Challenges in Near-Infrared Stellar Population Synthesis: CO Absorptions and Zero-Point Problem

Understanding of galaxy evolution is rooted in our comprehension of their stellar content. Stellar population synthesis (SPS) models are essential tools in this study, providing insights into the ages, metallicities, and star formation histories of galaxies. However, their accuracy and reliability across different spectral domains, especially in the relatively less-explored near-infrared (NIR) spectrum, present significant challenges. The NIR spectrum offers a wealth of information about the stellar content of galaxies. With the advent of the James Webb Space Telescope and the future NIR-focused telescopes and instruments, the need for reliable NIR SPS models to analyze the stellar content of galaxies is increasingly evident. This presentation outlines two key challenges with NIR SPS models in understanding stellar systems. The first concerns the unexplained strong CO absorption features in early-type galaxies (ETGs) observed at NIR wavelengths. We propose that massive ETGs may contain old stellar populations with enhanced [C/Fe] abundance. This hypothesis presents an alternative to the long-standing belief that the discrepancies between predicted and observed CO line strengths are due to intermediate-age stellar populations. The second challenge we discuss is the “zero-point problem,” where the spectroscopically inferred ages of globular clusters (GCs) exceed the age of the universe. GCs serve as crucial testbeds for the accuracy of SPS models. In this light, I will present the LIBERTY survey, a NIR spectral library of 21 extragalactic GCs from Centaurus A galaxy (NGC 5128). This library will be employed to extend the discussion of the “zero-point problem” to extragalactic GCs and the NIR spectral domain for the first time.