Catastrophic Cooling in Superwinds: Line Emission and Non-equilibrium
  Ionization
  Outflows are a pervasive feature of mechanical feedback from super star
clusters (SSC) in starburst galaxies, playing a fundamental role in galaxy
evolution. Observations are now starting to confirm that outflows can undergo
catastrophic cooling, suppressing adiabatic superwinds. Here we present a suite
of one-dimensional, hydrodynamic simulations that study the ionization
structure of these outflows and the resulting line emission generated by the
cooling gas. We use the non-equilibrium atomic chemistry package within MAIHEM,
our modified version of FLASH, which evolves the ionization state of the gas
and computes the total cooling rate on an ion-by-ion basis. We find that
catastrophically cooling models produce strong nebular line emission compared
to adiabatic outflows. We also show that such models exhibit non-equilibrium
conditions, thereby generating more highly ionized states than equivalent
equilibrium models. When including photoionization from the parent SSC,
catastrophically cooling models show strong C IV {\lambda}1549 and O VI
{\lambda}1037 emission. For density bounded photoionization, He II
{\lambda}1640, {\lambda}4686, C III] {\lambda}1908, Si IV {\lambda}1206, and Si
III {\lambda}1400 are also strongly enhanced. These lines are seen in extreme
starbursts where catastrophic cooling is likely to occur, suggesting that they
may serve as diagnostics of such conditions. The higher ionization generated by
these flows may help to explain line emission that cannot be attributed to SSC
photoionization alone.
