Analysis of spin directions of galaxies in the DESI Legacy Survey

TL;DR

This study analyzes the spin directions of nearly 1.3 million spiral galaxies from the DESI Legacy Survey, revealing hemispheric asymmetries and a potential large-scale dipole alignment in galaxy angular momentum.

Contribution

It provides a large-scale, model-free analysis of galaxy spin directions using a vast dataset, confirming hemispheric asymmetries and a dipole alignment with minimal bias.

Findings

Higher number of counterclockwise galaxies in the Northern hemisphere

Higher number of clockwise galaxies in the Southern hemisphere

Evidence of a dipole axis alignment with P<10^{-5}

Abstract

The DESI Legacy Survey is a digital sky survey with a large footprint compared to other Earth-based surveys, covering both the Northern and Southern hemispheres. This paper shows the distribution of the spin directions of spiral galaxies imaged by DESI Legacy Survey. A simple analysis of dividing nearly 1.3$\cdot10^6$ spiral galaxies into two hemispheres shows a higher number of galaxies spinning counterclockwise in the Northern hemisphere, and a higher number of galaxies spinning clockwise in the Southern hemisphere. That distribution is consistent with previous observations, but uses a far larger number of galaxies and a larger footprint. The larger footprint allows a comprehensive analysis without the need to fit the distribution into an a priori model, making this study different from all previous analyses of this kind. Fitting the spin directions of the galaxies to cosine dependence shows a dipole axis alignment with probability of $P<10^{-5}$. The analysis is done with a trivial selection of the galaxies, as well as simple explainable annotation algorithm that does not make use of any form of machine learning, deep learning, or pattern recognition. While further work will be required, these results are aligned with previous studies suggesting the possibility of a large-scale alignment of galaxy angular momentum.