Transient spiral arms from far out of equilibrium gravitational evolution

Abstract

We describe how a simple class of out of equilibrium, rotating and asymmetrical mass distributions evolve under their self-gravity to produce a quasi-planar spiral structure surrounding a virialized core, qualitatively resembling a spiral galaxy. The spiral structure is transient, but can survive tens of dynamical times, and further reproduces qualitatively noted features of spiral galaxies as the predominance of trailing two-armed spirals and large pitch angles. As our models are highly idealized, a detailed comparison with observations is not appropriate, but generic features of the velocity distributions can be identified to be potential observational signatures of such a mechanism. Indeed, the mechanism leads generically to a characteristic transition from predominantly rotational motion, in a region outside the core, to radial ballistic motion in the outermost parts. Such radial motions are excluded in our Galaxy up to 15 kpc, but could be detected at larger scales in the future by GAIA. We explore the apparent motions seen by external observers of the velocity distributions of our toy galaxies, and find that it is difficult to distinguish them from those of a rotating disc with sub-dominant radial motions at levels typically inferred from observations. These simple models illustrate the possibility that the observed apparent motions of spiral galaxies might be explained by non-trivial non-stationary mass and velocity distributions without invoking a dark matter halo or modification of Newtonian gravity. In this scenario the observed phenomenological relation between the centripetal and gravitational acceleration of the visible baryonic mass could have a simple explanation.

14 pages, 9 figures, The Astrophysical Journal in press. Movies of the simulation are available at http://goo.gl/L1fRzZ. The replaced version matches the small changes in the published one

References in corpus (9)

• The Gaia mission
• The Radial Acceleration Relation in Rotationally Supported Galaxies
• Dawes Review 4: Spiral Structures in Disc Galaxies
• Spiral arms in the disk of HD 142527 from CO emission lines with ALMA
• Rotation and Mass in the Milky Way and Spiral Galaxies
• Violent and mild relaxation of an isolated self-gravitating uniform and spherical cloud of particles
• Universal properties of violently relaxed gravitational structures
• Violent relaxation of ellipsoidal clouds
• Formation of satellites from cold collapse

Cited by in corpus (9)

• Gaia-DR2 extended kinematical maps. Part II: Dynamics in the Galactic disk explaining radial and vertical velocities
• Alternative ideas in cosmology
• Gaia-DR2 extended kinematical maps. Part III: Rotation curves analysis, dark matter, and Modified Newtonian dynamics tests
• Long-lived transient structure in collisionless self-gravitating systems
• Radial velocities in the outermost disk toward the anticenter
• Violent relaxation in the Hamiltonian Mean Field model: I. Cold collapse and effective dissipation
• Formation of disks with long-lived spiral arms from violent gravitational dynamics
• Properties of self-gravitating quasi-stationary states
• Formation of spiral structure from the violent relaxation of self-gravitating disks