Three Populous Clusters Discovered to be in the LMC Age Gap

Abstract

In the process of searching the Hubble Space Telescope archive, we have serendipitously discovered three populous Large Magellanic Cloud (LMC) clusters with ages that place them in the LMC `age gap.'. These clusters - NGC 2155, SL663, and NGC 2121 - turn out to have [Fe/H] ~ -1.0 and ages of ~4 Gyr. This puts them in the age gap between the intermediate-age LMC clusters, the oldest of which are ~2.5 Gyr old, and ESO121-SC03, which has an age of ~9 Gyr. The addition of these three clusters to the LMC age - metallicity relation has reduced the discrepancy between the age distribution of the LMC clusters and the field stars. Furthermore, it indicates that searches to find more clusters older than ~2.5 Gyr in the LMC are crucial to a better understanding of its global star formation history.