# Prospects for Observing the low-density Cosmic Web in Lyman-alpha Emission

## TL;DR

Mapping the cosmic web in Lyman-alpha emission through simulations reveals the potential and challenges of detecting intergalactic filaments, emphasizing the dominance of halo emission and the importance of observational strategies at different redshifts.

## Contribution

This study uses cosmological hydrodynamical simulations to predict Lyman-alpha emission from the IGM, providing new insights into observational prospects and the dominant sources of emission.

## Key findings

- Filament emission is mainly from halos and galaxies, not the low-density IGM.
- Detecting filament gas requires long exposures with advanced instruments like ELT.
- High-redshift protoclusters are promising targets for observing the cosmic web.

## Abstract

Mapping the intergalactic medium (IGM) in Lyman-$\alpha$ emission would yield unprecedented tomographic information on the large-scale distribution of baryons and potentially provide new constraints on the UV background and various feedback processes relevant to galaxy formation. Here, we use a cosmological hydrodynamical simulation to examine the Lyman-$\alpha$ emission of the IGM due to collisional excitations and recombinations in the presence of a UV background. We focus on gas in large-scale-structure filaments in which Lyman-$\alpha$ radiative transfer effects are expected to be moderate. At low density the emission is primarily due to fluorescent re-emission of the ionising UV background due to recombinations, while collisional excitations dominate at higher densities. We discuss prospects of current and future observational facilities to detect this emission and find that the emission of filaments of the cosmic web will typically be dominated by the halos and galaxies embedded in them, rather than by the lower density filament gas outside halos. Detecting filament gas directly would require a very long exposure with a MUSE-like instrument on the ELT. Our most robust predictions that act as lower limits indicate this would be slightly less challenging at lower redshifts ($z \lesssim 4$). We also find that there is a large amount of variance between fields in our mock observations. High-redshift protoclusters appear to be the most promising environment to observe the filamentary IGM in Lyman-$\alpha$ emission.