Abstract
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.
Highlights
The traditional theory of disc galaxy formation holds that isolated discs form through the dissipational collapse of gaseous protogalaxies embedded in cold dark matter (CDM) haloes (White & Rees 1978; Fall & Efstathiou 1980; Peebles 1984)
Spatially-resolved observational estimates of the star formation histories (SFHs) within spirals can provide powerful constraints on the simulations and their sub-grid physical models. These constraints are more robust against degeneracies when they come from colour-magnitude diagrams (CMDs) of resolved stars than when they rely on integrated, unresolved starlight
The recovered SFHs are in line with our qualitative expectations from §3
Summary
The traditional theory of disc galaxy formation holds that isolated discs form through the dissipational collapse of gaseous protogalaxies embedded in cold dark matter (CDM) haloes (White & Rees 1978; Fall & Efstathiou 1980; Peebles 1984). Spatially-resolved observational estimates of the star formation histories (SFHs) within spirals can provide powerful constraints on the simulations and their sub-grid physical models. These constraints are more robust against degeneracies when they come from colour-magnitude diagrams (CMDs) of resolved stars than when they rely on integrated, unresolved starlight. The small field-of-view of ACS often makes it unclear which galactic component, or mix of components has been imaged (thin disc, thick disc, bulge, halo, accreted substructure, disturbed disc, etc.) This problem is acute in MW-type spirals, which have non-negligible bulges and are more likely to have experienced massive satellite accretion, which can hinder efforts to study purely dissipative disc formation and secular evolution
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