Can HI 21-cm lines trace the missing baryons in the filamentary structures?

Abstract

A large fraction of baryons predicted from the standard cosmology is still missing observationally. Although previous numerical simulations have indicated that most of the missing baryons reside in large-scale filaments in the form of the 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 the 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 a comparison of 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 WHIM in the filamentary structures at redshifts z = 0.5–3 are too small to detect with the SKA1-mid. Our results suggest that a sensitivity about 10 times higher than the SKA1-mid is possibly enough to detect the WHIM at z = 0.5–3. However, we also show that the signals of the filamentary structures including the contribution from galaxies are detectable with the SKA1-mid if we assume the angular resolution of θ ≥ 10 and the observing time of tobs ≥ 100 hr.