Asymmetry between galaxies with clockwise handedness and counterclockwise handedness

TL;DR

This study demonstrates that photometric data from SDSS can distinguish between clockwise and counterclockwise spiral galaxies with about 64-65% accuracy, revealing an asymmetry that challenges the assumption of symmetry in galaxy characteristics.

Contribution

The paper introduces a method to classify galaxy handedness using photometric data, showing that galaxy asymmetry can be detected automatically and may bias citizen science classifications.

Findings

Photometric data can predict galaxy handedness with ~64-65% accuracy.

Automatic classifications show consistent asymmetry, indicating it's not due to human bias.

Galaxy morphology classifications may be biased by galaxy handedness.

Abstract

While it is clear that spiral galaxies can have different handedness, galaxies with clockwise patterns are assumed to be symmetric in all of their other characteristics to galaxies with counterclockwise patterns. Here we use data from SDSS DR7 to show that photometric data can distinguish between clockwise and counterclockwise galaxies. Pattern recognition algorithms trained and tested using the photometric data of a clean manually crafted dataset of 13,440 spiral galaxies with z<0.25 can predict the handedness of a spiral galaxy in ~64% of the cases, significantly higher than mere chance accuracy of 50% (P<10^{-5}). Experiments with a different dataset of 10,281 automatically classified galaxies showed similar results of $~65% classification accuracy, suggesting that the observed asymmetry is consistent also in datasets annotated in a fully automatic process, and without human intervention. That shows that the photometric data collected by SDSS is sensitive to the handedness of the galaxy. Also, analysis of the number of galaxies classified as clockwise and counterclockwise by crowdsourcing shows that manual classification between spiral and elliptical galaxies can be affected by the handedness of the galaxy, and therefore galaxy morphology analyzed by citizen science campaigns might be biased by the galaxy handedness. Code and data used in the experiment are publicly available, and the experiment can be easily replicated.