2025ApJ...989..169M

We explore the neutral hydrogen (H I) gas around 1.9 < z < 3.5 Lyman alpha emitters (LAEs) from the Hobby-Eberly Telescope Dark Energy Experiment using faint Lyα absorption. This absorption is the result of H I in the halo of the LAE scattering Lyα photons from the integrated light of background galaxies along the line of sight. We stack millions of spectra from regions around ∼88,000 LAEs, in order to focus on the physics of the gas at large radii. The extensive number of fiber spectra contributing to the stacks ensures a sufficient signal-to-noise ratio to detect the faint Lyα absorption, which would otherwise be buried within the noise. We detect absorption out to a projected ∼350 kpc around an average LAE at z ∼ 2.5. We use these results to create an empirical radial W_λ(Lyα) profile around LAEs. Comparison with numerical simulations reveals a profile similar to the empirical one within this region. Compared to previous studies, the profile is similar but modestly higher. We also outline a simple physical picture motivated by the observed trends in the data. We plan to quantify this radial profile as a function of redshift, local density, and Lyα luminosity, to explore the relationship between LAE environments and H I distribution.