UNCOVER: Illuminating the Early Universe -- JWST/NIRSpec Confirmation of $z > 12$ Galaxies

TL;DR

This paper reports the spectroscopic confirmation of two galaxies at redshifts greater than 12 using JWST, providing new insights into the properties and sizes of the earliest galaxies during the epoch of reionization.

Contribution

First spectroscopic confirmation of galaxies at z > 12 with detailed analysis of their properties and sizes using JWST data.

Findings

Confirmed galaxies at z=12.39 and z=13.08.

Galaxies are low mass, young, and star-forming.

Galaxies have larger sizes than similar high-redshift systems.

Abstract

Observations of high-redshift galaxies provide a critical direct test to the theories of early galaxy formation, yet to date, only three have been spectroscopically confirmed at $z>12$. Due to strong gravitational lensing over a wide area, the galaxy cluster field A2744 is ideal for searching for the earliest galaxies. Here we present JWST/NIRSpec observations of two galaxies: a robust detection at $z_{\rm spec} = 12.393^{+0.004}_{-0.001}$, and a plausible candidate at $z_{\rm spec} = 13.079^{+0.013}_{-0.001}$. The galaxies are discovered in JWST/NIRCam imaging and their distances are inferred with JWST/NIRSpec spectroscopy, all from the JWST Cycle 1 UNCOVER Treasury survey. Detailed stellar population modeling using JWST NIRCam and NIRSpec data corroborates the primeval characteristics of these galaxies: low mass ($\sim 10^8~{\rm M_\odot}$), young, rapidly-assembling, metal-poor, and star-forming. Interestingly, both galaxies are spatially resolved, having lensing-corrected rest-UV effective radii on the order of 300-400 pc, which are notably larger than other spectroscopically confirmed systems at similar redshifts. The observed dynamic range of $z \gtrsim 10$ sizes spans over 1 order of magnitude, implying a significant scatter in the size-mass relation at early times. Deep into the epoch of reionization, these discoveries elucidate the emergence of the first galaxies.