O VI EMISSION IMAGING of A GALAXY with the HUBBLE SPACE TELESCOPE: A WARM GAS HALO SURROUNDING the INTENSE STARBURST SDSS J115630.63+500822.1

We report results from a new Hubble Space Telescope campaign that targets the O vi λλ1032, 1038 Å doublet in emission around intensely star-forming galaxies. The program aims to characterize the energy balance in starburst galaxies and gas cooling in the difficult-to-map coronal temperature regime of K. We present the first resolved image of gas emission in the O vi line. Our target, SDSS J115630.63+500822.1, is very compact in the continuum but displays O vi emission to radii of 23 kpc. The surface brightness profile is well fit by an exponential with a scale length of 7.5 kpc. This is 10 times the size of the photoionized gas, and we estimate that about 1/6 the total O vi luminosity comes from resonantly scattered continuum radiation. Spectroscopy - which closely resembles a stacked sample of archival spectra - confirms the O vi emission, and determines the column density and outflow velocity from blueshifted absorption. The combination of measurements enables a large number of calculations with few assumptions. The O vi regions fill only of the volume. By comparing the cooling time with the cloud sound-crossing time, the cooling distance with the size, and the pressure in the O vi and nebular gas, we conclude that the O vi-bearing gas cannot have been lifted to the scale height at this temperature, and must be cooling in situ through this coronal temperature regime. The coronal phase contains ∼1% of the ionized mass, and its kinetic energy at a given instant is ∼1% of the budget set by supernova feedback. However, a much larger amount of the gas must have cooled through this phase during the star formation episode. The outflow exceeds the escape velocity and the gas may become unbound, but it will recombine before it escapes and become visible to Lyman (and O i) spectroscopy. The mapping of this gas represents a crucial step in further constraining galaxy formation scenarios and guiding the development of future astronomical satellites.