Title: Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups,
Abstract: The demographics of dwarf galaxy populations have long been in tension with
predictions from the Cold Dark Matter (CDM) paradigm. If primordial density
fluctuations were scale-free as predicted, dwarf galaxies should themselves
host dark matter subhaloes, the most massive of which may have undergone star
formation resulting in dwarf galaxy groups. Ensembles of dwarf galaxies are
observed as satellites of more massive galaxies, and there is observational and
theoretical evidence to suggest that these satellites at z=0 were captured by
the massive host halo as a group. However, the evolution of dwarf galaxies is
highly susceptible to environment making these satellite groups imperfect
probes of CDM in the low mass regime. We have identified one of the clearest
examples to date of hierarchical structure formation at low masses: seven
isolated, spectroscopically confirmed groups with only dwarf galaxies as
members. Each group hosts 3-5 known members, has a baryonic mass of ~4.4 x 10^9
to 2 x 10^10 Msun, and requires a mass-to-light ratio of <100 to be
gravitationally bound. Such groups are predicted to be rare theoretically and
found to be rare observationally at the current epoch and thus provide a unique
window into the possible formation mechanism of more massive, isolated
galaxies.