Galaxy Zoo: comparing the demographics of spiral arm number and a new method for correcting redshift bias

Abstract

The majority of galaxies in the local Universe exhibit spiral structure with\na variety of forms. Many galaxies possess two prominent spiral arms, some have\nmore, while others display a many-armed flocculent appearance. Spiral arms are\nassociated with enhanced gas content and star-formation in the disks of\nlow-redshift galaxies, so are important in the understanding of star-formation\nin the local universe. As both the visual appearance of spiral structure, and\nthe mechanisms responsible for it vary from galaxy to galaxy, a reliable method\nfor defining spiral samples with different visual morphologies is required. In\nthis paper, we develop a new debiasing method to reliably correct for\nredshift-dependent bias in Galaxy Zoo 2, and release the new set of debiased\nclassifications. Using these, a luminosity-limited sample of ~18,000 Sloan\nDigital Sky Survey spiral galaxies is defined, which are then further\nsub-categorised by spiral arm number. In order to explore how different spiral\ngalaxies form, the demographics of spiral galaxies with different spiral arm\nnumbers are compared. It is found that whilst all spiral galaxies occupy\nsimilar ranges of stellar mass and environment, many-armed galaxies display\nmuch bluer colours than their two-armed counterparts. We conclude that\ntwo-armed structure is ubiquitous in star-forming disks, whereas many-armed\nspiral structure appears to be a short-lived phase, associated with more\nrecent, stochastic star-formation activity.\n