Galaxy Evolution: Modeling the Role of Non-thermal Pressure in the
  Interstellar medium
  Galaxy evolution depends strongly on the physics of the interstellar medium
(ISM). Motivated by the need to incorporate the properties of the ISM in
cosmological simulations we construct a simple method to include the
contribution of non-thermal components in the calculation of pressure of
interstellar gas. In our method we treat three non-thermal components -
turbulence, magnetic fields and cosmic rays - and effectively parametrize their
amplitude. We assume that the three components settle into a quasi-steady-state
that is governed by the star formation rate, and calibrate their magnitude and
density dependence by the observed Radio-FIR correlation, relating synchrotron
radiation to star formation rates of galaxies. We implement our model in single
cell numerical simulation of a parcel of gas with constant pressure boundary
conditions and demonstrate its effect and potential. Then, the non-thermal
pressure model is incorporated into RAMSES and hydrodynamic simulations of
isolated galaxies with and without the non-thermal pressure model are presented
and studied. Specifically, we demonstrate that the inclusion of realistic
non-thermal pressure reduces the star formation rate by an order of magnitude
and increases the gas depletion time by as much. We conclude that the
non-thermal pressure can prolong the star formation epoch and achieve
consistency with observations without invoking artificially strong stellar
feedback.
