Cake Talk by Umran Koca from Caltech

Abstract: In the current concordance cosmological model, galaxies and clusters are thought to be formed through hierarchical structure formation in matter overdensities. The formation and evolution of galaxies and in particular their efficiency at producing stars and initiating the large-scale reionization of the universe is expected to predominantly occur in these regions. Identifying these overdensities of cold, neutral gas needed to fuel this process, is significantly limited due to the inaccessibility of the hyperfine 21-cm transition from neutral atomic hydrogen (HI) at these redshifts. However, HI can be probed in an alternative way through the Lyman-alpha features, imposed in absorption on bright background sources such as quasars or gamma-ray bursts, or in rare cases on galaxy spectra as well. With the James Webb Space Telescope (JWST), we have identified several galaxies in close projected distance but in the background of a proto-cluster at z=5.4 showing prominent Lyman-alpha absorption features. There is preliminary evidence that the strong Lyman-alpha damping feature observed in these galaxies are evidence of HI in the foreground cluster opposed to gas in the galaxies themselves. To solve this, I use Bayesian inference in conjunction with least squares and Nelder Mead fitting methods to constrain the redshift of the absorption feature, modeling it as a Voigt profile. Further, using the photometric template fitting code, EAZY, I extend these models to include reconstructed spectra with an added Lyman-alpha damping wing. Based on these results I identify 3 galaxies that show damped Lyman-alpha absorption features (N_HI > 10^20 cm^-2) consistent with the proto-cluster redshift, excluding a local origin at > 3 sigma. From this, I estimate the size and mass of the HI in this cluster overdensity. This is the first such detection, and provides valuable insight into the early processes of galaxy and structure formation when the universe was less than 1 Gyr old.