Secular Attrition of Classical Bulges by Stellar Bars

TL;DR

This paper demonstrates through simulations and observations that stellar bars can secularly transform classical bulges into disk-like structures by resonant trapping, explaining their scarcity in the universe.

Contribution

It provides the first quantitative analysis of how stellar bars can reconfigure classical bulges via resonant trapping, supported by N-body simulations and galaxy observations.

Findings

Up to 50% of bulge stars become resonantly trapped and adopt disk-like kinematics.

Slow bars are linked to weaker bulges in observed galaxies.

Long-lived bars can significantly reshape classical bulges, reducing their observational prominence.

Abstract

Classical bulges and stellar bars are common features in disk galaxies and serve as key tracers of galactic evolution. Angular momentum exchange at bar resonances drives secular morphological changes throughout the disk, including bar slowing and lengthening, and affects the structure of accompanying bulges. In this study, using a suite of N-body simulations, we quantify the secular reconfiguration of classical bulges through resonant trapping by evolving stellar bars. We use orbital frequency analysis to identify bar-resonant populations and find that up to 50% of the initial bulge stars become trapped in 2:1 resonant orbits and adopt disk-like kinematics. This transformation renders much of the classical bulge observationally indistinguishable from the disk. We compare these results with a sample of 210 MaNGA disk galaxies, finding that slow bars--indicative of older systems--are preferentially associated with weaker bulges. These results suggest that long-lived bars can significantly reshape classical bulges, potentially explaining their scarcity in the local universe and the low classical bulge fraction found in the Milky Way.