The small boxy/peanut structure of the Milky Way traced by old stars

TL;DR

This study reveals a peanut-shaped structure in the Milky Way's central region traced by RR Lyrae stars, showing a smaller physical size than previously observed, consistent with bar orbit fractionation.

Contribution

The paper provides new evidence for a peanut-shaped structure in the Milky Way's core using RR Lyrae stars and links it to bar orbit dynamics through simulations.

Findings

Detection of a peanut-shaped overdensity near the Galactic center

Measured physical separation of the peanut structure is ~0.7 kpc

Simulation supports formation via bar orbit fractionation

Abstract

We analyse the positions of RR Lyrae stars in the central region of the Milky Way. In addition to the overall bar shape detected previously, we find evidence for a peanut shaped structure, in form of overdensities near $\ell=-2$ deg and $1$ deg at $b\sim-3.5$ deg. The corresponding physical distance between the two peaks of the peanut is $\sim0.7$ kpc, significantly shorter than that found from near-IR images (3.3 kpc) and red-clump stars. Qualitatively this is expected from `fractionation' of bar orbits, which we demonstrate to be matched in a simulation of an inside-out growing disc subsequently forming a bar.