Dynamics of barred galaxies

Abstract

Some 30% of disc galaxies have a pronounced central bar feature in the disc plane and many more have weaker features of a similar kind. Kinematic data indicate that the bar constitutes a major non-axisymmetric component of the mass distribution and that the bar pattern tumbles rapidly about the axis normal to the disc plane. The observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis. A barred galaxy may also contain a spheroidal bulge at its centre, spirals in the outer disc and, less commonly, other features such as a ring or lens. Mild asymmetries in both the light and kinematics are quite common.We review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure. Bars are probably formed through a global dynamical instability of a rotationally supported galactic disc. Studies of the orbital structure seem to indicate that most stars in the bar follow regular orbits but that a small fraction may be stochastic. Theoretical work on the three-dimensional structure of bars is in its infancy, but first results suggest that bars should be thicker in the third dimension than the disc from which they formed. Gas flow patterns within bars seem to be reasonably well understood, as are the conditions under which straight offset dust lanes are formed. However, no observation so far supports the widely held idea that the spiral arms are the driven response to the bar, while evidence accumulates that the spiral patterns are distinct dynamical features having a different pattern speed. Both the gaseous and stellar distributions are expected to evolve on a time-scale of many bar rotation periods.

A Unified Framework for the Orbital Structure of Bars and Triaxial Ellipsoids

“…The disk formed a bar thatsubsequently experienced vertical thickening via the buckling instability (Toomre 1966, p. 111;Raha et al 1991;Sellwood & Wilkinson 1993) and developed the peanut-shaped bulge that is characteristic of this instability. After the disk reached a nearly steady state (simulation time t 1 =700), a central point mass 5 representing an SMBH of mass 0.2% M d was grown adiabatically (for details see Brown et al 2013).…”

“…In the characteristic diagrams (e.g., Sellwood & Wilkinson 1993) the prograde x2 family is found at low E J and is elongated along the y-axis of the bar, while the x4 family is retrograde and is found over a wide entire range of E J values. Both the x2 and x4 orbits are elongated along the yaxis at small E J .…”

“…For two-dimensional bars it is customary to construct an SoS by plotting the velocity component V y as a function of y every time an orbit intersects the x-axis with a negative value of V x (e.g., Sellwood & Wilkinson 1993). In three-dimensional bars one might choose to restrict orbits plotted on an SoS to those that are confined to the equatorial (i.e., x-y) plane of the model (e.g., Shen & Sellwood 2004).…”

“…The most important periodic families in two-dimensional bars were identified as the prograde x1 family, which is elongated along the major axis of the bar,andthe prograde stable x2and unstable x3 families, which are elongated perpendicular to the bar (primarily found at small radii). The retrograde x4 (stable) orbit family is also elongated perpendicular to the bar at small radii, but becomes rounder as it extends to larger radii (for detailed description of orbit families and how they are identified see Contopoulos & Grosbol 1989;Sellwood & Wilkinson 1993;Binney & Tremaine 2008;Sellwood 2014b). In the frame of reference rotating with the bar, all of these families are characterized by a 1:2 resonance between the tangential oscillation frequency (Ω f ) and the radial or epicyclic frequency (Ω R ).…”

Bars, Spiral Structure, and Secular Evolution in Disk Galaxies

“…They can become more squared off at the ends, or boxy, as a result of increasing populations of 4:1 orbits. Orbit families are described in Sellwood & Wilkinson (1993) and Athanassoula (2005). Gas accretion to the center and subsequent star formation can lead to a dense nucleus, which either forms an ILR or shifts the existing ILR to a larger radius.…”