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

Abstract

We report new photometry and main-sequence turnoff ages for seven populous star clusters in the SMC with M_V<-6 and age greater than 1 Gyr, using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. In contrast to the accepted picture, these clusters appear to have formed in two brief intervals, the oldest 8+/-2 Gyr ago and one during a more recent burst 2+/-0.5 Gyr ago. When the ridgelines of the four clusters (NGC 339, 361, and 416 and Kron 3) in the 8 Gyr burst are aligned, the dispersion in turnoff luminosities is less than 0.2 mag, corresponding to a maximum age spread of +/-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 +/-0.2 Gyr. Within each group of clusters, the entire cluster loci (including red giant branches and clumps) are nearly identical, consistent with a very small spread in metallicity and age. 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 oldest SMC clusters.