PRISMS/U13151. A Glimpse into the Earliest Phases of Galaxy Formation through an Ultra-Faint System about 500 Myr after the Big Bang

Detailed UV-to-optical studies of galaxies at z >= 10 have largely focused on bright systems (MUV <= -20 mag), leaving intrinsically faint galaxies virtually unexplored to date. Combining NIRCam, NIRSpec, and MIRI imaging and spectroscopy, we investigate UNCOVER-ABELL2744-13151 (U13151), an ultra-faint galaxy at z = 9.7995 (MUV = -17.44 mag), magnified by Abell 2744 (μ = 12.8). U13151 is very blue (β-2.64), ultra-low mass (log10(M*/Msun) 6.68), young (5.5 Myr), metal-poor (Z 3-6% Zsun), and compact (Re 122 pc). It exhibits negligible dust attenuation, extreme ionization conditions, and intense star formation (sSFR(Ha) 94 Gyr-1, or equivalently M*/SFR(Ha) 10.6 Myr). Despite similarities with much brighter galaxies, its low C/O, approximately solar Ne/O, and undetected nitrogen emission suggest enrichment dominated by core-collapse supernovae, in stark contrast with similar nitrogen-enhanced systems. U13151 might be undergoing its first major episode of star formation, assembling most (if not all) of its M* through five unresolved clumps. Its high ionizing photon production efficiency (log10(ξ ion / (Hz erg-1)) 25.64) suggests, through a simple toy model, suggests, through a simple toy model, that similarly short-lived episodes could contribute significantly to the ionizing photon budget of low-mass galaxies during Cosmic Reionization. We explore different evolutionary scenarios, from U13151-like galaxies potentially representing the progenitors of Blue Monsters at higher redshifts to the possibility that such systems could be the early building blocks of present-day Milky Way-like galaxies. Our results highlight the fundamental role of MIRI in studying galaxies in the early Universe and the urgent need to investigate the faintest, lowest-mass systems to shed light on the fundamental physics driving the earliest phases of galaxy formation.