Nearby dwarf galaxies with extreme star formation rates: a window into dwarf-galaxy evolution in the early Universe

We study a sample of nearby (z ∼ 0.2) low-luminosity dwarf (107 M⊙ < M⋆ < 108 M⊙) galaxies which have extreme (0.1 – 3 M⊙ yr−1) star formation rates (SFRs) for this mass regime, making them plausible analogues of dwarfs at z ∼ 5.5. We compare the properties of these analogues to control samples of ‘normal’ dwarfs, which reside on the star formation main sequence (SFMS) at z ∼ 0.2 and are matched in their stellar mass and redshift distributions to the analogue population. The analogue and normal populations do not show differences, either in their half-light radii or the projected distances to nodes, filaments and massive galaxies. This suggests that the comparatively extreme SFRs in the analogues are not driven by them being anomalously compact or because they reside in specific environments which might provide a larger gas supply. However, the fractions of interacting galaxies and those that have early-type morphology are significantly elevated (by factors of ∼5.6 and ∼9 respectively) in the analogues compared to the normal population. Extrapolation of the redshift evolution of the star formation main sequence into our mass range of interest appears to underestimate the SFRs of observed dwarfs at z ∼ 5.5. Since current SFMS measurements remain dominated by low and intermediate redshift data (especially at low stellar masses), our study suggests that this underestimation may be driven by interactions (which are more frequent at earlier epochs) boosting the SFRs in the high-redshift dwarf population. Our results are consistent with a picture where higher gas availability, augmented by interactions, drives much of the stellar mass assembly of dwarf galaxies in the early Universe.