Abstract
Purely collisionless Dark Matter Only (DMO) structure formation simulations
predict that Dark Matter (DM) haloes are typically prolate in their centres and
spheroidal towards their outskirts. The addition of gas cooling transforms the
central DM shape to be rounder and more oblate. It is not clear, however,
whether such shape transformations occur in `ultra-faint' dwarfs, which have
extremely low baryon fractions. We present the first study of the shape and
velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions
of $f_{\rm gas}(r<R_{\rm half}) < 0.06$. These dwarfs are drawn from the
Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high
resolution simulations (spatial and mass resolution of 3 pc and $120$
M$_\odot$, respectively) that allow us to resolve DM halo shapes within the
half light radius ($\sim 100$ pc). We show that gas-poor ultra-faints ($M_{\rm
200c} \leqslant 1.5\times10^9$ M$_\odot$; $f_{\rm gas} < 10^{-5}$) retain their
pristine prolate DM halo shape even when gas, star formation and feedback are
included. This could provide a new and robust test of DM models. By contrast,
gas-rich ultra-faints ($M_{\rm 200c} > 3\times10^9$ M$_\odot$; $f_{\rm gas} >
10^{-4}$) become rounder and more oblate within $\sim 10$ half light radii.
Finally, we find that most of our simulated dwarfs have significant radial
velocity anisotropy that rises to $\tilde{\beta} > 0.5$ at $R \gtrsim 3 R_{\rm
half}$. The one exception is a dwarf that forms a rotating gas/stellar disc
because of a planar, major merger. Such strong anisotropy should be taken into
account when building mass models of gas-poor ultra-faints.