Why the Milky Way’s bulge is not only a bar formed from a cold thin disk

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
e-mail: paola.dimatteo@obspm.fr
² LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
³ Institut d’Astrophysique de Paris, UMR 7095, CNRS, 98bis boulevard Arago, 75014 Paris, France
⁴ Sorbonne Universités, UPMC Univ. Paris 6, UMR 8112, LERMA, 75005 Paris, France
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA

Received: 24 June 2014
Accepted: 5 November 2014

Abstract

By analyzing an N-body simulation of a bulge formed simply via a bar instability mechanism operating on a kinematically cold stellar disk, and by comparing the results of this analysis with the structural and kinematic properties of the main stellar populations of the Milky Way bulge, we conclude that the bulge of our Galaxy is not a pure stellar bar formed from a pre-existing thin stellar disk, as some studies have recently suggested. On the basis of several arguments emphasized in this paper, we propose that the bulge population that, in the Milky Way, is observed to not be part of the peanut structure corresponds to the old Galactic thick disk, thus implying that the Milky Way is a pure thin+thick disk galaxy, with only a possible limited contribution by a classical bulge.