The Detection of Inside-Out Disk Growth in M33

We present resolved stellar photometry of four fields along the major axis of the M33 disk from images taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The photometry provides a detailed census of the red clump in all fields and reaches the ancient main sequence in the outermost field. Through detailed modeling of the color-magnitude diagrams, we find that the percentage of the stellar mass formed prior to z = 1 changes from 71 ± 9% in the innermost field to 16 ± 6% in the outermost field. The disk shows a clear trend of increasing scale length with time, evolving from r_s = 1.0 ± 0.1 kpc 10 Gyr ago to r_s = 1.8 ± 0.1 at times more recent than 5 Gyr ago, in agreement with analytical predictions for disk growth. Beyond the disk truncation radius, however, the stellar density profile steepens with time and the age gradient reverses, in agreement with recent simulations. The late and slow growth of the stellar disk may be due in part to the low mass of M33.