Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03

We present the results of photometric measurements from images of the Large Magellanic Cloud (LMC) cluster ESO 121-SC03 taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Our resulting colour-magnitude diagram (CMD) reaches 3 mag below the main-sequence turn-off, and represents by far the deepest observation of this cluster to date. We also present similar photometry from ACS imaging of the accreted Sagittarius dSph cluster Palomar 12, used in this work as a comparison cluster. From analysis of its CMD, we obtain estimates for the metallicity and reddening of ESO 121-SC03: [Fe/H] = - 0.97 +/- 0.10 E(V - I) = 0.04 +/- .02, in excellent agreement with previous studies. The observed horizontal branch (HB) level in ESO 121-SC03 suggests this cluster may lie 20 per cent closer to us than does the centre of the LMC. ESO 121-SC03 also possesses a significant population of blue stragglers, which we briefly discuss. Our new photometry allows us to undertake a detailed study of the age of ESO 121-SC03 relative to Palomar 12 and the Galactic globular cluster 47 Tuc. We employ both vertical and horizontal differential indicators on the CMD, calibrated against isochrones from the Victoria-Regina stellar models. These models allow us to account for the different alpha-element abundances in Palomar 12 and 47 Tuc, as well as the unknown run of alpha-elements in ESO 121-SC03. Taking a straight error-weighted mean of our set of age measurements yields ESO 121-SC03 to be 73 +/- 4 per cent the age of 47 Tuc, and 91 +/- 5 per cent the age of Palomar 12. Palomar 12 is 79 +/- 6 per cent as old as 47 Tuc, consistent with previous work. Our result corresponds to an absolute age for ESO 121-SC03 in the range 8.3-9.8 Gyr, depending on the age assumed for 47 Tuc, therefore confirming ESO 121-SC03 as the only known cluster to lie squarely within the LMC age gap. We briefly discuss a suggestion from earlier work that ESO 121-SC03 may have been accreted into the LMC system.