorption in NGC 1705 are not consistent with the simple superbubbl e model, in which the O VI would arise in a conductive interface inside the superbubble's outer shell. The relative outflow speed of the O VI is too h igh and the observed column density is much too (log N-OVI = 14.3) large. W e argue that the superbubble has begun to blow out of the ISM of NGC 1705. During this blowout phase the superbubble shell accelerates and fragments. The resulting hydrodynamical interaction as hot outrushing gas flows betwee n the cool shell fragments will create intermediate-temperature coronal gas that can produce the observed O VI absorption. For the observed flow speed of similar to 10(2) km s(-1), the observed O VI column density is just wha t is expected for gas that has been heated and which then cools radiatively . Assuming that the coronal-phase gas is in rough pressure balance with the warm photoionized gas, we estimate a cooling rate of order similar to0.1 M -circle dot yr(-1) and similar to 10(39) ergs s(-1) in the coronal gas. The latter represents less than 10% of the supernova heating rate. Independent of the assumed pressure, the lack of observed redshifted O VI emission fro m the back side of the outflow leads to upper limits on the cooling rate of less than or equal to 20% of the supernova heating rate. Since the X-ray l uminosity of NGC 1705 is negligible, we conclude that radiative losses are insignificant in the outflow. The outflow should therefore be able to fully blow out of the ISM of NGC 1705 and vent its metals and kinetic energy. Th is process has potentially important implications for the evolution of dwar f galaxies and the intergalactic medium.