Insights on the Milky Way bulge formation from the correlations between kinematics and metallicity ^*

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France e-mail: [Carine.Babusiaux;Ana.Gomez;Frederic.Royer;Frederic.Arenou]@obspm.fr
² Laboratoire CASSIOPEE, University of Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France e-mail: vanessa.hill@oca.eu
³ P. Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile e-mail: [mzoccali;dante]@astro.puc.cl
⁴ Observatoire de Genève, Université de Genève, 51 Ch des Maillettes, 1290 Sauverny, Switzerland
⁵ Observatoire de Besançon, CNRS UMR6091, BP 1615, 25010 Besançon, France e-mail: mathias.schultheis@obs-besancon.fr
⁶ Universidade de São Paulo, IAG, Rua do Matão 1226, São Paulo 05508-900, Brazil e-mail: barbuy@astro.iag.usp.br
⁷ Vatican Observatory, V00120 Vatican City State, Italy
⁸ Universita di Padova,Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: sergio.ortolani@unipd.it

Received: 4 March 2010
Accepted: 12 May 2010

Abstract

Context. Two main scenarios for the formation of the Galactic bulge are invoked, the first one through gravitational collapse or hierarchical merging of subclumps, the second through secular evolution of the Galactic disc.

Aims. We aim to constrain the formation of the Galactic bulge through studies of the correlation between kinematics and metallicities in Baade's Window (l = 1°, b = -4°) and two other fields along the bulge minor axis (l = 0°, b = -6° and b = -12°).

Methods. We combine the radial velocity and the [Fe/H] measurements obtained with FLAMES/GIRAFFE at the VLT with a spectral resolution of R = 20 000, plus for the Baade's Window field the OGLE-II proper motions, and compare these with published N-body simulations of the Galactic bulge.

Results. We confirm the presence of two distinct populations in Baade's Window found in Hill et al. (2010, A&A, submitted): the metal-rich population presents bar-like kinematics while the metal-poor population shows kinematics corresponding to an old spheroid or a thick disc. In this context the metallicity gradient along the bulge minor axis observed by Zoccali et al. (2008, A&A, 486, 177), visible also in the kinematics, can be related to a varying mix of these two populations as one moves away from the Galactic plane, alleviating the apparent contradiction between the kinematic evidence of a bar and the existence of a metallicity gradient.

Conclusions. We show evidence that the two main scenarios for the bulge formation co-exist within the Milky Way bulge.