Hubble Space TelescopeObservations of NGC 121: First Detection of Blue Stragglers in an Extragalactic Globular Cluster

Abstract

We have resolved stars in the core of the old SMC globular cluster NGC 121 with images from the refurbished Hubble Space Telescope and Wide Field Planetary Camera 2. Our photometry reaches 1.5 mag below the main-sequence turnoff, with negligible field contamination. A blue straggler star (BSS) sequence of 42 candidates is clearly seen—the first such detection in an extragalactic globular cluster. The BSSs are more centrally concentrated than the subgiant, red giant, and horizontal branch stars. Various blending and completeness tests demonstrate that at least 23 of our candidates are genuine BSSs. The BSSs extend about 1.8 mag brighter than the main-sequence turnoff, in accord with similar sequences in Galactic globular clusters. One BSS candidate is 2.4 mag brighter than the turnoff, as luminous as the brightest BSS seen in Galactic globulars. The cluster red horizontal branch is clearly detected and well populated, with a modest blueward extension. The relaxation times of the cluster stars and the BSSs are such that most BSSs are in energy equipartition with the other stars. We find ΔV= 3.3 ± 0.1 mag, corresponding to a cluster age that is 2 Gyr younger than that of most Galactic globular clusters and of one of the oldest LMC globulars.