The Milky Way bar and bulge revealed by APOGEE and Gaia EDR3

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: aqueiroz@aip.de
² Laboratório Interinstitucional de e-Astronomia – LIneA, Rua Gal. José Cristino 77, Rio de Janeiro, RJ 20921-400, Brazil
³ Department of Astronomy, Universidade de São Paulo, São Paulo 05508-090, Brazil
⁴ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 106, C.P. 22800, Ensenada, BC, Mexico
⁵ Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Carrer Martí i Franquès 1, 08028 Barcelona, Spain
⁶ Instituto de Física, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, Porto Alegre, RS 91501-970, Brazil
⁷ University of Arizona, Tucson, AZ 85719, USA
⁸ Observatório Nacional, Sao Cristóvao, Rio de Janeiro, Brazil
⁹ Observatoire de la Côte d’Azur, Laboratoire Lagrange, 06304 Nice Cedex 4, France
¹⁰ Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325, USA
¹¹ Dept. of Physical Sciences, Faculty of Exact Sciences, Universidad Andres Bello, Fernandez Concha 700, Santiago, Chile
¹² Vatican Observatory, 00120 Vatican City State, Italy
¹³ The Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹⁶ Instituto de Astronomía y Ciencias Planetarias de Atacama, Universidad de Atacama, Copayapu 485, Copiapó, Chile
¹⁷ Instituto de Astrofisica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹⁸ Universidad de La Laguna (ULL), Departamento de Astrofisica, 38206 La Laguna, Tenerife, Spain
¹⁹ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
²⁰ Instituto de Investigación Multidisciplinario en Ciencia y Tecnología, Universidad de La Serena, Avenida Raúl Bitrán s/n, La Serena, Chile
²¹ Departamento de Física, Facultad de Ciencias, Universidad de La Serena, Cisternas 1200, La Serena, Chile
²² Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
²³ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta 1270300, Chile
²⁴ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile
²⁵ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
²⁶ National Optical Astronomy Observatories, Tucson, AZ 85719, USA

Received: 25 July 2020
Accepted: 18 September 2021

Abstract

We investigate the inner regions of the Milky Way using data from APOGEE and Gaia EDR3. Our inner Galactic sample has more than 26 500 stars within |X_Gal|< 5 kpc, |Y_Gal|< 3.5 kpc, |Z_Gal|< 1 kpc, and we also carry out the analysis for a foreground-cleaned subsample of 8000 stars that is more representative of the bulge–bar populations. These samples allow us to build chemo-dynamical maps of the stellar populations with vastly improved detail. The inner Galaxy shows an apparent chemical bimodality in key abundance ratios [α/Fe], [C/N], and [Mn/O], which probe different enrichment timescales, suggesting a star formation gap (quenching) between the high- and low-α populations. Using a joint analysis of the distributions of kinematics, metallicities, mean orbital radius, and chemical abundances, we can characterize the different populations coexisting in the innermost regions of the Galaxy for the first time. The chemo-kinematic data dissected on an eccentricity–|Z|_max plane reveal the chemical and kinematic signatures of the bar, the thin inner disc, and an inner thick disc, and a broad metallicity population with large velocity dispersion indicative of a pressure-supported component. The interplay between these different populations is mapped onto the different metallicity distributions seen in the eccentricity–|Z|_max diagram consistently with the mean orbital radius and V_ϕ distributions. A clear metallicity gradient as a function of |Z|_max is also found, which is consistent with the spatial overlapping of different populations. Additionally, we find and chemically and kinematically characterize a group of counter-rotating stars that could be the result of a gas-rich merger event or just the result of clumpy star formation during the earliest phases of the early disc that migrated into the bulge. Finally, based on 6D information, we assign stars a probability value of being on a bar orbit and find that most of the stars with large bar orbit probabilities come from the innermost 3 kpc, with a broad dispersion of metallicity. Even stars with a high probability of belonging to the bar show chemical bimodality in the [α/Fe] versus [Fe/H] diagram. This suggests bar trapping to be an efficient mechanism, explaining why stars on bar orbits do not show a significant, distinct chemical abundance ratio signature.