Tidal stripping and the structure of dwarf galaxies in the Local Group

Azadeh Fattahi*, Julio F. Navarro, Carlos S. Frenk, Kyle A. Oman, Till Sawala, Matthieu Schaller

*Corresponding author for this work

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Abstract

The shallow faint-end slope of the galaxy mass function is usually reproduced in Λ cold dark matter (ΛCDM) galaxy formation models by assuming that the fraction of baryons that turn into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities Vmax < 20-30 km s-1. Dark-matter-dominated dwarfs should therefore have characteristic velocities of about that value, unless they are small enough to probe only the rising part of the halo circular velocity curve (i.e. half-mass radii, r1/2 ≪ 1 kpc). Many dwarfs have properties in disagreement with this prediction: they are large enough to probe their halo Vmax but their characteristic velocities are well below 20 km s-1. These `cold faint giants' (an extreme example is the recently discovered Crater 2 Milky Way satellite) can only be reconciled with our ΛCDM models if they are the remnants of once massive objects heavily affected by tidal stripping. We examine this possibility using the APOSTLE cosmological hydrodynamical simulations of the Local Group. Assuming that low-velocity-dispersion satellites have been affected by stripping, we infer their progenitor masses, radii, and velocity dispersions, and find them in remarkable agreement with those of isolated dwarfs. Tidal stripping also explains the large scatter in the mass discrepancy-acceleration relation in the dwarf galaxy regime: tides remove preferentially dark matter from satellite galaxies, lowering their accelerations below the amin ̃ 10-11 m s-2 minimum expected for isolated dwarfs. In many cases, the resulting velocity dispersions are inconsistent with the predictions from Modified Newtonian Dynamics, a result that poses a possibly insurmountable challenge to that scenario.
Original languageEnglish
Pages (from-to)3816-3836
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Volume476
Issue number3
DOIs
Publication statusPublished - May-2018

Keywords

  • galaxies: dwarf
  • galaxies: evolution
  • galaxies: kinematics and dynamics
  • Local Group
  • dark matter
  • COLD DARK-MATTER
  • MODIFIED NEWTONIAN DYNAMICS
  • DIGITAL SKY SURVEY
  • SPHEROIDAL GALAXIES
  • MILKY-WAY
  • LAMBDA-CDM
  • SATELLITE GALAXIES
  • EAGLE SIMULATIONS
  • SCALING RELATIONS
  • MASS FUNCTION

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