The Gaia-ESO survey: Metal-rich bananas in the bulge
astro-ph.GA
/ Authors
Angus A. Williams, N. W. Evans, Matthew Molloy, Georges Kordopatis, M. C. Smith, J. Shen, G. Gilmore, S. Randich, T. Bensby, P. Francois
and 18 more authors
S. E Koposov, A. Recio-Blanco, A. Bayo, G. Carraro, A. Casey, T. Costado, E. Franciosini, A. Hourihane, P. de Laverny, J. Lewis, K. Lind, L. Magrini, L. Monaco, L. Morbidelli, G. G Sacco, C. Worley, S. Zaggia, S. Mikolaitis
/ 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.