[Paper Review] Mapping the stellar age of the Milky Way bulge with the VVV. II. Deep JKs catalogs release based on PSF photometry

F. Surot, E. Valenti+7 more|arXiv (Cornell University)|Jul 3, 2019

Stellar, planetary, and galactic studies33 references12 citations

TL;DR

This paper presents a deep, high-cadence J and Ks photometric catalog of nearly 600 million stars across ~300 deg² of the Milky Way bulge using PSF-fitting photometry on multi-epoch VVV survey data. The catalog achieves unprecedented depth and completeness—sampling stars down to ~1–2 mag below the old main sequence turnoff—enabling detailed studies of stellar ages, kinematics, and structure, with full public release via the ESO Science Archive.

ABSTRACT

The bulge represents the best compromise between old and massive Galactic component, and as such its study is a valuable opportunity to understand how the bulk of the Milky Way formed and evolved. In addition, being the only bulge in which we can individually resolve stars in all evolutionary sequences, the properties of its stellar content provide crucial insights on the formation of bulges at large. We aim at providing a detailed and comprehensive census of the Milky Way bulge stellar populations by producing deep and accurate photometric catalogs of the inner ~300 sqdeg of the Galaxy. We perform DAOPHOT/ALLFRAME PSF-fitting photometry of multi-epochs J and Ks images provided by the VVV survey to obtain deep photometric catalogs. Artificial star experiments have been conducted on all images to properly assess the completeness and the accuracy of the photometric measurements. We present a photometric database containing nearly 600 million stars across the bulge area surveyed by the VVV. Through the comparison of derived color-magnitude diagrams of selected fields representative of different levels of extinction and crowding, we show the quality, completeness and depth of the new catalogs. With the exception of the fields located along the plane, this new photometry samples stars down to ~1-2 mag below the MS-TO with unprecedented accuracy. To demonstrate the tremendous potential inherent to this new dataset, we give few examples of possible applications such as: i) star counts studies through the dataset completeness map; ii) surface brightness map; and iii) cross-correlation with Gaia DR2. The database presented here represents an invaluable collection for the whole community, and we encourage its exploitation. The photometric catalogs including completeness information are publicly available through the ESO Science Archive.

Motivation & Objective

• To produce a comprehensive, deep, and accurate photometric catalog of the Milky Way bulge to enable detailed studies of its stellar populations.
• To overcome limitations of previous VVV data releases by using PSF-fitting photometry to improve accuracy and completeness, especially in crowded and highly extincted regions.
• To assess photometric completeness and accuracy across the entire VVV bulge field using artificial star experiments.
• To provide a public, community-accessible dataset for future studies of stellar ages, metallicity, kinematics, and bulge structure.

Proposed method

• Perfected PSF-fitting photometry using DAOPHOT/ALLFRAME on multi-epoch J and Ks images from the VVV survey.
• Conducted artificial star experiments on all 3,912 individual VVV images to calibrate photometric completeness and accuracy per field.
• Generated 196 individual photometric catalogs covering the full ~300 deg² VVV bulge region with limiting magnitudes of Ks ≈ 20 and J ≈ 21.
• Produced a detailed completeness map to enable reliable star count and surface brightness studies.
• Cross-matched the new VVV photometry with Gaia DR2 to assess the synergy between Gaia and ground-based near-IR photometry.
• Released the full photometric catalog, including completeness information, via the ESO Science Archive for public use.

Experimental results

Research questions

• RQ1 What is the photometric completeness and accuracy of deep J and Ks photometry across the crowded, highly extincted Milky Way bulge?
• RQ2 To what magnitude limit does the PSF-fitting photometry sample the old main sequence turnoff in different regions of the bulge?
• RQ3 How does the photometric depth and completeness compare between Gaia DR2 and the new VVV PSF photometry across varying extinction and crowding levels?
• RQ4 Can the new photometric catalog enable robust stellar age determination through synthetic color-magnitude diagram fitting?
• RQ5 What is the impact of extinction and crowding on the detectability of red clump and main sequence turnoff stars in the bulge?

Key findings

• The catalog contains nearly 600 million stars across ~300 deg² of the Milky Way bulge, with photometry reaching Ks ≈ 20 and J ≈ 21.
• The photometric completeness for the red clump population ranges from nearly 100% to 70% across the bulge, dropping to 50% in the innermost field near the Galactic center.
• The old main sequence turnoff is sampled with >50% completeness across the outer bulge (|b| > 3.5°), enabling age and star formation history studies.
• Gaia DR2 photometry is insufficiently deep to sample the main sequence turnoff in the central bulge (b ≈ -4°), but remains effective in less extincted and less crowded regions.
• The combination of Gaia and the new VVV PSF photometry enables improved separation of bulge and disk populations via proper motions and deeper CMDs.
• The entire dataset, including completeness maps, is publicly available through the ESO Science Archive for community-wide use.