---
title: 'GADGET4-Osaka: Advanced Galaxy Simulation Framework'
url: https://www.emergentmind.com/topics/gadget4-osaka
type: topic
---

# GADGET4-Osaka: Advanced Galaxy Simulation Framework

GADGET4-OSAKA is a modified version of the public GADGET-4 code developed by the Osaka group for galaxy-formation and gas-dynamical simulations spanning cosmological volumes, cosmological zoom-ins, isolated galaxies, explicit dust evolution, and AGN jet propagation. In the published implementations, it combines large-scale PM gravity with hierarchical short-range multipole methods, SPH hydrodynamics, GRACKLE cooling and chemistry, CELib-based stellar evolution and enrichment, and Osaka feedback modules; study-specific branches further add grain-size–resolved dust physics and reservoir-based jet launching [2401.06324] [2510.26513] [2410.21430] [2508.21282]. Its recent uses include the CROCODILE cosmological suite, the CROCODILE-DWARF zoom-ins of field dwarfs, AGORA validation runs, and isolated-galaxy calculations of dust radial structure, PAH indicators, and attenuation curves [2512.18945] [2402.02659] [2508.21157].

## 1. Code lineage and computational scope

Published descriptions present GADGET4-OSAKA as the “Osaka branch” of GADGET-4, retaining the parent code’s parallel architecture while augmenting it with improved subgrid physics and chemistry modules [2510.26513]. In CROCODILE-DWARF it is described as a “highly parallel Tree–PM + SPH code” built on the public framework and coupled to \textsc{CELib} and \textsc{grackle} [2510.26513]. In the AGORA validation study it appears as the implementation of the CROCODILE model inside the AGORA calibration workflow, where the resulting simulations were contributed to the AGORA CosmoRun suite [2512.18945].

The scope of applications is unusually broad. Cosmological galaxy-formation runs in CROCODILE include supernova feedback with a metallicity- and redshift-dependent, top-heavy IMF and an AGN feedback model [2401.06324]. Cosmological dwarf-galaxy zoom-ins target isolated halos of $\sim 10^{10}\,M_\odot$ at $z=0$ and analyze structural and kinematic diversity [2510.26513]. Isolated-galaxy studies add explicit grain-size evolution and SKIRT post-processing to connect simulated ISM dust physics to PAH diagnostics, dust surface densities, and attenuation curves [2402.02659] [2410.21430] [2508.21157]. A separate configuration simulates purely kinetic bipolar AGN jets and benchmarks SPH-based jet propagation against self-similar analytic expectations [2508.21282].

This body of work suggests that GADGET4-OSAKA is best understood as a simulation framework rather than a single immutable parameter set. The common substrate is the GADGET-4-derived gravity and SPH infrastructure together with Osaka feedback, CELib enrichment, and GRACKLE thermochemistry, while neighbor numbers, star-formation thresholds, feedback tunings, and auxiliary physics are configured according to the scientific problem [2401.06324] [2512.18945].

## 2. Gravity, SPH, and time integration

The gravity solver is described in closely related but not identical ways across publications. CROCODILE-DWARF specifies a hybrid Tree–PM Poisson solver in which a long-range Particle–Mesh computation on a regular grid handles low-frequency modes and a hierarchical oct-tree handles short-range forces through a multipole expansion up to quadrupole order [2510.26513]. The same study states that GADGET4-OSAKA employs an adaptive opening criterion tuned to ensure better force accuracy in the dense inner regions of dwarf halos, thereby reducing two-body noise without an appreciable increase in cost [2510.26513]. In the AGORA comparison, the gravity module is summarized as “FMM–PM (4th order)” for GADGET4-OSAKA, in contrast to “TreePM (2nd order)” for its predecessor [2512.18945]. CROCODILE additionally reports 3D Peano–Hilbert domain decomposition and hybrid MPI + OpenMP parallelization [2401.06324].

