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

TLDR

Dark Milky Way subhaloes have infall circular velocities of 30–70 km s⁻¹ and masses of 0.2–4 × 10¹⁰ M⊙, contrasting with the monotonic luminosity–halo mass relation seen for larger haloes. The study investigates whether baryonic feedback or alternative dark matter physics can reduce the central densities of massive subhaloes by an order of magnitude on 0.3–1 kpc scales. Using dissipationless ΛCDM simulations, the authors model the density profiles of massive subhaloes and compute their expected dark matter annihilation flux. They find that most massive subhaloes are too dense to host bright satellites, implying stochastic galaxy formation at these masses, and that at least two (often four) of them would yield a larger dark matter annihilation flux than Draco.

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.