Simba: Cosmological Simulations with Black Hole Growth and Feedback (1901.10203v2)

Abstract: We introduce the Simba simulations, the next generation of the Mufasa cosmological galaxy formation simulations run with Gizmo's meshless finite mass hydrodynamics. Simba includes updates to Mufasa's sub-resolution star formation and feedback prescriptions, and introduces black hole growth via the torque-limited accretion model of Angl\'es-Alc\'azar et al. (2017) from cold gas and Bondi accretion from hot gas, along with black hole feedback via kinetic bipolar outflows and X-ray energy. Ejection velocities are taken to be ~10^3 km/s at high Eddington ratios, increasing to ~8000 km/s at Eddington ratios below 2%, with a constant momentum input of 20L/c. Simba further includes an on-the-fly dust production, growth, and destruction model. Our Simba run with (100 Mpc/h)^3 and 1024^3 gas elements reproduces numerous observables, including galaxy stellar mass functions at z=0-6, the stellar mass--star formation rate main sequence, HI and H2 fractions, the mass-metallicity relation at z=0 and z=2, star-forming galaxy sizes, hot gas fractions in massive halos, and z=0 galaxy dust properties. However, Simba also yields an insufficiently sharp truncation of the z=0 mass function, and too-large sizes for low-mass quenched galaxies. We show that Simba's jet feedback is primarily responsible for quenching massive galaxies.

• The paper introduces Simba, a simulation framework that integrates torque-limited accretion with kinetic bipolar outflows and X-ray feedback for realistic black hole growth.
• The paper employs advanced meshless finite mass hydrodynamics to accurately reproduce galaxy stellar mass functions, star formation rates, and gas fraction trends across cosmic time.
• The paper further incorporates on-the-fly dust production, naturally generating black hole scaling and metallicity relations while identifying areas for refinement in massive galaxy modeling.

Overview of Cosmological Simulations with Black Hole Growth and Feedback

The paper presents the Simba simulations, a state-of-the-art cosmological framework designed to model galaxy formation. Leveraging meshless finite mass hydrodynamics, the Simba simulations introduce significant advancements over previous simulation suites, such as incorporating sophisticated physical processes for black hole growth and feedback mechanisms. These advancements include torque-limited accretion models, bipolar kinetic outflows, and X-ray energy feedback, aimed at capturing the complex dynamics that govern galaxy evolution.

Key Developments in Simba

1. Black Hole Growth and Feedback: The Simba simulations primarily improve upon previous efforts by integrating a detailed model of black hole growth. This is achieved through torque-limited accretion, where gas inflow in galactic disks, mediated by gravitational instabilities, significantly influences black hole growth. This approach differs from the traditional Bondi accretion models by providing a more realistic estimation of accretion rates, especially in cold gas environments.
2. Feedback Mechanisms: Simba includes several feedback process innovations. Kinetic bipolar outflows and X-ray feedback mechanisms are integral in regulating galaxy and black hole co-evolution. These feedback mechanisms are crucial for quenching star formation in massive galaxies, maintaining the balance of energy within the galactic ecosystem.
3. Dust Production Model: A novel feature of Simba is the on-the-fly modeling of dust production and destruction. This aspect is pivotal for understanding the role of dust in metal cycles and its effects on various scaling relations, such as the mass-metallicity relation.

Significant Results

Simba demonstrates robustness in replicating several critical observables:

• Galaxy Stellar Mass Functions: Simba matches the observed galaxy stellar mass functions across a wide redshift range (z=0-6), capturing the steep decline at high masses reasonably well. However, there is a noted excess in the production of the most massive galaxies by z=0.
• Star Formation Rates: The simulations reproduce the star-forming main sequence across cosmic time, addressing prior discrepancies between model predictions and observed amplitudes during peak epochs of star formation.
• Gas Fractions: Simba accurately models both molecular and neutral gas fractions as functions of stellar mass, corresponding well with existing data.
• Metallicity Relations: Both gas-phase and stellar mass-metallicity relations evolve in close alignment with observational trends, demonstrating Simba's effectiveness in modeling metal enrichment across different phases.
• Galaxy Sizes: While Simba appropriately matches the sizes of star-forming galaxies, it faces challenges in replicating the expected compact sizes of quenched galaxies at low masses.
• Black Hole Scaling Relations: Black hole mass-stellar mass relationships emerge naturally in the simulations, aligning with observational samples of bulge-dominated and spiral galaxies.

Implications and Future Directions

Simba’s ability to replicate numerous observed properties of galaxies suggests that the implemented physical models, particularly for AGN feedback, are on the right track. However, the persistent overproduction of the most massive galaxies by z=0 points towards potential refinements in feedback efficiency or additional processes influencing late-stage galaxy evolution.

The paper provides a compelling framework for future investigations into galaxy formation, offering a platform for testing further cosmological and astrophysical hypotheses. Upcoming research may focus on detailed examinations of jet feedback’s impact on the intracluster medium, the role of circum-galactic gas in galaxy evolution, and the evolution of dust properties in the interstellar medium.

Simba's comprehensive approach to integrating black hole dynamics with galaxy formation physics marks a significant step forward, establishing a foundation for more precise and predictive models in astrophysical research.