The HETDEX Survey: Probing neutral hydrogen in the circumgalactic medium of ~88,000 Lyman Alpha Emitters

TL;DR

This study uses stacking of spectra from ~88,000 Lyman Alpha Emitters at redshifts 1.9 to 3.5 to detect and analyze faint neutral hydrogen absorption in their circumgalactic medium, revealing the gas distribution at large radii.

Contribution

It provides the first empirical radial profile of H I around LAEs at high redshift using a large spectroscopic dataset, and compares it with simulations to understand gas distribution.

Findings

Detection of Lyα absorption out to ~350 kpc around LAEs

Empirical radial Wλ(Lyα) profile consistent with simulations

Profile slightly higher than previous measurements

Abstract

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 (HETDEX) using faint Ly$\alpha$ absorption. This absorption is the result of H I in the halo of the LAE scattering Ly$\alpha$ photons from the integrated light of background galaxies along the line of sight. We stack millions of spectra from regions around ~88,000 LAEs to focus on the physics of the gas at large radii. The extensive number of fiber spectra contributing to the stacks ensures significant signal-to-noise ratio (S/N) to detect the faint Ly$\alpha$ 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 $W_\lambda$(Ly$\alpha$) 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 Ly$\alpha$ luminosity to explore the relationship between LAE environments and H I distribution.