On the relation between Lya absorbers and local galaxy filaments

Abstract: 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. Lya absorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Lya absorption, galaxies and the large-scale structure is observationally not well-constrained. In this study, we aim at characterising the relation between Lya absorbers and nearby overdense cosmological structures (galaxy filaments) at recession velocities Delta v \leq 6700 km/s by using archival observational data from various instruments. We analyse 587 intervening Lya absorbers in the spectra of 302 extragalactic background sources obtained with the COS installed on the HST. We combine the absorption-line information with galaxy data of five local galaxy filaments originally mapped by Courtois et al. (2013). Along the 91 sightlines that pass close to a filament, we identify 215 (227) Lya absorption systems (components). Among these, 74 Lya 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 Lya absorbers have a ~90 percent higher rate of incidence (dN/dz=189 for log N HI \geq 13.2) and a mildly shallower slope (-beta = -1.47) of the column density distribution function than the general Lya population at z=0, reflecting the filaments' matter overdensity. The strongest Lya 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 KS test), but the clustering signal is less pronounced than for the galaxies in the filaments.