Vertical pressure balance

I showed that vertical pressure balance is achieved and that it explains the origin of the KS relation in the FIRE simulations. ADS link

If the gas in galaxies were allowed to collapse freely it would be entirely converted into stars within just a few hundred million years (i.e. 100% of the gas would be converted to stars per free-falll time). Instead, we see that stars form very inefficiently over the course of billions of years. This inefficiency is parameterized by the observed Kennicutt-Schmidt (KS) relation, a well known observational fact that the surface density of molecular gas and the star formation rate surface density in star forming galaxies is tightly correlated. The normalization of the KS relation reveals that star formation proceeds very inefficiently, at a rate of approximately 1-2% per free-fall time. However, the physics by which star formation is regulated to this inefficient state are uncertain.

One class of “dynamical equilibrium” models posits that the star formation rate is connected to the gas surface density by means of pressure. One can derive the KS relation (with both the correct slope and normalization) by equating the weight of the overlying gas at the midplane to the pressure generated by stellar feedback from recent star formation. One benefit of this class of models over other explanations for the KS relation is that they naturally predict variation in the slope of the KS relation (a poorly understood but observed phenomenon) as a function of galaxy composition (in particular, the gas fraction).

Using the FIRE simulations to test the of dynamical equilibrium theory, we find that the weight is balanced by the pressure across the face of the disk. I find that the total pressure is dominated by turbulent pressure from the warm and cold phases near the midplane and thermal pressure from the hot phase at above a few (4-5) gas-density-scale-heights. Intriguingly, the FIRE simulations may additionally show that scatter in the KS relation at fixed gas surface density may correspond 1-1 with deviations from vertical pressure balance though direct evidence of a hard link is difficult to prove.

For more details, see the paper.