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

Williams, Angus A.; Evans, N. W.; Molloy, Matthew; Kordopatis, Georges; Smith, M. C.; Shen, J.; Gilmore, G.; Randich, S.; Bensby, T.; Francois, P.; Koposov, S. E; Recio-Blanco, A.; Bayo, A.; Carraro, G.; Casey, A.; Costado, T.; Franciosini, E.; Hourihane, A.; de Laverny, P.; Lewis, J.; Lind, K.; Magrini, L.; Monaco, L.; Morbidelli, L.; Sacco, G. G; Worley, C.; Zaggia, S.; Mikolaitis, S.
The Astrophysical Journal Letters, Volume 824, Issue 2, article id. L29, pp. (2016).


We analyze the kinematics of ∼2000 giant stars in the direction of the Galactic bulge, extracted from the Gaia-ESO survey in the region -10^\circ ≲ {\ell }≲ 10^\circ and -11^\circ ≲ b≲ -3^\circ . We find distinct kinematic trends in the metal-rich ([{{M}}/{{H}}]\gt 0) and metal-poor ([{{M}}/{{H}}]\lt 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.