The Milky Way's bright satellites as an apparent failure of LCDM (1111.2048v2)

Abstract: We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos are also among the most massive at earlier times, and significantly exceed the UV suppression mass back to z ~ 10. No LCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way luminosity function, but doing so requires the dwarf spheroidals to have dark matter halos that are a factor of ~5 more massive than is observed. Independent of the difficulty in explaining the absence of these dense, massive subhalos, there is a basic tension between the derived properties of the bright Milky Way dwarf spheroidals and LCDM expectations. The inferred infall masses of these galaxies are all approximately equal and are much lower than standard LCDM predictions for systems with their luminosities. Consequently, their implied star formation efficiencies span over two orders of magnitude, from 0.2% to 20% of baryons converted into stars, in stark contrast with expectations gleaned from more massive galaxies. We explore possible solutions to these problems within the context of LCDM and find them to be unconvincing. In particular, we use controlled simulations to demonstrate that the small stellar masses of the bright dwarf spheroidals make supernova feedback an unlikely explanation for their low inferred densities.

• The paper reveals a significant mismatch between simulated massive subhalos and the observed dynamics of the Milky Way’s bright dwarf spheroidals.
• It finds that expected subhalos have circular velocities above 25 km/s, while observed satellites reside in halos with 12–25 km/s, challenging current star formation efficiency models.
• The paper suggests that stochastic galaxy formation, alternative dark matter models, or revised Milky Way mass estimates may explain the discrepancies in subhalo densities.

Analysis of the Milky Way's Dwarf Spheroidal Population and Implications for ΔΛCDM

The paper by Boylan-Kolchin, Bullock, and Kaplinghat presents a thorough investigation into the dynamics and characteristics of the Milky Way's bright dwarf spheroidal (dSph) satellites in the context of ΛCDM cosmology. Utilizing the high-resolution Aquarius simulations, the authors identify significant discrepancies between the predicted and observed properties of these satellite galaxies, challenging the conventional ΛCDM paradigm on sub-galactic scales.

Key Findings

The primary focus is on the massive subhalos within simulated Milky Way-like halos, which are anticipated to host the brightest dSphs. Instead, the authors find that the subhalos that match the known satellites' dynamics are significantly less massive than those predicted by ΛCDM simulations. Specifically, the subhalos expected to host these dSphs exhibit circular velocities (V) in the range of 12–25 km/s, whereas the simulations predict the presence of at least ten subhalos with V>25 km/s in every Milky Way-mass halo.

This paper presents robust numerical results demonstrating a mismatch between the expected and observed densities of the Milky Way’s bright dSphs. The authors illustrate that the predicted star formation efficiencies, inferred from the simulations, span over two orders of magnitude (0.2% to 20%), in stark contrast with the theoretical expectations for their luminosities.

Implications and Hypotheses

1. Galactic Baryon Dynamics: The paper explores baryonic feedback mechanisms, notably supernova-driven outflows, examining their efficacy in explaining the reduced dark matter densities observed in these subhalos. Despite efforts to demonstrate plausible scenarios, the paper suggests that feedback processes are an unlikely solution given the small stellar masses involved.
2. Galaxy Formation Stochasticity: The discrepancy implies a level of stochasticity in galaxy formation not adequately captured by current models. The bright dSphs seem to inhabit subhalos expected to be more massive, suggesting that the relationship between halo mass and galaxy luminosity is not as robust at these small scales.
3. Alternative Dark Matter Models: The discussion extends to considering alternatives to collisionless CDM, such as self-interacting dark matter (SIDM) or modifications in the thermal history of the Universe impacting subhalo populations.
4. Mass of the Milky Way Halo: Another hypothesis considered is the potential overestimation of the Milky Way's total dark matter halo mass. Yet, even with a revised mass estimate, the problem persists, especially concerning the "massive failures" — subhalos simulated but unexplained by any known satellite.

Future Directions

The findings suggest intriguing avenues for future research, such as more comprehensive simulations including hydrodynamical effects and different dark matter interactions, to address unresolved questions about galaxy formation's stochastic nature in low-mass halos. Additionally, observational advancements, particularly in understanding the ultra-faints and low surface brightness galaxies, could provide further insight into the formation and evolution of galaxy halos.

This paper underscores the necessity for revised models or new physics on small scales within the framework of ΛCDM to accurately describe and predict the complex dynamics and formation history of satellite galaxies like the Milky Way's dwarf spheroidals. The investigation calls for an integration of improved data and modeling to align theoretical expectations with observed phenomena in contemporary cosmological studies.