On the nature and correction of the spurious S-wise spiral galaxy winding bias in Galaxy Zoo 1

Abstract

The Galaxy Zoo 1 catalog displays a bias towards the S-wise winding direction in spiral galaxies which has yet to be explained. The lack of an explanation confounds our attempts to verify the Cosmological Principle, and has spurred some debate as to whether a bias exists in the real universe. The bias manifests not only in the obvious case of trying to decide if the universe as a whole has a winding bias, but also in the more insidious case of selecting which galaxies to include in a winding direction survey. While the former bias has been accounted for in a previous image-mirroring study, the latter has not. Furthermore, the bias has never been corrected in the GZ1 catalog, as only a small sample of the GZ1 catalog was re-examined during the mirror study. We show that the existing bias is a human selection effect rather than a human chirality bias. In effect, the excess S-wise votes are spuriously "stolen" from the elliptical and edge-on-disk categories, not the Z-wise category. Thus, when selecting a set of spiral galaxies by imposing a threshold T so that max(P S , P Z ) > T or P S +P Z > T , we spuriously select more S-wise than Z-wise galaxies. We show that when a provably unbiased machine selects which galaxies are spirals independent of their chirality, the Swise surplus vanishes, even if humans are still used to determine the chirality. Thus, when viewed across the entire GZ1 sample (and by implication, the Sloan catalog), the winding direction of arms in spiral galaxies as viewed from Earth is consistent with the flip of a fair coin.

Enhancing Automatic Prediction of Spirality using SpArcFiRe's Spiral Arm Analysis and Random Forests

“…Our results show that it is possible to have a solid model that is in agreement with human classifiers above 90% of the time and also deal with the winding bias problem which was addressed in more detail in [7]. In this sense, we "filter" the errors made by humans while still retaining the useful knowledge provided by the Galaxy Zoo.…”

“…This value, between 0 and 1, is represented by the percentage of humans that agree that a certain galaxy has visible spiral structure. We represent this idea by making the sum of GZ1 values P S + P Z as our target variable, and this is what we train our machine to reproducewhile simultaneously striving to eliminate the known P S bias [6,7].…”

“…Since humans introduce certain types of biases into the classification scheme (for example the chirality bias [5][6][7]), we also wish to "dilute" such biases even though we train our method on human classifications. We do this by carefully choosing which inputs we allow our code to use.…”

“…We do this by carefully choosing which inputs we allow our code to use. For example, we allow SpArcFiRe's measured pitch angle of spiral arms to be used as input to our machine learning classifier, but not the sign of the pitch angle [7], thus reducing chirality discrepancies to about 2 parts in 10,000 [3,7]. Our work follows up on existing work published in the astronomical literature [8-13].…”

“…The main difference here is that they are not concerned with the bias present in the dataset, so the smaller their Root Mean Squared Error (RMSE) is, "the better" their results are. In the recent Galaxy Zoo dataset releases, there has been an increased effort to eliminate human biases, but Hayes et al [7] have proven that these datasets, in particular, GZ1, still contain biases so there is a trade-off between lowering the RMSE of a model and avoiding the introduction of such biases on the prediction. Table 1.…”