Comparison of galaxy spiral arm pitch angle measurements using manual and automated techniques

Abstract

The pitch angle (PA) of arms in spiral galaxies has been found to correlate with a number of important parameters that are normally time intensive and difficult to measure. Accurate PA measurements are therefore important in understanding the underlying physics of disc galaxies. We introduce a semi-automated method that improves upon a parallelized twodimensional fast Fourier transform algorithm (P2DFFT) to estimate PA. Rather than directly inputting deprojected, star subtracted, and galaxy centred images into P2DFFT, our method (P2DFFT:TRACED) takes visually traced spiral arms from deprojected galaxy images as input. The tracings do not require extensive expertise to complete. This procedure ignores foreground stars, bulge and/or bar structures, and allows for better discrimination between arm and interarm regions, all of which reduce noise in the results.We compare P2DFFT:TRACED to other manual and automated methods of measuring PA using both simple barred and non-barred spiral galaxy models and a small sample of observed spiral galaxies with different representative morphologies.We find that P2DFFT:TRACED produces results that, in general, are more accurate and precise than the other tested methods and it strikes a balance between total automation and time-consuming manual input to give reliable PA measurements.