Spirality: A Novel Way to Measure Spiral Arm Pitch Angle
Deanna Shields,
Benjamin Boe,
Casey Pfountz,
Benjamin L. Davis,
Matthew Hartley,
Ryan Miller,
Zac Slade,
M. Shameer Abdeen,
Daniel Kennefick,
Julia Kennefick
Affiliations
Deanna Shields
Department of Physics & Arkansas Center for Planetary Sciences, University of Arkansas, 226 Physics Building, 835 West Dickson Street, Fayetteville, AR 72701, USA
Benjamin Boe
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
Casey Pfountz
Department of Medicinal Chemistry, The University of Kansas, 1251 Wescoe Hall Drive, 4070 Malott Hall, Lawrence, KS 66045-7582, USA
Benjamin L. Davis
Center for Astro, Particle and Planetary Physics (CAP<sup>3</sup>), New York University Abu Dhabi, Abu Dhabi 129188, United Arab Emirates
Matthew Hartley
Physics Department, California Institute of Technology, 103-33, Pasadena, CA 91125, USA
Ryan Miller
NASA Headquarters, 300 E. Street SW, Washington, DC 20546, USA
Zac Slade
Free Geek of Arkansas, 521 W Ash Street, Fayetteville, AR 72703, USA
M. Shameer Abdeen
Department of Physics and Astronomy, Georgia Southern University, Statesboro, GA 30458, USA
Daniel Kennefick
Department of Physics & Arkansas Center for Planetary Sciences, University of Arkansas, 226 Physics Building, 835 West Dickson Street, Fayetteville, AR 72701, USA
Julia Kennefick
Department of Physics & Arkansas Center for Planetary Sciences, University of Arkansas, 226 Physics Building, 835 West Dickson Street, Fayetteville, AR 72701, USA
We present the MATLAB code Spirality, a novel method for measuring spiral arm pitch angles by fitting galaxy images to spiral templates of known pitch. Computation time is typically on the order of 2 min per galaxy, assuming 8 GB of working memory. We tested the code using 117 synthetic spiral images with known pitches, varying both the spiral properties and the input parameters. The code yielded correct results for all synthetic spirals with galaxy-like properties. We also compared the code’s results to two-dimensional Fast Fourier Transform (2DFFT) measurements for the sample of nearby galaxies defined by DMS PPak. Spirality’s error bars overlapped 2DFFT’s error bars for 26 of the 30 galaxies. The two methods’ agreement correlates strongly with galaxy radius in pixels and also with i-band magnitude, but not with redshift, a result that is consistent with at least some galaxies’ spiral structure being fully formed by z=1.2, beyond which there are few galaxies in our sample. The Spirality code package also includes GenSpiral, which produces FITS images of synthetic spirals, and SpiralArmCount, which uses a one-dimensional Fast Fourier Transform to count the spiral arms of a galaxy after its pitch is determined. All code is freely available.
