The Variation of Galaxy Morphological Type with the Shear of Environment

TL;DR

This study investigates how galaxy morphological types correlate with the shear of their large-scale environment, revealing that galaxy shapes are influenced by the tidal shear, which affects galaxy formation and evolution.

Contribution

It provides the first observational evidence linking galaxy morphology to large-scale shear, extending understanding beyond density dependence.

Findings

Ellipticals prefer low-shear regions at moderate densities.

Late-type spirals are found in high-shear regions at low densities.

No significant galaxy-shear correlation in extreme density regions.

Abstract

Recent N-body simulations have shown that the assembly history of galactic halos depend on the density of large-scale environment. It implies that the galaxy properties like age and size of bulge may also vary with the surrounding large-scale structures, which are characterized by the tidal shear as well as the density. By using a sample of 15,882 well-resolved nearby galaxies from the Tully Catalog and the real space tidal field reconstructed from the 2Mass Redshift Survey (2MRS), we investigate the dependence of galaxy morphological type on the shear of large-scale environment where the galaxies are embedded. We first calculate the large scale dimensionless overdensities (d) and the large-scale ellipticities (e) of the regions where the Tully galaxies are located and classify the Tully galaxies according to their morphological types and create subsamples selected at similar value of d but span different ranges in e. We calculate the mean ellipticity, <e>, averaged over each subsample and find a signal of variation of <e> with galaxy morphological type: For the case of 0.5<=d<=1.0, the ellipticals are found to be preferentially located in the regions with low ellipticity. For the case of -0.3<=d<=0.1, the latest-type spirals are found to be preferentially located in the regions with high ellipticity. The null hypothesis that the mean ellipticities of the regions where the ellipticals and the latest type spirals are located are same as the global mean ellipticity averaged over all types is rejected at 3 sigma level when -0.3<=d<=0.1. Yet, no signal of galaxy-shear correlation is found in the highly overdense/underdense regions. The observed trend suggests that the formation epochs of galactic halos might be a function not only of halo mass and large-scale density but also of large-scale shear.