The hydrodynamics layer is SPH-based throughout, but the exact formulation depends on the application. Published descriptions include the improved density-independent SPH formulation of GADGET-4 [2510.26513], pressure–entropy SPH [2402.02659] [2508.21282], pressure–energy SPH [2512.18945], and density-entropy SPH with improved artificial-viscosity switches [2508.21157]. The Wendland C4 kernel is repeatedly used, with reported neighbor numbers of $\sim 50$, $100$, $120\pm2$, $128\pm4$, and $200$ in different setups [2402.02659] [2510.26513] [2401.06324] [2512.18945] [2508.21282]. Artificial viscosity is likewise problem-dependent: the code is reported with a Cullen–Dehnen switch, Monaghan–Gingold–Lattanzio viscosity, a Balsara shear-limiter, and artificial conduction terms introduced to suppress pressure blips or smooth contact discontinuities [2510.26513] [2402.02659] [2508.21282].

Representative SPH discretizations are given explicitly in the dwarf-galaxy study:
$$
\rho_i = \sum_j m_j\,W(|r_i-r_j|,h_i),
$$
and
$$
\frac{d\mathbf v_i}{dt} =
-\sum_j m_j
\left[
\frac{P_i}{\rho_i^2}
+
\frac{P_j}{\rho_j^2}
+
\Pi_{ij}
\right]
\nabla_i W_{ij},
$$
with corresponding energy equations including radiative cooling and UV-background heating terms [2510.26513].

Time integration is hierarchical and adaptive across the code family. CROCODILE-DWARF uses GADGET-4’s global kick–drift–kick leapfrog integrator with individual subcycling in powers of two, and chooses each particle timestep as the minimum of a CFL condition, an acceleration criterion, and a synchronization constraint from neighboring particles [2510.26513]. CROCODILE emphasizes hierarchical individual time-steps with a second-order predictor-corrector and a signal-velocity time-step limiter, with mutual “wake-up” of neighboring particles when strong feedback is injected [2401.06324]. Isolated dust simulations also use individual hierarchical timesteps with subcycling for rapid dust growth and destruction [2402.02659].

## 3. Thermochemistry, star formation, and feedback

GRACKLE provides the thermochemical backbone in multiple GADGET4-OSAKA applications. Published configurations include non-equilibrium primordial chemistry for species such as H, H$^+$, He, He$^+$, He$^{2+}$, $e^-$, H$_2$, H$_2^+$, H$^-$, D, D$^+$, and HD, combined with metal-line cooling from precomputed \textsc{Cloudy} tables and UV-background photo-heating [2510.26513]. CROCODILE uses non-equilibrium chemistry of 12 species, \textsc{Cloudy} metal cooling, and a Haardt & Madau UVB turning on at $z\approx 7$ [2401.06324], whereas CROCODILE-DWARF reports the Haardt & Madau (2012) UVB turned on at $z=15$ together with self-shielding corrections [2510.26513].

Star formation follows a Schmidt-type or volumetric Kennicutt–Schmidt law, but the threshold depends on the application. In CROCODILE-DWARF, gas above $n_{\rm H,thres}=1\,{\rm cm^{-3}}$ and below $T_{\rm thres}=10^4\,{\rm K}$ is eligible, with
$$
\frac{d\rho_\star}{dt}=c_\star\frac{\rho_{\rm gas}}{t_{\rm ff}},
\qquad c_\star=0.01,
$$
and stochastic conversion of gas particles into collisionless star particles of the same mass [2510.26513]. The isolated-galaxy dust study uses stochastic star formation once $n_{\rm H}>10\,{\rm cm^{-3}}$ with efficiency $\epsilon_{\star}=0.05$ [2410.21430]. The MW-like attenuation-curve study adopts $n_{\rm H}>20\,{\rm cm^{-3}}$, $T<10^4\,{\rm K}$, and $\epsilon_\star=0.05$ [2508.21157]. CROCODILE uses $\epsilon_\star=0.01$ with thresholds $n>0.1\,{\rm cm^{-3}}$ and $T<10^4\,{\rm K}$ [2401.06324]. AGORA validation reports $n_{\rm H,th}=10\,{\rm cm^{-3}}$ in the isolated run and $1\,{\rm cm^{-3}}$ in the cosmological run, together with a Jeans pressure floor
$$
P_{\rm Jeans} = \frac{1}{\gamma\pi} G N_J^2 \rho^2 \Delta x^2,
\qquad N_J=4
$$
[2512.18945].

