2021ApJ...920..105C

The recent star formation histories (SFHs) of post-starburst galaxies have been determined almost exclusively from detailed modeling of their composite starlight. This has provided important but limited information on the number, strength, and duration of bursts of star formation. In this work, we present a direct and independent measure of the recent SFH of the post-starburst galaxy S12 (plate-mjd-fiber for SDSS 623-52051-207; designated EAS12 in Smercina et al.) from its star cluster population. We detect clusters from high-resolution, UBR optical images taken with the Hubble Space Telescope and compare their luminosities and colors with stellar population models to estimate the ages and masses of the clusters. No clusters younger than ∼70 Myr are found, indicating star formation shut off at this time. Clusters formed ∼120 Myr ago reach masses up to a ∼few x 10⁷ M_☉, several times higher than similar-age counterparts formed in actively merging galaxies like the Antennae and NGC 3256. We develop a new calibration based on known properties for eight nearby galaxies to estimate the star formation rate (SFR) of a galaxy from the mass of the most massive cluster, M_max. The cluster population indicates that S12 experienced an extremely intense but short-lived burst ∼120 Myr ago, with an estimated peak of 500_–250⁺⁵⁰⁰M_☉yr^–1 and duration of 50 ± 25 Myr, one of the highest SFRs estimated for any galaxy in the modern universe. The cluster population also allows us to fill in more of the backstory of S12. Prior to the recent, intense burst, S12 was forming stars at a moderate rate of ∼3-5 M_☉ yr^–1, typical of spiral galaxies, but the system experienced an earlier burst at some point, approximately 1-3 Gyr ago. While fairly uncertain, we estimate that the SFR during this earlier burst was ∼20-30 M_☉ yr^–1, similar to the current SFR in the Antennae and NGC 3256.