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

TL;DR

The paper shows that dissipationless LCDM simulations predict many massive Milky Way subhaloes that are too dense to host the observed bright satellites, challenging monotonic galaxy formation at V_infall ≲50 km/s. By linking subhalo inner densities to MW dwarfs via M1/2 and using V_infall as a key variable, the authors identify a substantial population of 'massive dark subhaloes' and show that several could dominate indirect dark matter signals relative to Draco. These findings imply that galaxy formation at low masses is effectively stochastic and raise testable predictions for gamma-ray searches and disk perturbations, with implications for alternate DM physics or MW-specific assembly. The work provides a framework to falsify or confirm LCDM predictions about subhalo populations through kinematics and indirect detection.

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.

Figures (5)

• Figure 1: Constraints on the $V_{\rm{max}}-R_{\rm{max}}$ values (assuming NFW profiles) of the hosts of the nine bright ($L_V > 10^5\,L_{\odot}$) MW dwarf spheroidal galaxies. The colored bands show $1\,\sigma$ confidence intervals based on measured values of $R_{1/2}$ and $M_{1/2}$ from wolf2010.
• Figure 2: Subhaloes from all six Aquarius simulations (circles) and Via Lactea II (triangles), color-coded according to $V_{\rm infall}$. The shaded gray region shows the $2\,\sigma$ confidence interval for possible hosts of the bright MW dwarf spheroidals (see Fig. \ref{['fig:dwarfs']}).
• Figure 3: Cumulative $V_{\rm infall}$ function of massive subhaloes at $z=0$ that cannot host any MW satellite brighter than $L_V=10^{5}\,L_{\odot}$, including the Magellanic Clouds. Each of the seven high resolution simulations studied here has at least six such subhaloes with $V_{\rm infall} > 30\,{\rm km \, s}^{-1}$, and at least four with $V_{\rm infall} > 40\,{\rm km \, s}^{-1}$.
• Figure 4: Relation between $V_{\rm infall}$ and $L_V$ for Milky Way dwarf spheroidals (red points) and massive dark subhaloes (black points) for one representative halo realization (Aq-A).
• Figure 5: Distribution of annihilation fluxes from dark subhaloes, normalized to a typical scenario for the annihilation flux from Draco. Error bars reflect 68% confidence levels for varying the specific angular location of the observer on the solar circle. The typical halo has approximately four dark subhaloes with annihilation fluxes exceeding that of Draco.