Physics of a partially ionized gas relevant to galaxy formation
  simulations -- the ionization potential energy reservoir
  Simulation codes for galaxy formation and evolution take on board as many
physical processes as possible beyond the standard gravitational and
hydrodynamical physics. Most of this extra physics takes place below the
resolution level of the simulations and is added in a sub-grid fashion.
However, these sub-grid processes affect the macroscopic hydrodynamical
properties of the gas and thus couple to the on-grid physics that is explicitly
integrated during the simulation. In this paper, we focus on the link between
partial ionization and the hydrodynamical equations. We show that the energy
stored in ions and free electrons constitutes a potential energy term which
breaks the linear dependence of the internal energy on temperature. Correctly
taking into account ionization hence requires modifying both the equation of
state and the energy-temperature relation. We implemented these changes in the
cosmological simulation code Gadget2. As an example of the effects of these
changes, we study the propagation of Sedov-Taylor shock waves through an
ionizing medium. This serves as a proxy for the absorption of supernova
feedback energy by the interstellar medium. Depending on the density and
temperature of the surrounding gas, we find that up to 50% of the feedback
energy is spent ionizing the gas rather than heating it. Thus, it can be
expected that properly taking into account ionization effects in galaxy
evolution simulations will drastically reduce the effects of thermal feedback.
To the best of our knowledge, this potential energy term is not used in current
simulations of galaxy formation and evolution.
