Comparing the ancient star formation histories of the Magellanic Clouds

We present preliminary results from a new Hubble Space Telescope (HST) archival program aimed at tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs) of the field populations of the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC). We demonstrate the quality of the archival data by constructing HST/Wide Field Planetary Camera 2-based colour-magnitude diagrams (CMDs; M_F555W ∼ +8) for seven spatially diverse fields in the SMC and eight fields in the LMC. The HST-based CMDs are >2 mag deeper than any from ground-based observations, and are particularly superior in high surface brightness regions, e.g. the LMC bar, which contain a significant fraction of star formation and are crowding limited from ground-based observations. To minimize systematic uncertainties, we derive the SFH of each field using an identical maximum likelihood CMD fitting technique. We then compute an approximate mass weighted average SFH for each galaxy. From the average SFHs, we find that both galaxies lack a dominant burst of early star formation, which suggests either a suppression or an underfuelling of ancient star formation in the Magellanic Clouds (MCs). From 10 to 12 Gyr ago, we find that the LMC experienced a period of enhanced stellar mass growth relative to the SMC. Similar to some previous studies, we find two notable peaks in the SFH of the SMC at ∼4.5 and 9 Gyr ago, which could be due to repeated close passages with the LMC, implying an interaction history that has persisted for at least 9 Gyr. We find little evidence for strong periodic behaviour in the lifetime SFHs of both MCs, suggesting that repeated encounters with the Milky Way are unlikely. Beginning ∼3.5 Gyr ago, both galaxies show sharp increases in their SFHs, in agreement with previous studies. Subsequently, the SFHs track either remarkably well. Spatial variations in the SFH of the SMC are consistent with a picture where gas was driven into the centre of the SMC ∼3.5 Gyr ago, which simultaneously shut down star formation in the outer regions while dramatically increasing the star formation rate in the centre. In contrast, the LMC shows little spatial variation in its ancient SFH. The planned additional analysis of HST pointings at larger galactocentric radii will allow us to make more confident statements about spatial variations in the SFHs of the SMC and LMC.