The Fates of the Circumgalactic Medium in the FIRE Simulations
  We analyze the different fates of the circumgalactic medium (CGM) in FIRE-2
cosmological simulations, focusing on the redshifts z=0.25 and z=2
representative of recent surveys. Our analysis includes 21 zoom-in simulations
covering the halo mass range Mh(z=0) ~ 10^10 - 10^12 Msun. We analyze both
where the gas ends up after first leaving the CGM (its "proximate" fate), as
well as its location at z=0 (its "ultimate" fate). Of the CGM at z=2, about
half is found in the ISM or stars of the central galaxy by z=0 in Mh(z=2) ~
5e11 Msun halos, but most of the CGM in lower-mass halos ends up in the IGM.
This is so even though most of the CGM in M_h(z=2) ~ 5e10 Msun halos first
accretes onto the central galaxy before being ejected into the IGM. On the
other hand, most of the CGM mass at z=0.25 remains in the CGM by z=0 at all
halo masses analyzed. Of the CGM gas that subsequently accretes onto the
central galaxy in the progenitors of Mh(z=0) ~10^12 Msun halos, most of it is
cool (T~10^4 K) at z=2 but hot (~Tvir) at z=0.25, consistent with the expected
transition from cold mode to hot mode accretion. Despite the transition in
accretion mode, at both z=0.25 and z=2 >~80% of the cool gas in Mh >~ 10^11
Msun halos will accrete onto a galaxy. We find that the metallicity of CGM gas
is typically a poor predictor of both its proximate and ultimate fates. This is
because there is in general little correlation between the origin of CGM gas
and its fate owing to substantial mixing while in the CGM.
