A massive galaxy that formed its stars at $z \sim 11$ (2308.05606v3)

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.

• The paper demonstrates that the massive galaxy ZF-UDS-7329 formed its stars at z ~11 using detailed JWST spectroscopic observations.
• It applies both FAST++ and Prospector models to derive a stellar mass of approximately 1.24×10¹¹ M☉ and identify key spectral features like the 4000 Å break.
• The findings challenge standard theories of dark matter halo assembly, prompting a re-evaluation of early galaxy formation models.

Analysis of a Massive Galaxy with Stellar Formation at $z \sim 11$

The paper in question investigates a massive quiescent galaxy, ZF-UDS-7329, identified at a redshift of $z=3.205 \pm 0.005$. Through spectroscopic observations using the James Webb Space Telescope (JWST), the authors provide evidence that this galaxy formed its stars around the epoch of $z \sim 11$, a time where the hierarchical co-assembly of baryons within cold dark matter halos is understood to be insufficiently developed to host such massive structures. This finding challenges the prevailing models of galaxy formation that predict a stark decline in the abundance of massive galaxies during early cosmic times.

Spectroscopic Observations and Analysis

The spectroscopic analysis reveals that ZF-UDS-7329 possesses characteristics typical of older stellar populations, prominently featuring the 4000 Å break and Mg\,b 5174 Å absorption lines. These features suggest an age greater than 1 Gyr, indicative of a mature stellar population. The paper employs both FAST++ parametric models and Prospector non-parametric models to fit the galaxy's spectral and photometric data, resulting in a derived stellar mass of approximately $1.24 \pm 0.09 \times 10^{11} \, M_{\odot}$, and a metallicity significantly less than supersolar values ($[\text{Fe/H}] = 0.021 \pm 0.005$).

Implications on Theoretical Models

The finding that the galaxy's stellar population dates back to $z \sim 11$ suggests a potential discrepancy in current understanding of dark matter halo assembly, as simulations predict that halos capable of hosting such massive galaxies do not form until $z \sim 6$. The paper calls into question assumptions related to baryon conversion efficiency in early galaxies, highlighting the necessity of revising models that may currently underestimate the role of non-standard stellar formation histories or alternative dark matter compositions.

Systematic Considerations and Alternate Hypotheses

The authors address possible systematic errors rigorously, confirming negligible emission lines that might suggest active galactic nuclei contributions, verifying spectral fit accuracy via Monte Carlo simulations, and considering stellar mass variation under different hypothetical Initial Mass Functions (IMFs). The results are robust against these checks; even when assuming high metallicity ([Fe/H] = 0.05), the requirement for young stellar populations remains unmet.

Furthermore, alternate cosmological models like Warm Dark Matter (WDM) or Early Dark Energy (EDE) were proposed, albeit without definitive capacity to account for the early formation of massive galaxies. Exceptional scenarios such as primordial black hole seeds are suggested as possible, albeit unlikely, reconciliations.

Prospects for Future Research

The observation of ZF-UDS-7329 suggests significant gaps in the understanding of early stellar populations and galaxy formation. The detection underscores the need for further spectral analysis and larger sample sizes for verification. Future investigations with JWST, employing higher resolution and broader survey coverage, will be crucial to understanding these discrepancies in the $\Lambda$CDM formation paradigm. The potential discovery of more galaxies like ZF-UDS-7329 will inform the development of models that might accommodate or refute these early massive galaxy formations.

In summary, ZF-UDS-7329 serves as a critical observational milestone, challenging existing frameworks and advocating for deeper exploration into cosmic origins and dark matter properties. The paper exemplifies the insights achievable through advanced spectroscopic techniques and signals a paradigm evaluation in the astrophysical community regarding early galaxy formation processes.