The stellar feedback model is one of the code’s defining features. CELib supplies Type II SNe, Type Ia SNe, and AGB mass return, yields, and delay-time distributions [2510.26513] [2410.21430]. CROCODILE-DWARF tracks 16 isotopic species and injects feedback through a thermal channel based on stochastic heating of $N_{\rm ngb,FB}=120$ neighbors to a target entropy $K_{\rm OF}=10^8\,k_{\rm B}\,{\rm K\,cm^2}$ and a momentum channel calibrated on high-resolution superbubble simulations [2510.26513]. In that suite, the default CELib energy per solar mass formed is multiplied by 2, to $\zeta_{\rm SN}=2.3\times10^{49}\,{\rm erg}\,M_\odot^{-1}$, to mimic missing early stellar feedback and compensate for finite resolution [2510.26513].

The AGORA validation isolates two feedback ingredients as essential. Mechanical momentum injection distributes a terminal momentum per SN calibrated from high-resolution SN remnant simulations,
$$
\hat p(n_0,Z)=1.75\times10^5\,
\bigl(M_\odot\,{\rm km\,s^{-1}}\bigr)\,
n_0^{-0.05}\,\Lambda_{6,-22}^{-0.17},
$$
while stochastic thermal heating raises selected neighbors to a target entropy
$$
S_{\rm OF}=10^8\,k_{\rm B}\,{\rm K\,cm^2}
$$
with probability based on the available thermal budget and the required entropy jump [2512.18945]. In CROCODILE, the supernova model is further modified by a metallicity- and redshift-dependent, top-heavy IMF and a hot galactic-wind channel, while AGN feedback follows a stochastic heating model with $\Delta T_{\rm AGN}=10^{8.5}\,{\rm K}$ [2401.06324].

## 4. Dust, radiative transfer, and jet modules

A major extension of GADGET4-OSAKA is its explicit treatment of dust mass and grain-size distributions in isolated-galaxy studies. These implementations track grain radii between $a=3\times10^{-4}$ and $10\,\mu{\rm m}$ in 30 logarithmic bins [2410.21430] [2402.02659] [2508.21157]. Dust is fully coupled to gas, with no separate drift term, and the evolution of each bin includes stellar dust production, accretion, shattering, coagulation, sputtering, and astration [2410.21430]. The radial-properties study uses a two-phase subgrid ISM model in which each gas particle is split into a dense cloud with $n_{\rm H,cloud}=10^3\,{\rm cm^{-3}}$ and $T=50\,{\rm K}$ and a diffuse ambient phase, with dense-cloud mass fraction
$$
f_{\rm cloud}(n_{\rm H})=\min(\alpha\,n_{\rm H},\,1)
$$
for tunable $\alpha=0.012$–$0.12$ [2410.21430]. Dust diffusion can be included through a Smagorinsky-type turbulent mixing model [2410.21430].

The dust module is routinely coupled to SKIRT for post-processing radiative transfer. In the PAH-indicator study, GADGET4-OSAKA provides per-particle grain-size distributions, which are mapped onto a SKIRT grid where carbonaceous grains smaller than $1.3\,{\rm nm}$ are treated as neutral or ionized PAHs and larger grains are split into silicate and graphite according to local Si and C abundances [2402.02659]. The attenuation-curve study uses the same pipeline to analyze the optical–UV slope, the $2175\,\AA$ bump, scattering, and star–dust geometry [2508.21157].

