Galaxy alignment as a probe of large-scale filaments

TL;DR

This paper introduces a novel location-alignment-method (LAM) that uses the orientations of red galaxies to detect large-scale filaments around galaxy clusters, demonstrating its effectiveness with the Coma cluster.

Contribution

The paper presents the first use of red galaxy orientations as probes for filament detection, providing a new, redshift-independent method applicable to high-redshift galaxy surveys.

Findings

LAM effectively detects filaments with 3σ confidence.

LAM reveals filament orientations and structures around clusters.

Background galaxy lensing can enhance filament detection.

Abstract

The orientations of the red galaxies in a filament are aligned with the orientation of the filament. We thus develop a location-alignment-method (LAM) of detecting filaments around clusters of galaxies, which uses both the alignments of red galaxies and their distributions in two-dimensional images. For the first time, the orientations of red galaxies are used as probes of filaments. We apply LAM to the environment of Coma cluster, and find four filaments (two filaments are located in sheets) in two selected regions, which are compared with the filaments detected with the method of \cite{Falco14}. We find that LAM can effectively detect the filaments around a cluster, even with $3\sigma$ confidence level, and clearly reveal the number and overall orientations of the detected filaments. LAM is independent of the redshifts of galaxies, and thus can be applied at relatively high redshifts and to the samples of red galaxies without the information of redshifts. We also find that the images of background galaxies (interlopers) which are lensed by the gravity of foreground filaments are amplifiers to probe the filaments.