Abstract
Driven by the increasingly complete observational knowledge of systems of satellite galaxies, mutual spatial alignments and relations in velocities among satellites belonging to a common host have become a productive field of research. Numerous studies have investigated different types of such phase-space correlations and were met with varying degrees of attention by the community. The Planes of Satellite Galaxies issue is maybe the best-known example, with a rich field of research literature and an ongoing, controversial debate on how much of a challenge it poses to the ΛCDM model of cosmology. Another type of correlation, the apparent excess of close pairs of dwarf galaxies, has received considerably less attention despite its reported tension with ΛCDM expectations. With the fast expansion of proper motion measurements in recent years, largely driven by the Gaia mission, other peculiar phase-space correlations have been uncovered among the satellites of the Milky Way. Examples are the apparent tangential velocity excess of satellites compared to cosmological expectations, and the unexpected preference of satellites to be close to their pericenters. At the same time, other kinds of correlations have been found to be more in line with cosmological expectations—specifically, lopsided satellite galaxy systems and the accretion of groups of satellite galaxies. The latter has mostly been studied in cosmological simulations thus far, but it offers the potential to address some of the other issues by providing a way to produce correlations among the orbits of a group’s satellite galaxy members. This review is the first to provide an introduction to the highly active field of phase-space correlations among satellite galaxy systems. The emphasis is on summarizing existing, recent research and highlighting interdependencies between the different, currently almost exclusively individually considered types of correlations. Future prospects in light of upcoming observational facilities and our ever-expanding knowledge of satellite galaxy systems beyond the Local Group are also briefly discussed.
Highlights
Advances in observational facilities and successful observational surveys detecting increasingly fainter dwarf galaxies around the Milky Way (MW) and other nearby galaxies, coupled with improvements in numerical modeling of the cosmic evolution of matter and the formation of galaxies, have opened opportunities to investigate the highly non-linear regime of structure and galaxy formation
The comparison of expectations derived from cosmological simulations with observed galaxies and their satellites has revealed several ‘small-scale problems’ for the Λ Cold Dark Matter (ΛCDM) model of cosmology
Several different types of phase-space correlations among systems of satellite galaxies have been studied. Some of these, such as the planes of satellite galaxies, are recognized as either a problem, or at least a challenge, for the ΛCDM model of cosmology. Other such challenges include the apparent overabundance of close pairs of satellites, which, interestingly, are found to be members of the planes of satellites, the excess of tangential velocities for the observed MW satellites compared to satellites in cosmological simulations, and potentially the closeness of MW satellites to their pericenters
Summary
Advances in observational facilities and successful observational surveys detecting increasingly fainter dwarf galaxies around the Milky Way (MW) and other nearby galaxies, coupled with improvements in numerical modeling of the cosmic evolution of matter and the formation of galaxies, have opened opportunities to investigate the highly non-linear regime of structure and galaxy formation. There is the Too Big To Fail Problem (TBTF, [6]), i.e., that the dark matter halo density deduced from the dynamics of the most luminous satellite galaxies is lower than that of the most massive dark matter sub-halos in simulated MW analogs, which should be too large to have failed to form stars While they are deemed catastrophic for ΛCDM by some (e.g., [7]), a less extreme approach has been claimed to be adequate to address some of these problems: they do not necessarily indicate fundamental issues with the underlying cosmological model, but could be caused by lacking modeling of baryonic effects or by observational shortcomings that were not fully considered. To keep this review sufficiently brief and accessible, these aspects will not be covered in detail in the following
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