We investigate the impact of the far ultraviolet ( FUV ) heating rate on the stability of the three-phase interstellar medium using three-dimensional simulations of a 1 \mathrm { kpc } ^ { 2 } , vertically-extended domain .
The FUV heating rate sets the range of thermal pressures across which the cold ( \sim 10 ^ { 2 } \textrm { K } ) and warm ( \sim 10 ^ { 4 } \textrm { K } ) neutral media ( CNM and WNM ) can coexist in equilibrium .
Even absent a variable star formation rate regulating the FUV heating rate , the gas physics keeps the pressure in the two-phase regime : because radiative heating and cooling processes happen on shorter timescales than sound wave propagation , turbulent compressions tend to keep the interstellar medium within the CNM-WNM pressure regime over a wide range of heating rates .
The thermal pressure is set primarily by the heating rate with little influence from the hydrostatics .
The vertical velocity dispersion adjusts as needed to provide hydrostatic support given the thermal pressure : when the turbulent pressure \langle \rho \rangle \sigma _ { z } ^ { 2 } is calculated over scales \gtrsim 500 \textrm { pc } , the thermal plus turbulent pressure approximately equals the weight of the gas .
The warm gas volume filling fraction is 0.2 < f _ { w } < 0.8 over a factor of less than three in heating rate , with f _ { w } near unity at higher heating rates and near zero at lower heating rates .
We suggest that cosmological simulations that do not resolve the CNM should maintain an interstellar thermal pressure within the two-phase regime .