Spin-up of low mass classical bulges in barred galaxies

TL;DR

This study uses high-resolution N-body simulations to show how a low-mass classical bulge in barred galaxies can absorb angular momentum from the bar, transforming into a rotating, triaxial structure with cylindrical rotation, indicating secular evolution.

Contribution

It demonstrates the dynamical transformation of a non-rotating classical bulge into a rotating, triaxial structure through angular momentum exchange with the bar, revealing new insights into secular galaxy evolution.

Findings

Classical bulge gains angular momentum via resonances.

Bulge develops cylindrical rotation over time.

Bulge transforms into a fast rotating, triaxial object.

Abstract

Secular evolution is one of the key routes through which galaxies evolve along the Hubble sequence. Not only the disk undergoes morphological and kinematic changes, but also a preexisting classical bulge may be dynamically changed by the secular processes driven primarily by the bar. We study the influence of a growing bar on the dynamical evolution of a low mass classical bulge such as might be present in galaxies like the Milky Way. Using self-consistent high resolution {\it N}-body simulations, we study how an initially isotropic non-rotating small classical bulge absorbs angular momentum emitted by the bar. The basic mechanism of this angular momentum exchange is through resonances and a considerable fraction of the angular momentum is channeled through Lagrange point (-1:1) and ILR (2:1) orbits. In the phase of rapid dynamical growth, also retrograde non-resonant orbits absorb significant angular momentum. As a result of this angular momentum gain, the initially non-rotating classical bulge transforms into a fast rotating, radially anisotropic and triaxial object, embedded in the similarly fast rotating boxy bulge formed from the disk. Towards the end of the evolution, the classical bulge develops cylindrical rotation. By that time, its inner regions host a "classical bulge-bar" whose distinct kinematics could serve as direct observational evidence for the secular evolution in the galaxy. Some implications of these results are discussed briefly.