The Hidden Mass and Large Spatial Extent of a Poststarburst Galaxy Outflow

Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, but key aspects of these outflows have not been probed with observations. Using ultraviolet absorption spectroscopy, we show that "warm-hot" plasma at 10^{5.5} K contains 10-150 times more mass than the cold gas in a poststarburst galaxy wind. This wind extends to distances >68 kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of the cold matter with a hotter (unseen) phase at >>10^{6} K. Such multiphase winds can remove substantial masses and alter the evolution of poststarburst galaxies.