--- title: FLAMINGO Hydrodynamic Simulations url: https://www.emergentmind.com/topics/flamingo-hydrodynamic-simulations type: topic --- # FLAMINGO Hydrodynamic Simulations FLAMINGO—“Full-hydro Large-scale structure simulations with All-sky Mapping for the Interpretation of Next-Generation Observations”—is a suite of cosmological hydrodynamical simulations developed by the Virgo Consortium to connect galaxy-formation modelling with large-scale-structure cosmology. The suite is designed primarily to support late-time Universe probes—weak lensing, SZ, CMB lensing—and studies of rare objects such as galaxy clusters, while also modelling the galaxy population on large scales. Its defining combination is large volume, calibrated baryonic physics, explicit neutrino treatments, and on-the-fly lightcone products, with subgrid prescriptions for stellar and AGN feedback calibrated to the observed low-redshift galaxy stellar mass function and cluster gas fractions [2306.04024]. ## 1. Project remit and run hierarchy FLAMINGO was introduced to ensure that hydrodynamical simulations are sufficiently realistic for studies of large-scale structure while retaining enough volume to model survey observables and sufficiently flexible physics to explore uncertainties in galaxy formation. The scientific goals stated for the suite include robust predictions for large-scale-structure observables at $k\approx0.1$–$10\,h\,\mathrm{Mpc}^{-1}$ and $\ell\approx10^2$–$10^4$, provision of light-cone outputs for lensing, SZE, X-ray, dark matter, gas, stars, black holes, and neutrinos, and direct study of baryonic effects and neutrino free-streaming on the halo mass function and matter power spectrum [2306.04024]. The hydrodynamical component consists of cubic boxes of side length $L=1\,\mathrm{Gpc}$ at three resolutions and a large $2.8\,\mathrm{Gpc}$ box at intermediate resolution. The fiducial hydrodynamical simulations span three numerical resolutions that have each been calibrated to reproduce the present-day galaxy stellar mass function and gas fractions in low-redshift clusters. The public release contains 22 hydrodynamical simulations and 16 gravity-only simulations, including the $10080^3$ particles FLAMINGO-10k run, with initial conditions that match those of the corresponding hydrodynamical runs [2604.24324]. | Run | Volume and resolution | Role in the suite | |---|---|---| | **L1_m8** | $(1\,\mathrm{Gpc})^3$, $m_{\rm gas}=1.34\times10^8\,M_\odot$ | High-resolution galaxy-formation and high-$z$ studies | | **L1_m9** | $(1\,\mathrm{Gpc})^3$, $m_{\rm gas}=1.07\times10^9\,M_\odot$ | Intermediate-volume fiducial and feedback/cosmology variants | | **L2p8_m9** | $(2.8\,\mathrm{Gpc})^3$, $m_{\rm gas}=1.07\times10^9\,M_\odot$ | Flagship large-volume hydrodynamical run | At fixed “D3A” cosmology, $m9$ resolution, and $L=1\,\mathrm{Gpc}$, the project includes gas-fraction variants $f_{\rm gas}\pm\{2,4,8\}\sigma$, an SMF variant $M_\ast-\sigma$, a combined $M_\ast-\sigma$ and $f_{\rm gas}-4\sigma$ model, and two Jet variants using kinetic AGN feedback. Four additional cosmology variants are provided at the same resolution and box size, including Planck, PlanckNu0p24Var, PlanckNu0p24Fix, and LS8 [2306.04024]. The suite is organized around a DES Y3–based fiducial flat $\Lambda$CDM cosmology. In the flagship suite description this is quoted as $h=0.681$, $\Omega_m=0.306$, $\Omega_b=0.0486$, $\Sigma m_\nu=0.06\,\mathrm{eV}$, $A_s=2.099\times10^{-9}$, $n_s=0.967$, and $\sigma_8=0.807$, with additional Planck-based, low-$S_8$, and varied-neutrino cosmologies implemented in matched runs [2306.04024]. ## 2. Numerical realization FLAMINGO is run with the open-source SWIFT code and uses the SPHENIX flavour of modern SPH for gas dynamics. Across the project descriptions, the gravity solver is specified as a Tree–Particle–Mesh or 4th-order Fast Multipole Method plus PM implementation, while the hydrodynamics uses Wendland or quintic-kernel SPHENIX formulations with pressure-entropy coupling, conductivity, viscosity and time-step limiters, and adaptive individual timesteps [2306.05492]. Massive neutrinos are followed explicitly with particles using the $\delta f$ method. In the public description, the number of neutrino particles is smaller than the CDM count by a factor of $1.8^3$, and in the largest hydrodynamic box the particle load is $2\times5040^3$ for dark matter plus initial gas