The propagation of uncertainties in stellar population synthesis modeling II: The challenge of comparing galaxy evolution models to observations

TL;DR

This paper investigates how uncertainties in stellar evolution, initial mass function, and interstellar medium properties affect the comparison of galaxy evolution models with observations, highlighting significant impacts on synthetic galaxy colors.

Contribution

It systematically analyzes the impact of key uncertainties in stellar population synthesis on the translation of models to observable galaxy properties.

Findings

Uncertainties significantly affect UV, optical, and near-IR galaxy colors.

Key uncertain factors include IMF slope, dust law, and stellar evolution stages.

Systematic and correlated uncertainties complicate model-observation comparisons.

Abstract

Models for the formation and evolution of galaxies readily predict physical properties such as the star formation rates, metal enrichment histories, and, increasingly, gas and dust content of synthetic galaxies. Such predictions are frequently compared to the spectral energy distributions of observed galaxies via the stellar population synthesis (SPS) technique. Substantial uncertainties in SPS exist, and yet their relevance to the task of comparing galaxy evolution models to observations has received little attention. In the present work we begin to address this issue by investigating the importance of uncertainties in stellar evolution, the initial stellar mass function (IMF), and dust and interstellar medium (ISM) properties on the translation from models to observations. We demonstrate that these uncertainties translate into substantial uncertainties in the ultraviolet, optical, and near-infrared colors of synthetic galaxies. Aspects that carry significant uncertainties include the logarithmic slope of the IMF above 1 Msun, dust attenuation law, molecular cloud disruption timescale, clumpiness of the ISM, fraction of unobscured starlight, and treatment of advanced stages of stellar evolution including blue stragglers, the horizontal branch, and the thermally-pulsating asymptotic giant branch. The interpretation of the resulting uncertainties in the derived colors is highly non-trivial because many of the uncertainties are likely systematic, and possibly correlated with the physical properties of galaxies. We therefore urge caution when comparing models to observations.