Cooling in coronal gas in the M82 starburst superwind

We have used the Far Ultraviolet Spectroscopic Explorer to search for O VI λ1031.926 emission at four locations in the starburst superwind of M82. No O VI emission was detected at any of the four pointings, with upper limits less than or equal to the 0.3-2 keV X-ray flux. These observations limit the energy lost through radiative cooling of coronal phase (T ∼ 10 ^5.5 K) gas to roughly the same magnitude as that lost in the hot phase through X-ray emission, which has been shown to be small. The wind material retains most of its energy and should be able to escape from the gravitational potential of M82, enriching the intergalactic medium with energy and metals. The lack of coronal gas in the wind and observations of spatially correlated X-ray and Hα emission are consistent with a scenario in which the hot wind material overruns cold clouds in the halo or with one in which the Hα and X-ray emission arise at the interface between the hot wind and a cool shell of swept-up interstellar medium, as long as the shock velocity is ≲150 km s^-1. The observed limits on the O VI/Hα and C III/Hα flux ratios rule out shock heating as the source of the T = 10 ⁴ K gas unless the shock velocity is ≲90 km s^-1.