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_HI=(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^-1₇₀ Mpc if interpreted as the Hubble flow (Ω_m=0.3, Ω_Λ=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^-1₇₀ 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_HI 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_HI>10¹⁹ atoms cm^-2. Consequently, this rare configuration of four high-N_HI 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^-1₇₀ 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.

Based on data obtained from the Sloan Digital Sky Survey and on observations made with the Hubble Space Telescope operated by STScI-AURA, Inc., for NASA, the MMT (using NOAO public access time) operated by the Smithsonian Institution and the University of Arizona, and the Infrared Telescope Facility operated by the University of Hawaii, Institute for Astronomy, for NASA.