Scattering, Migration, Re-circularization and Relaxation to Build Out Galaxy Disks with Exponential Profiles

TL;DR

This paper demonstrates that star scattering by interstellar clouds leads to a steady-state eccentricity distribution, which helps maintain exponential stellar disks by recircularizing orbits and regulating disk heating.

Contribution

It introduces a simplified model showing how cloud-star scattering naturally produces and sustains exponential galactic disks through recircularization and equilibrium eccentricity distribution.

Findings

Scattering reaches a steady-state eccentricity distribution.

Recircularization maintains stellar velocity dispersion within limits.

Disk evolution bifurcates, leading to different end states.

Abstract

Scattering of stars by interstellar clouds or massive clumps increases the stellar velocity dispersion and promotes a radial disk profile that is exponential. Here we show that such scattering reaches a steady-state distribution function of stellar eccentricity, after which eccentricity increases and decreases occur at equal rates. The implication is that clump/cloud scattering recircularizes eccentric stellar orbits, keeping the stellar velocity dispersion in a limited range. This re-circularization regulates disk heating and maintains kinematic coherence, contributing to the longevity of disk structures. The eccentricity distribution function and the presence of recircularizing cloud-star interactions are independent of cloud mass but the timescale to reach equilibrium decreases with increasing mass. The calculations are made in the simplest possible disk system to highlight the effects of scattering without contamination from spiral waves, star formation, and other processes. The calculations also reveal a bifurcation in the disk evolutions whereby in a minority of cases temporary asymmetries in the clump spatial distribution drive the disks to an end state of increased velocity dispersion and orbital eccentricity corresponding to early type disks. Overall the models emphasize an important physical process that can make and maintain an exponential stellar disk in all galaxies with a cloudy interstellar medium.