Branch, Spur, and Feather Formation in Spiral Galaxies

Abstract

We use hydrodynamic simulations to investigate the response of geometrically thin, self-gravitating, singular isothermal disks of gas to imposed rigidly rotating spiral potentials. By minimizing reflection-induced feedback from boundaries, and by restricting our attention to models where the swing parameter X $ 10, we minimize the swing amplification of global normal modes even in models where Toomre's Q g $ 1 2 in the gas disk. We perform two classes of simulations: short-term ones over a few galactic revolutions where the background spiral forcing is large, and long-term ones over many galactic revolutions where the spiral forcing is considerably smaller. In both classes of simulations, the initial response of the gas disk is smooth and mimics the driving spiral field. At late times, many of the models evince substructure akin to the so-called branches, spurs, and feathers observed in real spiral galaxies. We comment on the parts played respectively by ultraharmonic resonances, reflection off internal features produced by nonlinear dredging, and local, transient, gravitational instabilities within spiral arms in the generation of such features. Our simulations reinforce the idea that spiral structure in the gaseous component becomes increasingly flocculent and disordered with the passage of time, even when the background population of old disk stars is a grand-design spiral. We speculate that truly chaotic behavior arises when many overlapping ultraharmonic resonances develop in reaction to an imposed spiral forcing that has itself a nonlinear, yet smooth, wave profile.

Giant molecular filaments in the Milky Way

“…Inter-arm cloud populations have been scrutinised observationally in external galaxies (Elmegreen 1980;Aalto et al 1999;Scoville et al 2001;Shetty et al 2007;Foyle et al 2010;Hughes et al 2013;Colombo et al 2014), in the Milky Way (Roman-Duval et al 2010;Moore et al 2012;Eden et al 2013) and in numerical simulations (e.g., Kim & Ostriker 2002;Chakrabarti et al 2003;Dobbs et al 2006;Shetty & Ostriker 2006;Smith et al 2014). The history of inter-arm clouds is related to so called "spurs" or "feathers" emanating from the spiral arms, which have been credited to either the growth of gravitational or magneto-Jeans instabilities preferentially perpendicular to the arm (Balbus 1988) or alternatively, to the rotational shearing of over-densities in the spiral arm itself (Dobbs et al 2006;Shetty & Ostriker 2006).…”

Dawes Review 4: Spiral Structures in Disc Galaxies

“…Chakrabarti et al (2003) showed the development of more varied features, including branches (bifurcations) and shorter leading and trailing features (spurs / feathers), again occurring primarily near the 4:1 resonance, with the morphology of the feature dependent primarily on the level of forcing of the spiral potential. Chakrabarti et al (2003) suppose that flocculence in spiral galaxies could be due largely to such resonant features, an idea recently followed up by Lee & Shu (2012), where they investigate the possibility that higher order resonances lead to the formation of multiple spurs along the arms. There is a notable difference between the work of Lee & Shu (2012), and GMC formation by gravitational instabilities in the gas or cloud-cloud collisions (which are subsequently sheared into spurs).…”

Globular Clusters and Spur Clusters in NGC 4921, the Brightest Spiral Galaxy in the Coma Cluster

“…Numerical simulation studies suggested several mechanisms to explain the origin of spurs. First, self-gravity was proposed as a mechanism to form spurs by Chakrabarti et al (2003). Later, Kelvin-Helmholtz instability was introduced to explain wiggle structures in the arms seen in the simulations by Wada & Koda (2004).…”