# Can HI 21 cm line trace the Missing Baryons in the Filamentary Structures?

## TL;DR

This study investigates whether the 21-cm line of neutral hydrogen can detect the elusive Warm Hot Intergalactic Medium in cosmic filaments, using simulations and SKA1-mid observational predictions.

## Contribution

It demonstrates the potential detectability of filamentary structures via 21-cm signals with current and future radio telescope sensitivities, highlighting the challenge of isolating WHIM signals.

## Key findings

- 21-cm signals from filaments are detectable with SKA1-mid at certain resolutions and observation times.
- Most detected signals originate from galaxies, not the diffuse WHIM.
- Detecting WHIM may require sensitivity ten times higher than SKA1-mid.

## Abstract

A large fraction of baryons predicted from the standard cosmology has been missing observationally. Although previous numerical simulations have indicated that most of the missing baryons reside in large-scale filaments in the form of Warm Hot Intergalactic Medium (WHIM), it is generally very difficult to detect signatures from such a diffuse gas. In this work, we focus on the hyperfine transition of neutral hydrogen (HI) called 21-cm line as a tool to trace the WHIM. For the purpose, we first construct the map of the 21-cm signals by using the data provided by the state-of-the-art cosmological hydrodynamics simulation project, Illustris, in which detailed processes affecting the dynamical and thermal evolution of the WHIM are implemented. From the comparison with the constructed 21-cm signal map with the expected noise level of the Square Kilometre Array phase 1 mid-frequency instrument (SKA1-mid), we find that the 21-cm signals from the filamentary structures at redshifts z=0.5-3 are detectable with the SKA1-mid if we assume the angular resolution of \Delta\theta > 10 arcmin and the observing time of t_obs > 100 hours. However, it also turns out that the signals mainly come from galaxies residing in the filamentary structures and the contribution from the WHIM is too small to detect with the SKA1-mid. Our results suggest that about 10 times higher sensitivity than the SKA1-mid is possibly enough to detect the WHIM at z=0.5-3.