Dissecting the stellar content of Leo I: a dwarf irregular caught in
  transition
  Leo I is considered one of the youngest dwarf spheroidals (dSph) in the Local
Group. Its isolation, extended star formation history (SFH), and recent
perigalacticon passage (~1 Gyr ago) make Leo~I one of the most interesting
nearby stellar systems. Here, we analyse deep photometric Hubble Space
Telescope data via colour-magnitude diagram fitting techniques to study its
global and radially-resolved SFH. We find global star formation enhancements in
Leo I ~13, 5.5, 2.0, and 1.0 Gyr ago, after which it was substantially
quenched. Within the context of previous works focused on Leo I, we interpret
the most ancient and the youngest ones as being linked to an early formation
(surviving reionisation) and the latest perigalacticon passage (transition from
dIrr to dSph), respectively. We clearly identify the presence of very metal
poor stars ([Fe/H]~-2) ageing ~5-6 and ~13 Gyr old. We speculate with the
possibility that this metal-poor population in Leo I is related to the merging
with a low mass system (possibly an ultra-faint dwarf). This event would have
triggered star formation (peak of star formation ~5.5 Gyr ago) and accumulated
old, metal poor stars from the accreted system in LeoI. Some of the stars born
during this event would also form from accreted gas of low-metallicity (giving
rise to the 5-6 Gyr low-metallicity tail). Given the intensity and extension of
the 2.0 Gyr burst, we hypothesise that this enhancement could also have an
external origin. Despite the quenching of star formation around 1 Gyr ago (most
probably induced by ram pressure stripping with the Milky Way halo at
pericentre), we report the existence of stars as young as 300-500 Myr. We also
distinguish two clear spatial regions: the inner ~190 pc presents an
homogeneous stellar content (size of the gaseous star forming disc in LeoI from
~4.5 to 1 Gyr ago), whereas the outer regions display a clear positive age
gradient.
