Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets (1412.4783v2)

Abstract: We use cosmological simulations to study a characteristic evolution pattern of high redshift galaxies. Early, stream-fed, highly perturbed, gas-rich discs undergo phases of dissipative contraction into compact, star-forming systems (blue nuggets) at z~4-2. The peak of gas compaction marks the onset of central gas depletion and inside-out quenching into compact ellipticals (red nuggets) by z~2. These are sometimes surrounded by gas rings or grow extended dry stellar envelopes. The compaction occurs at a roughly constant specific star-formation rate (SFR), and the quenching occurs at a constant stellar surface density within the inner kpc ($\Sigma_1$). Massive galaxies quench earlier, faster, and at a higher $\Sigma_1$ than lower-mass galaxies, which compactify and attempt to quench more than once. This evolution pattern is consistent with the way galaxies populate the SFR-radius-mass space, and with gradients and scatter across the main sequence. The compaction is triggered by an intense inflow episode, involving (mostly minor) mergers, counter-rotating streams or recycled gas, and is commonly associated with violent disc instability. The contraction is dissipative, with the inflow rate >SFR, and the maximum $\Sigma_1$ anti-correlated with the initial spin parameter, as predicted by Dekel & Burkert (2014). The central quenching is triggered by the high SFR and stellar/supernova feedback (possibly also AGN feedback) due to the high central gas density, while the central inflow weakens as the disc vanishes. Suppression of fresh gas supply by a hot halo allows the long-term maintenance of quenching once above a threshold halo mass, inducing the quenching downsizing.

• The paper demonstrates that gas-rich inflows and dissipative processes drive the compaction of high-z galaxies into blue nuggets before quenching into red nuggets.
• Simulations show that quenching begins when central gas inflow falls below the combined rate of star formation and outflows at peak compaction.
• Results imply a mass-dependent quenching process where more massive galaxies transition earlier, supported by critical halo mass and CGM insulation.

Compaction and Quenching of High-Redshift Galaxies

The paper "Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets" by Zolotov et al. provides an in-depth analysis of the compaction and quenching processes experienced by high-redshift galaxies, based on cosmological simulations. The authors aimed to model and understand the evolutionary stages that lead to the formation of compact star-forming and quenched galaxies termed "blue nuggets" and "red nuggets," respectively. The paper emphasizes the role of dissipative processes in driving these galaxies into compact configurations and subsequent quenching.

Key Findings and Results

The simulations reveal a characteristic sequence of evolutionary phases for massive high-redshift galaxies:

• Diffuse Star-Forming Phase: Early on, galaxies undergo a phase dominated by intense star formation in a diffuse configuration. They are fed by gas-rich inflows, often leading to violent disk instability.
• Compaction Phase: This is followed by a critical compaction phase where the galaxy contracts into a denser, more compact structure. This often results in "blue nuggets," defined by significant star formation activity concentrated in small volumes.
• Quenching Phase: Post-compaction, these galaxies experience central gas depletion and reduced star formation rates, transitioning into "red nuggets" — compact, quenched ellipticals. The authors observe that the transition to quenching is triggered at the peak of gas compaction.

The paper notes that the onset of quenching is linked to a shift in balance, where the gas inflow rate drops below the rate of star formation plus outflows. A significant finding is the notion of quenching downsizing, where more massive galaxies quench earlier and more completely than their lower-mass counterparts. This insight supports observational analyses showing massive galaxies transitioning to passive evolutions earlier in cosmic history.

Mechanisms and Theoretical Implications

Several factors are postulated to trigger the compaction, such as minor mergers, counter-rotating streams, and potentially intense inflow episodes, including interactions with recycled gas. During compaction, the interplay of high specific star-formation rates with self-gravitating baryons in the central regions leads to the observed density peaks. The simulations highlight that post-compaction quenching often occurs in a self-gravitating phase, providing insight into the processes required for a galaxy to cease forming stars on a massive scale.

As for maintenance of quenching, the paper underscores the role of a hot circumgalactic medium (CGM) present in halos exceeding a critical mass threshold (~$10^{11.5-12} M_\odot$), reinforcing theoretical models that link halo mass with the sustainable suppression of cold gas supply. This mechanism effectively insulates the central regions from external gas inflows, maintaining the quenched state.

Implications and Future Directions

Practically, these results articulate a comprehensive framework for understanding the transitional phases of galaxies across cosmic time, and theoretically enrich our comprehension of galaxy growth and decline mechanisms. The insights garnered serve as a bridge linking simulation results with observations, providing testable predictions related to galaxy morphology, star formation efficiencies, kinematic profiles, and their time evolution.

Moving forward, the paper suggests enticing areas for exploration, such as the integration of AGN feedback within these models to potentially resolve any discrepancies in quenching completeness and timescales. Additionally, future studies may explore the interplay between internal structural changes and external environmental factors such as satellite interactions, further elaborating the nuanced pathways through which galaxies undergo their life cycles.

This academic endeavor reflects a laudable effort to unpack the complex processes that govern galaxy evolution, illuminating pivotal phases in their transformation from vibrant star factories to dormant elliptical structures. The simulations, while highlighting certain inherent limitations, lay the groundwork for more nuanced investigations that will incrementally refine our understanding of the cosmos.