Blanco DECam Bulge Survey (BDBS)

III. A new view of the double red clump in the Milky Way bulge through luminosity and color distribution

¹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: dongwook.lim@uni-heidelberg.de
² Center for Galaxy Evolution Research & Department of Astronomy, Yonsei University, Seoul 03722, Republic of Korea
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ College of Arts & Sciences, St Martin’s University, Ernsdor Center 130 5000 Abbey Way SE, Lacey, WA 98503, USA
⁵ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 20030, PR China
⁶ Department of Physics & Astronomy, University of California Los Angeles, 430 Portola Plaza, Box 951547 Los Angeles, CA 90095-157, USA
⁷ Department of Natural Sciences, University of Michigan-Dearborn, 4901 Evergreen Rd, Dearborn, MI 48128, USA
⁸ Indiana University Department of Astronomy, SW319, 727 E 3rd Street, Bloomington, IN 47405, USA
⁹ Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile
¹⁰ Indiana University, University Information Technology Services, CIB 2709 E 10th Street, Bloomington, IN 47401, USA

Received: 21 November 2020
Accepted: 29 December 2020

Abstract

Red clump (RC) stars are one of the best stellar tracers of the structure of the Milky Way (MW) bulge. Here we report a new view of the double RC through luminosity and color distributions of RC stars in nine bulge fields (l = 0.0°, ±4.5°; b = −6.0°, −7.5°, −9.0°) from the Blanco DECam Bulge Survey (BDBS), which covers near-ultraviolet to near-infrared bandpasses. The bright and faint RCs show contrasting distributions in (u − g)₀ and (u − i)₀ colors but similar distributions in (J − K_s)₀ with a variation depending on the Galactic longitude, where the bright RC is typically redder than the faint RC. In particular, the RC stars are clearly divided into the bluer and redder populations when using the (u − g)₀ color ((u − g)₀ < 2.5 for the bluer RC; (u − g)₀ ≥ 2.5 for the redder RC). The bluer stars show a single clump on the faint RC regime, whereas the redder stars form double clumps on both the bright and faint RCs. The bright clump of the redder stars is dominant in the positive longitude fields, while the faint clump of those red stars is significant at negative longitudes. We also confirm that the bluer and redder stars have different peak metallicity through comparison with spectroscopy (Δ[Fe/H] ∼ 0.45 dex). Therefore, our results support a scenario whereby the MW bulge is composed of a spheroid of metal-poor stars and a boxy/peanut shape (X-shape) predominantly made up of metal-rich stars.