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Large scale filaments, with lengths that can reach tens of Mpc, are the mostprominent features in the cosmic web. These filaments have only been observedindirectly through the positions of galaxies in large galaxy surveys or throughabsorption features in the spectra of high redshift sources. In this study wepropose to go one step further and directly detect intergalactic mediumfilaments through their emission in the HI 21cm line. We make use of highresolution cosmological simulations to estimate the intensity of this emissionin low redshift filaments and use it to make predictions for the directdetectability of specific filaments previously inferred from galaxy surveys, inparticular the Sloan Digital Sky Survey. Given the expected signal of thesefilaments our study shows that HI emission from large filaments can be observedby current and next generation radio telescopes. We estimate that gas infilaments of length $l \gtrsim$ 15 $h^{-1}$Mpc with relatively smallinclinations to the line of sight ($\lesssim 10^\circ$) can be observed in$\sim40-100$ hours with telescopes such as GMRT or EVLA, potentially providinglarge improvements over our knowledge of the astrophysical properties of thesefilaments. Due to their large field of view and sufficiently long integrationtimes, upcoming HI surveys with the Apertif and ASKAP instruments will be ableto detect large filaments independently of their orientation and curvature.Furthermore, our estimates indicate that a more powerful future radio telescopelike SKA-2 can be used to map most of these filaments, which will allow them tobe used as a strong cosmological probe.

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