The Stellar Populations of M33's Outer Regions III: Star Formation   History
  We present a detailed analysis of the star formation history (SFH) of three fields in M33 located ~ 4 - 6 visual scale lengths from its nucleus. These fields were imaged with the Advanced Camera for Surveys on the Hubble Space Telescope and reach ~ 2.5 magnitudes below the red clump of core helium burning stars. The observed color-magnitude diagrams are modeled as linear combinations of individual synthetic populations with different ages and metallicities. To gain a better understanding of the systematic errors we have conducted the analysis with two different sets of stellar evolutionary tracks which we designate as Padova (Girardi et al. 2000) and Teramo (Pietrinferni et al. 2004). The precise details of the results depend on which tracks are used but we can make several conclusions that are fairly robust despite the differences. Both sets of tracks predict the mean age to increase and the mean metallicity to decrease with radius. Allowing age and metallicity to be free parameters and assuming star formation began ~ 14 Gyr ago, we find that the mean age of all stars and stellar remnants increases from ~ 6 Gyr to ~ 8 Gyr and the mean global metallicity decreases from ~ -0.7 to ~ -0.9. The fraction of stars formed by 4.5 Gyr ago increases from ~ 65% to ~ 80%. The mean star formation rate 80 - 800 Myr ago decreases from ~ 30% of the lifetime average to just ~ 5%. The random errors on these estimates are ~ 10%, 1.0 Gyr, and 0.1 dex. By comparing the results of the two sets of stellar tracks for the real data and for test populations with known SFH we have estimated the systematic errors to be 15%, 1.0 Gyr, and 0.2 dex. These do not include uncertainties in the bolometric corrections or variations in alpha-element abundance which deserve future study. 
Michael K. Barker, Ata Sarajedini (Univ. of FL), Doug Geisler (Univ.   de Concepcion), Paul Harding (Case Western Res. Univ....