# Stellar Properties of z ~ 8 Galaxies in the Reionization Lensing Cluster Survey

## TL;DR

This study analyzes high-redshift galaxies at z~8 using HST and Spitzer data, revealing diverse stellar properties and evidence of early star formation, with some galaxies showing evolved stellar populations just 100 million years after the Big Bang.

## Contribution

It incorporates Spitzer/IRAC imaging into spectral energy distribution fitting for z~8 galaxy candidates, providing new insights into their stellar masses, ages, and star formation histories.

## Key findings

- Most z~8 candidates are confirmed at high redshift after Spitzer data inclusion.
- Detected evolved stellar populations in some galaxies imply early star formation within 100 Myr of the Big Bang.
- Deep Spitzer data supports the high-redshift nature of a z~10 candidate.

## Abstract

Measurements of stellar properties of galaxies when the universe was less than one billion years old yield some of the only observational constraints of the onset of star formation. We present here the inclusion of \textit{Spitzer}/IRAC imaging in the spectral energy distribution fitting of the seven highest-redshift galaxy candidates selected from the \emph{Hubble Space Telescope} imaging of the Reionization Lensing Cluster Survey (RELICS). We find that for 6/8 \textit{HST}-selected $z\gtrsim8$ sources, the $z\gtrsim8$ solutions are still strongly preferred over $z\sim$1-2 solutions after the inclusion of \textit{Spitzer} fluxes, and two prefer a $z\sim 7$ solution, which we defer to a later analysis. We find a wide range of intrinsic stellar masses ($5\times10^6 M_{\odot}$ -- $4\times10^9$ $M_{\odot}$), star formation rates (0.2-14 $M_{\odot}\rm yr^{-1}$), and ages (30-600 Myr) among our sample. Of particular interest is Abell1763-1434, which shows evidence of an evolved stellar population at $z\sim8$, implying its first generation of star formation occurred just $< 100$ Myr after the Big Bang. SPT0615-JD, a spatially resolved $z\sim10$ candidate, remains at its high redshift, supported by deep \textit{Spitzer}/IRAC data, and also shows some evidence for an evolved stellar population. Even with the lensed, bright apparent magnitudes of these $z \gtrsim 8$ candidates (H = 26.1-27.8 AB mag), only the \textit{James Webb Space Telescope} will be able further confirm the presence of evolved stellar populations early in the universe.