# On the detectability of visible-wavelength line emission from the local circumgalactic and intergalactic medium

## TL;DR

This paper proposes a new method using narrow-band imaging to detect low surface brightness Hα emission from the local circumgalactic and intergalactic medium, enabling insights into gas flows and UV background constraints.

## Contribution

It introduces a simulation-based approach to predict Hα emission brightness and demonstrates the feasibility of detecting these signals with current and future telescopic arrays.

## Key findings

- Hα emission extends up to 100 kpc with brightness >700 photons cm$^{-2}$ sr$^{-1}$ s$^{-1}$.
- Current telescopes can image these structures in about 10 hours.
- Detecting the cosmic web's Hα emission requires a hypothetical 1000-lens array.

## Abstract

We describe a new approach to studying the intergalactic and circumgalactic medium in the local Universe: direct detection through narrow-band imaging of ultra-low surface brightness visible-wavelength line emission. We use the hydrodynamical cosmological simulation EAGLE to investigate the expected brightness of this emission at low redshift ($z$ $\lesssim$ 0.2). H$\alpha$ emission in extended halos (analogous to the extended Ly$\alpha$ halos/blobs detected around galaxies at high redshifts) has a surface brightness of $\gtrsim700$ photons cm$^{-2}$ sr$^{-1}$ s$^{-1}$ out to $\sim$100 kpc. Mock observations show that the Dragonfly Telephoto Array, equipped with state-of-the-art narrow-band filters, could directly image these structures in exposure times of $\sim$10 hours. H$\alpha$ fluorescence emission from this gas can be used to place strong constraints on the local ultra-violet background, and on gas flows around galaxies. Detecting H$\alpha$ emission from the diffuse intergalactic medium (the "cosmic web") is beyond current capabilities, but would be possible with a hypothetical 1000-lens Dragonfly array.