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

• Published in: The Astrophysical journal Vol. 863; no. 1; p. 16
• Main Authors: Renzini, Alvio, Gennaro, Mario, Zoccali, Manuela, Brown, Thomas M., Anderson, Jay, Minniti, Dante, Sahu, Kailash C., Valenti, Elena, VandenBerg, Don A.
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 10.08.2018
• Subjects: Astrophysics; Bulging; Contaminants; Galactic bulge; Galactic clusters; Galaxies; Hubble Space Telescope; Luminosity; Metallicity; Metals; Milky Way; Open clusters; Peanuts; Photometry; Red shift; Space telescopes; Star formation; Stars & galaxies; Stellar age
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aad09b

Abstract The Hubble Space Telescope /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 (LF) 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 LFs, hence turnoff luminosity and age of the metal-rich and metal-poor components therefore appear essentially coeval. In particular, we find that no more than ∼3% of the metal-rich component can be ∼5 Gyr old, or younger. Conversely, theoretical LFs match well 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 of the measured stars in the four fields.