Galaxies - Группа авторов

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m = 2 n =±1, and the outer and inner 4:1 resonances (O4R and I4R, respectively), for which m = 4 and n =±1. In any normal galaxy, there will likely be two ILRs, with the innermost one called the inner ILR (or IILR) and the outermost one called the outer ILR (or OILR). Another important (but not necessarily ring-forming) resonance is the corotation resonance (CR), where Ωp = Ω. The locations of all of these resonances can be determined by plotting the curves Ω and Ω ± κ/m versus galactocentric radius and reading the radii where these functions equal Ωp.

      Support for the resonance idea comes principally from (a) the morphology of galactic rings (Buta and Crocker 1991; Buta 1995b); (2) statistical studies of intrinsic shapes and orientations of galactic rings with respect to bars (Buta 1995b; Comerón et al. 2014); and (3) detailed studies and numerical modeling of individual cases such as NGC 3081 (Buta and Purcell 1998) and NGC 6782 (Lin et al. 2008). Rings are relatively narrow features, most frequently associated with gas. In particular, nuclear rings are often conspicuous sites of star formation. In general, rings form when gas is accumulating in the bar resonance locations, since the gravity torques from the bar are then canceling out. The angular momentum transport is discussed in detail in Buta and Combes (1996).

Photos depict three galaxies showing outer resonant subclass features.

      The theoretical work of Zhang (2018 and references therein) challenges previous views on galaxy dynamics and provides a real mechanism for the secular evolution of the stellar distribution of a spiral galaxy, not just the interstellar gas distribution. The galaxy-disk mass distribution, contributed mostly by stellar mass, is part of the so-called “basic state” used to calculate spiral density wave perturbations. In addition to the axisymmetric (i.e. azimuthally averaged) mass distribution, determined observationally from disk surface brightness and color, the basic state specification includes also the axisymmetric rotation speed (with contributions from the halo and bulge, in addition to the contribution from the disk) and velocity dispersion, with all three usually specified as a function of galactocentric radius. The key factor for the evolution of the basic-state mass distribution is an azimuthal phase shift (phase offset in angles) between a self-consistent spiral perturbation potential, and the density this potential gives rise to. The spiral can arise as a mode in an initially featureless disk, and if it can achieve a quasi-steady state, angular momentum can be taken away from the inner disk, transferred outward and deposited onto the outer disk, leading to the slow buildup of a central mass concentration and an extended outer disk.

      The main departure of Zhang’s work from previous studies of galactic dynamics is the elevated role of collective effects on the dynamical evolution of galaxies. This refers to the correlated small-angle scatterings stars would experience as they move across the spiral density wave crest. The density wave itself was shown in Zhang’s work to be collisionless shocks related to “dissipative structures”. Another important aspect of the work is that the phase shift is not simply an abstract concept from potential theory, but is manifestly measurable from any image which approximates the stellar mass distribution. From such images, one can derive a graph of the phase shift as a function of radius, and in such a graph a single mode appears as a single positive bump followed by a single negative bump (e.g. Buta and Zhang 2009). The point where the phase shift changes from positive to negative is the location of the mode’s CR, which is generally considered the most important resonance for any model pattern in a galaxy. This location is also the divide between radial mass inflow and outflow patterns.

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