---
title: 'The Three Hundred Project: Cluster ReSimulations'
url: https://www.emergentmind.com/topics/the-three-hundred-project
type: topic
---

# The Three Hundred Project: Cluster ReSimulations

The Three Hundred Project is a suite of 324 galaxy-cluster zoom re-simulations drawn from the MultiDark Planck 2 dark-matter-only run and re-simulated with full baryonic physics, together with matched semi-analytic realizations on the same halo backbone, for cosmological and astrophysical applications [1809.04622]. Its stated objectives include providing a mass-complete sample of the most massive clusters at \(z=0\) and their environments, enabling precision cosmology via cluster abundance and internal structure, advancing understanding of cooling, star formation, and AGN/SN feedback on cluster scales, and offering direct comparisons between hydrodynamical and semi-analytic models [1809.04622]. Subsequent work has used the same framework to study baryon fractions, gas and stellar profiles, cluster outskirts, mergers, hydrostatic-mass bias, radio relics, mass mapping, splashback structure, and spectroscopic mass estimators [2505.21624].

## 1. Scope, sample construction, and scientific remit

The baseline project re-simulates the 324 most massive \(z=0\) clusters in the MDPL2 parent simulation, a \(1\,h^{-1}\,\mathrm{Gpc}\) cube with \(3840^3\) particles in a Planck-2015 cosmology [1809.04622]. For each target, a \(15\,h^{-1}\,\mathrm{Mpc}\)-radius region is regenerated with multi-level refinement, yielding a mass-complete sample of the most massive clusters with \(M_{200}>6.4\times10^{14}\,h^{-1}\,M_\odot\) at \(z=0\), while also resolving \(\simeq 5500\) haloes down to \(M_{200}=10^{13}\,h^{-1}\,M_\odot\) without low-resolution contamination [1809.04622]. Snapshots have been stored at 128 epochs between \(z=17\) and \(z=0\), so the project supports both low-redshift statistical analyses and progenitor tracking over a large fraction of cosmic time [1809.04622].

The project is not restricted to a single hydrodynamical realization. Later extensions add the \textsc{Gizmo-Simba} run, which places the SIMBA galaxy-formation model in the same zoom framework and emphasizes the co-evolution of galaxies, gas, and black holes in cluster environments [2202.14038]. Other derivative studies explicitly treat the suite as a platform for cluster astrophysics, cluster-galaxy evolution, mass-observable calibration, and survey forecasting [1911.07878]. This suggests that “The Three Hundred Project” functions less as a single simulation product than as a common experimental infrastructure for cross-code, cross-method, and survey-facing cluster research.

## 2. Numerical implementations and data infrastructure

The original hydrodynamical realizations were run with \textsc{GADGET-X} and \textsc{GADGET-MUSIC}, both using the same initial conditions but different SPH formulations and sub-grid prescriptions [1809.04622]. \textsc{GADGET-X} uses a “modern” SPH scheme with a Wendland-C4 kernel, artificial conductivity and viscosity, metal-dependent cooling, star formation and SN feedback, and dual-mode AGN feedback; \textsc{GADGET-MUSIC} uses an entropy-conserving SPH formulation, metal-independent cooling, Springel & Hernquist star formation, and no AGN feedback [1809.04622]. The \textsc{Gizmo-Simba} extension instead uses the GIZMO meshless finite-mass solver with SIMBA sub-grid physics, including \(\mathrm{H}_2\)-regulated star formation, two-phase winds, torque-limited cold-gas accretion, Bondi hot accretion, kinetic AGN winds, and X-ray heating [2202.14038].

| Implementation | Type | Distinctive features |
|---|---|---|
| \textsc{GADGET-X} | Full hydro | Modern SPH, metal-dependent cooling, AGN feedback |
| \textsc{GADGET-MUSIC} | Full hydro | Classical SPH, no AGN feedback |
| \textsc{Gizmo-Simba} | Full hydro | MFM hydro, SIMBA feedback, kinetic jets |
| Galacticus / SAG / SAGE | SAMs | Same MDPL2 dark-matter backbone |

The baseline high-resolution masses in the original hydrodynamical project are \(m_{\rm DM}=1.27\times10^9\,h^{-1}\,M_\odot\) and \(m_{\rm gas}=2.36\times10^8\,h^{-1}\,M_\odot\), with physical Plummer-equivalent softening \(\epsilon_{\rm phys}=6.5\,h^{-1}\,\mathrm{kpc}\) below \(z=0\)–2 and comoving above [1809.04622]. The emulator work later introduces “3K”, “7K”, and “15K” dark-matter-only realizations, with corresponding GIZMO hydrodynamical runs at 3K and 7K, to transfer cluster-galaxy statistics from hydro to higher-resolution DMO calculations [2504.03519].

