The cosmic web's Lyman-$\alpha$ glow at $z\approx 2.5$; hydrodynamic models, dust, and wide-field, narrow-band detection

Abstract

The diffuse Lyman-$\alpha$ glow of the cosmic web has long been predicted but has so far eluded direct detection over cosmologically significant volumes. We construct synthetic Lyman-$\alpha$ surface-brightness maps using five state-of-the-art hydrodynamic simulations (\texttt{IllustrisTNG, EAGLE, CROCODILE, SIMBA, and Sherwood}), modeling recombination, collisional excitation, star formation, and localized dust attenuation. Our study focuses on the redshift range $2.0<z<2.7$, motivated by the numerous detailed studies of the COSMOS region. Significant variations are seen in the results obtained from these independent simulations. Using the Anderson-Darling statistic to probe these statistical differences, we demonstrate that a $5\sigma$ statistical detection of the total intergalactic and circumgalactic Lyman-$\alpha$ emission is achievable with current facilities at flux thresholds brighter than $\sim 8\times 10^{-17}{\text{ erg s}}^{-1}{\text{ cm}}^{-2}{\text{ arcsec}}^{-2}$. Conversely, isolating the underlying low-density component of the cosmic web requires ultra-deep sensitivity, with the most optimistic simulation (IllustrisTNG) reaching a 5$\sigma$ detection only for background noise levels below $\sigma \sim 2\times 10^{-19}{\text{ erg s}}^{-1}{\text{ cm}}^{-2}{\text{ arcsec}}^{-2}$. These quantitative limits validate the feasibility of ongoing wide-field narrow-band campaigns, opening a new era of empirical intergalactic cartography.

21 pages, 10 figures