The ACS LCID Project: Quantifying the Delayed Star Formation in Leo A

We have obtained deep images of the Local Group dwarf irregular galaxy Leo A using the Hubble Space Telescope Advanced Camera for Surveys. From these images we have constructed the deepest ever color-magnitude diagram (CMD) for an irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of Leo A s star formation has occurred more recently than 8 Gyr ago. Our measurements are not inconsistent with a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, with subsequent suppression until lookback times corresponding to redshift z 1. However, given Leo A s strongly isolated state and its dynamical mass of 108 M, its large gas supply was more likely to have been merely heated than driven out of the galaxy altogether.