Below you will find a list of BDBS-related publications by team members. Please indicate the name of the authors, a brief overview of co-authorship (ala "Core members", "Open to all" etc), and the status (e.g., first draft circulated, submitted, accepted, or publication details).
R.M. Rich, C.I. Johnson, M. Young, et al. 2020, MNRAS: The Blanco DECam Bulge Survey. I. The Survey Description and Early Results
Status: published in MNRAS, 499, 2340
Abstract: The Blanco Dark Energy Camera (DECam) Bulge survey is a Vera Rubin Observatory (LSST) pathfinder imaging survey, spanning ⇠ 200 sq. deg. of the Southern Galactic bulge, 2 <b< 13 and 11 <l< +11 . We have employed the CTIO-4m telescope and the Dark Energy Camera (DECam) to image a contiguous ⇠ 200 sq. deg. region of the relatively less reddened Southern Galactic bulge, in SDSS u + Pan-STARRSgrizY. Optical photometry with its large color 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 ⇠ 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 show that putative young, massive "blue loop" stars proposed as a recent star forming population instead lie at ⇠ 2 kpc from the Sun and are likely red clump giants in the old disk. A bright red clump near (l, b) = (+8 , 4 ) 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 show previously reported new bulge globular clusters cannot be confirmed as real.
C.I. Johnson, R.M. Rich, M.D. Young, et al. 2020, MNRAS: Blanco DECam Bulge Survey (BDBS) II: Project Performance, Data Analysis, and Early Science Results
Status: published in MNRAS, 499, 2357
Abstract: The Blanco DECam Bulge Survey (BDBS) imaged more than 200 square degrees of the Southern Galactic bulge using the ugrizY filters of the Dark Energy Camera, and produced point spread function photometry of approximately 250 million unique sources. In this paper, we present details regarding the construction and collation of survey catalogs, and also discuss the adopted calibration and dereddening procedures. Early science results are presented with a particular emphasis on the bulge metallic- ity distribution function and globular clusters. A key result is the strong correlation (σ ∼ 0.2 dex) between (u−i)o and [Fe/H] for bulge red clump giants. We utilized this relation to find that interior bulge fields may be well described by simple closed box enrichment models while fields exterior to b ∼ −6◦ seem to require a secondary metal-poor component. Applying scaled versions of the closed box models to the outer bulge fields is shown to significantly reduce the strengths of any additional metal-poor components when compared to Gaussian mixture models. Additional results include: a confirmation that the u-band splits the sub-giant branch in M 22 as a function of metallicity, the detection of possible extra-tidal stars along the orbits of M 22 and FSR 1758, and additional evidence that NGC 6569 may have a small but discrete He spread, as evidenced by red clump luminosity variations in the reddest bands. We do not confirm previous claims that FSR 1758 is part of a larger extended structure.
D. Lim, A. J. Koch-Hansen, C. Chung, et al. 2021, A&A: Blanco DECam Bulge Survey (BDBS) III: A new view of the double red clump in the Milky Way bulge through luminosity and color distribution
Status: published in A&A, 647, 34
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 - Ks)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 (∆[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.
J.A. Kader, C.A. Pilachowski, C.I. Johnson, et al. 2021, ApJ: Blanco DECam Bulge Survey (BDBS) IV: Multiple Populations in Globular Clusters of the Galactic Bulge
Status: submitted
Abstract: We present photometric evidence for multiple stellar populations in 14 globular clusters (GCs) toward the southern Galactic bulge. The photometric data come as part of the Blanco DECam Bulge Survey, which is a deep, wide-field ugrizY survey of the southern Galactic bulge. Here we present the first systematic study of bulge GC multiple populations (MPs) with deep photometry including the u-band which is a crucial indicator of the abundance of CNO-bearing molecules in stellar atmospheres. We identify cluster members using Gaia EDR3 proper motion measurements, and then isolate red giant branch stars using traditional r vs. u − r CMDs. We use the pseudo-color index Cugi, which is highly sensitive to light element abundance variations, to create pseudo-CMDs that allow for the identification of MPs along the red giant branch. We find clear evidence that all 14 clusters host at least two populations, and NGC 6441 and NGC 6656 appear to have three populations. Many of these clusters are not part of the HST surveys nor do they have comprehensive spectroscopic analyses so we are presenting the first evidence of MPs in several clusters. Not only do we find a strong anticorrelation between the fraction of first generation stars and cluster absolute V-magnitude, but the correlation coefficient and cluster-to-cluster scatter are similar to the results obtained from HST. Our groundbased data extend to much larger radial distances than similar HST observations, enabling a reliable estimate of the global fraction of first generation stars in each cluster. This study demonstrates that groundbased u-band photometry as provided by DECam and eventually by the Vera C. Rubin Observatory, will prove powerful in the study of multiple populations in resolved globular clusters.
C.I. Johnson, R.M. Rich, I.T. Simion, et al. 2022, MNRAS: Blanco DECam Bulge Survey (BDBS) V: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars
Status: submitted
Abstract: We present photometric metallicity measurements, based on an empirical calibration between (u−i)o and [Fe/H], for a sample of 2.6 million bulge red clump stars extracted from the Blanco DECam Bulge Survey (BDBS). The data span |l| < 10◦ and −10◦ < b < −3.5◦, and have an [Fe/H] precision of ∼ 0.20 dex. Similar to previous studies, we find that the bulge exhibits a strong vertical metallicity gradient, and that at least two peaks in the metallicity distribution functions appear at b < −5◦. We can discern a metal-poor ([Fe/H] ∼ −0.3) and metal-rich ([Fe/H] ∼ +0.2) abundance distribution that each show clear systematic trends with latitude, and may be best understood by changes in the bulge’s star formation/enrichment processes. Both groups exhibit significant, asymmetric tails that are inconsistent with Gaussian distributions, and as a result we argue that a star’s proximity to either peak in [Fe/H] space is not necessarily an affirmation of group membership. The metal-poor peak shifts to lower [Fe/H] values at larger distances from the plane while the metal-rich tail truncates. Close to the plane, the metal-rich tail appears broader along the minor axis than in off-axis fields. We also posit that the bulge has two metal-poor populations – one that belongs to the metal-poor tail of the low latitude and predominantly metal-rich group, and another belonging to the metal-poor group that dominates in the outer bulge. We detect the X-shape structure in fields with |Z| > 0.7 kpc and for stars with [Fe/H] > −0.5. Stars with [Fe/H] < −0.5 may form a spheroidal or “thick bar” distribution while those with [Fe/H] >∼ −0.1 are strongly concentrated near the plane and produce a boxy/peanut morphology.