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 scalelengths and straddle the break in M33's surface brightness profile. The colour–magnitude diagrams (CMDs) reach the ancient main-sequence turn-off 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 SFH of S1 unambiguously reveals how the inside-out growth previously measured for M33's inner disc out to Graphic extends out to the disc edge at Graphic. 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.