---
title: SiMBA Cosmological Simulations
url: https://www.emergentmind.com/topics/simba
type: topic
---

# SiMBA Cosmological Simulations

Simba refers to a comprehensive suite of cosmological hydrodynamic simulations and the underlying codebase designed to study galaxy formation, supermassive black hole (SMBH) growth, AGN feedback, and the resulting observable properties of galaxies and large-scale cosmic structures. The Simba simulations represent a major advance over earlier works by integrating an H₂-based star-formation law, torque-limited and Bondi black-hole accretion channels, a dual-mode kinetic AGN feedback model (winds and jets), and on-the-fly tracking of dust production and evolution. Simba is constructed on the GIZMO gravity+hydrodynamics code in its Meshless Finite Mass (MFM) mode and incorporates extensive sub-grid physics modules with parameters calibrated to match a wide array of low- and high-redshift observations [1901.10203].

## 1. Code Framework and Numerical Methods

The Simba simulations use the public GIZMO code (Hopkins 2015, 2018) in MFM mode, which combines adaptive Lagrangian particle-based hydrodynamics with Riemann-solver-based shock capturing and strict mass conservation. Radiative cooling, non-equilibrium primordial and metal-line cooling, and photoionization heating are computed on the fly using GRACKLE-3.1, including integrated self-shielding via Rahmati et al. (2013) [1901.10203]. Star formation is implemented as an H₂-based Schmidt law following Krumholz & Gnedin (2011): SFR = ε_* ρ_H₂ / t_dyn, with ε_* = 0.02, and local H₂ fractions assigned explicitly using metallicity and column density prescriptions.

Chemical enrichment tracks 11 species (H, He, C, N, O, Ne, Mg, Si, S, Ca, Fe), with yields from classic supernova and AGB sources. Simba augments the standard galaxy formation toolkit by including an on-the-fly model for dust production, grain growth, sputtering, and destruction based on the local interstellar environment [1901.10203].

## 2. Black Hole Accretion and Feedback Mechanisms

### 2.1. Accretion Prescriptions

Simba advances SMBH growth modeling by combining two accretion regimes:
- **Torque-limited accretion** for cold gas (T < 10⁵ K), scaling as $\dot M_{\rm torque} \sim \epsilon_T\, f_d^{5/2}\, M_{\rm BH}^{1/6}\, M_{\rm enc}(R_0)\, (R_0/100\,\mathrm{pc})^{-3/2}\, [1+f_0/f_{\rm gas}]^{-1}$,
- **Bondi accretion** for hot gas (T > 10⁵ K): $\dot M_{\rm Bondi} = \epsilon_m\,4\pi\,G^2\,M_{\rm BH}^2\,\rho /(c_s^2)^{3/2}$,

with key parameters set to match the local $M_{\rm BH}$–$M_*$ relation. Total SMBH accretion combines both channels, capped at 3×Eddington for torque-limited inflow and 1×Eddington for Bondi [1905.02741].

### 2.2. Feedback Channels

Black hole feedback is kinetic and bipolar, aligned with the rotational axis of the gas:
- **Radiative mode winds** are launched at high Eddington ratios ($f_{\rm Edd}\gtrsim0.2$) with $v_w = 500 + 500\,(\log_{10}(M_{\rm BH}/10^6\,M_\odot))/3$ km/s (Perna et al. 2017).
- **Jet mode** is triggered below $f_{\rm Edd}<0.2$ and $M_{\rm BH}>10^{7.5}\,M_\odot$, where velocities increase up to $v_w + 7000\,\log_{10}(0.2/f_{\rm Edd})$ km/s, capped at an additional 7000 km/s. Both modes enforce a momentum injection rate of $20L/c$ for self-regulation and galactic quenching [1911.02041, 2010.11225].

- **X-ray feedback** (active when $f_{\rm gas}<0.2$ in jet mode) deposits energy/momentum into gas, powering central gas evacuation and maintenance-mode AGN feedback in massive galaxies [1911.02041].

## 3. Simulation Suite and Physical Scaling

Simba's flagship run occupies a $(100\,h^{-1}\,\mathrm{Mpc})^3$ cosmological volume with $1024^3$ dark matter and gas particles ($m_{\rm gas}=1.82\times10^7\,M_\odot$), force softening $\epsilon_{\rm min}=0.5\,h^{-1}$ kpc, and Planck 2016 cosmological parameters. Ancillary volumes of 50, 25, 12.5 $h^{-1}$ Mpc enable resolution convergence and rare object studies [1901.10203]. Galaxy finding and halo cataloging occur on-the-fly using star+gas FOF and standard linking lengths.

