Why the Milky Way's bulge is not only a bar formed from a cold thin disk

TL;DR

This paper uses N-body simulations and observational data to argue that the Milky Way's bulge is not solely a bar formed from a thin disk, but also includes a thick disk component, challenging previous models.

Contribution

It provides evidence that the Milky Way's bulge comprises both a bar from a thin disk and a thick disk component, refining our understanding of galactic bulge formation.

Findings

The bulge is not a pure bar from a thin disk.

The old thick disk contributes to the bulge population.

The Milky Way is a thin+thick disk galaxy with limited classical bulge.

Abstract

By analyzing a N-body simulation of a bulge formed simply via a bar instability mechanism operating on a kinematically cold stellar disk, and by comparing the results of this analysis with the structural and kinematic properties of the main stellar populations of the Milky Way bulge, we conclude that the bulge of our Galaxy is not a pure stellar bar formed from a pre-existing thin stellar disk, as some studies have recently suggested. On the basis of several arguments emphasized in this paper, we propose that the bulge population which, in the Milky Way, is observed not to be part of the peanut structure corresponds to the old galactic thick disk, thus implying that the Milky Way is a pure thin+thick disk galaxy, with only a possible limited contribution of a classical bulge.