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Constraints on dark matter models from the stellar cores observed in ultra-faint dwarf galaxies: Self-interacting dark matter

It has been proposed that the stellar cores observed in ultra-faint dwarf (UFD) galaxies reflect underlying dark matter (DM) cores that cannot be formed by stellar feedback acting on collisionless cold dark matter (CDM) halos. Assuming this claim is correct, we investigate the constraints that arise if such cores are produced by self-interacting dark matter (SIDM). We derive the range of SIDM cross-sections (sigma/m) required to reproduce the observed core sizes. These can result from halos in either the core-formation phase (low sigma/m) or the core-collapse phase (high sigma/m), yielding a wide allowed range (sim 0.3 -- 200 cm2/g) consistent with values reported in the literature for more massive galaxies. We also construct a simple model relating stellar mass to core radius - two observables likely connected in SIDM. This model reproduces the stellar core sizes and masses in UFDs with sigma/m consistent with those derived above. It also predicts a trend of increasing core radius with stellar mass, in agreement with observations of more massive dwarf galaxies. The model central DM densities match observations when assuming the SIDM profile to originate from an initial CDM halo that follows the mass-concentration relation. Since stellar feedback is insufficient to form cores in these galaxies, UFDs unbiasedly anchor sigma/m at low velocities. If the core-collapse scenario holds (i.e., high sigma/m), UFD halos are thermalized on kpc scales, approximately two orders of magnitude larger than the stellar cores. These large thermalization scales could potentially influence substructure formation in more massive systems.