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

Abstract

Collisionless dark matter only (DMO) structure formation simulations predict that dark matter (DM) haloes are prolate in their centres and triaxial towards their outskirts. The addition of gas condensation 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 fgas(r < Rhalf) < 0.06. These dwarfs are drawn from the Engineering Dwarfs at Galaxy formation’s Edge (EDGE) project, using high-resolution simulations that allow us to resolve DM halo shapes within the half-light radius (∼100 pc). We show that gas-poor ultra-faints (M200c ≤ 1.5 × 109 M⊙; fgas < 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 (M200c > 3 × 109 M⊙; fgas > 10−4) become rounder and more oblate within ∼10 half-light radii. Finally, we find that most of our simulated dwarfs have significant radial velocity anisotropy that rises to $\tilde{\beta } \gt 0.5$ at R ≳ 3Rhalf. 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.

Closing the Gap between Observed Low-mass Galaxy H i Kinematics and Cold Dark Matter Predictions

“…Given the bursty nature of star formation in dwarfs (e.g., McQuinn et al 2010aMcQuinn et al , 2010bWeisz et al 2012;Kauffmann 2014), it is not clear that H I morphologies and kinematics are traced by the stars, and thus it not clear that using the optically derived inclinations is applicable to correcting the H I velocities. Moreover, dwarf galaxies appear to become thicker relative to their size with decreasing mass (e.g., Dalcanton et al 2004;Xu et al 2023), and stellar shapes may be a better tracer of the underlying (prolate) dark matter halo shapes in dwarfs (Xu & Randall 2020;Orkney et al 2023). Both effects make an inclination measurement determined from optical axis ratios more uncertain.…”