We analyze the cold dark matter density profiles of 54 galaxy halos simulated with FIRE-2 galaxy formation physics, each resolved within $0.5%$ of the halo virial radius. These halos contain galaxies with masses that range from ultra-faint dwarfs ($M_star simeq 10^{4.5} M_{odot}$) to the largest spirals ($M_star simeq 10^{11} M_{odot}$) and have density profiles that are both cored and cuspy. We characterize our results using a new analytic density profile that extends the standard Einasto form to allow for a pronounced constant-density core in the resolved innermost radius. With one additional core-radius parameter, $r_{c}$, this core-Einasto profile is able to characterize the shape and normalization of our feedback-impacted dark matter halos. In order to enable comparisons with observations, we provide fitting functions for $r_{c}$ and other profile parameters as a function of both $M_star$ and $M_{star}/M_{rm halo}$. In agreement with similar studies done in the literature, we find that dark matter core formation is most efficient at the characteristic stellar-mass to halo-mass ratio $M_star/M_{rm halo} simeq 5 times 10^{-3}$, or $M_{star} sim 10^9 , M_{odot}$, with cores that are roughly the size of the galaxy half-light radius, $r_{c} simeq 1-5$ kpc. Furthermore, we find no evidence for core formation at radii $gtrsim 100 rm pc$ in galaxies with $M_{star}/M_{rm halo} < 5times 10^{-4}$ or $M_star lesssim 10^6 , M_{odot}$. For Milky Way-size galaxies, baryonic contraction often makes halos significantly more concentrated and dense at the stellar half-light radius than dark matter only runs. However, even at the Milky Way scale, FIRE-2 galaxy formation still produces small dark matter cores of $simeq 0.5-2$ kpc in size. Recent evidence for a ${sim} 2$ kpc core in the Milky Ways dark matter halo is consistent with this expectation.
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