The disc origin of the Milky Way bulge

Dissecting the chemo-morphological relations using N-body simulations and APOGEE

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
e-mail: ffrag@mpa-garching.mpg.de
² GEPI, Observatoire de Paris, PSL University, CNRS, Place Jules Janssen, 92195, Meudon, France
³ Laboratoire Lagrange, Université Côte-d’Azur, Observatoire de la Côte-d’Azur, CNRS, Bd de l’Observatoire, 06304 Nice, France
⁴ Observatoire de Paris, LERMA, CNRS, PSL Univ., UPMC, Sorbonne Univ., 75014 Paris, France
⁵ Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France

Received: 20 December 2017
Accepted: 7 May 2018

Abstract

There is a long-standing debate over the origin of the metal-poor stellar populations of the Milky Way (MW) bulge, with the two leading scenarios being that these populations are either (i) part of a classical metal-poor spheroid or (ii) the same population as the chemically defined thick disc seen at the solar neighbourhood. Here we test whether the latter scenario can reproduce the observed chemical properties of the MW bulge. To do so we compare an N-body simulation of a composite (thin+thick) stellar disc – which evolves secularly to form a bar and a boxy/peanut (b/p) bulge – to data from APOGEE DR13. This model, in which the thick disc is massive and centrally concentrated, can reproduce the morphology of the metal-rich and metal-poor stellar populations in the bulge, as well as the mean metallicity and [α/Fe] maps as obtained from the APOGEE data. It also reproduces the trends, in both longitude and latitude, of the bulge metallicity distribution function (MDF). Additionally, we show that the model predicts small but measurable azimuthal metallicity variations in the inner disc due to the differential mapping of the thin and thick disc in the bar. We therefore see that the chemo-morphological relations of stellar populations in the MW bulge are naturally reproduced by mapping the thin and thick discs of the inner MW into a b/p.