Radiative cooling of swept up gas in AGN-driven galactic winds and its
  implications for molecular outflows
  We recently used hydro-chemical simulations to demonstrate that molecular
outflows observed in luminous quasars can be explained by molecule formation
within the AGN wind. However, these simulations cover a limited parameter
space, due to their computational cost. We have therefore developed an analytic
model to follow cooling in the shocked ISM layer of an AGN wind. We explore
different ambient densities ($1-10^{4} \, \rm{cm}^{-3}$), density profile
slopes ($0-1.5$), AGN luminosities ($10^{44}-10^{47} \, \rm{erg} \,
\rm{s}^{-1}$), and metallicities ($0.1-3 \rm{Z}_{\odot}$). The swept up gas
mostly cools within ~1 Myr. Based on our previous simulations, we predict that
this gas would produce observable molecular outflows. The instantaneous
momentum boost initially increases as the outflow decelerates. However, it
reaches a maximum of $\approx$20, due to work done against the gravitational
potential. The predicted time-averaged observational estimate of the molecular
outflow momentum boost reaches a maximum of $\approx1-2$, partly due to our
assumed molecular fraction, 0.2, but also because the instantaneous and
observational, time-averaged definitions are not equivalent. Thus recent
observational estimates of order unity momentum boosts do not necessarily rule
out energy-driven outflows. Finally, we find that dust grains are likely to
re-form by accretion of metals after the shocked ISM layer has cooled, assuming
that a small fraction of dust grains swept up after this layer has cooled are
able to mix into the cool phase, and assuming that grain growth remains
efficient in the presence of the strong AGN radiation field. This would enable
rapid molecule formation, as assumed in our models.
