The X-shaped Bulge of the Milky Way revealed by WISE

TL;DR

This paper uses WISE imaging to confirm the X-shaped structure of the Milky Way bulge, supporting theories that such morphology results from dynamical instabilities in the galaxy's disk.

Contribution

The study provides clear imaging evidence of the Milky Way's X-shaped bulge, reinforcing the link between bulge morphology and galaxy formation processes.

Findings

WISE images reveal the X-shape of the Milky Way bulge.

Supports the dynamical instability formation scenario.

Implications for galaxy formation theories.

Abstract

The Milky Way bulge has a boxy/peanut morphology and an X-shaped structure. This X-shape has been revealed by the `split in the red clump' from star counts along the line of sight toward the bulge, measured from photometric surveys. This boxy, X-shaped bulge morphology is not unique to the Milky Way and such bulges are observed in other barred spiral galaxies. N-body simulations show that boxy and X-shaped bulges are formed from the disk via dynamical instabilities. It has also been proposed that the Milky Way bulge is not X-shaped, but rather, the apparent split in the red clump stars is a consequence of different stellar populations, in an old classical spheroidal bulge. We present a WISE image of the Milky Way bulge, produced by downsampling the publicly available "unWISE" coadds. The WISE image of the Milky Way bulge shows that the X-shaped nature of the Milky Way bulge is self-evident and irrefutable. The X-shape morphology of the bulge in itself and the fraction of bulge stars that comprise orbits within this structure has important implications for the formation history of the Milky Way, and, given the ubiquity of boxy X-shaped bulges, spiral galaxies in general.