The BoRG-JWST Survey: Analogs at z ∼ 8 to the UV-luminous Galaxy Population at z ≳ 10
April 2026 • 2026ApJ..1000..285R
Abstract • The population of bright galaxies at z ≳ 10 discovered by JWST, including the so-called "blue monsters," has been difficult to reconcile with standard galaxy evolution models. To shed light on this extraordinary population, we study the z ∼ 8 galaxies discovered by the BoRG-JWST survey. These slightly lower redshift analogs are comparable in UV luminosity to the blue monsters, and their lower redshift makes it much easier to access key rest frame optical diagnostics with NIRspec. We find that BoRG-JWST galaxies are consistent with being dust-poor based on their blue UV slopes and Balmer decrement ratios. We find no strong evidence for dominant active galactic nuclei contribution to the UV brightness, based on line-ratio diagnostics, though some contribution cannot be excluded. We further infer the stellar mass, star formation, and UV-brightness history of the BoRG-JWST galaxies by fitting their rest-frame UV-optical spectra. We see evidence for stochastic episodes of star formation for all the BoRG-JWST galaxies, providing a temporal boosting of UV luminosity in short timescales. The UV-bright blue monsters at z ≳ 10 can be explained by the presence of stars with ages below 100 Myr.
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