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Abstract: The outer regions of disc galaxies are becoming increasingly recognized askey testing sites for models of disc assembly and evolution. Important issuesare the epoch at which the bulk of the stars in these regions formed and howdiscs grow radially over time. To address these issues, we use Hubble SpaceTelescope Advanced Camera for Surveys imaging to study the star formationhistory SFH of two fields at 9.1 and 11.6 kpc along M33-s northern majoraxis. These fields lie at ~ 4 and 5 V-band disc scale-lengths and straddle thebreak in M33-s surface brightness profile. The colour-magnitude diagrams CMDsreach 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 bothfields 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 starformation history of S1 unambiguously reveals how the inside-out growthpreviously measured for M33-s inner disc out to ~ 6 kpc extends out to the discedge 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 ~ 30times less stellar mass. These results provide the most compelling evidence yetthat M33-s age gradient reverses at large radii near the disc break and thatthis reversal is accompanied by a break in stellar mass surface density. Wediscuss several possible interpretations of this behaviour including radialstellar mixing, warping of the gaseous disc, a change in star formationefficiency, and a transition to another structural component. These resultsoffer one of the most detailed views yet of the peripheral regions of any discgalaxy and provide a much-needed observational constraint on the last majorepoch of star formation in the outer disc.

Autor: M. K. Barker, A. M. N. Ferguson U. of Edinburgh, A. A. Cole U. of Tasmania, R. Ibata Obs. Strasbourg, M. Irwin Cambridge U., G. F


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