Too big to fail? The puzzling darkness of massive Milky Way subhaloes

TL;DR

This paper reveals that many massive Milky Way subhaloes predicted by simulations are too dense to host known bright satellites, challenging the conventional galaxy-halo relation and suggesting stochastic galaxy formation at these scales.

Contribution

It demonstrates that most massive subhaloes are dark and dense, conflicting with established galaxy luminosity relations and highlighting potential new physics or feedback effects.

Findings

Most massive subhaloes are too dense to host bright satellites.

Several dark subhaloes could produce significant dark matter annihilation signals.

Galaxy formation may be stochastic at high subhalo masses.

Abstract

We show that dissipationless LCDM simulations predict that the majority of the most massive subhaloes of the Milky Way are too dense to host any of its bright satellites (L_V > 10^5 L_sun). These dark subhaloes have circular velocities at infall of 30-70 km/s and infall masses of [0.2-4] x 10^10 M_sun. Unless the Milky Way is a statistical anomaly, this implies that galaxy formation becomes effectively stochastic at these masses. This is in marked contrast to the well-established monotonic relation between galaxy luminosity and halo circular velocity (or halo mass) for more massive haloes. We show that at least two (and typically four) of these massive dark subhaloes are expected to produce a larger dark matter annihilation flux than Draco. It may be possible to circumvent these conclusions if baryonic feedback in dwarf satellites or different dark matter physics can reduce the central densities of massive subhaloes by order unity on a scale of 0.3 - 1 kpc.