  As part of a major program to use isolated Local Group dwarf galaxies as
near-field probes of cosmology, we have obtained deep images of the dwarf
irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble
Space Telescope. From these images we have constructed a color-magnitude
diagram (CMD) reaching apparent [absolute] magnitudes of (M475, M814) > (29.0
[+4.4], 27.9 [+3.4]), the deepest ever achieved for any 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 all the
star formation that ever occurred in Leo A happened more recently than 8 Gyr
ago. The CMD shows only a very small amount of star formation in the first few
billion years after the Big Bang; a possible burst at the oldest ages cannot be
claimed with high confidence. The peak SFR occurred ~1.5-4 Gyr ago, at a level
5-10 times the current value. Our modelling indicates that Leo A has
experienced very little metallicity evolution; the mean inferred metallicity is
consistent with measurements of the present-day gas-phase oxygen abundance. We
cannot exclude a scenario in which ALL of the ancient star formation occurred
prior to the end of the era of reionization, but it seems unlikely that the
lack of star formation prior to ~8 Gyr ago was due to early loss or exhaustion
of the in situ gas reservoir.
