Abstract
ABSTRACT Spiral structure is ubiquitous in the Universe, and the pitch angle of arms in spiral galaxies provide an important observable in efforts to discriminate between different mechanisms of spiral arm formation and evolution. In this paper, we present a hierarchical Bayesian approach to galaxy pitch angle determination, using spiral arm data obtained through the Galaxy Builder citizen science project. We present a new approach to deal with the large variations in pitch angle between different arms in a single galaxy, which obtains full posterior distributions on parameters. We make use of our pitch angles to examine previously reported links between bulge and bar strength and pitch angle, finding no correlation in our data (with a caveat that we use observational proxies for both bulge size and bar strength which differ from other work). We test a recent model for spiral arm winding, which predicts uniformity of the cotangent of pitch angle between some unknown upper and lower limits, finding our observations are consistent with this model of transient and recurrent spiral pitch angle as long as the pitch angle at which most winding spirals dissipate or disappear is larger than 10°.
Highlights
Spiral structure is present in a majority of massive galaxies (e.g. Buta 1989, Lintott et al 2008) yet the formation mechanisms through which spiral structure originates are still hotly debated (e.g. Dobbs & Baba 2014)
We see a slight drop in the expectation values of galaxy pitch angle (E[φgal]) compared to the expectation of arm pitch angles (E[φarm]) at small galaxy pitch angles, which is caused by a combination of the truncation of φgal at 0◦ and the large spread
This paper presents a new Bayesian approach to estimate galaxy pitch angle, making use of citizen science results to measure spiral arms through photometric modelling
Summary
Spiral structure is present in a majority of massive galaxies (e.g. Buta 1989, Lintott et al 2008) yet the formation mechanisms through which spiral structure originates are still hotly debated (e.g. Dobbs & Baba 2014). The Hubble classification scheme (Hubble 1926) and its revisions and expansions (Sandage 1961; de Vaucouleurs et al 1991) contain detailed variations of different types of spiral galaxy, divided by the presence of a bar and ordered by the openness of the spiral arms, the degree of resolution into condensations, and the prominence of a central bulge. Elmegreen & Elmegreen (1982) found that flocculent spirals are more prevalent in unbarred, isolated galaxies. The presence of a bar, a binary companion or group membership result in a higher fraction of observed grand design spiral patterns.
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