Public accessibility is a formal part of the project. The released products include roughly \(50\,\mathrm{TB}\) of simulations and \(4\,\mathrm{TB}\) of halo catalogues, with full-physics snapshots, AHF halo catalogues, merger trees, stellar-population outputs, project website access, and CosmoSim database entries; the introductory paper states that all simulations and derived data products are publicly available [1809.04622].

## 3. Operational definitions and theoretical quantities

The project uses standard overdensity definitions,
\[
M_{\Delta}\equiv \frac{4\pi}{3}\,\Delta\,\rho_c(z)\,R_{\Delta}^3,
\qquad
\rho_c(z)=\frac{3H(z)^2}{8\pi G},
\]
with common choices \(\Delta=200\) or \(500\) [1809.04622]. Baryonic fractions are defined as
\[
f_b\equiv \frac{M_b}{M_{\rm tot}},
\qquad
f_{\rm gas}\equiv \frac{M_{\rm gas}}{M_{\rm tot}},
\qquad
f_{\rm star}\equiv \frac{M_{\rm star}}{M_{\rm tot}},
\]
and scaling relations are generally written as \(Y\propto M^\alpha E(z)^\beta\), with \(E(z)=H(z)/H_0\) [1809.04622].

Several later analyses extend this common language to the intracluster medium and cluster outskirts. The entropy profile is defined by
\[
K(r)\equiv \frac{k_B T(r)}{n_e(r)^{2/3}},
\]
and two operational definitions of the shock radius are used: the peak-entropy radius \(r_{\rm shock,p}\), where \(K(r)\) is maximal, and the slope-minimum radius \(r_{\rm shock,m}\), where \(d\ln K/d\ln r\) attains its minimum [2412.09864]. The splashback radius \(r_{\rm splash}\) is measured from the spherically averaged 3D mass-density profile as the radius where \(d\ln\rho/d\ln r\) is minimal [2412.09864]. In baryon-fraction work, the hot-gas fraction is defined using gas particles with \(T>0.3\,\mathrm{keV}\) that are not flagged as star-forming, and the relative fraction is written as
\[
Y_\Delta \equiv \log_{10}\!\left[\frac{f_\Delta}{f_{b,\rm cosmic}}\right]
\]
with \(f_{b,\rm cosmic}=\Omega_b/\Omega_m\approx0.157\) [2505.21624].

The project has also been used to study the cluster fundamental plane,
\[
T\propto M_s^\alpha\,r_s^\beta,
\]
where \(r_s\) and \(M_s\equiv M(<r_s)\) are the NFW scale radius and scale mass, respectively [2108.13498]. Across these applications, the project repeatedly uses profile minima, profile curvature, and self-similar normalization to define physically motivated cluster boundaries and observables.

## 4. Baryonic structure, scaling relations, and self-similarity

The introductory analysis found that the modelled clusters are generally in reasonable agreement with observations with respect to baryonic fractions and gas scaling relations at \(z=0\), while also showing model-dependent differences such as overly massive central galaxies and galaxy colours shifted to bluer \(g-r\) by about \(0.2\) dex at the peak position [1809.04622]. Gas scaling relations down to \(10^{13}\,h^{-1}\,M_\odot\) are a particularly robust result: the \(T\)–\(M\) slopes are \(B\simeq0.63\) for \textsc{GADGET-MUSIC} and \(0.57\) for \textsc{GADGET-X}, versus the non-radiative self-similar value \(2/3\), and the \(Y_{\rm SZ}\)–\(M\) slopes are \(B\simeq1.62\)–\(1.63\), consistent with Planck and weak-lensing measurements [1809.04622]. The same study argues that gas scaling relations are remarkably robust to details of feedback prescriptions, at least above \(10^{13}\,h^{-1}\,M_\odot\) [1809.04622].

A more profile-oriented analysis shows that gas density profiles in both hydrodynamical runs match observed trends well, including reduced scatter at large radii, while \textsc{GADGET-X} reproduces the observed temperature-profile shape more successfully than \textsc{GADGET-MUSIC}; by contrast, cumulative stellar density profiles from semi-analytic models are in better agreement with SDSS than the hydrodynamical runs, which show relatively higher profiles [2005.06135]. The same paper finds that gas density and metallicity display the strongest self-similarity beyond \(0.2\,R_{500}\), whereas temperature and stellar mass density show larger scatter and stronger dependence on dynamical state and cool-core status [2005.06135].

