A ~6 Mpc overdensity at z = 2.7 detected along a pair of quasar sight lines: filament or protocluster?

TL;DR

This study reports the detection of a large-scale (~6 Mpc) gas overdensity at z=2.7 along two quasar sight lines, suggesting the presence of a cosmic filament or protocluster, based on absorption features in the intergalactic medium.

Contribution

First observational evidence of a ~6 Mpc IGM overdensity at high redshift along a quasar pair, indicating filamentary or protocluster structures.

Findings

Detected three main absorption regions spanning ~2000 km/s

Two regions are mild overdensities with low metallicity

One region is a sub-DLA likely associated with a galaxy

Abstract

Simulations predict that gas in the intergalactic medium (IGM) is distributed in filamentary structures that connect dense galaxy clusters and form the cosmic web. These structures of predominantly ionized hydrogen are difficult to observe directly due to their lack of emitting regions. We serendipitously detected an overdensity of log N(HI) > 18.0 absorbers at z = 2.69 along the lines of sight toward a pair of background quasars. Three main absorption regions spanning ~2000 km/s (corresponding to 6.4 (h_70)^(-1) Mpc proper) are coincident in the two lines of sight, which are separated by ~90 (h_70)^(-1) kpc transverse proper distance. Two regions have [Fe/H] < -1.9 and correspond to mild overdensities in the IGM gas. The third region is a sub-DLA with [Fe/H] = -1.1 that is probably associated with a galaxy. We discuss the possibility that the lines of sight probe along the length of a filament or intercept a galaxy protocluster.