EDGE – Dark matter or astrophysics? Breaking dark matter heating degeneracies with HI rotation in faint dwarf galaxies

Low-mass dwarf galaxies are expected to reside within dark matter haloes that have a pristine, `cuspy' density profile within their stellar half-light radii. This is because they form too few stars to significantly drive dark matter heating through supernova-driven outflows. Here, we study such simulated faint systems (10^4 ≤ M_⋆≤ 2× 10^6 M_⊙) drawn from high-resolution (3 pc) cosmological simulations from the `Engineering Dwarf Galaxies at the Edge of galaxy formation' (EDGE) project. We confirm that these objects have steep and rising inner dark matter density profiles at z=0, little affected by galaxy formation effects. But five dwarf galaxies from the suite also showcase a detectable HI reservoir (M_HI≈ 10^5-10^6 M_⊙), analogous to the observed population of faint, HI-bearing dwarf galaxies. These reservoirs exhibit episodes of ordered rotation, opening windows for rotation curve analysis. Within actively star-forming dwarfs, stellar feedback easily disrupts the tenuous HI discs (v_ϕ≈ 10 km s^-1), making rotation short-lived (≪ 150 Myr) and more challenging to interpret for dark matter inferences. In contrast, we highlight a long-lived (≥ 500 Myr) and easy-to-interpret HI rotation curve extending to ≈ 2 r_1/2, 3D in a quiescent dwarf, that has not formed new stars since z=4. This stable gas disc is supported by an oblate dark matter halo shape that drives high-angular momentum gas flows. Our results strongly motivate further searches for HI in rotation curves in the observed population of HI-bearing low-mass dwarfs, that provide a key regime to disentangle the respective roles of dark matter microphysics and galaxy formation effects in driving dark matter heating.