Pixel Color Magnitude Diagrams for Semi-Resolved Stellar Populations: The Star Formation History of Regions within the Disk and Bulge of M31

TL;DR

This paper introduces pixel color magnitude diagrams (pCMDs) as a new method to analyze semi-resolved stellar populations, enabling detailed star formation histories in regions like M31's disk and bulge.

Contribution

The paper develops pCMDs as a novel technique for semi-resolved stellar populations, allowing extraction of star formation histories from imaging data.

Findings

pCMDs are sensitive to star formation history, age, metallicity, and dust.

Applied to HST data of M31, pCMDs reveal smooth, exponential decay star formation histories.

pCMDs can distinguish different stellar population properties in crowded regions.

Abstract

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 vs. 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, $N_{\rm pix}$. Simulated pCMDs demonstrate a strong sensitivity to the star formation history (SFH) and allow one to break degeneracies between age, metallicity and dust based on two filter data for values of $N_{\rm pix}$ up to at least $10^4$. We extract pCMDs from Hubble Space Telescope (HST) optical imaging of M31 and derive non-parametric SFHs from $10^6$ yr to $10^{10}$ yr for both the crowded disk and bulge regions (where $N_{\rm pix}\approx30-10^3$). From analyzing a small region of the disk we find a non-parametric 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.