2019ApJ...879L...1V

Cosmic gas cycles in and out of galaxies, but outside of galaxies it is difficult to observe except for the absorption lines that circumgalactic clouds leave in the spectra of background quasars. Using photoionization modeling of those lines to determine cloud pressures, we find that galaxies are surrounded by extended atmospheres that confine the clouds and have a radial pressure profile that depends on galaxy mass. Motivated by observations of the universe's most massive galaxies, we compare those pressure measurements with models predicting the critical pressure at which cooler clouds start to precipitate out of the hot atmosphere and rain toward the center. We find excellent agreement, implying that the precipitation limit applies to galaxies over a wide mass range.