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

R. J. Bouwens, G. D. Illingworth+4 more|arXiv (Cornell University)|Jun 15, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy159 references34 citations

TL;DR

This study analyzes lensed, low-luminosity galaxies at z ≈ 6–8 using Hubble Frontier Fields data to compare their sizes and luminosities with nearby star cluster complexes. It finds that the smallest, faintest high-redshift galaxies have sizes (10–50 pc) and UV magnitudes comparable to massive star cluster complexes like 30 Doradus, suggesting they may host proto-globular clusters; these results constrain formation models, ruling out scenarios with high post-formation mass loss (ξ = 10), and indicate we may be near detecting bona fide globular cluster formation in the early universe.

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.

Motivation & Objective

• To compare the sizes and luminosities of faint, lensed z ≈ 6–8 galaxies with nearby stellar systems such as star cluster complexes and proto-globular clusters.
• To assess whether the smallest, faintest high-redshift galaxies could be proto-globular clusters or star cluster complexes.
• To use observed counts and sizes to constrain high-redshift proto-globular cluster luminosity functions and formation models.
• To evaluate the robustness of size measurements given lensing model uncertainties and their implications for identifying forming globular clusters.

Proposed method

• Utilized size and luminosity measurements from 333 z ≈ 6–8 galaxies in the Hubble Frontier Fields, derived from deep HST imaging and public parametric lensing models.
• Applied median linear magnification and shear corrections (µ and S) from v3/v4 lensing models to convert observed angular sizes to physical sizes.
• Compared observed sizes and magnitudes with those of nearby star cluster complexes (e.g., 30 Doradus) and evolved systems to identify analogs.
• Used the observed number density and size distribution of faint sources to derive upper limits on the proto-globular cluster luminosity function at z ≈ 6.
• Tested consistency with theoretical models including E-MOSAICS, Boylan-Kolchin (2017), and Pozzetti et al. (2019) to constrain formation scenarios.
• Accounted for lensing model uncertainties by applying conservative magnification limits (linear µ ≤ 30, total µ ≤ 50), acknowledging their impact on size estimates for the faintest sources.

Experimental results

Research questions

• RQ1 Do the smallest, faintest z ≈ 6–8 galaxies have physical sizes and luminosities comparable to nearby star cluster complexes like 30 Doradus?
• RQ2 Can the observed size and luminosity distribution of high-redshift galaxies be explained by proto-globular cluster formation models?
• RQ3 What constraints do the observed counts and sizes of faint lensed galaxies place on the high-redshift proto-globular cluster luminosity function?
• RQ4 Are extreme formation scenarios—such as those with substantial post-formation mass loss (ξ = 10)—consistent with current observations?
• RQ5 Are we approaching the detection of bona fide forming globular clusters in the early universe based on current Hubble data?

Key findings

• The faintest z ≈ 6–8 galaxies have half-light radii as small as 10–30 pc, with several sources in the 10–15 pc range under conservative magnification limits.
• The sizes and luminosities of these low-luminosity galaxies are statistically indistinguishable from those of nearby star cluster complexes such as 30 Doradus, which has a half-light radius of ∼100 pc.
• Twenty-three sources in the sample have median sizes ≤40 pc, and three are likely ≤20 pc, placing them in the size range of super star clusters and proto-globular clusters.
• The observed number of compact, faint sources is consistent with predictions from the E-MOSAICS model, which expects ∼1.5 proto-globular clusters at −17.5 mag and ∼1 at −16.5 mag in the HFF fields.
• The results rule out proto-globular cluster formation models with high post-formation mass loss (ξ = 10), as such scenarios would produce far more detectable sources than observed.
• The data suggest we are very close to identifying bona fide forming globular clusters in the early universe, with some candidates already detected at <−15 mag in the same luminosity range predicted by models.