Photometric Asymmetry Between Clockwise and Counterclockwise Spiral Galaxies in SDSS

Abstract

While galaxies with clockwise and counterclockwise handedness are visually different, they are expected to be symmetric in all of their other characteristics. Previous experiments using both manual analysis and machine vision have shown that the handedness of Sloan Digital Sky Survey (SDSS) galaxies can be predicted with accuracy significantly higher than mere chance using its photometric data alone, showing that the photometric pipeline of SDSS is sensitive to the handedness of the galaxy. However, some of these previous experiments were based on manually classified galaxies, and the results may therefore be subjected to bias originated from the human perception. This paper describes an experiment based on a set of 162,514 celestial objects classified as clockwise and counterclockwise spiral galaxies in a fully automatic process, showing that the source of the asymmetry in SDSS database is not the human perception bias. The results are compared to two smaller datasets, and confirm the observation that the handedness of galaxies in SDSS database can be predicted by the way SDSS measures their photometry, and show that the position angle of counterclockwise galaxies computed by SDSS photometry pipeline is consistently higher than the position angle computed for galaxies with clockwise patterns. The experiment also shows statistically significant differences in the measured magnitude of SDSS galaxies, according which galaxies with clockwise patterns imaged by SDSS are brighter than galaxies with counterclockwise patterns. The magnitude of that difference changes across RA ranges, and exhibits a strong correlation with the cosine of the right ascension.

Asymmetry between galaxies with different spin patterns: A comparison between COSMOS, SDSS, and Pan-STARRS

“…Mean, standard error of the mean, and the one-tail P value of the differences between the magnitude of clockwise galaxies and counterclockwise galaxies in COSMOS such that the 5% highest values and the 5% lowest values were removed. The SDSS data were taken from previous work (Shamir 2017c). The number of SDSS galaxies in that field is 5447 counterclockwise galaxies and 5774 clockwise galaxies.…”

“…More recent experiments with a much larger dataset (Kuminski & Shamir 2016;Paul et al 2018) of ∼ 1.62 • 10 5 galaxies showed a clear statistically significant difference (P< 10 −5 ) between the magnitude of galaxies with clockwise spin patterns and galaxies with counterclockwise spin patterns (Shamir 2017c). The experiments also showed that the photometric differences changed with the direction of observation (Shamir 2017c). These observations are consistent across SDSS and Pan-STARRS, both showing the same profile of the asymmetry (Shamir 2017b).…”

“…The manual annotation of clockwise and counterclockwise galaxies also led to substantial bias, as humans tend to misidentify galaxies that spin counterclockwise as elliptical galaxies (Hayes et al 2016), leading to a systematic bias in the dataset. Analysis (Shamir 2017b) of the average brightness of the clockwise and counterclockwise galaxies classified by Galaxy Zoo as "superclean" (meaning that 95% or more of the voters agreed on the annotation) showed that the difference was in agreement with the direction and magnitude of the difference observed in (Shamir 2017c), but due to the small size of the Galaxy Zoo "superclean" dataset no statistically significant difference or indifference could be determined (Shamir 2017b). Longo (2011) used four undergraduate students to sort SDSS galaxies by their spin pattern, and showed possible differences between the number of clockwise and counterclockwise galaxies in the dataset.…”

Analysis of the Alignment of Non-Random Patterns of Spin Directions in Populations of Spiral Galaxies

“…The dataset that was used in this study is a dataset of SDSS galaxies designed for experiments related to photometry of galaxies [21]. Here the dataset is tested for the identification of non-random distribution of galaxy spin directions, and for that purpose photometric objects that are part of the same galaxies were removed.…”

“…Since such subtle biases are often difficult to identify, the analysis method needs to be a symmetric method. More information about Ganalyzer is available in [19], and the process of the galaxy annotation is described in [21,22].…”