NIHAO III: the constant disc gas mass conspiracy

We show that the cool gas masses of galactic discs reach a steady state that lasts many Gyr after their last major merger in cosmological hydrodynamic simulations. The mass of disc gas, M-gas, depends mostly upon a galaxy virial mass and halo's spin, and less upon stellar feedback. Haloes with low spin have high star formation efficiency and lower disc gas mass. Similarly, lower stellar feedback leads to more star formation so the gas mass ends up being nearly the same regardless of stellar feedback strength. Rather than regulating cool gas mass, stellar feedback regulates the mass of stars that forms. Even considering spin, the M-gas relation with halo mass, M-200 only shows a factor of 3 scatter. The simulated M-gas-M-200 relation shows a break at M-200 = 2 x 10(11) M-circle dot that corresponds to an observed break in the M-gas-M-star relation. The galaxies that maintain constant disc masses share a common halo gas density profile shape in all the simulated galaxies. In their outer regions, the profiles are isothermal. Where the profile rises above n = 10(-3) cm(-3), the gas readily cools and the profile steepens. Inside the disc, rotation supports gas with a flatter density profile. Energy injection from stellar feedback provides pressure support to the halo gas to prevent runaway cooling flows. The constant gas mass makes simpler models for galaxy formation possible, either using a 'bathtub' model for star formation rates or when modelling chemical evolution.