The Variation of Galaxy Morphological Type with Environmental Shear

Abstract

Recent N-body simulations indicate that the assembly history of galactic halos depends on the density of the large-scale environment. This implies that galaxy properties such as age and size of the bulge may also vary with the surrounding large-scale structures, which are characterized by tidal shear as well as density. 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, we investigate the dependence of galaxy morphological type on the shear of the large-scale environment in which they are embedded. We first calculate the large-scale dimensionless overdensities (δ) and ellipticities (e) of the regions where the galaxies are located, classify them according to morphological type, and create subsamples selected at similar δ-values but spanning different ranges in e. We calculate the mean ellipticity, e, averaged over each subsample and find a signal of variation of e with morphological type: for 0.5 ≤ δ ≤ 1.0, elliptical galaxies are preferentially located in regions with low ellipticity; for –0.3 ≤ δ ≤ 0.1, the latest-type spirals are preferentially located in 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 the same as the global mean ellipticity averaged over all types is rejected at the 3 σ level when –0.3 ≤ δ ≤ 0.1. Yet, no signal of a galaxy-shear correlation is found in highly overdense or underdense regions. The observed trend suggests that the formation epochs of galactic halos might be a function of not only halo mass and large-scale density but also large-scale shear. Since the statistical significance of the overall trend is low, a sample of at least 100,000 galaxies is required to verify this correlation.