0 Assigned ID: 7508 ----- Attempting Submission 1 (Thu Apr 16 17:47:49 GMT 2026) ----- The age distribution of stars in the Galactic bulge remains one of the most important unresolved problems in galaxy formation. While early HST studies suggest a uniformly old (>10 Gyr) population, recent spectroscopic results indicate that a substantial fraction of bulge stars may be significantly younger, leading to a fundamental tension that cannot be resolved with current methods alone. We propose to exploit archival HST/WFC3 Stromgren and infrared imaging of the low-extinction SWEEPS field to directly measure the age distribution of bulge main-sequence turnoff (MSTO) stars. These data uniquely combine precise proper-motion cleaning with metallicity and age sensitive medium-band photometry, enabling a breakthrough in disentangling the age-metallicity degeneracy. We will derive new metallicity calibrations tailored to HST filters and use Stromgren [m] and [c] indices to separately constrain [M/H] and age. This approach delivers robust individual ages and the first statistically meaningful age distribution for a large, clean bulge sample below the MSTO. The results will decisively test whether the bulge is uniformly old or hosts a significant intermediate-age population, providing a critical constraint on bulge formation scenarios. In addition, we will release improved Stromgren metallicity calibrations and a comprehensive catalog of reddening, metallicity, and age measurements for the community. Dr. Christian Johnson Background: Dr. Johnson is an expert in globular cluster spectroscopy, stellar populations, and large sample photometric analyses. Role: Dr. Johnson will work with Dr. Calamida to derive PSF photometry from all WFC3 images, manage the resulting catalogs, work on the photometric calibration in comparison to the globular clusters, and will be the lead author of at least one of the papers. Dr. Annalisa Calamida Background: Dr. Calamida is an expert in the Galactic bulge and globular cluster population studies, particularly with applications related to Stromgren photometry. Role: Dr. Calamida will perform the new emprical and theoretical calibrations and will be the lead author of a paper. Dr. Meridith Joyce Background: Dr. Joyce has been a member of the MESA (Modules for Experiments in Stellar Astrophysics) software development team for more than five years and is a current contributor to the second generation MIST (MESA Isochrones and Stellar Tracks) project. Role: Dr. Joyce will generate custom MIST isochrones for the project and lead the effort in extracting ages from the isochrone fitting. Dr. Niall Miller Background: Dr. Miller has extensive experience working with large photometric data sets in the Galactic bulge, and has published a paper about the bulge's age distribution. Role: Dr. Miller will work with Dr. Joyce and Tayar on determing age estimates and errors for bulge stars, and will utilize his MESA Custom Colors module to produce custom isochrones for the HST filters. Dr. Jamie Tayar Background: Dr. Tayar is an expert in stellar evolution and chemical abundances, and works closely with Drs. Johnson, Joyce, and Miller on Galactic bulge projects. Role: Dr. Tayar will work with Dr. Joyce to determine age estimates and errors for bulge stars. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 0 false false false false false false false SUPPORTED