Spirals in Galaxies

Abstract

Spirals in galaxies have long been thought to be caused by gravitational instability in the stellar component of the disk, but discerning the precise mechanism had proved elusive. Tidal interactions, and perhaps bars, may provoke some spiral responses, but spirals in many galaxies must be self-excited. We survey the relevant observational data and aspects of disk dynamical theory. The origin of the recurring spiral patterns in simulations of isolated disk galaxies has recently become clear, and it is likely that the mechanism is the same in real galaxies, although evidence to confirm this supposition is hard to obtain. As transient spiral activity increases random motion, the patterns must fade over time unless the disk also contains a dissipative gas component. Continuing spiral activity alters the structure of the disks in other ways: reducing metallicity gradients and flattening rotation curves are two of the most significant. The overwhelming majority of spirals in galaxies have two- or three-fold rotational symmetry, indicating that the cool, thin disk component is massive. Spirals in simulations of halo-dominated disks instead manifest many arms and, consequently, do not capture the expected full spiral-driven evolution. We conclude by identifying areas where further work is needed. Expected final online publication date for the Annual Review of Astronomy Volume 60 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Effect of Environment on Galaxy Spiral Arms, Bars, Concentration, and Quenching

“…Yu & Ho (2020) suggest that in galaxies with small bulges, spiral patterns are instead produced by random gravitational instabilities in the disk. Models of spiral structure produced by self-gravity in isolated differentially rotating disks are able to reproduce the multiple short arm fragments seen in flocculent galaxies, supporting this interpretation (Sellwood 2011;Grand et al 2012;D'Onghia et al 2013;Dobbs et al 2018;Sellwood & Masters 2022).…”

“…For galaxies with small bulges, Yu & Ho (2020) suggest that the spiral patterns are instead produced by random gravitational instabilities in the disk. Models of this process (Sellwood 2011;Grand et al 2012;D'Onghia et al 2013;Dobbs et al 2018;Sellwood & Masters 2022) produce multiple short arm fragments similar to those seen in flocculent galaxies. Flocculent galaxies tend to have smaller bulges Bittner et al 2017;Díaz-García et al 2019), consistent with this idea.…”

“…The galaxies marked by open magenta diamonds were classified by Yu & Ho (2020) as either S0-, S0, or SB0. (Binney & Tremaine 2008;Fujii et al 2011;Sellwood & Masters 2022). We do not find strong evidence for a time lag between the buildup of the bulge and the weakening of the arms, or vice versa.…”

Spiral instabilities: Mode saturation and decay

“…It is to be hoped that the same mechanism also accounts for the origin of self-excited spiral patterns in galaxies, but supporting observational evidence for this, or any other suggested excitation mechanism for spirals in galaxies, remains flimsy. See Sellwood & Masters (2022) for a full review of both the observed properties of spirals in galaxies and theoretical ideas to account for them.…”

“…However the quiescent "basic state" they invoke for the disc is at variance with the unrelaxed phase space in the local disc of the Milky Way that was recently reported by from an actionangle analysis of Gaia DR2 data (Gaia collaboration 2018). Furthermore, this theory disregards the more vigorous insta-1 Reproduced as http://www.physics.rutgers.edu/∼sellwood/supp material.html, and in the supplementary material of Sellwood & Masters (2022).…”