[Paper Review] A massive galaxy that formed its stars at $z \sim 11$

Karl Glazebrook, Themiya Nanayakkara+16 more|arXiv (Cornell University)|Aug 10, 2023

Astronomy and Astrophysical Research40 references4 citations

TL;DR

This paper presents spectroscopic evidence from the James Webb Space Telescope (JWST) for a massive, quiescent galaxy (ZF-UDS-7329) with a stellar population formed at redshift z ≈ 11, implying a formation time of just 1.5 billion years after the Big Bang. The galaxy's old stellar population, confirmed by strong 4000Å break and near-infrared molecular features, challenges standard hierarchical galaxy formation models, as such massive halos are not expected to have assembled at this epoch.

ABSTRACT

The formation of galaxies by gradual hierarchical co-assembly of baryons and cold dark matter halos is a fundamental paradigm underpinning modern astrophysics and predicts a strong decline in the number of massive galaxies at early cosmic times. Extremely massive quiescent galaxies (stellar masses $>10^{11}$ M$_\odot$) have now been observed as early as 1-2 billions years after the Big Bang; these are extremely constraining on theoretical models as they form 300-500 Myr earlier and only some models can form massive galaxies this early. Here we report on the spectroscopic observations with the James Webb Space Telescope of a massive quiescent galaxy ZF-UDS-7329 at redshift 3.205 $\pm$ 0.005 that eluded deep ground-based spectrscopy, is significantly redder than typical and whose spectrum reveals features typical of much older stellar populations. Detailed modelling shows the stellar population formed around 1.5 billion years earlier in time (z ~ 11) at an epoch when dark matter halos of sufficient hosting mass have not yet assembled in the standard scenario. This observation may point to the presence of undetected populations of early galaxies and the possibility of significant gaps in our understanding of early stellar populations, galaxy formation and/or the nature of dark matter.

Motivation & Objective

• To determine the formation epoch and stellar population age of the massive quiescent galaxy ZF-UDS-7329 using JWST NIRSpec spectroscopy.
• To test whether the observed old stellar population is consistent with standard hierarchical galaxy formation models, particularly the timing of massive halo assembly.
• To assess the galaxy's dust content, metallicity, and structural properties using multi-wavelength photometry and spectral fitting.
• To investigate whether the observed spectral features—especially the 0.94 µm absorption bump—indicate a stellar population older than 1 Gyr, as expected for old stellar populations.
• To evaluate the implications of this early massive galaxy for theories of early galaxy formation, dark matter halo assembly, and the completeness of the observed early galaxy population.

Proposed method

• Spectroscopic observations of ZF-UDS-7329 were conducted with the JWST NIRSpec prism mode (R ≈ 51), covering rest-frame 0.2–0.7 µm, to measure absorption features indicative of old stellar populations.
• Spectral energy distribution (SED) fitting was performed using the FAST++ code with a grid of parametric star formation histories, incorporating stellar population models and dust attenuation to derive stellar mass, metallicity, and formation time.
• Independent modeling was conducted using the Prospector code, which employs different stellar tracks and spectral libraries, to test robustness of age and mass estimates.
• The rest-frame near-infrared spectrum was compared to local analogs (e.g., NGC 5850 and NGC 205) to validate the presence of molecular features (ZrO, CN, TiO) associated with old stellar populations.
• Halo mass functions were computed using the hmf Python module with cosmological parameters Ωₘ = 0.3, ΩΛ = 0.7, H₀ = 70 km s⁻¹ Mpc⁻¹, to assess the theoretical likelihood of such massive halos at z ≈ 11.
• Photometry from JWST NIRCAM (F115W–F444W) was used to derive rest-frame magnitudes and confirm the galaxy’s compact, edge-on disk morphology and low dust content.

Figure 1: The JWST NIRSpec spectrum of galaxy ZF-UDS-7329. This covers rest frame 0.2–0.7 $\mu$ m and is compared to the best fit FAST++ model which is shown binned to the varying pixel size and resolution of the PRISM spectrum as well as at its original native resolution. Wavelengths of common abso

Experimental results

Research questions

• RQ1 At what redshift did the stellar population of ZF-UDS-7329 form, and how old is it based on spectral features?
• RQ2 Is the observed stellar population age of ~1.5 Gyr (t₅₀) consistent with the expected timing of massive halo assembly in the standard ΛCDM cosmology?
• RQ3 Do the near-infrared spectral features (e.g., 0.94 µm bump) confirm the presence of old, cool stars consistent with a population older than 1 Gyr?
• RQ4 How does the inferred stellar mass (1.24 × 10¹¹ M⊙) and halo mass compare to theoretical predictions for galaxy formation at z ≈ 11?
• RQ5 What are the implications of this galaxy’s existence for the completeness of early galaxy surveys and potential gaps in our understanding of early stellar populations or dark matter?

Key findings

• The galaxy ZF-UDS-7329 has a stellar population that formed at z ≈ 11, with a 50% mass formation time (t₅₀) of 1.52 ± 0.16 Gyr after the Big Bang, significantly older than other quiescent galaxies observed at similar redshifts.
• The rest-frame spectrum shows a well-developed 4000Å break and strong molecular absorption features (e.g., 0.94 µm bump from ZrO, CN, TiO), both of which are diagnostic of stellar populations older than 1 Gyr.
• Stellar mass is measured as 1.24 ± 0.09 × 10¹¹ M⊙, consistent with photometric estimates, and metallicity is [Fe/H] = 0.021 ± 0.005, indicating solar-like metallicity.
• The galaxy is compact (effective radius 1.15 ± 0.08 kpc at 1 µm) and shows no signs of star-forming clumps, supporting its quiescent nature, with low dust attenuation (<0.3 mag in V-band).
• Halo mass function analysis shows that massive halos of this mass (Mₕ ≈ 10¹³ M⊙) are not expected to have assembled at z ≈ 11 in standard ΛCDM models, indicating a significant tension with theoretical expectations.
• The agreement between FAST++ and Prospector modeling results confirms the robustness of the age and mass estimates, reinforcing the conclusion that this galaxy formed earlier than predicted by current models.

Figure 2: The effect of age and metallicity on stellar populations. Simple Stellar Population models [ 21 ] are compared with the JWST spectrum of ZF-UDS-7329. It can be seen how the transition from a post-starburst type spectrum with a Balmer break and Balmer lines from young A stars to an evolved