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

Understand

The shallow faint-end slope of the galaxy mass function is usually reproduced in $\Lambda$CDM galaxy formation models by assuming that the fraction of baryons that turns into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities $V_{\rm max}<20$-$30$ km/s.

• 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, $r_{1/2}\ll 1$ kpc).
• Many dwarfs have properties in disagreement with this prediction: they are large enough to probe their halo $V_{\rm max}$ but their characteristic velocities are well below $20$ km/s.
• These `cold faint giants' (an extreme example is the recently discovered Crater 2 Milky Way satellite) can only be reconciled with our $\Lambda$CDM models if they are the remnants of once massive objects heavily affected by tidal stripping.