Gas Cooling in Hydrodynamic Simulations with An Exact Time Integration
  Scheme
  We implement and test the exact time integration method proposed by Townsend
2009 for gas cooling in cosmological hydrodynamic simulations. The errors using
this time integrator for the internal energy are limited by the resolution of
the cooling tables and are insensitive to the size of the timestep, improving
accuracy relative to explicit or implicit schemes when the cooling time is
short. We compare results with different time integrators for gas cooling in
cosmological hydrodynamic simulations. We find that the temperature of the gas
in filaments before accreting into dark matter halos to form stars, obtained
with the exact cooling integration, lies close to the equilibrium where
radiative cooling balances heating from the UV background. For comparison, the
gas temperature without the exact integrator shows substantial deviations from
the equilibrium relation. Galaxy stellar masses with the exact cooling
technique agree reasonably well, but are systematically lower than the results
obtained by the other integration schemes, reducing the need for feedback to
suppress star formation. Our implementation of the exact cooling technique is
provided and can be easily incorporated into any hydrodynamic code.
