Double-damped Lyα Absorption: A Possible Large Neutral Hydrogen Gas Filament near Redshift z = 1

Abstract

We report the discovery of two damped Lyα systems (DLAs) near redshift z = 1 along a single quasar sight line (Q1727+5302) with neutral hydrogen column densities of N = (1.45 ± 0.15) × 1021 and (2.60 ± 0.20) × 1021 atoms cm-2. Their sight line velocity difference of 13,000 km s-1 corresponds to a proper separation of 106 h Mpc if interpreted as the Hubble flow (Ωm = 0.3, ΩΛ = 0.7). The random probability of such an occurrence is significantly less than 3%. Follow-up spectroscopy reveals neutral gas-phase Zn abundances of [Zn/H] = -0.58 ± 0.15 (26.5% solar) and -1.32 ± 0.28 (4.7% solar), respectively. The corresponding Cr abundances are [Cr/H] = -1.26 ± 0.15 (5.5% solar) and -1.77 ± 0.28 (1.7% solar), respectively, which is evidence for depletion onto grains. Follow-up IR images show the two most likely DLA galaxy candidates to have impact parameters of ≈22 and ≈32 h kpc if near z = 1. They are significantly underluminous relative to the galaxy population at z = 1. To investigate the possibility of additional high-N absorbers, we have searched the Sloan Digital Sky Survey database for z > 1 quasars within 30' of the original sight line. Five were found, and two show strong Mg II-Fe II absorption near z = 1, consistent with classical damped Lyα absorption ≈37% of the time, but almost always N > 1019 atoms cm-2. Consequently, this rare configuration of four high-N absorbers with a total sight line velocity extent of 30,600 km s-1 may represent a large filament-like structure stretching over a proper distance of 241 h Mpc along our sight line, and a region in space capable of harboring excessive amounts of neutral gas. Future studies of this region of the sky are encouraged.