Low-luminosity galaxies in the early universe have observed sizes similar to star cluster complexes

TL;DR

This study compares the sizes and luminosities of faint high-redshift galaxies with local star-forming regions, revealing many are as small as star cluster complexes, and provides insights into proto-globular cluster formation.

Contribution

It demonstrates that faint z=6-8 galaxies are extremely compact, similar to star clusters, and constrains models of globular cluster formation at high redshift.

Findings

Faint high-redshift galaxies have sizes as small as 10-30 pc.

Many of these galaxies are comparable in size to local star cluster complexes.

Results set upper limits on proto-globular cluster formation scenarios.

Abstract

We compare the sizes and luminosities of faint $z=6$-8 galaxies magnified by the Hubble Frontier Fields (HFF) clusters with star-forming regions, as well as more evolved objects, in the nearby universe. Our high-redshift comparison sample includes 333 z=6-8 galaxies, for which size measurements were made as part of a companion study where lensing magnifications were estimated from various public models. Accurate size measurements for these sources are complicated by the lens model uncertainties, but other results and arguments suggest that faint galaxies are small, as discussed in a companion study. The measured sizes for sources in our comparison sample range from <50 pc to ~500 pc. For many of the lowest luminosity sources, extremely small sizes are inferred, reaching individual sizes as small as 10-30 pc, with several sources in the 10-15 pc range with our conservative magnification limits. The sizes and luminosities are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and -- in a few cases -- super star clusters. The identification of these compact, faint star-forming sources in the z~6-8 universe also allows us to set upper limits on the proto-globular cluster LF at z~6. By comparisons of the counts and sizes with recent models, we rule out (with some caveats) proto-globular cluster formation scenarios favoring substantial (xi=10) post-formation mass loss and set useful upper limits on others. Our size results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift.