OVI emission in the halos of low redshift star-forming galaxies

We report results of ultraviolet imaging of four low-redshift (z ~ 0.25) star-forming galaxies with the Hubble Space Telescope. Using synthetic narrowband imaging we isolate emission from log(T[K])=5.5 gas in and around galaxies by isolating the OVI1032,1038Å doublet. Only one of the galaxies shows significant OVI emission, and we place upper limits on the surface brightness less than 8 erg s^-1 cm^-2 arcsec^-2 (3σ) for the remaining three SFGs. Ultraviolet spectroscopy of the brightest star-forming regions of the galaxies shows the λ ~ 1000 Å continuum to be well-detected, and the OVI doublet is detected in absorption in three of the four galaxies, showing unequivocally that the gas is present. We derive column densities of log(N_OVI[cm^-2]) ~ 14.5 in each case, and outflow velocities from the first velocity moment of the absorption of 200-300 km s^-1. Using the measured N_OVI in combination with limits on the surface brightness in emission, we place further limits on the physical density of electrons (n_e < 1.2 cm^-3) and the column lengths of the OVI-bearing warm gas (D_OVI ~ 20 pc). We contrast these results with those of Otte et al. (2003) and Hayes et al. (2016), where circumgalactic OVI was detected in emission. We conclude that radiative cooling from the warm phase is a minor energetic drain on the winds from starburst galaxies. We report various other properties of the sample based upon nebular line emission in the optical and modeling of the stellar spectral energy distribution, and discuss under what conditions a galaxy may display significant OVI emission.