There is No Missing Satellites Problem (1711.06267v3)

Abstract: A critical challenge to the cold dark matter (CDM) paradigm is that there are fewer satellites observed around the Milky Way than found in simulations of dark matter substructure. We show that there is a match between the observed satellite counts corrected by the detection efficiency of the Sloan Digital Sky Survey (for luminosities $L \gtrsim$ 340 L$\odot$) and the number of luminous satellites predicted by CDM, assuming an empirical relation between stellar mass and halo mass. The "missing satellites problem", cast in terms of number counts, is thus solved. We also show that warm dark matter models with a thermal relic mass smaller than 4 keV are in tension with satellite counts, putting pressure on the sterile neutrino interpretation of recent X-ray observations. Importantly, the total number of Milky Way satellites depends sensitively on the spatial distribution of satellites, possibly leading to a "too many satellites" problem. Measurements of completely dark halos below $10^8$ M$\odot$, achievable with substructure lensing and stellar stream perturbations, are the next frontier for tests of CDM.

• The paper demonstrates that applying completeness corrections reconciles observed satellite counts with Cold Dark Matter predictions.
• It employs detailed models of baryonic processes to explain why only a fraction of dark subhalos form visible galaxies.
• The study challenges warm dark matter models by showing that lower thermal relic masses are inconsistent with satellite observations.

Insights into the Milky Way's Satellite Population and the Non-Existence of the Missing Satellites Problem

The paper "There is No Missing Satellites Problem" presents a detailed analysis of the discrepancy between the observed number of satellite galaxies around the Milky Way (MW) and the predictions made by cold dark matter (CDM) models. Historically, this disparity has been referred to as the "missing satellites problem" (MSP), which emerged when simulations predicted a higher count of satellite galaxies than those observed within the MW. However, recent advancements in astronomical surveys and a deeper understanding of galaxy formation processes challenge this issue's relevance. This paper provides a critical evaluation by incorporating improved detection techniques and a framework that accounts for various physical factors.

The core assertion of this paper is that when detection efficiencies of surveys like the Sloan Digital Sky Survey (SDSS) are considered, along with an empirical relationship between stellar mass and halo mass, the observed satellite counts align with CDM model predictions. Specifically, the focus is on galaxy luminosities above 340 L$_\odot$, starting from the ultra-faint dwarfs like Segue I. This alignment is achieved after applying necessary completeness corrections, which account for the portion of the MW's halo volume that remains unsurveyed. The authors argue that the perceived discrepancy in satellite numbers dissipates, thereby resolving the MSP under these considerations.

Key aspects of the research include:

1. Completeness Correction Methodology: The authors employ a comprehensive model to correct for unobserved satellites, factoring in the limits of observed surveys. This involves assessing the survey coverage, sensitivity, and the three-dimensional spatial distribution of the satellites. The corrected counts bridge the gap between observed numbers and theoretical predictions.
2. Impact of Baryonic Processes: The paper emphasizes the role of baryonic physics in galaxy formation, where only a subset of dark matter subhalos forms luminous galaxies. Processes like feedback and reionization are pivotal in halting star formation in low-mass halos, leaving many dark subhalos that do not host visible galaxies.
3. Constraints on Dark Matter Models: In scenarios involving warm dark matter (WDM) or sterile neutrino interpretations, the paper demonstrates tension with the observations. Specifically, extrapolating the models, thermal relic masses below 4 keV appear inconsistent with the satellite counts, challenging the viability of these models compared to CDM.
4. Radial Distribution and Lensing Techniques: The radial distribution of satellites is critically analyzed due to its impact on completeness corrections and potential evidence of a "too many satellites" issue instead of a deficit. The paper also notes the future potential of substructure lensing and stellar stream perturbations to reveal dark matter properties below $10^8$ M$_\odot$.
5. Predictive Framework for Satellite Populations: By merging observational data with theoretical galaxy models, the authors provide a robust framework for predicting the satellite populations of the MW. This encompasses halo mass functions and considers the intricate phase of galaxy formation.

Though the paper dismisses the MSP in the MW context, it opens avenues for investigations into the lower mass scales of halo substructures and the matter power spectrum in dark matter models. The results hold significant implications for future astronomical surveys and the refinement of galaxy formation theories. As observational capacities expand with advanced instrumentation, expectations for refining these theoretical models grow.

In summary, this research significantly shifts the narrative around the missing satellites problem by providing strong evidence that the existing satellite population can indeed be reconciled with CDM predictions when appropriate factors are included. The methodologies and findings present detailed insights into the interaction of dark matter, baryonic processes, and observational strategies, offering clarity and direction for future explorations in cosmology and astrophysics.