Asymmetry in the distribution of HSC galaxy spin directions: comment on arXiv:2410.18884v1

TL;DR

This paper discusses the observed asymmetry in galaxy spin directions across large sky surveys, critiques a new statistical method that fails to detect this asymmetry, and emphasizes the importance of conventional statistics in understanding large-scale cosmic structures.

Contribution

It critically evaluates a new complex statistical method claiming to show random galaxy spin distributions, demonstrating it cannot detect known asymmetries, thus reaffirming the significance of traditional statistical approaches.

Findings

Traditional statistics confirm asymmetry in galaxy spin directions.

New complex method fails to detect known asymmetries.

Asymmetry may be due to galaxy physics, not large-scale anomalies.

Abstract

In the past decade, an asymmetry in the large-scale distribution of galaxy spin directions has been observed in data from all relevant digital sky surveys, all showing a higher number of galaxies rotating in the opposite direction relative to the Milky Way as observed from Earth. Additionally, JWST deep fields have shown that the asymmetry is clear and obvious, and can be sensed even by the naked human eye. These experiments were performed using two separate statistical methods: standard binomial distribution and simple $\chi^2$ statistics. Stiskalek \& Desmond (2024) suggested that the asymmetry in the distribution of galaxy spin directions is due to the use of binomial or $\chi^2$ statistics. Instead, they developed a new complex ad-hoc statistical method that shows random distribution in galaxy spin directions, and specifically in data from HSC. Source code for the method was also made available. The primary downside of the new method is that it is not able to identify asymmetry in the distribution of galaxy spin directions. Even when the new method is provided with synthetic data with extreme and obvious asymmetry, it still reports a null-hypothesis Universe with random distribution. That shows empirically that the method cannot sense asymmetry in the distribution of the directions of rotation of galaxies. While this further concludes that the distribution of galaxy spin direction as observed from Earth is not symmetric, it is not necessarily an indication of an anomaly in the large-scale structure. The excessive number of galaxies that rotate in the opposite direction relative to the Milky Way can also be driven by the internal structure of galaxies and the physics of galaxy rotation. The phenomenon can be related to other puzzling anomalies such the Ho tension. Data are publicly available, and no code is needed to reproduce the results since only conventional statistics is used.