A re-assessment of SpArcFiRe’s performance on toy spiral galaxies

Abstract

ABSTRACT Recently, a study (H&T in this work) compared the pitch angle measurements of several methods on toy spiral galaxies. They claimed that their method outperformed the other methods. In particular, they claimed that our method, SpArcFiRe, either failed completely or produced erroneous results on many of their toy galaxies. In this paper, we assess the validity of that claim with respect to: (1) their toy models as realistic approximations of galaxies, and (2) the handling of SpArcFiRe’s parameters to correctly trace the arms of the toy galaxies. We demonstrate that the structure displayed in their toy galaxies deviates significantly from those of real galaxies. Given that SpArcFiRe was designed for and tested against hundreds of thousands of images of real galaxies, SpArcFiRe could not have been expected to work well with default parameters on H&T’s unrealistic toy galaxies. We then review the parameters that affected SpArcFiRe’s ability to measure pitch angles in the toy galaxies, and show that with minor parameter adjustments, SpArcFiRe easily measures the pitch angles of the toy galaxies with mean error less than 1° and maximum error less than 4°, completely eliminating the failures and outliers discussed by H&T.