1711.02090
Very low-luminosity galaxies in the early universe have observed sizes similar to single star cluster complexes
Bouwens, van Dokkum, et al
Compare the sizes and luminosities of 307 faint z=6-8 sources revealed by HFF with sources in the nearby universe. Making use of the latest lensing models and data from the first four HFF clusters with an extensive suite of public lens models, measure both the sizes and luminosities for 153 z~6, 101 z~7, and 53 z~8 galaxies. The sizes range over more than a decade from ~500 to <50 pc. Extremely small sizes are inferred for many of the lowest luminosity sources, reaching individual sizes as small as 10-30 pc (the smallest is 11-6+28 pc). The uncertainty in these measures ranges from 80 pc for the largest sources to typically about 20 pc for the smallest. Such sizes are smaller than extrapolations of the size-luminosity relation, and expectations for the completeness of the faint samples, suggesting a likely break in the size-luminosity relation at ~-17 mag with size proportional to L^(0.5-0.11+0.10). The sizes and luminosities of the lowest-luinosity sources are similar to those of single star cluster complexes like 30 Doradus in the lower-z universe and -- in a few cases -- super star clusters. Remarkably, the identification of these compact, faint star-forming sources in the z~6-8 universe also allow to set upper limits on the porto-globular cluster LF at z~6. Comparisons with recent models allows ruling out (with some caveats) some scenarios for porto-globular cluster formation and set useful upper limits on other less extreme ones. The results suggest that discovering a bone-fide population of forming globular clusters at high-z is very close.
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