The Statistical Properties of Spiral Arms in Nearby Disk Galaxies

Abstract

Abstract We analyze 4378 nearby bright disk (S0 and spiral) galaxies selected from the Sloan Digital Sky Survey to characterize their spiral structure and systematically investigate the relationships between spiral arm properties (strength, pitch angle, and number) and the global properties of the host galaxies. This is to date the largest sample of galaxies with comprehensive measurements of spiral properties based on Fourier decomposition. We find that spiral pitch angle decreases (arms become more tightly wound) toward galaxies of earlier Hubble type, higher stellar mass, redder color, and greater degree of central concentration. The dependence of pitch angle on stellar mass is mainly driven by color, and for a given concentration, spiral arms are more tightly wound in redder galaxies. Spiral pitch angles show a bimodal distribution, peaking at φ = 12° ± 3.°4 and 23° ± 4.°3. Spiral arms weaken in galaxies of earlier type, redder color, and higher concentration, but for a given concentration, spiral arms become more prominent in more massive and bluer galaxies. We confirm that spiral arm strength loosely correlates with pitch angle, although the scatter in this relation is considerable and depends systematically on mass and color; for a given pitch angle, more massive or bluer galaxies have stronger arms. Galaxies of earlier Hubble type, higher mass concentration, or redder color have fewer spiral arms. Lastly, galaxies with stronger bars have fewer but stronger arms. We discuss the possible origins of these various relationships, which provide useful empirical guidance to theories of spiral structure. Depending on the galaxy, both the density wave and swing amplification mechanisms operate.