The Blanco DECam Bulge Survey: Perspectives, results, and implications

Abstract

The Blanco DECam Bulge Survey (BDBS) program has surveyed 200 sq. deg. of the Southern Galactic bulge in ugrizY using the Dark Energy Camera on the Blanco 4m telescope at Cerro Tololo, with photometry for : 500 M stars. This multicolor survey is a Rubin Observatory pathfinder; the availability of blue optical colors has enabled the derivation of metallicity for 2.3M red clump giant stars; matching these to the Gaia catalog gives chemistry and kinematics. Calibrating the ( u–i ) ₀ photometric color vs [Fe/H] measured from high resolution spectroscopy, the structure of the bulge as a function of metallicity has been discerned for –10° < l < + 10° and –2.5° < b < + 10°. Well-known features such as the X-shaped bulge can be seen in the metal rich population, but additionally, a striking concentration of metal rich stars toward the Galactic Plane is clearly visible. There is a steep vertical metallicity gradient, and no radial gradient can be discerned over the inner 2 kpc. Confirming many previous studies, the bulge metallicity distribution is bimodal, but at b < =–6° a sub-solar metallicity peak at –0.4 dex is dominant, strongly pointing to a different formation history for that population. Signatures of the bar (the double red clump, and kinematic signatures) are clearly evident at [Fe/H] > –0.4.