The Baryon Content of the Local Lyα Forest

Abstract

Our recent survey of low column density (NHI ≤ 1014.5 cm-2) HI Lyα absorbers made with the Hubble Space Telescope (HST) has allowed us a first look at gas in the local intergalactic medium (IGM). Because the cosmic evolution of the number density (dN/dz) of lower column density absorbers found by our survey is much slower than that of the high column density absorbers, the baryon content of the Lyα forest is even more dominated by low column density absorbers at z ∼ 0 than at high-z. Using two different methods for modelling these clouds, we find that the photoionized local Lyα forest can account for 32±6% of all local baryons. This number can increase significantly if these absorbers are hotter than expected or if there are a significant number of collisionally ionized, ‘warm-hot’ clouds. While this number is consistent with the amount of baryons found in the photoionized forest by numerical simulations, it is difficult to determine which Lyα absorptions correspond to which population of absorbers in the simulations, so that exact comparisons are not yet possible.