Phase-Space Correlations Among Systems of Satellite Galaxies

Abstract

Driven by 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 LCDM 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 LCDM expectations. With the expansion of proper motion measurements, largely driven by Gaia, other peculiar phase-space correlations were uncovered among the Milky Way satellites. Examples are the tangential velocity excess of satellites compared to cosmological expectations, and the unexpected preference of satellites to be close to pericenter. 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 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. It summarizes existing, recent research and highlights interdependencies between the different, currently almost exclusively individually considered types of correlations. Future prospects are also briefly discussed. (abridged)