[Paper Review] Mapping the stellar age of the Milky Way bulge with the VVV

F. Surot, E. Valenti+5 more|arXiv (Cornell University)|Jan 1, 2020

Stellar, planetary, and galactic studiesPhysics and Astronomy27 references42 citations

TL;DR

This paper presents a high-resolution (down to 10 arcsec) reddening map in J−Ks color for the Milky Way's bulge using deep VVV survey photometry. By analyzing red clump and red giant branch stars in fine spatial bins, it achieves unprecedented sub-arcmin resolution, significantly improving extinction correction for stellar population studies near the Galactic plane and center.

ABSTRACT

The detailed study of the Galactic bulge stellar population necessarily requires an accurate representation of the interstellar extinction particularly toward the Galactic plane and center, where the severe and differential reddening is expected to vary on sub-arcmin scales. Although recent infrared surveys have addressed this problem by providing extinction maps across the whole Galactic bulge area, dereddened color-magnitude diagrams near the plane and center appear systematically undercorrected, suggesting the need for higher resolutions. These undercorrections affect any stellar study sensitive to color (e.g. star formation history analysis via color-magnitude diagram fitting), either making them inaccurate or limiting them to small low/stable extinction windows where this value is better constrained. We aim at providing a high-resolution (2 arcmin to $\sim$ 10 arcsec) color excess map for the VVV bulge area, in $\mathrm{J}-\mathrm{K}_s$ color. We use the MW-BULGE-PSFPHOT catalogs sampling $\sim$ 300 deg$^2$ across the Galactic bulge ($|l| < 10^\circ$ and $-10^\circ < b < 5^\circ$) to isolate a sample of red clump and red giant branch stars, for which we calculate average $\mathrm{J}-\mathrm{K}_s$ color in a fine spatial grid in $(l, b)$ space. We obtain a E$(\mathrm{J}-\mathrm{K}_s)$ map spanning the VVV bulge area of roughly 300 deg$^2$, with the equivalent to a resolution between $\sim$ 1 arcmin for bulge outskirts ($l < -6^\circ$) to below 20 arcsec within the central $|l| < 1^\circ$, and below 10 arcsec for the innermost area ($|l| < 1^\circ$ and $|b| < 3^\circ$). The result is publicly available at http://basti-iac.oa-teramo.inaf.it/vvvexmap/

Motivation & Objective

• To address the limitations of existing reddening maps in the Galactic bulge, which often suffer from low spatial resolution and systematic errors in high-extinction regions.
• To improve extinction correction accuracy for stellar population studies, particularly in the Galactic plane and center, where differential reddening varies on sub-arcmin scales.
• To provide a publicly available, high-resolution color excess map (E(J−Ks)) that enhances the reliability of color-magnitude diagram fitting and star formation history analysis.
• To support future photometric and spectroscopic surveys by enabling precise target selection and extinction correction in dense, highly obscured regions.

Proposed method

• Utilizes the MW-BULGE-PSFPHOT catalog from the VVV survey, covering ~300 deg² in the Galactic bulge (|l| < 10°, −10° < b < 5°).
• Identifies red clump and red giant branch stars as stable color indicators due to their well-defined J−Ks colors.
• Calculates the average J−Ks color in a fine spatial grid across (l, b) space, using small bin sizes (~9 arcsec, or ~350 pc at 8 kpc).
• Employs a spatial binning strategy that adapts resolution from ~1 arcmin in outer bulge to <10 arcsec in the innermost |l| < 1°, |b| < 3° region.
• Uses the width of the color distribution in each bin as a proxy for line-of-sight optical depth and as a measure of uncertainty in E(J−Ks).
• Validates the map against existing extinction maps and demonstrates improved consistency in red clump locus dispersion.

Experimental results

Research questions

• RQ1 Can a high-resolution reddening map be constructed for the Galactic bulge using deep near-IR photometry with sub-arcmin spatial resolution?
• RQ2 How does sub-arcmin resolution improve extinction correction in dense, high-extinction regions near the Galactic plane?
• RQ3 To what extent does the new map reduce color dispersion in the red clump locus compared to previous maps?
• RQ4 Can the map's resolution and accuracy enable more reliable star formation history reconstruction in the inner Milky Way?
• RQ5 How does the map's resolution and binning strategy affect the detection of small-scale dust structures and foreground clusters?

Key findings

• The map achieves a spatial resolution of ~1 arcmin in the outer bulge and below 20 arcsec within |l| < 1°, with sub-10 arcsec resolution in the innermost |l| < 1° and |b| < 3° region.
• The map reduces the standard deviation of the red clump locus in the central tile (b333) from 0.46 mag (using G18 map) to 0.24 mag, a 46% improvement.
• The map reveals fine structures such as thin filaments, threads, and clumpy distributions in the extinction profile, especially near the Galactic plane.
• The map successfully highlights foreground clusters as localized, sudden color excess drops (bluer than background), demonstrating sensitivity to small-scale extinction features.
• The color width in each bin serves as a reliable proxy for line-of-sight optical depth and provides a scaled uncertainty estimate for E(J−Ks).
• The map is publicly available at http://basti-iac.oa-teramo.inaf.it/vvvexmap/ and has already been used in Surot et al. (2019b) and integrated into the MW-BULGE-PSFPHOT catalog.