How to bend galaxy disc profiles II: stars surfing the bar in Type-III discs

TL;DR

This paper investigates how strong, long-lived bars in disc galaxies can cause stars to migrate outward and develop anti-truncated (Type-III) profiles, linking galaxy dynamics to observed stellar distributions.

Contribution

It identifies the physical mechanism involving bar-driven stellar migration that leads to Type-III galaxy profiles, supported by detailed simulation analysis.

Findings

Stars in Type-III outskirts are on eccentric orbits.

Stars were born on near-circular orbits at smaller radii.

Bar interactions cause outward migration and increased eccentricity.

Abstract

The radial profiles of stars in disc galaxies are observed to be either purely exponential (Type-I), truncated (Type-II) or anti-truncated (Type-III) exponentials. Controlled formation simulations of isolated galaxies can reproduce all of these profile types by varying a single parameter, the initial halo spin. In this paper we examine these simulations in more detail in an effort to identify the physical mechanism that leads to the formation of Type-III profiles. The stars in the anti-truncated outskirts of such discs are now on eccentric orbits, but were born on near-circular orbits at much smaller radii. We show that, and explain how, they were driven to the outskirts via non-linear interactions with a strong and long-lived central bar, which greatly boosted their semi-major axis but also their eccentricity. While bars have been known to cause radial heating and outward migration to stellar orbits, we link this effect to the formation of Type-III profiles. This predicts that the anti-truncated parts of galaxies have unusual kinematics for disc-like stellar configurations: high radial velocity dispersions and slow net rotation. Whether such discs exist in nature, can be tested by future observations.