Observations of Outflowing OVI in the Dwarf Starburst Galaxy NGC 1705

We report F U SE far-UV spectroscopy of the prototypical dwarf starburst galaxy NGC 1705. These data allow us for the first time to directly probe the coronal-phase (T = few ×10 5 K) gas that may dominate the radiative cooling of the supernova-heated ISM and thereby determine the dynamical evolution of the starburst-driven outflows in dwarf galaxies. We detect a broad (∼100 km s −1 FWHM) and blueshifted (∆v = 97 km s −1) OV Iλ1032 absorption-line arising in the previously-known galactic outflow. The mass and kinetic energy in the outflow we detect is dominated by the warm (T ∼ 10 4 K) photoionized gas which is also seen through its optical line-emission. The kinematics of this warm gas are compatible with a simple model of the adiabatic expansion of a superbubble driven by the collective effect of the kinetic energy supplied by supernovae in the starburst. However, the observed properties of the OV I absorption in NGC 1705 are not consistent with the simple superbubble model, in which the OV I would arise in a conductive interface inside the superbubble's outer shell. The relative outflow speed of the OV I is too high and the observed column density (logN OV I = 14.3) is much too large. The lack of appreciable absorption at the systemic velocity along the line-of-sight implies that the superbubble has begun to blow out of the ISM of NGC 1705. During – 2 – this blow-out phase the superbubble shell accelerates and fragments. The resulting hydrodynamical interaction as hot outrushing gas flows between the cool shell fragments will create intermediate-temperature coronal gas that can produce the observed OV I absorption. For the observed flow speed of ∼ 10 2 km s −1 , the observed OV I column density is just what 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 ∼0.1 M ⊙ per year and ∼10 39 erg s −1 in the coronal gas. The latter represents <10% of the supernova heating rate. Independent of the assumed pressure, the lack of observed redshifted OV I emission from the backside of the outflow leads to upper limits on the cooling rate of ≤ 20% of the supernova heating rate. Since the X-ray luminosity of NGC 1705 is negligible, …