We present the results of photometric measurements from images of the LMC cluster ESO 121-SC03 taken with the Advanced Camera for Surveys on the Hubble Space Telescope .
Our resulting colour-magnitude diagram ( CMD ) reaches 3 magnitudes 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 \pm 0.10 and E ( V - I ) = 0.04 \pm 0.02 , in excellent agreement with previous studies .
The observed horizontal branch 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 Pal .
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 Pal .
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 \pm 4 per cent the age of 47 Tuc , and 91 \pm 5 per cent the age of Pal .
12 .
Pal .
12 is 79 \pm 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 .