The relation between Lyα absorbers and local galaxy filaments

Abstract

Context. The intergalactic medium (IGM) is believed to contain the majority of baryons in the universe and to trace the same dark matter structure as galaxies, forming filaments and sheets. Lyα absorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Lyα absorption, galaxies, and the large-scale structure is observationally not well constrained. Aims. In this study, we aim at characterising the relation between Lyα absorbers and nearby over-dense cosmological structures (galaxy filaments) at recession velocities Δv ≤ 6700 km s−1 by using archival observational data from various instruments. Methods. We analyse 587 intervening Lyα absorbers in the spectra of 302 extragalactic background sources obtained with the Cosmic Origins Spectrograph (COS) installed on the Hubble Space Telescope (HST). We combine the absorption line information with galaxy data of five local galaxy filaments from the V8k catalogue. Results. Along the 91 sightlines that pass close to a filament, we identify 215 (227) Lyα absorption systems (components). Among these, 74 Lyα systems are aligned in position and velocity with the galaxy filaments, indicating that these absorbers and the galaxies trace the same large-scale structure. The filament-aligned Lyα absorbers have a ∼90% higher rate of incidence (d𝒩/dz = 189 for log N(H I) ≥ 13.2) and a slightly shallower column density distribution function slope (−β = −1.47) relative to the general Lyα population at z = 0, reflecting the filaments’ matter over-density. The strongest Lyα absorbers are preferentially found near galaxies or close to the axis of a filament, although there is substantial scatter in this relation. Our sample of absorbers clusters more strongly around filament axes than a randomly distributed sample would do (as confirmed by a Kolmogorov–Smirnov test), but the clustering signal is less pronounced than for the galaxies in the filaments.