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

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.

• The paper reveals that massive subhaloes with peak circular velocities of 30–70 km/s do not host bright satellites, suggesting a stochastic process in galaxy formation.
• It employs dissipationless simulations from the Aquarius and Via Lactea II projects to demonstrate a density–luminosity mismatch that questions standard CDM predictions.
• The results indicate that these dark subhaloes are promising targets for indirect dark matter detection, pushing for refinements in current cosmological models.

Analysis of the Puzzling Darkness of Massive Milky Way Subhaloes

This paper titled "Too big to fail? The puzzling darkness of massive Milky Way subhaloes" by Boylan-Kolchin et al., investigates the dynamics and characteristics of massive subhaloes within the Milky Way (MW) that are seemingly devoid of bright satellite galaxies. Utilizing dissipationless simulations from the Aquarius project and the Via Lactea II simulation, the authors tackle one of the intriguing issues in cosmology related to the missing satellite problem in cold dark matter (CDM) models.

Key Findings

The primary quandary addressed in this study is the apparent absence of luminous satellites within some of the Milky Way's most massive subhaloes. This disparity confronts the conventional premise that galaxy formation and luminosity maintain a monotonic relation with halo mass or circular velocity. Key results and observations include:

1. Subhalo Density vs. Luminosity Discrepancy: The simulations indicate that most massive subhaloes, characterized by peak circular velocities ranging from 30 to 70 km/s, are too dense to host bright satellites with V-band luminosity $L_V > 10^{5}$. This finding implies a stochastic nature of galaxy formation in such haloes.
2. Potential for Dark Matter Detection: Two to four of these dark subhaloes can exceed the dark matter annihilation flux of the dwarf galaxy Draco, suggesting them as potential targets for gamma-ray detections relevant to dark matter research.
3. Implications of Massive, Dark Subhaloes: Subhaloes with high peak circular velocities but scarce visible counterpart galaxies challenge established models, pushing the need to refine our understanding of galaxy formation or consider non-standard cosmological models.
4. Baryonic Effects and Non-standard Dark Matter Models: The study suggests that the discrepancies could be reconciled if baryonic effects lowered the central densities of these subhaloes or if alternative dark matter models, such as warm dark matter, influenced halo characteristics.

Theoretical and Practical Implications

The presence of massive, dark subhaloes has significant theoretical and observational implications. If confirmed, this would necessitate a revision of current galaxy formation models, recognizing a non-monotonic relationship between stellar mass and subhalo mass for $\lessapprox 50 \, \text{km/s}$. The stochastic nature of galaxy formation might stem from subhalo environmental factors and specific histories affecting star formation, challenging straightforward halo-based parametrizations.

Such dark subhaloes also serve as intriguing targets for indirect dark matter detection efforts, providing potential observational handles on the properties of dark matter particles. Furthermore, the existence of these structures could potentially be probed through their dynamical effects on the Milky Way itself, such as perturbations of the galactic disk.

Future Perspectives

The findings underscore the need for comprehensive models integrating small-scale baryonic physics and dark matter interactions. Future work could benefit from improved observational data, particularly concerning satellite dynamics in other galaxies such as Andromeda, to enhance statistical assessments of typical halo properties. Further, these results could motivate more detailed simulations that incorporate baryonic processes, potentially bridging the gap between CDM predictions and observational realities.

This paper contributes to the intricate mosaic of cosmology, characterizing the invisible complexity of subhaloes and challenging researchers to rethink aspects of both galaxy formation and dark matter phenomenology within the prevalent ΛCDM framework.