The alignment of galaxy spin with the shear field in observations

TL;DR

This study investigates the alignment between galaxy spins and the cosmic shear field using observed peculiar velocities, revealing significant alignments for elliptical galaxies and weak signals for spirals, thus providing insights into galaxy formation and evolution.

Contribution

First to analyze galaxy spin alignments with the velocity-shear field using observational data, confirming alignments in ellipticals and weak signals in spirals.

Findings

Elliptical galaxies align with principal axes of shear field.

Spiral galaxies show no initial alignment, but a weak signal emerges in a subsample.

Alignment signals are stronger when considering galaxies used in velocity field reconstruction.

Abstract

Tidal torque theory suggests that galaxies gain angular momentum in the linear stage of structure formation. Such a theory predicts alignments between the spin of haloes and tidal shear field. However, non-linear evolution and angular momentum acquisition may alter this prediction significantly. In this paper, we use a reconstruction of the cosmic shear field from observed peculiar velocities combined with spin axes extracted from galaxies within $115\, \mathrm{Mpc} $ ($\sim8000 \, {\mathrm {km}}{\mathrm s}^{-1}$) from 2MRS catalog, to test whether or not galaxies appear aligned with principal axes of shear field. Although linear reconstructions of the tidal field have looked at similar issues, this is the first such study to examine galaxy alignments with velocity-shear field. Ellipticals in the 2MRS sample, show a statistically significant alignment with two of the principal axes of the shear field. In general, elliptical galaxies have their short axis aligned with the axis of greatest compression and perpendicular to the axis of slowest compression. Spiral galaxies show no signal. Such an alignment is significantly strengthened when considering only those galaxies that are used in velocity field reconstruction. When examining such a subsample, a weak alignment with the axis of greatest compression emerges for spiral galaxies as well. This result indicates that although velocity field reconstructions still rely on fairly noisy and sparse data, the underlying alignment with shear field is strong enough to be visible even when small numbers of galaxies are considered - especially if those galaxies are used as constraints in the reconstruction.