The James Webb Space Telescope has just confirmed that a disputed type of ageing star—the thermally pulsating asymptotic-giant-branch (TP-AGB) phase—dominates the near-infrared light of distant, massive galaxies, but even the best models cannot fully match the telescope's sharp new spectra. This matters because astronomers rely on stellar population models to date galaxies and reconstruct how the universe built its structures. The two-decade-old debate over TP-AGB stars has skewed results: models that underestimate their contribution make galaxies appear older and heavier than they really are, even suggesting some galaxies formed impossibly early. The JWST data now demand a fix. The researchers will build new models that incorporate how varying abundances of carbon, oxygen, and other elements alter the near-infrared spectrum, especially for young, metal-rich systems. If successful, these models will let astronomers accurately interpret JWST observations of galaxies at cosmic noon—roughly 10 billion years ago—and resolve tensions with the standard dark-matter-driven picture of galaxy formation. This is fundamental science. It will sharpen our understanding of how stars evolve and how galaxies assemble, but has no immediate practical application. Past improvements in stellar models, however, have underpinned everything from cosmic distance measurements to predictions of gravitational-wave sources.
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Evolutionary population synthesis models describing the energy emission of galaxies and star clusters are an essential tool in astrophysics and cosmology, spanning from data interpretation, galaxy formation simulations, predictions of stellar remnants for gravitational wave studies. As astronomers depend on evolutionary population synthesis models to infer the timeline of galaxy formation and evolution, any inaccuracy in the models severely limits progress. Particularly controversial is the contribution from the thermally pulsating asymptotic-giant-branch (TP-AGB) stellar phase of intermediate-mass stars (1-10 Msun) , which is relevant to young (0.2-2 Gyr) stellar systems and is at the centre of a two-decades-long debate. The James Webb Space telescope (JWST) has just settled this debate, by detecting strong spectral features from Oxygen-rich and Carbon-rich stellar types in the near-infrared spectrum of massive (10^10M?) galaxies at large look-back time, z = 1–2. These unambiguously point to a sizable contribution from the TP-AGB and exclude widely used models with low TP-AGB contribution. Moreover, TP-AGB models point to younger and lighter galaxies, which impacts on the recent debate around galaxies that are too old and too massive to even exist in a lambda-Cold Dark Matter universe, as triggered by the analysis of JWST data with low TP-AGB models. A revision of published stellar population fitting results is therefore needed. However these observations reveal that - while matching the JWST high signal-to-noise spectra better than any other model - even our TP-AGB rich models are unable to reproduce the whole variety of Carbon, Oxygen, Titanium Oxide, Vanadium and Zirconium features. We argue that the residual mismatches are due to specific chemical abundance ratio effects (of C and O primarily) in these distant metal-rich systems. We propose to improve stellar population synthesis models by including abundance ratio effects extended to the near-infrared. Models as such and covering the parameter space of high-metallicity and young age plus TP-AGB are not yet available. They are our best bet to resolve the discrepancy revealed by the JWST. They are urgently required for the interpretation of JWST data. Remarkably, we have just been awarded Cycle 3 JWST time for obtaining high-quality spectra for 40 more high-redshift galaxies. This project will leverage our leading expertise in population synthesis, chemical evolution and galaxy spectra for setting a theoretical framework for the interpretation of galaxy data. Our work will enhance current understanding of not only galaxy evolution, but also stellar evolution and model atmosphere theories.
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