RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z \sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec (2405.01473v2)

Abstract: The identification of red, apparently massive galaxies at $z>7$ in early JWST photometry suggests a strongly accelerated timeline compared to standard models of galaxy growth. A major uncertainty in the interpretation is whether the red colors are caused by evolved stellar populations, dust, or other effects such as emission lines or AGN. Here we show that three of the massive galaxy candidates at $z=6.7-8.4$ have prominent Balmer breaks in JWST/NIRSpec spectroscopy from the RUBIES program. The Balmer breaks demonstrate unambiguously that stellar emission dominates at $\lambda_{\rm rest} = 0.4\,\mu$m, and require formation histories extending hundreds of Myr into the past in galaxies only 600--800 Myr after the Big Bang. Two of the three galaxies also show broad Balmer lines, with H$\beta$ FWHM $>2500~{\rm km\,s^{-1}}$, suggesting that dust-reddened AGN contribute to, or even dominate, the SEDs of these galaxies at $\lambda_{\rm rest}\gtrsim 0.6\,\mu$m. All three galaxies have relatively narrow [O III] lines, seemingly ruling out a high-mass interpretation if the lines arise in dynamically-relaxed, inclined disks. Yet, the inferred masses also remain highly uncertain. We model the high-quality spectra using Prospector to decompose the continuum into stellar and AGN components, and explore limiting cases in stellar/AGN contribution. This produces a wide range of possible stellar masses, spanning $M_\star \sim 10^9 - 10^{11}\,{\rm M_{\odot}}$. Nevertheless, all fits suggest a very early and rapid formation, most of which follow with a truncation in star formation. Potential origins and evolutionary tracks for these objects are discussed, from the cores of massive galaxies to low-mass galaxies with over-massive black holes. Intriguingly, we find all of these explanations to be incomplete; deeper and redder data are needed to understand the physics of these systems.

• The paper identifies pronounced Balmer breaks in candidate massive galaxies at z~7–8, indicating mature stellar populations formed hundreds of millions of years post-Big Bang.
• It employs JWST/NIRSpec spectroscopy and detailed Prospector modeling to estimate stellar masses from 10^9 to 10^11 M☉, supporting rapid early assembly followed by quenching.
• The detection of broad Hβ emission lines in two galaxies suggests significant AGN activity that may influence spectral features and accelerate galaxy evolution.

Evolved Stellar Populations at High Redshift: Insights from JWST Observations

The paper by Wang et al. investigates a notable population of massive galaxy candidates at redshifts $z \sim 7-8$, revealing significant insights into early galaxy formation and evolution, using data from the James Webb Space Telescope (JWST). The research focuses on three primary subjects identified through the RUBIES program, which exhibit notable Balmer breaks. The presence of these Balmer breaks, as detected by JWST's NIRSpec spectroscopy, indicates the existence of mature stellar populations formed hundreds of millions of years prior to their observation, thus challenging conventional models of galaxy evolution.

Balmer Breaks and Early Formation Histories

The identification of Balmer breaks in galaxies at such high redshifts is a critical finding. It unequivocally demonstrates that these galaxies possess evolved stellar populations. The strong Balmer breaks in these galaxies imply a considerable period of star formation and subsequent quenching, occurring merely 600–800 million years post-Big Bang. This prompts a reevaluation of galaxy formation timelines, suggesting they could evolve much more rapidly than previously modeled by standard evolutionary scenarios.

The Role of AGN in High-redshift Galaxies

Two of the three galaxies in the paper also exhibit broad Balmer emission lines (H$\beta$), which are typically indicative of active galactic nuclei (AGN) activity. The widths of these lines (FWHM $> 2500$ km s$^-1$) point to the presence of AGN, which may dominate or significantly influence the spectral energy distributions (SEDs) of these objects, particularly in the rest-frame optical wavelengths. This observation suggests a complex interplay between star formation, AGN activity, and the observed spectral features, including the possibility that AGN could obscure the underlying stellar continua or even propel rapid galaxy evolution.

Mass Estimates and Implications

Wang et al. performed detailed modeling using the Prospector software to decompose the contributions of stellar and AGN components to the observed spectra. The resultant stellar mass estimates span a broad range ($10^9 - 10^{11}$ M$_\odot$), indicating significant uncertainty in mass determination. Regardless, all fits suggest rapid stellar mass assembly shortly followed by a quenching phase. These early galaxies’ properties suggest they might evolve into the massive, quiescent galaxies observed in the local universe, consistent with the hypothesis that early galaxy formation can occur in bursts, driven by intense star formation and AGN feedback.

Theoretical and Practical Implications

These findings pose intriguing challenges to the current understanding of cosmic evolution, particularly in explaining how such high-mass systems can emerge so early. The implied high-efficiency star formation processes might indicate previously unaccounted-for mechanisms in early galaxy assembly or variations in star formation efficiency with redshift. These could involve non-standard initial mass functions (IMFs) or alternative feedback processes, both of which have profound implications for cosmological simulations and our understanding of stellar populations' evolution.

Future Directions

To fully capture the complexity of these early massive galaxies, further observational efforts are needed. Deeper, high-resolution spectroscopic data and mid-infrared observations, particularly through MIRI on JWST, could disentangle the contributions of dust, evolved stellar populations, and AGNs. Additionally, enhancing our models of the initial mass function and feedback processes in cosmological simulations will be crucial in reconciling these observations with theoretical expectations.

The paper by Wang et al. highlights the intricate dynamics of early universe structures and stresses the need for nuanced models that accommodate the unexpected rapidity of galaxy evolution. As JWST continues to probe these early epochs, it offers an unprecedented opportunity to refine our models and concepts of galaxy formation in the nascent universe.