A second major extension is the AGN jet module. In the jet-evolution study, GADGET4-OSAKA launches purely kinetic bipolar jets with fixed power $P_j=10^{46}\,{\rm erg\,s^{-1}}$, launch velocity $v_j=1.5\times10^4\,{\rm km\,s^{-1}}$, and cone half-angle $\theta_j=10^\circ$ [2508.21282]. Injection uses a pre-allocated reservoir of particles activated in pairs. Three spatial schemes are tested: Reservoir–Grid, Reservoir–Random, and Spawning–Random [2508.21282].

| Domain | Added module(s) | Representative use |
|---|---|---|
| Isolated galaxies | Grain-size–resolved dust evolution, two-phase ISM, SKIRT | Dust radial profiles, PAH indicators, attenuation curves |
| Cosmological galaxy formation | Osaka SN feedback, AGN feedback, CELib, GRACKLE | CROCODILE and CROCODILE-DWARF |
| AGN jet simulations | Reservoir-based kinetic jet launching | Self-similar jet-lobe evolution |

These extensions are not ancillary. They alter the code’s phenomenology in ways that are explicitly measured: dust studies track small-to-large grain ratios and PAH fractions, while the jet study shows that lobe morphology and energetics are highly sensitive to artificial viscosity and launch geometry [2410.21430] [2402.02659] [2508.21282].

## 5. Resolution scales, diagnostics, and computational performance

The numerical resolution reported for GADGET4-OSAKA spans several orders of magnitude. CROCODILE-DWARF uses effective zoom-in resolution $2\times 2048^3$ in a $(10\,h^{-1}\,{\rm Mpc})^3$ parent box, with $m_{\rm DM}=1.04\times10^4\,M_\odot$, initial gas particle mass $m_{\rm gas,IC}=2.09\times10^3\,M_\odot$, and Plummer-equivalent softenings $\epsilon_{\rm DM}=104\,{\rm pc}$ and $\epsilon_{\rm gas}=26\,{\rm pc}$ [2510.26513]. The CROCODILE fiducial cosmological run uses a $50\,h^{-1}\,{\rm cMpc}$ box with $2\times512^3$ particles, $m_{\rm DM}=6.74\times10^7\,h^{-1}\,M_\odot$, $m_{\rm gas}=1.26\times10^7\,h^{-1}\,M_\odot$, and $\epsilon_{\rm grav}=3.38\,h^{-1}\,{\rm ckpc}$ capped at $0.5\,h^{-1}\,{\rm pkpc}$ [2401.06324]. The AGORA isolated disk uses $\epsilon_{\rm grav}=80\,{\rm pc}$ with minimum SPH smoothing $0.2\,\epsilon_{\rm grav}$, while the AGORA cosmological zoom switches from $800$ comoving pc until $z=9$ to $80$ proper pc thereafter [2512.18945]. In isolated MW-like dust simulations, $\epsilon_{\rm grav}=40\,{\rm pc}$ and the minimum SPH smoothing is $\sim 4\,{\rm pc}$ [2508.21157].

The code employs an extensive diagnostic apparatus. For dwarf-galaxy kinematics, CROCODILE-DWARF uses orbital circularity
$$
\epsilon_{{\rm circ},i}=\frac{j_{z,i}}{j_{\rm circ}(E_i)}
$$
and the global disk fraction
$$
\kappa_{\rm rot}=
\frac{\sum_i \tfrac12 m_i (v_{\phi,i})^2}
{\sum_i \tfrac12 m_i |\mathbf v_i|^2},
$$
together with the gas kinematic ratio $V_{\rm rot,gas}/\sigma_{\rm gas}$ [2510.26513]. The jet study uses slice maps of temperature, density, entropy, pressure, velocity, and Mach number, as well as phase diagrams in the $\rho$–$T$, $P$–$K$, and thermal-versus-ram-pressure planes [2508.21282]. CROCODILE outputs on-the-fly FoF and Subfind catalogs and uniform mesh dumps of density, temperature, and metallicity for IGM analysis [2401.06324].

