Large-scale asymmetry in galaxy spin directions: evidence from the Southern hemisphere

TL;DR

This study analyzes a large dataset of galaxy images from the Southern hemisphere, revealing a significant large-scale asymmetry in galaxy spin directions that aligns with previous findings and suggests a possible cosmological anisotropy.

Contribution

It presents the first large-scale analysis of galaxy spin asymmetry in the Southern hemisphere using the DESI Legacy Survey, confirming previous asymmetry patterns with a significantly larger dataset.

Findings

Strong asymmetry in galaxy spin directions observed.

The dipole axis aligns near the CMB Cold Spot.

Asymmetry profiles are consistent across multiple telescopes.

Abstract

Recent observations using several different telescopes and sky surveys showed patterns of asymmetry in the distribution of galaxies by their spin directions as observed from Earth. These studies were done with data imaged from the Northern hemisphere, showing excellent agreement between different telescopes and different analysis methods. Here, data from the DESI Legacy Survey was used. The initial dataset contains $\sim2.2\cdot10^7$ galaxy images, reduced to $\sim8.1\cdot10^5$ galaxies annotated by their spin direction using a symmetric algorithm. That makes it not just the first analysis of its kind in which the majority of the galaxies are in the Southern hemisphere, but also by far the largest dataset used for this purpose to date. The results show strong agreement between opposite parts of the sky, such that the asymmetry in one part of the sky is similar to the inverse asymmetry in the corresponding part of the sky in the opposite hemisphere. Fitting the distribution of galaxy spin directions to cosine dependence shows a dipole axis with probability of 4.66$\sigma$. Interestingly, the location of the most likely axis is within close proximity to the CMB Cold Spot. The profile of the distribution is nearly identical to the asymmetry profile of the distribution identified in Pan-STARRS, and it is within 1$\sigma$ difference from the distribution profile in SDSS and HST. All four telescopes show similar large-scale profile of asymmetry.