Notes on the Missing Satellites Problem (1009.4505v1)

Abstract: The Missing Satellites Problem (MSP) broadly refers to the overabundance of predicted Cold Dark Matter (CDM) subhalos compared to satellite galaxies known to exist in the Local Group. The most popular interpretation of the MSP is that the smallest dark matter halos in the universe are extremely inefficient at forming stars. The question from that standpoint is to identify the feedback source that makes small halos dark and to identify any obvious mass scale where the truncation in the efficiency of galaxy formation occurs. Among the most exciting developments in near-field cosmology in recent years is the discovery of a new population satellite galaxies orbiting the Milky Way and M31. Wide field, resolved star surveys have more than doubled the dwarf satellite count in less than a decade, revealing a population of ultrafaint galaxies that are less luminous that some star clusters. For the first time, there are empirical reasons to believe that there really are missing satellite galaxies in the Local Group, lurking just beyond our ability to detect them, or simply inhabiting a region of the sky that has yet to have been surveyed. Both kinematic studies and completeness-correction studies seem to point to a characteristic potential well depth for satellite subhalos that is quite close to the mass scale where photoionization and atomic cooling should limit galaxy formation. Among the more pressing problems associated with this interpretation is to understand the selection biases that limit our ability to detect the lowest mass galaxies. The least massive satellite halos are likely to host stealth galaxies with very-low surface brightness and this may be an important limitation in the hunt for low-mass fossils from the epoch of reionization.

• The paper provides a comprehensive analysis of the mismatch between predicted dark matter subhalos and observed satellite galaxies.
• It employs high-resolution simulations and completeness-corrected surveys to identify a characteristic mass scale where star formation is truncated.
• The study emphasizes the role of baryonic processes such as supernova feedback and photoionization, suggesting many ultra-faint satellites remain undetected.

Overview of the Missing Satellites Problem

The "Notes on the Missing Satellites Problem" by James S. Bullock provides an in-depth analysis of the discrepancy between the predicted and observed number of satellite galaxies within the Local Group, commonly termed the Missing Satellites Problem (MSP). This phenomenon primarily arises from the theory of Cold Dark Matter (CDM), which suggests an overabundance of low-mass dark matter subhalos compared to the number of satellite galaxies observed.

Numerical and Observational Highlights

Recent observational advancements have significantly expanded our understanding of the MSP. Wide-field, resolved star surveys have identified a new population of ultrafaint dwarf galaxies around the Milky Way and M31, more than doubling the known satellites. These discoveries reinforce empirical claims that numerous undetected satellite galaxies likely exist, extending beyond current observational limitations.

Kinematic and completeness-correction studies suggest a characteristic mass scale for satellite subhalos. This is proximate to theoretical limits where photoionization and atomic cooling are expected to constrain galaxy formation, indicating a potential truncation scale for star formation in smaller halos. Yet, detecting these low-mass galaxies is impeded by selection biases, particularly for those with low surface brightness.

Theoretical Implications and Simulation-Based Studies

Bullock situates the MSP within the broader $\Lambda$CDM framework, which robustly explains cosmological structures from cosmic microwave background observations to galaxy clustering scales. However, on smaller scales, particularly within the Milky Way's halo, the predicted abundance of subhalos does not concur with direct observation. Theoretical models and high-resolution simulations of Milky-Way-sized halos, like those from the Via Lactea and Aquarius projects, have consistently reaffirmed this overabundance.

The paper emphasizes the importance of key physical processes underlying the suppression of star formation in small halos—such as supernova feedback and photoionization heating—as potential explanations for the observed lack of low-mass satellites. Such processes highlight distinct mass scales, aligning closely with the kinematic constraints observed in current satellite surveys.

Empirical Completeness and Prospective Surveys

The survey completeness and the radial distribution of dwarfs necessitate correction estimates for satellite counts. The empirical data, augmented with simulated subhalo distributions, imply that hundreds of faint satellites, termed 'stealth galaxies', could remain undiscovered due to current survey limitations. Forthcoming surveys like LSST and DES promise deeper and wider coverage, potentially illuminating these missing populations.

Conclusions and Future Directions

The document underlines a central hypothesis: the MSP is not merely a matter of unseen satellites but involves complex interactions between dark matter subhalos and baryonic processes inhibiting galaxy formation. The recorded kinematic properties of Milky Way satellites, alongside simulations, underscore a potential truncation mass scale associated with baryonic physics—and yet this very relationship challenges straightforward theoretical expectations like abundance matching.

Ultimately, resolving the MSP will likely necessitate a multi-faceted approach, involving improved observational capabilities and refined numerical models to better capture the complexities of galaxy formation and evolution in the smallest dark matter halos. The projected advancements in astronomical surveys and theoretical frameworks hold promise for addressing the MSP, enhancing our understanding of both the local Universe and the fundamental nature of dark matter.