Parallel performance is inherited from GADGET-4’s hybrid MPI + OpenMP design. CROCODILE-DWARF reports runs with $O(10^3)$ MPI ranks and 2–4 OpenMP threads per rank, and notes that GADGET-4, and hence GADGET4-OSAKA, is known to show near-ideal weak scaling to $>10^3$ ranks on modern supercomputers [2510.26513]. Typical computational cost per dwarf zoom-in is $\sim 2$–$5\times10^5$ CPU h [2510.26513], while the CROCODILE fiducial cosmological run costs $\sim 1.12\times10^5$ CPU-h on SQUID with AVX-512 SPH loops [2401.06324].

## 6. Validation, scientific outcomes, and limitations

The scientific return of GADGET4-OSAKA is documented through several validation programs. In CROCODILE-DWARF, the simulated galaxies reproduce the observed stellar-to-halo mass, mass–metallicity, and size–mass relations, yielding stellar masses of $10^6$–$10^8\,M_\odot$ and metallicities consistent with Local Group dwarfs [2510.26513]. The same suite finds that early-assembling, high-concentration halos form stars efficiently and become gas-poor by $z=0$, while late-assembling, low-concentration halos remain gas-rich because of delayed star formation and rejuvenated gas accretion; it also identifies a clear anti-correlation between rotational support and the cumulative merger mass fraction [2510.26513]. In some cases, late-time mergers induce extended gas disks by delivering fresh gas and angular momentum [2510.26513].

The AGORA validation study shows that mechanical momentum injection is necessary to suppress unphysical gas fragmentation and regulate star formation, while stochastic thermal heating is essential for driving a hot, metal-enriched gaseous halo and establishing a multiphase CGM [2512.18945]. Quantitatively, the isolated disk at $t=500\,{\rm Myr}$ has $M_\star\approx 1.2\times10^9\,M_\odot$ and $\lesssim 5$ clumps in GADGET4-OSAKA, compared with $M_\star\approx 2.1\times10^9\,M_\odot$ and $\sim 15$–20 self-bound clumps in the predecessor code [2512.18945]. In the cosmological AGORA Cal-4 run, the GADGET4-OSAKA halo contains cold filaments at $T\approx 10^4\,{\rm K}$, a warm–hot interface at $T\approx 10^5$–$10^6\,{\rm K}$, and a hot volume-filling halo at $T>10^6\,{\rm K}$ extending to $\sim 100\,{\rm kpc}$ [2512.18945].

At cosmological volume scale, CROCODILE reports that the stellar-mass function, star-formation main sequence, and mass–metallicity relation show promising agreement with observations, especially for the Fiducial run [2401.06324]. The same work concludes that SN feedback is a key driver of IGM chemical enrichment, while AGN feedback produces metal-rich bipolar outflows extending over a few Mpc scales [2401.06324].

The dust and jet studies also expose current limitations. The radial dust-properties calculation reproduces the radial profile of dust mass surface density in NGC628 but overestimates the small-to-large grain ratio, with simulated SLRs exceeding observed values by $\gtrsim \times 2$ [2410.21430]. The PAH-signature study finds that the NGC 628-like simulation underestimates the PAH mass fraction throughout the galaxy by a factor of $\sim 8$ on average, possibly because of efficient PAH loss by coagulation [2402.02659]. The attenuation-curve study shows that the slope–$A_V$ relation is driven by variations in star–dust geometry and the amount of scattered photons escaping the galaxy, with additional contributions from grain-size distribution and the fraction of obscured young stars [2508.21157]. The AGN jet benchmark demonstrates that global lobe length converges with resolution to within $\lesssim 10\%$ across a two-order-of-magnitude mass-resolution span, but also that lobe morphology and the partitioning of thermal and kinetic energy are highly sensitive to the artificial-viscosity prescription [2508.21282].

A recurring misconception would be to treat GADGET4-OSAKA as a single, fully fixed “model.” The published record points instead to a family of closely related implementations that share a GADGET-4-derived SPH and gravity infrastructure plus Osaka feedback, while differing in gravity order, SPH form, neighbor number, star-formation threshold, and optional dust or jet modules [2512.18945] [2510.26513]. This suggests that comparisons across results should track the exact configuration, not only the code name.

Source: https://www.emergentmind.com/topics/gadget4-osaka