together with $2800^3$ neutrino particles. The flagship $L2p8\_m9$ run follows $3\times10^{11}$ total particles to $z=0$ [2306.04024]. The initial conditions are generated with monofonIC using 3-fluid 3LPT in $N$-body gauge with back-scaling for massive neutrinos and panphasia phase-fixed initial conditions, with a starting redshift of $z=31$ in the flagship description. Specific clustering analyses of FLAMINGO also describe initial conditions set at $z\simeq127$ using second-order Lagrangian perturbation theory together with a pair-fixed Gaussian random field in which amplitudes of each $k$-mode are held equal to their ensemble mean and phases are chosen in paired opposite sets, reducing sample variance by up to $\sim50\times$ [2306.04024; 2407.18912]. Halo and subhalo finding in the main analysis pipeline is based on VELOCIraptor and SOAP. VELOCIraptor first runs a standard 3D Friend-of-Friend with linking length $b=0.2\times$ mean interparticle spacing to find parent haloes, then an iterative 6D FoF in phase space to isolate self-bound substructures. SOAP computes spherical-overdensity masses and galaxy properties such as stellar masses, star-formation rates, and metallicities in fixed 3D apertures of $50\,\mathrm{kpc}$ proper, excluding unbound particles and particles belonging to other substructures. In the data release, merger trees and cross-matching catalogues are supplied through HBT-HERONS alongside SOAP-based object catalogues [2407.18912; 2604.24324]. ## 3. Baryonic physics and machine-learning calibration All FLAMINGO hydrodynamical runs include radiative cooling, star formation, stellar mass loss and the resulting chemical enrichment, supernova feedback, and two implementations of AGN feedback. Radiative cooling and photo-heating are computed element by element using CLOUDY-based or Ploeckinger & Schaye tables, with photo-ionisation, self-shielding, dust, cosmic rays, and UV/X-ray backgrounds included in the suite descriptions. Star formation follows the pressure-law prescription of Schaye & Dalla Vecchia, calibrated to reproduce the Kennicutt–Schmidt relation above a metallicity-dependent density threshold [2306.05492]. In the calibration paper, the star-formation law is written \[ \dot m_* \;=\; m_{\rm g}\;A\,(1\,M_\odot\,{\rm pc}^{-2})^{-n}\;\biggl(\frac{\gamma}{G}\,f_{\rm g}\,P\biggr)^{(n-1)/2}, \] with $A=1.515\times10^{-4}\,M_\odot\,\mathrm{yr}^{-1}\,\mathrm{pc}^{-2}$, $n=1.4$, $\gamma=5/3$, and $f_g=1$. Stellar evolution and enrichment track multiple elements, with yields from SN II, SN Ia, and AGB channels, while stellar feedback is implemented through stochastic kinetic kicks or thermal injection depending on the run description, with core-collapse supernova energetics normalized to $10^{51}\,\mathrm{erg}$ per event [2306.05492]. Black holes are seeded in sufficiently massive haloes and grow through a boosted Bondi–Hoyle–Lyttleton prescription capped at the Eddington rate. In the calibration paper this is written \[ \dot m_{\rm accr} = \alpha\,\frac{4\pi G^2 m_{\rm BH}^2\,\rho}{(c_s^2+v_{\rm BH}^2)^{3/2}}, \qquad \alpha = \max\bigl[(n_H/n_{H,*})^{\beta_{\rm BH}},\,1\bigr], \] with $n_{H,*}=0.1\,\mathrm{cm}^{-3}$ and $\beta_{\rm BH}$ a calibrated parameter. The fiducial thermal AGN model stores a fraction $\epsilon_r\epsilon_f=0.1\times0.15=0.015$ of the rest-mass energy of accreted gas and injects it once enough energy is accumulated to heat one neighbour by $\Delta T_{\rm AGN}$. FLAMINGO also includes a kinetic jet implementation in which pairs of neighbours are kicked along the BH spin axis with target velocity $v_{\rm jet}$ [2306.05492]. A distinctive feature of FLAMINGO is the use of Gaussian-process emulators trained on Latin hypercubes of 32 smaller-volume simulations to calibrate the stellar and AGN feedback models. The training boxes are $(100\,\mathrm{Mpc})^3$ for high resolution, $(200\,\mathrm{Mpc})^3$ for intermediate resolution, and $(400\,\mathrm{Mpc})^3$ for low resolution, each with $2\times360^3$ particles. The emulators model how the galaxy stellar mass function and cluster gas fractions change as a function of the subgrid parameters and are then fit to observational data with MCMC, while allowing for observational random errors and biases in stellar masses, cosmic variance, and hydrostatic masses [2306.05492]. The calibration data comprise the $z=0$ GAMA DR4 stellar mass function and cluster gas fractions