## 4. Observational Validation and Main Results

Simba matches a wide array of observables:
- **Stellar Mass Functions**: The $z=0$–6 galaxy stellar mass function (GSMF) agrees closely with data and other leading simulations (e.g., EAGLE), accurately reproducing both star-forming and quenched galaxy abundances [1901.10203].
- **Star Formation Main Sequence**: z~0 sSFR–$M_*$ main sequence slope, amplitude, and scatter match GSWLC-X2 and higher-z comparators [1901.10203].
- **Gas and Metallicity**: Fractional H I and H₂ masses reproduce xCOLD GASS and GASS data; mass–metallicity trends match low- and high-z observations [1901.10203].
- **Black Hole and Quenching Demographics**: Simba recovers the observed $M_{\rm BH}$–$M_*$, $M_{\rm BH}$–$\sigma$ relations, and the black hole mass function out to $z\sim5$ [1905.02741]. AGN jet feedback produces realistic quenching at high $M_*$, shaping the bright-end cutoff of the GSMF [1901.10203].
- **Galaxy Sizes and Gas Profiles**: Star-forming R-band half-light radii trace SDSS trends, though passive sizes are overestimated at $z=0$ due to numerical heating. Simba uniquely reproduces central sSFR depressions (“holes”) in green valley galaxies at $z=0$, attributed to efficient X-ray AGN feedback [1911.02041].

| Observable            | Simba (z=0) Performance         | Reference/Benchmark         |
|-----------------------|---------------------------------|----------------------------|
| GSMF (0.1–10^12 M_⊙)  | Excellent agreement w/ data     | Bernardi 2017, Tomczak 2014|
| sSFR–$M_*$ main seq.  | Slope, scatter, norm match      | GSWLC-X2, Whitaker 2014    |
| MZR (gas & stars)     | Matches low/high-z obs          | Tremonti 2004, Sanders 2015|
| H₂ and H I fractions  | Consistent with xCOLD GASS/GASS | Saintonge 2017, Catinella 2012|
| $M_{\rm BH}$–$M_*$    | Correct slope, no high-z offset | Kormendy & Ho 2013         |
| Radio luminosity      | Observed RLF, HERG/LERG ratios  | Mauch & Sadler 2007, Best 2012|
| Galaxy sizes          | SF match, quenched too large    | Zhang & Yang 2017          |

## 5. Black Hole–Galaxy Co-Evolution and AGN Feedback Effects

Simba’s torque-limited + Bondi accretion naturally reproduces the observed invariance of the black hole accretion rate–star formation rate (BHAR–SFR) "main sequence," the anti-correlation of Eddington ratio $f_{\rm Edd}$ with $M_{\rm BH}$ at low masses, and the lognormal $f_{\rm Edd}$ distribution with a peak near 0.01 at $z=0$ [1905.02741]. As a function of redshift, the SMBH mass function decreases in normalization, with the high-mass end shifting from being SF-dominated at high z to quenched-dominated at $z=0$, in broad agreement with constraints.

Crucially, Simba’s two-mode kinetic black hole feedback—especially the onset of powerful jets above $M_{\rm BH}=10^{7.5} M_\odot$—is essential for quenching massive galaxies, steepening the GSMF high-mass end, producing radio-loud AGN demographics that match HERG/LERG population ratios and the 1.4 GHz radio luminosity function, and redistributing baryons in the IGM and haloes [2010.11225, 2507.16115]. The inclusion of X-ray feedback yields sharp central quenched regions ("holes") in sSFR and gas fraction profiles, in contrast with previous models [1911.02041].

## 6. Impact, Limitations, and Future Prospects

Simba stands out for its physical realism and multi-faceted validation against galaxy, SMBH, and IGM observables, including the size-luminosity relation, sSFR radial profiles, radio AGN phenomenology, baryon partitioning in the cosmic web, and dust evolution [2507.16115]. Noted limitations include excessive quenched-galaxy sizes at low mass due to numerical heating, possible overly sharp transitions at $M_*\sim2\times10^{10}\,M_\odot$, and mild deviations in scaling relations at the low-mass end [1901.10203, 1911.02041].

Extensions in the Simba project include baryon distribution studies with refined cosmic-web classification, improved AGN feedback calibration impacting FRB dispersion measure predictions, and further high-resolution, multi-wavelength synthetic survey outputs [2507.16115]. The Simba codebase and simulation data underpin a broad array of current and future large-scale structure and galaxy formation analyses, motivating their use in observational planning and theoretical exploration of feedback-regulated galaxy evolution.

Source: https://www.emergentmind.com/topics/simba