Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups

The demographics of dwarf galaxy populations have long been in tension with predictions from the Î? cold dark matter (Î? CDM) paradigm 1-4. If primordial density fluctuations were scale-free as predicted, dwarf galaxies should themselves host dark-matter subhaloes 5, the most massive of which may have undergone star formation resulting in dwarf galaxy groups. Ensembles of dwarf galaxies are observed as sateÂ-llites of more massive galaxies 6-9, and there is observational 10 and theoretical 11 evidence to suggest that these satellites at redshift z = 0 were captured by the massive host halo as a group. However, the evolution of dwarf galaxies is highly susceptible to environment 12-14, making these satellite groups imperfect probes of Î? CDM in the low-mass regime. Here we report one of the clearest examples yet of hierarchical structure formation at low masses: Using deep multi-wavelength data, we identify seven isolated, spectroscopically confirmed groups of only dwarf galaxies. Each group hosts three to five known members, has a baryonic mass of ∼4.4 × 10 9 to 2 × 10 10 solar masses (M âŠ(tm)), 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.