Circumgalactic Gas and the Precipitation Limit
  During the last decade, numerous and varied observations, along with
increasingly sophisticated numerical simulations, have awakened astronomers to
the central role the circumgalactic medium (CGM) plays in regulating galaxy
evolution. It contains the majority of the baryonic matter associated with a
galaxy, along with most of the metals, and must continually replenish the star
forming gas in galaxies that continue to sustain star formation. And while the
CGM is complex, containing gas ranging over orders of magnitude in temperature
and density, a simple emergent property may be governing its structure and
role. Observations increasingly suggest that the ambient CGM pressure cannot
exceed the limit at which cold clouds start to condense out and precipitate
toward the center of the potential well. If feedback fueled by those clouds
then heats the CGM and causes it to expand, the pressure will drop and the
"rain" will diminish. Such a feedback loop tends to suspend the CGM at the
threshold pressure for precipitation. The coming decade will offer many
opportunities to test this potentially fundamental principle of galaxy
evolution.
