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Oscar Agertz. Profile photo.

Oscar Agertz

Associate Professor / Senior university lecturer / Wallenberg Academy Fellow

Oscar Agertz. Profile photo.

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


  • Matthew D.A. Orkney
  • Ethan Taylor
  • Justin I. Read
  • Martin P. Rey
  • A. Pontzen
  • Oscar Agertz
  • Stacy Y. Kim
  • Maxime Delorme

Summary, in English

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 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.


  • Lund Observatory - Has been reorganised
  • Astrophysics
  • eSSENCE: The e-Science Collaboration

Publishing year







Monthly Notices of the Royal Astronomical Society





Document type

Journal article


Oxford University Press


  • Astronomy, Astrophysics and Cosmology


  • galaxies: dwarf
  • galaxies: evolution
  • galaxies: formation
  • galaxies: haloes
  • methods: numerical




  • ISSN: 0035-8711