Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration

TL;DR

This paper analytically investigates how the efficiency of radial migration in spiral galaxies depends on the angular momentum distribution and velocity dispersion of stellar populations, confirming that hotter populations migrate less effectively.

Contribution

It introduces an analytic criterion for stellar orbit trapping at corotation and applies it to quantify how migration efficiency varies with velocity dispersion and spiral pattern strength.

Findings

Radial migration efficiency decreases with higher velocity dispersion.

Higher amplitude spiral patterns increase the fraction of trapped stars.

Analytic results align with simulation findings on migration dependence.

Abstract

The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with 'hotter' kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations.