[Paper Review] Strong spectral features from asymptotic giant branch stars in distant quiescent galaxies

Shiying Lu, E. Daddi+28 more|arXiv (Cornell University)|Mar 12, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This study uses JWST/NIRSpec to detect strong thermally pulsing asymptotic giant branch (TP-AGB) star features—such as oxygen- and carbon-rich absorption, vanadium, and zirconium lines—in three massive, ~1 Gyr-old quiescent galaxies at z ≈ 1–2. The results show a dominant TP-AGB contribution inconsistent with standard models, necessitating revisions to stellar population synthesis models toward younger ages and lower masses, with implications for dust production and chemical enrichment.

ABSTRACT

Dating the ages and weighting the stellar populations in galaxies are essential steps when studying galaxy formation through cosmic times. Evolutionary population synthesis models with different input physics are used for this purpose. Moreover, the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) stellar phase, which peaks for intermediate-age 0.6-2 Gyr, has been debated for decades. Here we report the detection of strong cool-star signatures in the rest-frame near-infrared spectra of three young (~1Gyr), massive (~10^10Msun) quiescent galaxies at large look-back time, z=1-2, using JWST/NIRSpec. The coexistence of oxygen- and carbon-type absorption features, spectral edges and features from rare species, such as vanadium and possibly zirconium, reveal a strong contribution from TP-AGB stars. Population synthesis models with a significant TP-AGB contribution reproduce the observations better than those with a weak TP-AGB, which are commonly used. These findings call for revisions of published stellar population fitting results, as they point to populations with lower masses and younger ages and have further implications for cosmic dust production and chemical enrichment. New generations of improved models are needed, informed by these and future observations.

Motivation & Objective

• To determine the contribution of thermally pulsing asymptotic giant branch (TP-AGB) stars to the rest-frame near-infrared spectra of distant quiescent galaxies.
• To resolve long-standing uncertainties in evolutionary population synthesis models regarding TP-AGB phase energetics and spectral characteristics.
• To assess the impact of TP-AGB contributions on age and mass estimates in stellar population fitting of high-redshift galaxies.
• To evaluate the consistency of observed spectral features with current TP-AGB model prescriptions, particularly those from M05 and C09 models.
• To provide observational constraints for future generations of improved stellar population synthesis models.

Proposed method

• Acquired rest-frame near-infrared spectra of three massive, quiescent galaxies at z ≈ 1–2 using JWST/NIRSpec.
• Performed spectral energy distribution (SED) fitting using multiple population synthesis models: BC03, C09, M13, and M05, with varying TP-AGB prescriptions.
• Constrained model parameters including redshift, age, dust attenuation (Calzetti and BC03 laws), star formation timescale (τ), and metallicity (Z/Z⊙).
• Applied SNR-weighted relative deviation analysis to compare model fits to observed spectra, identifying best-fitting models.
• Excluded the 0.5–1 μm region in selected fits to test robustness and isolate the impact of TP-AGB features.
• Used the BT-SETTL and Aringer et al. (2009) spectral libraries for extrapolating TP-AGB stellar spectra in the models.

Experimental results

Research questions

• RQ1 What is the relative contribution of TP-AGB stars to the near-infrared light of massive, quiescent galaxies at z ≈ 1–2?
• RQ2 How do observed spectral features—such as O-rich and C-rich absorption, and rare elements like vanadium and zirconium—constrain TP-AGB model prescriptions?
• RQ3 To what extent do standard population synthesis models (e.g., C09) fail to reproduce the observed spectra compared to models with strong TP-AGB contributions (e.g., M05)?
• RQ4 How do the inferred stellar population parameters (age, mass, metallicity) change when a strong TP-AGB component is included?
• RQ5 What are the implications of a dominant TP-AGB contribution for galaxy mass-to-light ratios, dust production, and chemical enrichment histories?

Key findings

• Strong oxygen- and carbon-rich absorption features, along with spectral features from vanadium and possibly zirconium, are detected in the rest-frame near-infrared spectra of three massive, ~1 Gyr-old quiescent galaxies at z ≈ 1–2.
• The observed spectral features are best reproduced by population synthesis models with a strong TP-AGB contribution (e.g., M05), while models with weak TP-AGB (e.g., C09) fail to match the data.
• The best-fit models imply significantly younger ages and lower stellar masses than previously reported when using standard models with weak TP-AGB components.
• The inclusion of TP-AGB stars reduces the inferred mass-to-light ratio in the near-infrared, consistent with the observed high NIR flux from these galaxies.
• The detection of rare-element features (e.g., vanadium) supports the presence of evolved cool stars and validates the use of detailed TP-AGB spectral libraries in models.
• The results necessitate a revision of published stellar population fitting results for high-redshift quiescent galaxies, with implications for galaxy evolution, dust production, and chemical enrichment.