The evolution paper extends these conclusions to \(z=4\), finding that both \textsc{Gadget-X} and \textsc{Gizmo-Simba} agree with observations mostly at outer radii \(r\gtrsim0.3\,r_{500}\), consistent with self-similarity, but diverge in the centre, where \textsc{Gizmo-Simba} often produces large entropy cores and higher central temperatures while \textsc{Gadget-X} remains closer to low-redshift X-ray samples [2305.09629]. In the baryon-fraction analysis, simple power-law mass trends are found to be inadequate: high masses show a near-constant plateau, low masses show a steep decline, and the transition near \(M\sim2\)–\(3\times10^{14}\,M_\odot\) is better captured by either a parabolic or logarithmic form in the logarithmic plane, with stronger redshift evolution at smaller radii [2505.21624]. A plausible implication is that cluster baryon-content calibration depends on both mass scale and aperture, not merely on a single global slope.

## 5. Outskirts, accretion shocks, splashback, and infall processing

The project has been used extensively to characterize the physical boundary of clusters. In the shock–splashback study based on 324 \textsc{GIZMO-Simba-7k} clusters, the median stacked radii are
\[
r_{\rm shock,p}\simeq 2.58\,R_{200}\simeq 1.38\,r_{\rm splash},
\qquad
r_{\rm shock,m}\simeq 3.54\,R_{200}\simeq 1.91\,r_{\rm splash},
\]
and individual-cluster fits give
\[
r_{\rm shock,p}=0.64\,r_{\rm splash}+1.39,
\qquad
r_{\rm shock,m}=0.65\,r_{\rm splash}+2.38,
\]
in \(h^{-1}\,\mathrm{Mpc}\) [2412.09864]. Both \(r_{\rm shock}/R_{200}\) and \(r_{\rm splash}/R_{200}\) anti-correlate with \(M_{200}\) and with recent fractional mass growth, while \(r_{\rm shock}/r_{\rm splash}\) tends to be larger for clusters with higher recent accretion rates [2412.09864]. The paper summarizes this as the shock radius lying systematically outside the splashback radius by \(\simeq 40\)–\(90\%\) [2412.09864].

A complementary stellar-splashback analysis dynamically decomposes clusters into orbiting and infalling material and finds that the truncation radius \(r_t\), associated with the splashback feature, coincides for stars and dark matter, with \(\langle r_{t,\rm stars}/r_{t,\rm DM}\rangle=1.00\pm0.03\), while the stellar orbiting profile declines more steeply, with \(\beta_{\rm stars}\simeq 4.8\pm0.3\) versus \(\beta_{\rm DM}\simeq 3.5\pm0.2\) [2508.07232]. The fitted relation
\[
r_t/R_{200\rm m}\simeq A+B\,\exp[-\Gamma/\Gamma_0]
\]
has \(A\simeq0.45\pm0.02\), \(B\simeq0.40\pm0.03\), and \(\Gamma_0\simeq3.0\pm0.5\), and projected stellar profiles recover \(R_t\) with RMS scatter \(\simeq0.30\,R_{200\rm m}\) [2508.07232]. This suggests that diffuse stellar-light measurements can act as a proxy for recent mass accretion.

The same outer-cluster environment hosts strong preprocessing and backsplash phenomena. Infalling haloes within \(5R_{200}\) lose \(\approx30\%\) of their gas by \(\sim2R_{200}\) and then rapidly lose the remainder by \(\sim1.7R_{200}\) in 3D, with the projected 100% gas-loss radius shifting inward to \(\approx R_{200}\) [2101.01734]. Separately, backsplash galaxies—defined as systems that previously passed within \(R_{200}\) but are now outside it—constitute 58% of all galaxies between \(R_{200}\) and \(2R_{200}\) at \(z=0\), with 95% of clusters lying between 21% and 85%; the fraction is higher in dynamically relaxed clusters and builds rapidly at \(z\lesssim0.4\) [2001.11518]. Taken together, these studies treat the outskirts not as a single boundary but as a set of coupled transitions in gas thermodynamics, collisionless orbits, and galaxy environmental history.

