EDGE: The shape of dark matter haloes in the faintest galaxies

Matthew D.A Orkney , Ethan Taylor , Justin I Read , Martin P Rey , Andrew Pontzen , Oscar Agertz , Stacy Y Kim , Maxime Delorme (1)

1 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation

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 $f_{\rm gas}(r 3\times10^9$ 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.

Domaines: Astrophysique [astro-ph]

Matthew D.A Orkney, Ethan Taylor, Justin I Read, Martin P Rey, Andrew Pontzen, et al.. EDGE: The shape of dark matter haloes in the faintest galaxies. Monthly Notices of the Royal Astronomical Society, 2023, 525 (3), pp.3516-3532. ⟨10.1093/mnras/stad2516⟩. ⟨hal-04038922⟩