Using 3D and 2D analysis for analyzing large-scale asymmetry in galaxy spin directions

TL;DR

This study reanalyzes galaxy spin directions using 3D and 2D methods, revealing potential large-scale asymmetry that previous analyses might have missed due to methodological limitations.

Contribution

The paper demonstrates that using unfiltered data and exact redshift measurements can reveal significant asymmetry in galaxy spin directions, challenging prior conclusions of randomness.

Findings

Reanalysis shows >2σ asymmetry in galaxy spin directions.

Previous studies' limitations may have obscured real asymmetries.

Results align with other large-scale galaxy surveys.

Abstract

The nature of galaxy spin is still not fully known. Iye et al (2021) applied a 3D analysis to a dataset of bright SDSS galaxies that was used in the past for photometric analysis. They showed that the distribution of spin directions of spiral galaxies is random, providing a dipole axis with low statistical significance of 0.29$\sigma$. However, to show random distribution, two decisions were made, each can lead to random distribution regardless of the real distribution of the spin direction of galaxies. The first decision was to limit the dataset arbitrarily to z$<$0.1, which is a redshift range in which previous literature already showed that random distribution is expected. More importantly, while the 3D analysis requires the redshift of each galaxy, the analysis was done with the photometric redshift. If the asymmetry existed, its signal is expected to be an order of magnitude weaker than the error of the photometric redshift, and therefore the low statistical signal under these conditions is expected. When using the exact same data without limiting to $z_{phot}<0.1$ and without using the photometric redshift, the distribution of the spin directions in that dataset shows a statistical signal of $>2\sigma$. Code and data for reproducing the analysis are publicly available. These results are in agreement with other experiments with SDSS, Pan-STARRS, HST, and the DESI Legacy Survey. The paper also examines other previous studies that showed random distribution in galaxy spin directions. While further research will be required, the current evidence suggest that large-scale asymmetry between the number of clockwise and counterclockwise galaxies cannot be ruled out.