  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.
