Old Main Sequence Turnoff Photometry in the Small Magellanic Cloud. II.
  Star Formation History and Its Spatial Gradients
  We present a quantitative analysis of the SFH of 12 fields in the SMC. We
find that there are four main periods of enhancement of star formation: a young
one peaked at around 0.2-0.5 Gyr old, only present in the eastern and in the
central-most fields; two at intermediate ages present in all fields (a
conspicuous one peaked at 4-5 Gyr old, and a less significant one peaked at
1.5-2.5); and an old one, peaked at 10 Gyr in all fields but the western ones.
In the western fields, this old enhancement splits into two, one peaked at
around 8 Gyr old and another at around 12 Gyr old. This "two-enhancement" zone
seems to be a robust feature since it is unaffected by our choice of stellar
evolutionary library but more data covering other fields of the SMC are
necessary in order to ascertain its significancy.
  Some correlation could exist with encounters taken from the orbit
determination of Kallivayalil et al. (2006). But our results would be also fit
in a first pericenter passage scenario like the one claimed by Besla et al.
(2007). There is a strong dichotomy between East/Southeast and West in the
current irregular shape of the SMC. We find that this dichotomy is produced by
the youngest population and began about 1 Gyr ago or later. We do not find yet
a region dominated by an old halo at 4.5 kpc from the SMC center, indicating
either that this old stellar halo does not exist in the SMC or that its
contribution to the stellar populations, at the galactocentric distances of our
outermost field, is negligible. We derive the age-metallicity relation and find
that the metallicity increased continuously from early epochs until now.
