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

Abstract

The nature of galaxy spin is still not fully known. Iye, Yagi, and Fukumoto (2021, AJ, 907, 123) 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 0.29σ. However, to show random distribution, two decisions were made, each of which 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 a low statistical signal under these conditions is expected. When using the exact same data without limiting to zphot < 0.1 and without using the photometric redshift, the distribution of the spin directions in that dataset shows a statistical signal of >2σ. 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 suggests that large-scale asymmetry between the number of clockwise and counterclockwise galaxies cannot be ruled out.

Reanalysis of the Spin Direction Distribution of Galaxy Zoo SDSS Spiral Galaxies

“…Analysis with a higher number of more than 6•10 3 galaxies from the southern hemisphere showed that the distribution is random [13]. As explained in [17], given the expected magnitude of the parity violation, the number of galaxies used in that study was too small to show a statistically signifcant parity violation. On the other hand, analysis of a larger number of galaxies from SDSS showed evidence of parity violation that forms a statistically signifcant dipole axis [1,69].…”

“…Tat study, however, does not refer to a specifc paper that claimed for the presence of a dipole axis formed by the distribution of galaxy spin directions and also had duplicate objects in the data. Also, a simple analysis showed that the "clean" data used in [16] is in fact not random [17]. Code and data to reproduce the analysis are available at https://people.cs.ksu.edu/~lshamir/ data/iye_et_al.…”

“…Te statistical strength of a dipole axis in that specifc dataset is > rbin2σ and therefore agrees with previous experiments that showed a dipole axis exhibited by the large-scale distribution of galaxy spin directions. More information about experiments and analysis of that dataset is provided in [17].…”

“…As stated in the paper, "we choose our attributes to include some photometric attributes that were disjoint with those that Shamir (2016) found to be correlated with chirality, in addition to several SPARCFIRE outputs with all chirality information removed" [15]. While that implementation decision led to a random distribution of the annotated spin directions, it is also not clear whether the selection of the spiral galaxies was biased due to the removal of features that correlate with the spin direction, as it is expected that the removal of these features would lead to an even distribution of the annotations [17]. Tat is, the diferences between galaxies with opposite spin directions could also be the source of an astronomical reason, rather than a bias in the algorithm.…”

“…Reasons can include datasets that are not sufciently large or biases in the data that their corrections require modifcations to the data. Tese reasons are summarized in Section 2, and a more detailed discussion can be found in [11,17,18].…”