FIRE in the field: simulating the threshold of galaxy formation

We present a suite of 15 cosmological zoom-in simulations of isolated dark matter haloes, all with masses of M-halo approximate to 10(10)M(circle dot) at z = 0, in order to understand the relationship among halo assembly, galaxy formation and feedback's effects on the central density structure in dwarf galaxies. These simulations are part of the Feedback in Realistic Environments (FIRE) project and are performed at extremely high resolution (m(baryon) = 500M(circle dot), m(dm) = 2500 M-circle dot). The resultant galaxies have stellar masses that are consistent with rough abundance matching estimates, coinciding with the faintest galaxies that can be seen beyond the virial radius of the Milky Way (M*/M circle dot approximate to 10(5) - 10(7)). This non-negligible spread in stellar mass at z = 0 in haloes within a narrow range of virial masses is strongly correlated with central halo density or maximum circular velocity V-max, both of which are tightly linked to halo formation time. Much of this dependence of M* on a second parameter (beyond Mhalo) is a direct consequence of the M-halo similar to 10(10)M(circle dot) mass scale coinciding with the threshold for strong reionization suppression: the densest, earliest-forming haloes remain above the UV-suppression scale throughout their histories while late-forming systems fall below the UV-suppression scale over longer periods and form fewer stars as a result. In fact, the latest-forming, lowest-concentration halo in our suite fails to form any stars. Haloes that form galaxies with M-star greater than or similar to 2 x 10(6) M-circle dot have reduced central densities relative to dark-matter-only simulations, and the radial extent of the density modifications is well-approximated by the galaxy half-mass radius r(1/2). Lower-mass galaxies do not modify their host dark matter haloes at the mass scale studied here. This apparent stellar mass threshold of M-star approximate to 2 x 10(6) - 2 x 10(-4) M-halo is broadly consistent with previous work and provides a testable prediction of FIRE feedback models in Lambda cold dark matter.