The Gaia-ESO survey: Metal-rich bananas in the bulge

TL;DR

This study analyzes the kinematics of stars in the Galactic bulge, revealing that metal-rich stars support the boxy-peanut shape through resonant orbits, with distinct velocity dispersion profiles and velocity histogram features.

Contribution

It identifies the link between metal-rich star kinematics and resonant orbits supporting the bulge's shape, using Gaia-ESO data and simulations.

Findings

Metal-rich stars have a steeply decreasing velocity dispersion with latitude.

Peaky velocity features suggest stars on resonant orbits support the bulge shape.

Resonant orbit simulations match observed velocity histograms.

Abstract

We analyse the kinematics of $\sim 2000$ giant stars in the direction of the Galactic bulge, extracted from the Gaia-ESO survey in the region $-10^\circ \lesssim \ell \lesssim 10^\circ$ and $-11^\circ \lesssim b \lesssim -3^\circ$. We find distinct kinematic trends in the metal rich ($\mathrm{[M/H]}>0$) and metal poor ($\mathrm{[M/H]}<0$) stars in the data. The velocity dispersion of the metal-rich stars drops steeply with latitude, compared to a flat profile in the metal-poor stars, as has been seen previously. We argue that the metal-rich stars in this region are mostly on orbits that support the boxy-peanut shape of the bulge, which naturally explains the drop in their velocity dispersion profile with latitude. The metal rich stars also exhibit peaky features in their line-of-sight velocity histograms, particularly along the minor axis of the bulge. We propose that these features are due to stars on resonant orbits supporting the boxy-peanut bulge. This conjecture is strengthened through the comparison of the minor axis data with the velocity histograms of resonant orbits generated in simulations of buckled bars. The 'banana' or 2:1:2 orbits provide strongly bimodal histograms with narrow velocity peaks that resemble the Gaia-ESO metal-rich data.