Oscar Agertz

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 that 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 reionization. Sustained post-reionization star formation decreases central dark matter density through potential fluctuations; conversely, pre-reionization star formation is too short-lived to have such an effect. In fact, large stellar masses accrued prior to reionization are a strong indicator of early collapse, and therefore indicative of an increased central dark matter density. We parametrize 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 10⁹M⊙ < M₂₀₀ < 10¹⁰M⊙ (stellar masses in the range 10⁴M⊙ < M_∗ < 10⁸M⊙), 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 new-found dependence.