Measuring Star Formation Histories, Distances, and Metallicities with Pixel Color–Magnitude Diagrams. I. Model Definition and Mock Tests

Abstract

We present a comprehensive study of the applications of the pixel color-magnitude diagram (pCMD) technique for measuring star formation histories (SFHs) and other stellar population parameters of galaxies, and demonstrate that the technique can also constrain distances. SFHs have previously been measured through either the modeling of resolved-star CMDs or of integrated-light SEDs, yet neither approach can easily be applied to galaxies in the "semi-resolved regime". The pCMD technique has previously been shown to have the potential to measure stellar populations and star formation histories in semi-resolved galaxies. Here we present Pixel Color-Magnitude Diagrams with Python (PCMDPy), a GPU-accelerated package that makes significant computational improvements to the original code and including more realistic physical models. These advances include the simultaneous fitting of distance, modeling a Gaussian metallicity-distribution function, and an observationally-motivated dust model. GPU-acceleration allows these more realistic models to be fit roughly 7× faster than the simpler models in the original code. We present results from a suite of mock tests, showing that with proper model assumptions, the code can simultaneously recover SFH, [Fe/H], distance, and dust extinction. Our results suggest the code, applied to observations with HST -like resolution, should constrain these properties with high precision within 10 Mpc and can be applied to systems out to as far as 100 Mpc. pCMDs open a new window to studying the stellar populations of many galaxies that cannot be readily studied through other means.

Downsizing revised: Star formation timescales for elliptical galaxies with an environment-dependent IMF and a number of SNIa

“…More recent studies of highredshift galaxies by Saracco et al (2019) broadly agree with the downsizing relation suggested by McDermid et al (2015). Independent methods such as measuring the SFH with pixel colourmagnitude diagrams (CMD, Cook et al 2019) could also be helpful supplementary evidence supporting the decreasing SFHs of massive ETGs. The uncertainty of such a method is large, but better-resolved pixel CMD images have the potential to reveal the SFH fluctuations that are hidden in integrated-light observations.…”

An Older, More Quiescent Universe from Panchromatic SED Fitting of the 3D-HST Survey

“…A hierarchical Bayesian model would be a logical next step to craft an SFH prior which is simultaneously consistent with the observed SEDs and with observations such as the evolution of the stellar mass function with time. Thorough comparisons of SFHs derived from integrated light to those derived from resolved (e.g., Johnson et al 2013) and semi-resolved (Cook et al 2019) stellar populations in local galaxies is also a promising way forward.…”

Stellar Population Inference with Prospector

“…Another possibility is to use piece-wise constant or δfunction SFHs where the mass in each temporal bin be-comes a parameter of the model. These so-called nonparametric -actually many-parameter -models have become the standard in SFH inference from (semi-) resolved stellar color-magnitude diagrams (e.g., Dolphin 1997;Harris & Zaritsky 2001;Weisz et al 2011;Cook et al 2019) and have been used in maximum-likelihood "inversion" codes for unresolved spectra (e.g., Cappellari & Emsellem 2004;Cid Fernandes et al 2005;Ocvirk et al 2006;Tojeiro et al 2007;Koleva et al 2009). In principle any basis function can be used (e.g., Iyer & Gawiser 2017) though the piece-wise constant basis has the advantage of being readily interpreted.…”