PRISMS GHZ8: Evidence for a Young Burst and Potential Merger Activity at z=10.23

Abstract

UV-bright galaxies in the early Universe provide important constraints on the rapid build-up of stellar mass and the sources responsible for cosmic reionization. However, the physical processes that drive their high UV luminosities remain unclear. Here we report JWST/MIRI LRS spectroscopy of GHZ8, a UV-bright galaxy at z=10.23 with a magnification $\mu=1.23$. The spectrum shows a $\sim 5\sigma$ detection for the $\rm H\beta$-[OIII] complex and $\sim 4\sigma$ for $\rm H\alpha$ line, with fluxes of $3.2\pm0.9\times10^{-18}$ and $\rm 1.7\pm0.4\times10^{-18}\,erg\,s^{-1}\,cm^{-2}$, respectively. Based on JWST/NIRCam imaging, we find that GHZ8 consists of two components separated only by 0.16"($\approx600$ pc): a bright, extended main component and a faint, compact southeastern clump, hereafter referred to as GHZ8-main and GHZ8-SE. Through morphological decomposition, we find demagnified $M_{UV}=-19.91\pm0.10$ and $-18.03\pm0.15$ mag for GHZ8-main and GHZ8-SE, with blue UV slopes of $\beta_{UV}=-2.53\pm0.22$ and $-3.45\pm0.49$, respectively, indicating that both components host young stellar populations. In particular, the extremely blue UV continuum of GHZ8-SE requires an extremely young, ISM-poor stellar population with a high ionizing photon escape fraction, $f_{esc}= 0.6-1.0$. Combining the UV and $\rm H\alpha$ luminosities with the results from the spectro-photometric SED fitting, we constrain the star-formation activity of GHZ8-main, and find either a recent, short-lived burst of star formation that is now in its final phases, or continuous star formation over a relatively short period ($<50\,{\rm Myr}$). GHZ8 therefore provides observational evidence for bursty star formation in a UV-bright galaxy. Globally, GHZ8 may be undergoing a merger-induced starburst, although deeper imaging or spatially resolved kinematic information is required for a more definitive interpretation.