Core formation in dwarf haloes with self-interacting dark matter: no fine-tuning necessary

We investigate the effect of self-interacting dark matter (SIDM) on the density profiles of Vmax ≃ 40km s−1 isolated dwarf dark matter haloes – the scale of relevance for the too big to fail problem (TBTF) – using very high resolution cosmological zoom simulations. Each halo has millions of particles within its virial radius. We find that SIDM models with cross-sections per unit mass spanning the range σ/m = 0.5–50 cm2 g−1 alleviate TBTF and produce constant-density cores of size 300–1000 pc, comparable to the half-light radii of M⋆ ∼ 105 − 7 M⊙ dwarfs. The largest, lowest density cores develop for cross-sections in the middle of this range, σ/m ∼ 5–10 cm2 g−1. Our largest SIDM cross-section run (σ/m = 50 cm2 g−1) develops a slightly denser core owing to mild core-collapse behaviour, but it remains less dense than the cold dark matter case and retains a constant-density core profile. Our work suggests that SIDM cross-sections as large or larger than 50 cm2 g−1 remain viable on velocity scales of dwarf galaxies (vrms ∼ 40 km s−1). The range of SIDM cross-sections that alleviate TBTF and the cusp/core problem spans at least two orders of magnitude and therefore need not be particularly fine-tuned.