New photometry for the intermediate-age Large Magellanic Cloud globular cluster NGC 2121 and the nature of the LMC age gap

We report new photometry for the cluster NGC 2121 in the Large Magellanic Cloud, which shows a prominent hydrogen core exhaustion gap at the turnoff and a descending subgiant branch reminiscent of Galactic open clusters. We achieve an excellent fit using the Girardi isochrones, finding an age of 3.2 +/- 0.5 Gyr, with [Fe/H]= -0.6 +/- 0.2. The isochrones fit the color and shape of the turnoff and subgiant branch so precisely that we can constrain the metallicity, as well as the age. The same isochrones also fit SL 663 and NGC 2155, although our photometry for these clusters has much larger errors. We find these clusters to be 0.8 Gyr younger and 0.4 dex more metal-rich than recently reported in the literature. Consequently, we argue that NGC 2121, NGC 2155, and SL 663 are not properly assigned to the age gap in the LMC, but instead are among the first clusters to be have formed in the relatively metal-rich, younger group of LMC clusters. We propose a new definition of the LMC age gap as extending from 3.2 to 13 Gyr, with ESO 121-SC03 still the only remaining candidate for membership in the age gap.