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 $(M_{\star }\mathrm{/}{M}_{200})$ 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 $M_{\star ,\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}}\mathrm{/}{M}_{\star ,\mathrm{p}\mathrm{r}\mathrm{e}}$, where the numerator and denominator represent the amount of star formation after and before $z\sim 6.5$, respectively. Our study covers the halo mass range ${10}^9<M_{200}<{10}^{10}{M}_{\odot }$ (stellar masses between ${10}^4<M_{\star }<{10}^8{M}_{\odot }$), spanning both ultra-faint and classical dwarfs. In this regime, $M_{\star ,\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}}\mathrm{/}{M}_{\star ,\mathrm{p}\mathrm{r}\mathrm{e}}$ 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.