Reanalysis of the spin direction distribution of Galaxy Zoo SDSS spiral galaxies

TL;DR

This study reanalyzes the distribution of galaxy spin directions in SDSS data, confirming a non-random dipole pattern across different selection and analysis methods, supporting previous findings of large-scale cosmic anisotropy.

Contribution

It applies a consistent analysis pipeline to Galaxy Zoo data, demonstrating the robustness of the observed dipole pattern in galaxy spin directions regardless of selection method.

Findings

Galaxy spin directions show a non-random dipole pattern.

The probability of parity violation by chance is less than 0.01.

The dipole axis exhibits a statistical strength between 2.33σ and 3.97σ.

Abstract

The distribution of the spin directions of spiral galaxies in the Sloan Digital Sky Survey has been a topic of debate in the past two decades, with conflicting conclusions reported even in cases where the same data was used. Here we follow one of the previous experiments by applying the SpArcFiRe algorithm to annotate the spin directions in original dataset of Galaxy Zoo 1. The annotation of the galaxy spin directions is done after a first step of selecting the spiral galaxies in three different manners: manual analysis by Galaxy Zoo classifications, by a model-driven computer analysis, and with no selection of spiral galaxies. The results show that when spiral galaxies are selected by Galaxy Zoo volunteers, the distribution of their spin directions as determined by SpArcFiRe is not random, which agrees with previous reports. When selecting the spiral galaxies using a model-driven computer analysis or without selecting the spiral galaxies at all, the distribution is also not random. Simple binomial distribution analysis shows that the probability of the parity violation to occur by chance is lower than 0.01. Fitting the spin directions as observed from Earth to cosine dependence exhibits a dipole axis with statistical strength of 2.33$\sigma$ to 3.97$\sigma$. These experiments show that regardless of the selection mechanism and the analysis method, all experiments show similar conclusions. These results are aligned with previous reports using other methods and telescopes, suggesting that the spin directions of spiral galaxies as observed from Earth exhibit a dipole axis formed by their spin directions. Possible explanations can be related to the large-scale structure of the Universe, or to internal structure of galaxies. The catalogs of annotated galaxies generated as part of this study are available.