  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.
