Our Milky Way as a Pure-Disk Galaxy -- A Challenge for Galaxy Formation

TL;DR

This study models the Milky Way's bulge as a bar structure formed through internal dynamics, challenging the traditional view that bulges are primarily merger remnants, and shows the galaxy lacks a significant classical bulge.

Contribution

It provides the first detailed N-body model of the Milky Way's bulge as a pure disk bar, with no need for a classical bulge component, redefining galaxy formation understanding.

Findings

The Milky Way's bulge is a bar, not a classical bulge.

The classical bulge contribution is less than 8% of the disk mass.

The model fits stellar kinematic data remarkably well.

Abstract

Bulges are commonly believed to form in the dynamical violence of galaxy collisions and mergers. Here we model the stellar kinematics of the Bulge Radial Velocity Assay (BRAVA), and find no sign that the Milky Way contains a classical bulge formed by scrambling pre-existing disks of stars in major mergers. Rather, the bulge appears to be a bar, seen somewhat end-on, as hinted from its asymmetric boxy shape. We construct a simple but realistic N-body model of the Galaxy that self-consistently develops a bar. The bar immediately buckles and thickens in the vertical direction. As seen from the Sun, the result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar kinematic data covering the whole bulge strikingly well with no need for a merger-made classical bulge. The bar in our best fit model has a half-length of ~ 4kpc and extends 20 degrees from the Sun-Galactic Center line. We use the new kinematic constraints to show that any classical bulge contribution cannot be larger than ~ 8% of the disk mass. Thus the Galactic bulge is a part of the disk and not a separate component made in a prior merger. Giant, pure-disk galaxies like our own present a major challenge to the standard picture in which galaxy formation is dominated by hierarchical clustering and galaxy mergers.