[What is the AGB star?]  

During their lifetime, stars go through various stages of stellar evolution, featured by different internal burning, effective temperatures, and lifetimes (see Fig.1). The Asymptotic-Giant-Branch (AGB) phase is the last luminous phase of intermediate-mass stars (~ typical 3 M_☉, a few Myr in duration), characterized by alternate double-shell burning of Hydrogen and Helium, convection, mixing and mass loss. The whole AGB phase is composed of two periods, an Early AGB (E-AGB) and the subsequent and more substantial Thermal Pulsing AGB (TP-AGB). As the H or the He fuel is burned in inner shells, the energy injection makes the star expand and collapse like in a breathing cycle, which is called a thermal pulse. During thermal pulses, material from the interior is dredged outward, Carbon in particular. 

_{Fig.1 | The sketch of stellar evolution. The Hertzsprung-Russell diagram of stellar evolution, from Falk Herwig (2005).}

[What is the importance of AGB stars?]   

A galaxy consists of hundreds of billions of stars in various evolutionary stages, whose energy emission makes up the total spectrum of a galaxy. The history of stellar formation in a galaxy is therefore encoded in the integrated spectra: understanding galaxy spectra helps constrain the process of galaxy formation and evolution. This is obtained by comparing observed galaxy spectra to stellar population models, which are based on a detailed understanding of stars from theory and observations in our Galaxy and nearby ones. The modeling of most stellar phases in Fig.1 is uncontroversial, but the TP-AGB phase has been unclear for decades due to the complicated stellar physics described above, with different stellar population models being highly discrepant, especially in the NIR - 0.5-2 microns - where the cold AGB stars leave their imprints, with a predicted NIR contribution ranging from ~20% to 80% according to different models. This uncertainty has hampered a precise determination of galaxy properties, such as ages and masses, especially for galaxies around 1 billion years of age where the TP-AGB phase provides its maximum contribution. 

[What is the discovery?]  

Previous studies suggested that distant quiescent galaxies are ideal candidates for exploring the contribution of the TP-AGB, but these were based on photometry from broadband images which is more prone to degeneracies and indeed yielded inconsistent results. Fortunately, the near-infrared spectrograph (NIRSpec) onboard the JWST can obtain high-quality galaxy spectra spanning 0.5 to 5 microns, which should contain the broad absorption features from TP-AGB stars. 

Serendipitously, we found three quiescent galaxies with such TP-AGB star features from our Cosmic Evolution Early Release Science (CEERS) JWST/NIRSpec program (DD-ERS-1345) and a follow-up program (DD-2750), one of which, D36123, was much brighter than the others and provided a spectrum of excellent quality (see Fig.2). This is the FIRST detection of clear signatures from TP-AGB stars in distant galaxies, including numerous Oxygen- and Carbon-type absorption features, spectral discontinuities and some features from rare species.

We compared the observed spectra to a variety of stellar-population models with different TP-AGB prescriptions. We found that models with a substantial TP-AGB contribution perform better and point towards younger ages hence lower stellar masses. Our results have implications for galaxy and stellar evolution, cosmic dust production, and chemical enrichment, and call for revision of published stellar-population fitting results._{Fig.2 | NIR rest-frame spectrum of the quiescent galaxy D36123 at z=1.082.  The spectrum is plotted as a double histogram corresponding to the ±1σ noise, showing its exceptionally high quality. Numerous broad absorptions and spectral discontinuities in the NIR are specific spectral signatures from TP-AGB stars: multiple TiO absorptions accompanied by deep CN and C2 features are detected for the first time in the spectrum of distant galaxies. The presence of VO and ZrO features also indicates the prominence of the TP-AGB phase. The inset from JWST imaging observations with NIRCam presents the compact quiescent galaxy morphology of  D36123. (Credited: Shiying Lu).}

[What is the next?]

The spectrum of D36123 is at present unique, without any known analogs in the nearby Universe. But there is no reason to think this particular quiescent galaxy is peculiar in any respect: it seems to be pretty normal and representative. A larger sample of similar observations is needed to help exclude any peculiarity and gauge the range of TP-AGB star contributions in galaxies and the variety and strength of spectral features. Indeed, not even the best-fitting model can explain the full richness of features we detected in D36123 and we are working on the next generation of models.

Fortunately, we will obtain new JWST observations next year, which will help us to further understand all these aspects in the future, and explore the impact of TP-AGB stars on galaxy models, dust production, and chemical enrichment of the interstellar medium.