FIRE in the Field: Simulating the Threshold of Galaxy Formation
  We present a suite of 15 cosmological zoom-in simulations of isolated dark
matter halos, all with masses of $M_{\rm halo} \approx 10^{10}\,{\rm M}_\odot$
at $z=0$, in order to understand the relationship between 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. 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 ($M_\star/{\rm M}_\odot\approx 10^5-10^7$). This
non-negligible spread in stellar mass at $z=0$ in halos within a narrow range
of virial masses is strongly correlated with central halo density or maximum
circular velocity $V_{\rm max}$. Much of this dependence of $M_\star$ on a
second parameter (beyond $M_{\rm halo}$) is a direct consequence of the $M_{\rm
halo}\sim10^{10}\,{\rm M}_\odot$ mass scale coinciding with the threshold for
strong reionization suppression: the densest, earliest-forming halos 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. Halos that form galaxies with
$M_\star\gtrsim2\times10^{6}\,{\rm M}_\odot$ 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 $r_{1/2}$.
This apparent stellar mass threshold of $M_\star \approx 2\times 10^{6} \approx
2\times 10^{-4} \,M_{\rm halo}$ is broadly consistent with previous work and
provides a testable prediction of FIRE feedback models in LCDM.
