Associations of Dwarf Galaxies

TLDR

The Hubble Space Telescope Advanced Camera for Surveys measured the brightest red‑giant‑branch luminosities in 20 galaxies to obtain precise distances. Five previously identified dwarf‑galaxy associations are confirmed to be distance‑correlated, two additional associations are found, one appears unbound, and all exhibit bound‑structure kinematics with ~10¹¹ M⊙ and mass‑to‑light ratios of 100–1000, such that nearly all galaxies within 3 Mpc belong to these groups.

Abstract

The Hubble Space Telescope Advanced Camera for Surveys has been used to determine accurate distances for 20 galaxies from measurements of the luminosity of the brightest red giant branch stars. Five associations of dwarf galaxies that had originally been identified based on strong correlations on the plane of the sky and in velocity are shown to be equally well correlated in distance. Two more associations with similar properties have been discovered. Another association is identified that is suggested to be unbound through tidal disruption. The associations have the spatial and kinematic properties expected of bound structures with × 1011 M⊙. However, these entities have little light, with the consequence that the mass-to-light ratios are in the range 100-1000 M⊙ L. Within a well-surveyed volume extending to a 3 Mpc radius, all but one known galaxy lie within one of the groups or associations that have been identified.