from X-ray and weak-lensing samples at low redshift. The fitting machinery explicitly includes a systematic stellar-mass bias $b_\ast$, a cosmic-variance shift $b_{\rm cv}$ for the stellar mass function, and a hydrostatic-mass bias $b_{\rm HSE}$ for X-ray gas fractions. This enables model variations to be defined in terms of shifts in the calibration observables rather than in terms of the values of specific subgrid parameters. For the intermediate-resolution thermal model, the maximum-likelihood parameters are quoted as $f_{\rm SN}=0.238$, $\Delta v_{\rm SN}=562\,\mathrm{km\,s^{-1}}$, $\log\Delta T_{\rm AGN}/\mathrm{K}=7.95$, and $\beta_{\rm BH}=0.514$ [2306.05492]. ## 4. Catalogues, lightcones, and public data products The public FLAMINGO release describes more than $2.3$ petabytes of data. Snapshot outputs are provided at 13 redshifts, namely $z=5,4,3,2,1.5,1.0,0.75,0.5,0.4,0.3,0.2,0.1,0.0$, together with reduced snapshots containing all particles within $R_{100c}$ of selected massive haloes and downsampled snapshots containing $1\%$ of particles plus all black holes [2604.24324]. Halo and galaxy catalogues include spherical-overdensity masses such as $M_{200c}$ and $M_{500c}$, fixed physical apertures, inclusive and exclusive measurements, and projected apertures along $x/y/z$. The release also includes complete merger trees, cross-matching catalogues to link objects across hydro and gravity-only runs and across model variants, and initial conditions for selected runs [2604.24324]. Lightcone output is a central architectural feature of FLAMINGO. The original suite description states that lightcone output is produced on-the-fly for up to 8 different observers, with full-sky HEALPix maps and particle lists for lensing, SZE, X-ray, dark matter, gas, stars, black holes, and neutrinos. In the public release, particle lightcones record gas, dark matter, stars, black holes, and neutrinos crossing the past lightcone to $z_{\max}$, halo lightcones extend to $z=15$, and HEALPix shell maps provide mass by species, SFR, dispersion measure, kSZ Doppler $b$, thermal SZ $y$, X-ray emission in multiple bands, CMB lensing convergence, cosmic infrared background, and radio AGN emission [2306.04024; 2604.24324]. A practical innovation of the release is an HTTP-based HDF5 streaming service for partial remote access. Clients request filesystem metadata or dataset slices through URL endpoints, data are serialized using MessagePack and streamed in chunks, and a Python client named `hdfstream` mimics an `h5py` interface with lazy loading and slicing. The service is integrated with `swiftsimio` for high-level access to snapshots and SOAP catalogues, and a web-browser interface allows inspection of file and dataset metadata and generation of direct download links [2604.24324]. ## 5. Cluster and large-scale-structure predictions A primary outcome of FLAMINGO is a calibrated description of baryonic suppression in large-scale-structure statistics. By comparing hydrodynamical and dark-matter-only simulations, the project introduction reports that baryonic effects can suppress the halo mass function and the matter power spectrum by up to $\approx20$ per cent. The halo-mass-function suppression dips by up to $\approx20$ per cent at $M_{200m}\approx2\times10^{13}\,M_\odot$, while the matter power spectrum is approximately tracer-like at $k\lesssim0.1\,h\,\mathrm{Mpc}^{-1}$, is suppressed by up to $\approx10$–$20$ per cent at $k\approx10\,h\,\mathrm{Mpc}^{-1}$ due to gas ejection, and is boosted at $k\gtrsim100\,h\,\mathrm{Mpc}^{-1}$ by stellar condensation [2306.04024]. A more focused analysis of the gas field shows that while gas velocities do not differ from those of dark matter on large scales, the gas mass power spectrum is suppressed by up to $\approx8\%$ relative to matter even on gigaparsec scales, with $S(k)\approx0.92$–$0.95$ at $k\sim0.01\,h\,\mathrm{Mpc}^{-1}$ and stronger suppression on $k\ge0.1\,h\,\mathrm{Mpc}^{-1}$. The same study interprets this large-scale effect as a “density bias” caused by star formation removing gas from the densest, most clustered regions and leaving gas preferentially in lower-bias regions. Around haloes of mass $M_{200}\approx10^{13}\,h^{-1}M_\odot$, AGN-driven outflows produce bubbles extending to $r\sim(5$–$10)\,R_{200,\mathrm{GrO}}$ with peak velocities $v_r\sim(0.5$–$1)\,v_{200}$ [2412.09526]. Galaxy-cluster thermodynamics are