Age distribution of stars in boxy/peanut/X-shaped bulges formed without bar buckling

TL;DR

This study uses simulations to show that in some barred galaxies, the boxy/peanut/X-shaped bulge is mainly composed of stars older than the bar, formed via vertical resonance heating without buckling, providing a way to date bar formation.

Contribution

It demonstrates through simulations that BPX-shaped bulges formed without buckling are dominated by pre-existing stars, offering a new observable method to determine bar formation timing.

Findings

BPX bulge stars are mostly older than the bar formation.

Stars near the bar ends have higher Jacobi energy, preventing them from reaching the BPX region.

The age difference between the NSD and BPX bulge can reveal bar formation time.

Abstract

Some barred galaxies, including the Milky Way, host a boxy/peanut/X-shaped bulge (BPX-shaped bulge). Previous studies suggested that the BPX-shaped bulge can either be developed by bar buckling or by vertical inner Lindblad resonance (vILR) heating without buckling. In this paper, we study the observable consequence of an BPX-shaped bulge built up quickly after bar formation via vILR heating without buckling, using an N-body/hydrodynamics simulation of an isolated Milky Way-like galaxy. We found that the BPX-shaped bulge is dominated by stars born prior to bar formation. This is because the bar suppresses star formation, except for the nuclear stellar disc (NSD) region and its tips. The stars formed near the bar ends have higher Jacobi energy, and when these stars lose their angular momentum, their non-circular energy increases to conserve Jacobi energy. This prevents them from reaching the vILR to be heated to the BPX region. By contrast, the NSD forms after the bar formation. From this simulation and general considerations, we expect that the age distributions of the NSD and BPX-shaped bulge formed without bar buckling do not overlap each other. Then, the transition age between these components betrays the formation time of the bar, and is testable in future observations of the Milky Way and extra-galactic barred galaxies.