The BoRG-$JWST$ Survey: Analogs at $z\sim8$ to the UV-luminous Galaxy Population at $z\gtrsim10$

TL;DR

This study uses $z\\sim8$ galaxies from the BoRG-$JWST$ survey as analogs to understand the properties of the luminous galaxy population at $z\\gtrsim10$, revealing insights into their dust content, star formation, and stellar ages.

Contribution

It provides the first detailed analysis of $z\\sim8$ analogs to the $z\\gtrsim10$ blue monsters, offering new insights into their dust properties, star formation episodes, and stellar populations.

Findings

BoRG-$JWST$ galaxies are dust-poor with blue UV slopes.

No strong evidence for dominant AGN activity.

Star formation occurs in stochastic episodes, with young stellar populations below 100 Myr.

Abstract

The population of bright galaxies at $z\gtrsim10$ 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 $z\sim8$ 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 $z\gtrsim10$ can be explained by the presence of stars with ages below 100 Myr.