PIXEL COLOR MAGNITUDE DIAGRAMS FOR SEMI-RESOLVED STELLAR POPULATIONS: THE STAR FORMATION HISTORY OF REGIONS WITHIN THE DISK AND BULGE OF M31

摘要

The analysis of stellar populations has, by and large, been developed for two limiting cases: spatially resolved stellar populations in the color–magnitude diagram, and integrated light observations of distant systems. In between these two extremes lies the semi-resolved regime, which encompasses a rich and relatively unexplored realm of observational phenomena. Here we develop the concept of pixel color–magnitude diagrams (pCMDs) as a powerful technique for analyzing stellar populations in the semi-resolved regime. pCMDs show the distribution of imaging data in the plane of pixel luminosity versus pixel color. A key feature of pCMDs is that they are sensitive to all stars, including both the evolved giants and the unevolved main sequence stars. An important variable in this regime is the mean number of stars per pixel, . Simulated pCMDs demonstrate a strong sensitivity to the star formation history (SFH) and have the potential to break degeneracies between age, metallicity and dust based on two filter data for values of up to at least 104. We extract pCMDs from Hubble Space Telescope optical imaging of M31 and derive SFHs with seven independent age bins from 106 to 1010 year for both the crowded disk and bulge regions (where ). From analyzing a small region of the disk we find a SFH that is smooth and consistent with an exponential decay timescale of 4 Gyr. The bulge SFH is also smooth and consistent with a 2 Gyr decay timescale. pCMDs will likely play an important role in maximizing the science returns from next generation ground and space-based facilities.