Alignments of the Galaxies in and around the Virgo Cluster with the Local Velocity Shear

TL;DR

This study provides observational evidence that galaxy alignments around the Virgo cluster correlate with the local velocity shear and cosmic web structures, supporting the idea that velocity shear traces the cosmic web.

Contribution

It presents the first observational detection of galaxy alignments with the local velocity shear and cosmic web structures around the Virgo cluster.

Findings

Galaxies in Virgo are aligned with the local velocity shear axes.

Dwarf satellites show stronger alignment than normal satellites.

The local filament and sheet are aligned with specific shear axes.

Abstract

An observational evidence is presented for the alignment between the cosmic sheet and the principal axis of the velocity shear field at the position of the Virgo cluster. The galaxies in and around the Virgo cluster from the Extended Virgo Cluster Catalog recently constructed by Kim et al. are used to determine the direction of the local sheet. The peculiar velocity field reconstructed from the Sloan Digital Sky Survey Data Release 7 is analyzed to estimate the local velocity shear tensor at the Virgo center. Showing first that the minor principal axis of the local velocity shear tensor is almost parallel to the line of sight direction, we detect a clear signal of alignment between the positions of the Virgo satellites and the intermediate principal axis of the local velocity shear projected onto the plane of the sky. Furthermore, the dwarf satellites are found to appear more strongly aligned than the normal counterparts, which is interpreted as indication of the following: (i) The normal and the dwarf satellites fall in the Virgo cluster preferentially along the local filament and the local sheet, respectively. (ii) The local filament is aligned with the minor principal axis of the local velocity shear while the local sheet is in parallel to the plane spanned by the minor and the intermediate principal axes. Our result is consistent with the recent numerical claim that the velocity shear is a good tracer of the cosmic web.