Automated Quantitative Description of Spiral Galaxy Arm-Segment Structure

We describe a system that builds quantitative structural descriptions of spiral galaxies. This enables translation of sky survey images into data needed to help address funda-mental astrophysical questions such as the origin of spi-ral structure—a phenomenon that has eluded full theoreti-cal description despite 150 years of study (Sellwood 2011). The difficulty of automated measurement is underscored by the fact that, to date, only manually-guided efforts (such as the citizen science project Galaxy Zoo) have been able to extract structural information about spiral galaxies. An automated approach is needed to eliminate measurement subjectivity and handle the otherwise-overwhelming image quantities (up to billions of images) from near-future sur-veys. Our approach automatically describes spiral galaxy structure as a set of arcs fit to pixel clusters, precisely char-acterizing spiral arm segment arrangement while retaining the flexibility needed to accommodate the observed wide va-riety of spiral galaxy structure. The largest existing quan-titative measurements were manually-guided and encom-passed fewer than 100 galaxies, while we have already ap-plied our method to nearly 30,000 galaxies. Our output is consistent with previous information, both quantitatively over small existing samples, and qualitatively with human classifications. 1