All about baryons: revisiting SIDM predictions at small halo masses

We use cosmological hydrodynamic simulations to consistently compare the assembly of dwarf galaxies in both Λ dominated, cold dark matter (CDM) and self-interacting dark matter (SIDM) models. The SIDM model adopts a constant cross-section of 2 cm² g^-1, a relatively large value to maximize its effects. These are the first SIDM simulations that are combined with a description of stellar feedback that naturally drives potential fluctuations able to create dark matter (DM) cores. Remarkably, SIDM fails to significantly lower the central DM density within the central 500 pc at halo peak velocities V_max < 30 km s^-1. This is due to the fact that the central regions of very low mass field haloes have relatively low central velocity dispersion and densities, leading to time-scales for SIDM collisions greater than a Hubble time. CDM haloes with V_max < 30 km s^-1 have inefficient star formation, and hence weak supernova feedback. At a fixed 2 cm² g^-1 SIDM cross-section, the DM content of very low mass CDM and SIDM haloes differs by no more than a factor of 2 within 100-200 pc. At larger halo masses (∼10¹⁰ M_⊙), the introduction of baryonic processes creates field dwarf galaxies with DM cores and central DM+baryon distributions that are effectively indistinguishable between CDM and SIDM. Both models are in broad agreement with observed Local Group field galaxies across the range of masses explored. To significantly differentiate SIDM from CDM at the scale of faint dwarf galaxies, a velocity-dependent cross-section that rapidly increases to values larger than 2 cm² g^-1 for haloes with V_max < 25-30 km s^-1 needs to be introduced.