Star formation history of the Galactic bulge from deep HST imaging of low reddening windows

TL;DR

This study uses deep HST imaging to determine the star formation history of the Galactic bulge, revealing most stars are older than 8 Gyr but with a significant younger, metal-rich population.

Contribution

It provides the first detailed star formation history of the bulge using deep, proper-motion-cleaned CMDs from HST, highlighting the age distribution and metallicity evolution.

Findings

Over 80% of bulge stars formed before 8 Gyr ago.

10-25% of stars are younger than 5 Gyr, especially in the metal-rich population.

Metallicity distribution matches spectroscopic surveys, with [Fe/H] from -0.7 to 0.6.

Abstract

Despite the huge amount of photometric and spectroscopic efforts targetting the Galactic bulge over the past few years, its age distribution remains controversial owing to both the complexity of determining the age of individual stars and the difficult observing conditions. Taking advantage of the recent release of very deep, proper-motion-cleaned colour--magnitude diagrams (CMDs) of four low reddening windows obtained with the Hubble Space Telescope (HST), we used the CMD-fitting technique to calculate the star formation history (SFH) of the bulge at -2deg > b > -4deg along the minor axis. We find that over 80 percent of the stars formed before 8 Gyr ago, but that a significant fraction of the super-solar metallicity stars are younger than this age. Considering only the stars that are within reach of the current generation of spectrographs (i.e. V < 21), we find that 10 percent of the bulge stars are younger than 5 Gyr, while this fraction rises to 20-25 percent in the metal-rich peak. The age-metallicity relation is well parametrized by a linear fit implying an enrichment rate of dZ/dt ~ 0.005 Gyr$^{-1}$. Our metallicity distribution function accurately reproduces that observed by several spectroscopic surveys of Baade's window, with the bulk of stars having metal-content in the range [Fe/H] ~ -0.7 to ~0.6, along with a sparse tail to much lower metallicities.