The Blanco DECam Bulge Survey. I. The Survey Description and Early Results

TL;DR

The Blanco DECam Bulge Survey is a comprehensive imaging project mapping the Southern Galactic bulge to study its stellar populations, structure, and composition, providing new data and insights into the bulge's properties and constituents.

Contribution

This paper introduces the survey design, data acquisition, and initial results, including new reddening-corrected color-magnitude diagrams and population analyses of the Galactic bulge.

Findings

Identification of a population of red clump giants in the bulge.

Detection of a bright red clump feature possibly related to the foreground disk.

First map of the blue horizontal branch population in the surveyed region.

Abstract

The Blanco Dark Energy Camera (DECam) Bulge survey is a Vera Rubin Observatory (LSST) pathfinder imaging survey, spanning $\sim 200$ sq. deg. of the Southern Galactic bulge, $-2^\circ <$b$< -13^\circ$ and $-11^\circ <$l$ < +11^\circ$. We have employed the CTIO-4m telescope and the Dark Energy Camera (DECam) to image a contiguous $\sim 200$ sq. deg. region of the relatively less reddened Southern Galactic bulge, in SDSS $u$ + Pan-STARRS$grizy$. Optical photometry with its large colour baseline will be used to investigate the age and metallicity distributions of the major structures of the bulge. Included in the survey footprint are 26 globular clusters imaged in all passbands. Over much of the bulge, we have Gaia DR2 matching astrometry to $i\sim 18$, deep enough to reach the faint end of the red clump. This paper provides the background, scientific case, and description of the survey. We present an array of new reddening-corrected colour-magnitude diagrams that span the extent of Southern Galactic bulge. We argue that a population of massive stars in the blue loop evolutionary phase, proposed to lie in the bulge, are instead at $\sim 2$ kpc from the Sun and likely red clump giants in the old disk. A bright red clump near $(l,b)=(+8^\circ,-4^\circ)$ may be a feature in the foreground disk, or related to the long bar reported in earlier work. We also report the first map of the blue horizontal branch population spanning the BDBS field of regard, and our data does not confirm the reality of a number of proposed globular clusters in the bulge.