Abstract
Low-mass dwarf galaxies are expected to showcase pristine `cuspy' inner dark
matter density profiles compared to their stellar sizes, as they form too few
stars to significantly drive dark matter heating through supernovae-driven
outflows. Here, we study such simulated faint systems ($10^4 \leq M_{\star}
\leq 2\times 10^6 \, M_\mathrm{\odot}$) 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 showcase a detectable
HI reservoir ($M_{\mathrm{HI}}\approx 10^{5}-10^{6} \, M_\mathrm{\odot}$),
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 ($v_{\phi} \approx 10\, \mathrm{km} \,
\mathrm{s}^{-1}$), making rotation short-lived ($\ll 150 \, \mathrm{Myr}$) and
more challenging to interpret for dark matter inferences. Contrastingly, we
highlight a long-lived ($\geq 500 \, \mathrm{Myr}$) and easy-to-interpret HI
rotation curve extending to $\approx 2\, r_{1/2, \text{3D}}$ 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 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.