The Star Formation History in The Far Outer Disc of M33
  The outer regions of disc galaxies are becoming increasingly recognized as
key testing sites for models of disc assembly and evolution. Important issues
are the epoch at which the bulk of the stars in these regions formed and how
discs grow radially over time. To address these issues, we use Hubble Space
Telescope Advanced Camera for Surveys imaging to study the star formation
history (SFH) of two fields at 9.1 and 11.6 kpc along M33's northern major
axis. These fields lie at ~ 4 and 5 V-band disc scale-lengths and straddle the
break in M33's surface brightness profile. The colour-magnitude diagrams (CMDs)
reach the ancient main sequence turnoff with a signal-to-noise ratio of ~ 5.
From detailed modelling of the CMDs, we find that the majority of stars in both
fields combined formed at z < 1. The mean age in the inner field, S1, is ~ 3
+/- 1 Gyr and the mean metallicity is [M/H] ~ -0.5 +/- 0.2 dex. The star
formation history of S1 unambiguously reveals how the inside-out growth
previously measured for M33's inner disc out to ~ 6 kpc extends out to the disc
edge at ~ 9 kpc. In comparison, the outer field, S2, is older (mean age ~ 7 +/-
2 Gyr), more metal-poor (mean [M/H] ~ -0.8 +/- 0.3 dex), and contains ~ 30
times less stellar mass. These results provide the most compelling evidence yet
that M33's age gradient reverses at large radii near the disc break and that
this reversal is accompanied by a break in stellar mass surface density. We
discuss several possible interpretations of this behaviour including radial
stellar mixing, warping of the gaseous disc, a change in star formation
efficiency, and a transition to another structural component. These results
offer one of the most detailed views yet of the peripheral regions of any disc
galaxy and provide a much-needed observational constraint on the last major
epoch of star formation in the outer disc.
