Discrepancies Between JWST Observations and Simulations of Quenched Massive Galaxies at $z > 3$: A Comparative Study With IllustrisTNG and ASTRID (2406.02664v3)

Abstract: Recent JWST observations have uncovered an unexpectedly large population of massive quiescent galaxies at $z>3$. Using the cosmological simulations IllustrisTNG and ASTRID, we identify analogous galaxies and investigate their abundance, formation, quenching mechanisms, and post-quenching evolution for stellar masses $9.5 < \log_{10}{(M_\star/{\rm M}\odot)} < 12$. We apply three different quenching definitions and find that both simulations significantly underestimate the comoving number density of quenched massive galaxies at $z \gtrsim 3$ compared to JWST observations by up to $\sim 2$ dex. In both simulations, the high-$z$ quenched massive galaxies often host overmassive central black holes above the local $M{BH}-M_\star$ relation, implying that AGN feedback is key in quenching galaxies in the early Universe. The typical quenching timescales for these galaxies are $\sim 200-600$ Myr. IllustrisTNG primarily employs AGN kinetic feedback, while ASTRID relies on AGN thermal feedback at $z > 2.3$, which is less effective and has a longer quenching timescale. Although these simulations differ in many aspects, making a direct comparison challenging, our findings suggest the need for improved physical models of AGN feedback in galaxy formation simulations. At lower stellar masses, the quenched galaxies have denser local environments than the star-forming galaxies, suggesting that environmental quenching helps quench less massive galaxies. We also study the post-quenching evolution of the high-$z$ massive quiescent galaxies and find that many experience subsequent reactivation of star formation, evolving into primary progenitors of $z=0$ brightest cluster galaxies.

• The paper reveals that simulations underestimate quenched massive galaxies at z>3 by up to 2 orders of magnitude compared to JWST observations.
• The study shows that AGN feedback, particularly kinetic in IllustrisTNG, is pivotal in galaxy quenching, urging refinements in simulation models.
• The analysis finds that quenched galaxies host overmassive central black holes and may reactivate star formation via mergers, reshaping our view of early galaxy evolution.

Discrepancies Between JWST Observations and Simulations of Quenched Massive Galaxies at $z > 3$: A Comparative Study With IllustrisTNG and ASTRID

The paper investigates the notable incongruities between the latest James Webb Space Telescope (JWST) observations and current cosmological simulations, specifically IllustrisTNG and ASTRID, regarding massive quenched galaxies at redshifts greater than 3. These massive galaxies, after undergoing a star formation quenched state, pose a significant challenge to simulations that traditionally account for such quiescence based on AGN feedback and other mechanisms.

Key Findings and Methodological Approach

1. Quenched Galaxy Abundance: The paper identifies a substantial underestimation of quenched massive galaxies by the simulations when compared with notable JWST observations. The discrepancy sharply increases by up to 2 orders of magnitude above redshifts of 3. This significant difference underscores the necessity for refining models to accommodate a higher frequency of quenched galaxies in high-redshift environments.
2. AGN Feedback Mechanisms: AGN feedback emerges as the principal driver behind the quenching of these massive galaxies at high redshifts. In IllustrisTNG, AGN kinetic feedback is primarily responsible, whereas, in ASTRID, AGN thermal feedback prevails, albeit it underperforms compared to the kinetic feedback mechanism. The distinction in feedback mechanisms across the simulations suggests variability in quenching efficiency and highlights the need for further refinement in AGN modeling to achieve better alignment with observed data.
3. Quenching Timescales and Black Hole Dynamics: Quenching of galaxies is reported to occur over relatively brief timescales ranging from 200 to 600 Myr. Additionally, quenched massive galaxies possess overmassive central black holes, deviating from the standard $M_{BH}-M_{\star}$ relation. This indicates intensified AGN activity as a critical component of early quenching processes, especially in systems deviating from typical scaling relations.
4. Post-Quenching Evolution: Many of these galaxies, despite undergoing early quenching, experience reactivation of star formation as they evolve. This reactivation is attributed to mergers and accretion events, which replenish the gas reservoirs of these galaxies. Eventually, these reactivated galaxies often become precursors to low-redshift brightest cluster galaxies (BCGs), showcasing a transformation from quiescence to dominant intra-cluster entities.

Implications and Future Directions

The findings emphasize the vital role of AGN feedback, particularly its kinetic aspect in galaxy quenching at high redshifts, calling into question the adequacy of current simulation frameworks. The observational versus simulated discrepancies point toward a field of galaxy evolution physics that remains incompletely understood or represented, especially regarding AGN feedback efficacy across different feedback modalities.

Practically, this paper's insights are paramount for upcoming modifications and improvements in galaxy evolution models, which should aim to replicate the JWST's observed distribution and characteristics of quenched galaxies. The theoretical implications rest on refining theoretical frameworks to accommodate more nuanced quenching mechanisms that capture observed phenomena accurately.

In conclusion, the paper serves as a critical comparative analysis revealing the complexities of cosmic quenching processes while guiding future improvements to align cosmological simulations more closely with cutting-edge observational data. Through continued integration of new observational insights, particularly from instruments like JWST, and iterative advancements in simulation methodologies, researchers may unravel the intricacies of the early universe's galaxy evolution.