Mapping the Recent Star Formation History of the Disk of M51
  Using data acquired as part of a unique Hubble Heritage imaging program of
broadband colors of the interacting spiral system M51/NGC 5195, we have
conducted a photometric study of the stellar associations across the entire
disk of the galaxy in order to assess trends in size, luminosity, and local
environment associated with recent star formation activity in the system.
Starting with a sample of over 900 potential associations, we have produced
color-magnitude and color-color diagrams for the 120 associations that were
deemed to be single-aged. It has been found that main sequence turnoffs are not
evident for the vast majority of the stellar associations in our set,
potentially due to the overlap of isochronal tracks at the high mass end of the
main sequence, and the limited depth of our images at the distance of M51. In
order to obtain ages for more of our sample, we produced model spectral energy
distributions (SEDs) to fit to the data from the GALEXEV simple stellar
population (SSP) models of Bruzual and Charlot (2003). These SEDs can be used
to determine age, size, mass, metallicity, and dust content of each association
via a simple chi-squared minimization to each association's B, V, and I-band
fluxes. The derived association properties are mapped as a function of
location, and recent trends in star formation history of the galaxy are
explored in light of these results. This work is the first phase in a program
that will compare these stellar systems with their environments using
ultraviolet data from GALEX and infrared data from Spitzer, and ultimately we
plan to apply the same stellar population mapping methodology to other nearby
face-on spiral galaxies.
