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

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 $μ=1.23$. The spectrum shows a ~ 5σ detection for the Hβ-[OIII] complex and ~ 4σ for Hα line, with fluxes of 3.2±0.9×10⁻¹⁸ and 1.7±0.4×10⁻¹⁸\,erg\,s⁻¹\,cm⁻², respectively. Based on JWST/NIRCam imaging, we find that GHZ8 consists of two components separated only by 0.16" (≈600 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 MUV=-19.91±0.10 and -18.03±0.15 mag for GHZ8-main and GHZ8-SE, with blue UV slopes of βUV=-2.53±0.22 and -3.45±0.49, 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, fesc= 0.6-1.0. Combining the UV and Hα 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\, 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.