of the HR simulation, because it has not yet run to $z$$=$0.

The simulations include a metagalactic UV background (Haardt & Madau 1996), a model for shielding of UV radiation by neutral hydrogen (Cen et al. 2005), and metallicity-dependent radiative cooling (Cen et al. 1995). The fraction and density of neutral hydrogen is directly computed within the simulations. Star particles are created in gas cells that satisfy a set of criteria for star formation proposed by Cen & Ostriker (1992), and reiterated with regards to this simulation in Cen (2012). Once formed, the stellar particle loses mass through gas recycling from Type II supernovae feedback, and about 30% of the stellar particle mass is returned to the ISM within a time step. Supernovae feedback is implemented as described in Cen (2012): feedback energy and ejected metal-enriched mass are distributed into 27 local gas cells centered at the star particle in question, weighted by the specific volume of each cell, which is to mimic the physical process of supernova blastwave propagation that tends to channel energy, momentum, and mass into the least dense regions (with the least resistance and cooling). We allow the whole feedback process to be hydrodynamically coupled to 