Abstract
We study feedback-driven cold dark matter core creation in the EDGE suite of radiation-hydrodynamical dwarf galaxy simulations. Understanding this process is crucial when using observed dwarf galaxies to constrain the particle nature of dark matter. While previous studies have shown the stellar-mass to halo-mass ratio (M-star/M-200) determines the extent of core creation, we find that in low-mass dwarfs there is a crucial additional effect, namely the timing of star formation relative to reionisation. Sustained post-reionisation star formation decreases central dark matter density through potential fluctuations; conversely, pre-reionisation star formation is too short-lived to have such an effect. In fact, large stellar masses accrued prior to reionisation are a strong indicator of early collapse, and therefore indicative of an increased central dark matter density. We parameterise this differentiated effect by considering M-star,M-post/M-star,M-pre, where the numerator and denominator represent the amount of star formation after and before z similar to 6.5, respectively. Our study covers the halo mass range 10(9)<M-200<10(10)M(circle dot) (stellar masses between 10(4)<M-star<10(8)M(circle dot)), spanning both ultra-faint and classical dwarfs. In this regime, M-star,M-post/M-star,M-pre correlates almost perfectly with the central dark matter density at z=0, even when including simulations with a substantially different variant of feedback and cooling. We provide fitting formulae to describe the newfound dependence.