# Filament Hunting: Integrated HI 21cm Emission From Filaments Inferred by Galaxy Surveys

## TL;DR

This paper proposes a method to directly detect cosmic web filaments through their HI 21cm emission using current and upcoming radio telescopes, enhancing understanding of large-scale structures.

## Contribution

It provides simulation-based predictions for detecting intergalactic filaments via HI emission, outlining observational strategies with existing and future radio telescopes.

## Key findings

- HI emission from filaments can be detected with current telescopes like GMRT and EVLA.
- Upcoming surveys with Apertif and ASKAP can detect filaments regardless of orientation.
- Future telescopes like SKA-2 will enable comprehensive mapping of cosmic filaments.

## Abstract

Large scale filaments, with lengths that can reach tens of Mpc, are the most prominent features in the cosmic web. These filaments have only been observed indirectly through the positions of galaxies in large galaxy surveys or through absorption features in the spectra of high redshift sources. In this study we propose to go one step further and directly detect intergalactic medium filaments through their emission in the HI 21cm line. We make use of high resolution cosmological simulations to estimate the intensity of this emission in low redshift filaments and use it to make predictions for the direct detectability of specific filaments previously inferred from galaxy surveys, in particular the Sloan Digital Sky Survey. Given the expected signal of these filaments our study shows that HI emission from large filaments can be observed by current and next generation radio telescopes. We estimate that gas in filaments of length $l \gtrsim$ 15 $h^{-1}$Mpc with relatively small inclinations to the line of sight ($\lesssim 10^\circ$) can be observed in $\sim40-100$ hours with telescopes such as GMRT or EVLA, potentially providing large improvements over our knowledge of the astrophysical properties of these filaments. Due to their large field of view and sufficiently long integration times, upcoming HI surveys with the Apertif and ASKAP instruments will be able to detect large filaments independently of their orientation and curvature. Furthermore, our estimates indicate that a more powerful future radio telescope like SKA-2 can be used to map most of these filaments, which will allow them to be used as a strong cosmological probe.