Blanco DECam Bulge Survey (BDBS)

V. Cleaning the foreground populations from Galactic bulge colour-magnitude diagrams using Gaia EDR3

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: tommaso.marchetti@eso.org
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Kavli Institute for Theoretical Physics, University of Santa Barbara, California 93106, USA
⁴ Department of Physics and Astronomy, University of California Los Angeles, 430 Portola Plaza, Box 951547 Los Angeles, CA 90095-1547, USA
⁵ Shanghai Key Lab for Astrophysics, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, PR China
⁶ Indiana University, University Information Technology Services, CIB 2709 E 10th Street, Bloomington, IN 47401, 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 Saint Martin’s University, 5000 Abbey Way SE, Lacey, WA 98503, USA
¹⁰ Zentrum für Astronomie der Universitat Heidelberg, Astronomisches Rechen-Institut, Monchhofstr. 12, 69120 Heidelberg, Germany

Received: 2 May 2022
Accepted: 23 June 2022

Abstract

Aims. The Blanco DECam Bulge Survey (BDBS) has imaged more than 200 square degrees of the southern Galactic bulge, providing photometry in the ugrizy filters for ∼250 million unique stars. The presence of a strong foreground disk population, along with complex reddening and extreme image crowding, has made it difficult to constrain the presence of young and intermediate age stars in the bulge population.

Methods. We employed an accurate cross-match of BDBS with the latest data release (EDR3) from the Gaia mission, matching more than 140 million sources with BDBS photometry and Gaia EDR3 photometry and astrometry. We relied on Gaia EDR3 astrometry, without any photometric selection, to produce clean BDBS bulge colour-magnitude diagrams (CMDs). Gaia parallaxes were used to filter out bright foreground sources, and a Gaussian mixture model fit to Galactic proper motions could identify stars kinematically consistent with bulge membership. We applied this method to 127 different bulge fields of 1 deg² each, with |ℓ| ≤ 9.5° and −9.5° ≤b ≤ −2.5°.

Results. The astrometric cleaning procedure removes the majority of blue stars in each field, especially near the Galactic plane, where the ratio of blue to red stars is ≲10%, increasing to values ∼20% at higher Galactic latitudes. We rule out the presence of a widespread population of stars younger than 2 Gyr. The vast majority of blue stars brighter than the turnoff belong to the foreground population, according to their measured astrometry. We introduce the distance between the observed red giant branch bump and the red clump as a simple age proxy for the dominant population in the field, and we confirm the picture of a predominantly old bulge. Further work is needed to apply the method to estimate ages to fields at higher latitudes, and to model the complex morphology of the Galactic bulge. We also produce transverse kinematic maps, recovering expected patterns related to the presence of the bar and of the X-shaped nature of the bulge.