Little Blue Dots in the Hubble Space Telescope Frontier Fields: Precursors to Globular Clusters?

Abstract

Galaxies with stellar masses <107.4 M⊙ and specific star formation rates sSFR > 10-7.4 yr-1 were examined on images of the Hubble Space Telescope Frontier Field Parallels for Abell 2744 and MACS J0416.1-02403. They appear as unresolved "Little Blue Dots" (LBDs). They are less massive and have higher specific star formation rates (sSFRs) than "blueberries" studied by Yang et al. and higher sSFRs than "Blue Nuggets" studied by Tacchella et al. We divided the LBDs into three redshift bins and, for each, stacked the B435, V606, and I814 images convolved to the same stellar point-spread function (PSF). Their radii were determined from PSF deconvolution to be ∼80 to ∼180 pc. The high sSFRs suggest that their entire stellar mass has formed in only 1% of the local age of the universe. The sSFRs at similar epochs in local dwarf galaxies are lower by a factor of ∼100. Assuming that the star formation rate is ∈ffMgas/tff for efficiency ∈ff , gas mass Mgas, and free-fall time, tff, the gas mass and gas-to-star mass ratio are determined. This ratio exceeds 1 for reasonable efficiencies, and is likely to be ∼5 even with a high∈ff of 0.1. We consider whether these regions are forming today's globular clusters. With their observed stellar masses, the maximum likely cluster mass is ∼105 M⊙, but if star formation continues at the current rate for ∼10tff ∼ 50 Myr before feedback and gas exhaustion stop it, then the maximum cluster mass could become ∼106 M⊙.