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\nin spiral galaxies which has yet to be explained. The lack of an explanation\nconfounds our attempts to verify the Cosmological Principle, and has spurred\nsome debate as to whether a bias exists in the real universe. The bias\nmanifests not only in the obvious case of trying to decide if the universe as a\nwhole has a winding bias, but also in the more insidious case of selecting\nwhich galaxies to include in a winding direction survey. While the former bias\nhas been accounted for in a previous image-mirroring study, the latter has not.\nFurthermore, the bias has never been {\\em corrected} in the GZ1 catalog, as\nonly a small sample of the GZ1 catalog was re-examined during the mirror study.\nWe show that the existing bias is a human {\\em selection} effect rather than a\nhuman chirality bias. In effect, the excess S-wise votes are spuriously\n"stolen" from the elliptical and edge-on-disk categories, not the Z-wise\ncategory. Thus, when selecting a set of spiral galaxies by imposing a threshold\n$T$ so that $\\max(P_S,P_Z) > T$ or $P_S+P_Z>T$, we spuriously select more\nS-wise than Z-wise galaxies. We show that when a provably unbiased machine\nselects which galaxies are spirals independent of their chirality, the S-wise\nsurplus vanishes, even if humans are still used to determine the chirality.\nThus, when viewed across the entire GZ1 sample (and by implication, the Sloan\ncatalog), the winding direction of arms in spiral galaxies as viewed from Earth\nis consistent with the flip of a fair coin.\n