another core validation domain. FLAMINGO predicts $L_X$–$M_{500c}$, $T$–$M_{500c}$, $Y$–$M_{500c}$, $M_{\rm gas}$–$M_{500c}$, and $L_X$–$T$ relations that match observed samples across more than two dex in mass, and the evolution of the scaling relations is close to self-similar to $z\approx2$ after the standard $E(z)$ rescalings. Radial temperature, density, pressure, and entropy profiles of the fiducial run agree with de-projected XMM and Chandra measurements to within observational scatter for $r\gtrsim0.05\,R_{500}$, whereas core metallicities exceed observed values by $\sim0.3$ dex and pressure profiles are slightly too flat [2312.08277]. The suite also resolves dynamical systematics in cluster mass calibration. For hydrostatic masses derived from X-ray–weighted profiles, the median hydrostatic mass bias increases from $b_{\rm HSE}\approx-0.1$ on group scales to $-0.2$ on massive cluster scales, with minimum scatter $\sigma(b_{\rm HSE})\approx0.1$ around $M_{500c}\approx10^{14}\,M_\odot$. Analyses of the virial and Euler equations in the same work suggest that non-thermal motions, including rotation, account for most of the hydrostatic mass bias, and that X-ray luminosity weighted profiles strongly overestimate the deviations from hydrostatic equilibrium [2409.07849]. At the level of halo boundaries, FLAMINGO predicts a clear dark-matter splashback radius whose radius $R_{\rm sp}$ anti-correlates with the specific mass-accretion rate $\Gamma$, with $R_{\rm sp}/R_{200m}$ falling from $\approx1.4$ at $\Gamma\approx0.5$ to $\approx1.15$ at $\Gamma\approx4$. The minimum in the stacked 3D gas-density and pressure slopes and the maximum in the entropy slope broadly align with the dark-matter splashback feature, while projected weak-lensing surface-density minima trace $R_{\rm sp}$ to within $\lesssim10\%$ and X-ray/SZ minima occur at systematically larger radii [2312.05126]. For CMB lensing and neutrino-mass studies, FLAMINGO is used to measure a baryonic suppression function $SP(k,z)\equiv P_{\rm hydro}(k,z)/P_{\rm DMO}(k,z)$ and to validate fast predictions of the non-linear lensing power spectrum. In that application, clustering suppression due to small-scale baryonic phenomena such as feedback from active galactic nuclei can reduce the lensing power by of order $10\%$, while the scale-dependent suppressions due to neutrinos and baryons are found to approximately factorize, $F_{\nu+b}(L)\approx F_\nu(L)\times F_b(L)$, implying that careful baryonic modelling can limit bias in neutrino-mass constraints [2308.09755]. ## 6. High-redshift galaxies and SMBH populations Although FLAMINGO was designed primarily for late-time large-scale structure and cluster surveys, the highest-resolution $1\,\mathrm{cGpc}$ run is also used for early massive galaxies in the JWST era. In that configuration, the box contains $\sim10^3$ independent “JWST-sized” sub-boxes of $(100\,\mathrm{cMpc})^3$. At $z\approx6$, the total variance in the number of haloes with $M_{200}\simeq10^{11.5}\,M_\odot$ or galaxies with $M_\ast\simeq10^{10}\,M_\odot$ is $2$–$3$ times the Poisson expectation, and for the most massive halo per $(100\,\mathrm{cMpc})^3$ field the $1\sigma$ scatter in $\log M_{h,\max}$ exceeds the Poisson-only prediction by a factor $\simeq2$ at $z\gtrsim4$ [2511.09618]. The same analysis finds a pronounced large-scale conformity. When sub-boxes are ranked by the stellar mass of their most massive galaxy, $M_{\ast,\max}$, the stellar-to-halo mass relation and star-formation efficiency are coherently elevated or suppressed throughout the full $(100\,\mathrm{cMpc})^3$ volume. For $M_{200}\sim10^{12}\,M_\odot$, the offset is $\Delta\log(M_\ast/M_{200})\simeq0.2$–$0.3$ dex, and the signal survives only to radii $\lesssim50\,\mathrm{cMpc}$ once galaxies outside the original volume are included. In this framework, $M_{\ast,\max}$ is a better predictor of the volume-wide efficiency of massive galaxies than total number counts, and the stellar fraction of the most massive galaxies, \[ f_\ast \equiv \frac{M_\ast}{M_{200}f_{b,\mathrm{cosmic}}}, \] rises from $\langle f_\ast\rangle\approx0.05$ at $z\approx10$ to a peak $\langle f_\ast\rangle\approx0.2$ at $z\approx5$, with a log-normal dispersion $\sigma_{\log f_\ast}\approx0.2$–$0.4$ dex [2511.09618]. A different high-redshift application uses FLAMINGO to compare against JADES spectroscopy of massive quiescent galaxies at $2