  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.
