The WFC3 Galactic Bulge Treasury Program: Relative Ages of Bulge Stars of High and Low Metallicity

Abstract

The HST/WFC3 multiband photometry spanning from the UV to the near-IR of four fields in the Galactic bulge, together with that for six template globular and open clusters, are used to photometrically tag the metallicity [Fe/H] of stars in these fields after proper-motion rejecting most foreground disk contaminants. Color-magnitude diagrams and luminosity functions are then constructed, in particular for the most metal rich and most metal poor stars in each field. We do not find any significant difference between the $I$-band and $H$-band luminosity functions, hence turnoff luminosity and age, of the metal rich and metal poor components which therefore appear essentially coeval. In particular, we find that no more than $\sim 3\%$ of the metal-rich component can be $\sim 5$ Gyr old, or younger. Conversely, theoretical luminosity functions give a good match to the observed ones for an age of ~10 Gyr. Assuming this age is representative for the bulk of bulge stars, we then recall the observed properties of star-forming galaxies at 10 Gyr lookback time, i.e., at z~2, and speculate about bulge formation in that context. We argue that bar formation and buckling instabilities leading to the observed boxy/peanut, X-shaped bulge may have arisen late in the history of the Milky Way galaxy, once its gas fraction had decreased compared to the high values typical of high-redshift galaxies. This paper follows the public release of the photometric and astrometric catalogs for the measured stars in the four fields.

11 pages, 9 figures, to appear on the Astrophysical Journal

References in corpus (24)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The Dartmouth Stellar Evolution Database
• From rings to bulges: evidence for rapid secular galaxy evolution at z~2 from integral field spectroscopy in the SINS survey
• Combined CO & Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star Formation Rate and Stellar Mass
• Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies
• Evidence for Mature Bulges and an Inside-out Quenching Phase 3 Billion Years After the Big Bang
• Oxygen, Sodium, Magnesium and Aluminium as tracers of the Galactic Bulge Formation
• Made-to-Measure models of the Galactic Box/Peanut bulge: stellar and total mass in the bulge region
• Stellar Proper Motions in the Galactic Bulge from deep HST ACS/WFC Photometry
• The GIRAFFE Inner Bulge Survey (GIBS) III. Metallicity distributions and kinematics of 26 Galactic bulge fields
• Transiting extrasolar planetary candidates in the Galactic bulge
• Dense cores in galaxies out to z=2.5 in SDSS, UltraVISTA, and the five 3D-HST/CANDELS fields
• Bulge-forming galaxies with an extended rotating disk at z~2
• Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models for $-2.4\le$ [Fe/H] $\le +0.6$, $0.25\le Y\le 0.33$, and $-0.4\le$ [$\alpha$\Fe] $\le +0.4$
• Extinction ratios in the inner Galaxy as revealed by the VVV survey
• Connection Between Stellar Mass Distributions Within Galaxies and Quenching Since z =2
• Old and young bulges in late-type disk galaxies
• Star formation history of the Galactic bulge from deep HST imaging of low reddening windows
• Constraining Dust Extinction Properties via the VVV Survey
• A Galactic Center Origin for HE 0437-5439, the Hypervelocity Star near the Large Magellanic Cloud
• The WFC3 Galactic Bulge Treasury Program: A First Look at Resolved Stellar Population Tools
• Chemical Study of the Metal-rich Globular Cluster NGC 5927
• Was the Milky Way Bulge Formed From The Buckling Disk Instability, Hierarchical Collapse, Accretion of Clumps, or All of the Above?
• Chemically-dissected rotation curves of the Galactic Bulge from Main Sequence proper motions

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• The stellar mass versus stellar metallicity relation of star-forming galaxies at $1.6\le z\le 3.0$ and implications for the evolution of the $\alpha$-enhancement
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• Blanco DECam Bulge Survey (BDBS) II: Project Performance, Data Analysis, and Early Science Results
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XX. Ages of single and multiple stellar populations in seven bulge globular clusters
• Compact, bulge dominated structures of spectroscopically confirmed quiescent galaxies at z~3
• A census of new globular clusters in the Galactic bulge
• Search Instructions for Globular Clusters in Formation at High Redshifts
• The age distribution of stars in the Milky Way bulge
• The Blanco DECam Bulge Survey. I. The Survey Description and Early Results
• The imprint of clump formation at high redshift. II. The chemistry of the bulge
• Another relic bulge globular cluster: ESO 456-SC38 (Djorgovski~2)
• Evidence of an age gradient along the line of sight in the nuclear stellar disc of the Milky Way
• Modeling the chemical enrichment history of the Bulge Fossil Fragment Terzan 5
• Halo intruders in the Galactic bulge revealed by HST and Gaia: the globular clusters Terzan 10 and Djorgovski 1
• Chemical evolution of local post-starburst galaxies: Implications for the mass-metallicity relation
• Cobalt and copper abundances in 56 Galactic bulge red giants
• The Rich Lack Close Neighbours: The Dependence of Blue-Straggler Fraction on Metallicity
• Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. VIII. Carbon and oxygen
• Probing the Milky Way stellar and brown dwarf initial mass function with modern microlensing observations
• Blanco DECam Bulge Survey (BDBS) V. Cleaning the foreground populations from Galactic bulge colour-magnitude diagrams using Gaia EDR3
• He abundances in disc galaxies -- I. Predictions from cosmological chemodynamical simulations
• A MUSE study of the inner bulge globular cluster Terzan 9: a fossil record in the Galaxy
• Comparing Observed Stellar Kinematics and Surface Densities in a Low Latitude Bulge Field to Galactic Population Synthesis Models
• Predicted Trends in Milky Way Bulge Proper Motion Rotation Curves: future Prospects for HST and LSST
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• The Near-Infrared Spectrum of the Nuclear Star Cluster: Looking Below the Tip of the Iceberg and Comparisons with Extragalactic Nuclei
• Modelling the chemical evolution of the Milky Way
• The chemical and spatial variations of the bulge's velocity ellipsoids
• SDSS-IV MaNGA: Stellar rotational support in disk galaxies vs. central surface density and stellar population age
• The Formation of Globular Clusters as a Case of Overcooling
• Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars
• Sulfur abundances in the Galactic bulge and disk