EDGE: Dark matter core creation depends on the timing of star formation

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⋆/M200) 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⋆, post/M⋆, pre, where the numerator and denominator represent the stellar mass formed after and before z ∼ 6.5, respectively. Our study covers the halo mass range 109 < M200 < 1010 M⊙ (stellar masses between 104 < M⋆ < 108 M⊙), spanning both ultra-faint and classical dwarfs. In this regime, M⋆, post/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.