2024ApJ...966L..36X

Neutral hydrogen (H I) is the primary component of the cool interstellar medium (ISM) and is the reservoir of fuel for star formation. Owing to the sensitivity of existing radio telescopes, our understanding of the evolution of the ISM in galaxies remains limited, as it is based on only a few hundred galaxies detected in H I beyond the local Universe. With the high sensitivity of the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we carried out a blind H I search, the FAST Ultra-Deep Survey, which extends to redshifts up to 0.42 and a sensitivity of 50 μJy beam^–1. Here, we report the first discovery of six galaxies in H I at z > 0.38. For these galaxies, the FAST angular resolution of ∼4' corresponds to a mean linear size of $\sim 1.3\,{h}_{70}^{-1}\,$ Mpc. These galaxies are among the most distant H I emission detections known, with one having the most massive H I content (${10}^{10.93\pm 0.04}\,{h}_{70}^{-2}\,{M}_{\odot }$). Using recent data from the DESI survey and new observations with the Hale, Big Telescope Alt-azimuth, and Keck telescopes, optical counterparts are detected for all galaxies within the 3σ positional uncertainty ($0.5\,{h}_{70}^{-1}\,$Mpc) and 200 km s^–1 in recession velocity. Assuming that the dominant source of H I is the identified optical counterpart, we find evidence of evolution in the H I content of galaxies over the last 4.2 Gyr. Our new high-redshift H I galaxy sample provides the opportunity to better investigate the evolution of cool gas in galaxies. A larger sample size in the future will allow us to refine our knowledge of the formation and evolution of galaxies.