Two Groups of Nearly Coeval Star Clusters in the Small Magellanic Cloud

Abstract

We report new photometry of populous intermediate-age clusters in the SMC using the {\sl Hubble Space Telescope}. In contrast to the accepted picture, these clusters appear to have formed in two brief intervals, one $8\pm 2$ Gyr ago, and a more recent burst $2\pm 0.5$ Gyr ago. When the ridgelines of the four clusters (NGC 339, 361, 416, and Kron 3) in the 8 Gyr burst are aligned, the dispersion in turnoff luminosities is $<0.2$ mag, corresponding to a maximum age spread of $\pm 0.7$ Gyr. When the ridgelines of three clusters (NGC 152, 411, and 419) in the 2 Gyr burst are aligned, the maximum dispersion of 0.2 mag in turnoff luminosity corresponds to a permitted age spread of $\pm 0.2$ Gyr. Within each group of clusters, the entire cluster loci (including red giant branches and clumps) are nearly identical, consistent with indistinguishable metallicities and ages. In contrast to the wide dispersion in ages previously reported in the literature, our sample with more precise photometry and age measurements supports a burst-punctuated rather than a continuous cluster formation history for the 2 Gyr and 8 Gyr SMC clusters.