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

Sofía Rojas-Ruiz, Guido Roberts-Borsani, Takahiro Morishita, Antonello Calabrò, Micaela B. Bagley, Tommaso Treu, Steven L. Finkelstein, Massimo Stiavelli, Michele Trenti, L. Y. Aaron Yung

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.

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

Abstract

The population of bright galaxies at 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 galaxies discovered by the BoRG- 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- 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- galaxies by fitting their rest-frame UV-optical spectra. We see evidence for stochastic episodes of star formation for all the BoRG- galaxies, providing a temporal boosting of UV luminosity in short timescales. The UV-bright blue monsters at can be explained by the presence of stars with ages below 100 Myr.

Paper Structure

This paper contains 3 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Data and Methods
  3. Emission Line fit

Figures (1)

  • Figure 1: Example of galaxy 1747_817 showing the emission line fitting in the three spectral windows used in this work. The purple lines and shaded regions indicate the observed flux and 1$\sigma$ uncertainties, while the orangered lines show the Gaussian fits described in §\ref{['emission-fit']}. Signal-to-noise ratios for the emission lines are listed ordered left to right as they appear in the spectrum.