FIRE in the Field: Simulating the Threshold of Galaxy Formation

Fitts et al., available on arXiv.

Abstract: We present a suite of 15 cosmological zoom-in simulations of isolated dark matter halos, all with masses of Mh~10^10 Msun 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 (Mstar/Msun~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 Vmax. Much of this dependence of Mstar on a second parameter (beyond Mh) is a direct consequence of the Mh~10^10 Msun 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 Mstar>~2×10^6 Msun 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 Mstar~2×10^6~2×10^-4 Mh is broadly consistent with previous work and provides a testable prediction of FIRE feedback models in LCDM.