## 6. Mergers, radio relics, and galaxy evolution in dense environments

Cluster mergers are another major theme. A merger catalogue based on rapid mass jumps in the main-progenitor history identifies 178 merger events with \(\Delta M/M\ge1\) and \(N_{\rm particles}(z_{\rm start})\ge10\,000\), reduced to 164 after requiring \(z_{\rm end}\) and \(z_{\rm after}\) to be defined; the inferred mass ratios peak at \(\sim0.7\), confirming that the selected events are major mergers [2201.12252]. The same work finds that BCG stellar growth during mergers is dominated by ex-situ accretion, which provides \(\sim80\%\) of \(\Delta M_\ast\), while in-situ star formation provides \(\sim20\%\), and BCGs in mergers form in median around 70 per cent more stars than those in a control sample [2201.12252]. Observable consequences include a median increase in SDSS-\(u\) luminosity of about 20 per cent during mergers and a slower increase of the \(g-r\) colour [2201.12252].

A radio-emission analysis uses 555 major-merger events with \(M_{200,1}\ge10^{13}\,M_\odot\) to model merger-induced shock fronts and their synchrotron output [2409.09422]. The median radio luminosity light curve rises sharply after core passage, peaks after \(\sim0.1\)–\(0.8\,\mathrm{Gyr}\), and its peak enhancement relative to the emission at core passage is \(\lesssim10\times\) for groups and \(\sim10\)–\(50\times\) for massive clusters [2409.09422]. The peak relic power follows
\[
P_{1.4,\rm peak}=1.4\times10^{23}
\left(\frac{M_{200,1}}{10^{14}\,M_\odot}\right)^{2.05}
\,\mathrm{W\,Hz^{-1}},
\]
with scatter \(\sim0.5\)–1 dex at fixed mass [2409.09422]. Most merger orbits are fairly radial, with a median opening angle of \(\sim20^\circ\) before collision and a median impact parameter of about \(255\,\mathrm{kpc}\) [2409.09422].

These studies collectively connect the project’s dynamical merger histories to directly modelled observables. The evidence in the data is that mergers reshape both the stellar content of BCGs and the non-thermal emission of the outer intracluster medium, while the details depend on orbit, mass, and accretion geometry.

## 7. Inference, emulation, and survey-facing methodologies

A substantial part of the project’s later impact lies in methods development. For hydrostatic-equilibrium masses, an analysis of more than 300 simulated massive clusters finds raw median biases at \(R_{500}\) of \(1-b_{\rm HE,X}\approx0.90\) and \(1-b_{\rm HE,SZ}\approx0.90\), with the underestimation increasing in irregular, high-clumpiness systems [1911.07878]. By combining X-ray azimuthal scatter or ellipticity with the asymptotic density or pressure slope, the proposed corrections bring the medians to 1.00 and reduce the skewness of the SZ bias distribution from \(\approx0.7\) to \(\approx0.2\) [1911.07878]. The stated goal is to reduce and symmetrize the HE-mass bias distribution for cosmological analyses of X-ray and SZ surveys [1911.07878].

The suite has also been used as supervised-learning training data. A multiview U-Net trained on idealized SZ, X-ray, and stellar surface-density maps from The Three Hundred predicts total projected mass maps with median cylindrical mass bias at \(R_{200}\) of about 1% and scatter of about 3% for the best multiview model, while reproducing the 2D mass-map power spectrum to \(\sim\pm10\%\) down to \(\lambda\sim0.1R_{200}\) [2311.02469]. The authors explicitly note that the method depends on the physics implemented in the hydrodynamic simulations, but the near-unbiased integrated masses indicate that the simulated map pairs form a useful benchmark for non-lensing reconstruction strategies [2311.02469].

Semi-analytic emulation pushes this idea further. A calibrated emulator based on the SAMs SAGE and SAG is trained against 7K-GIZMO cluster galaxies and then run on DMO versions of The Three Hundred at three resolutions; SAG is found to emulate the hydrodynamical galaxy populations more successfully than SAGE because of its explicit treatment of orphan galaxies [2504.03519]. The resulting 15K-SAG realization extends the cumulative stellar mass function to \(M_\ast\approx10^7\,M_\odot\), about an order of magnitude below 7K-GIZMO, while a full 7K hydro region requires \(\sim10^5\) CPU-h and the SAM emulator runs in \(\sim10^3\) CPU-h per region [2504.03519]. This is presented as a fast and efficient emulator of hydrodynamical cluster simulations [2504.03519].

Finally, the project has been used to forecast spectroscopic mass-profile recovery for upcoming surveys. In a caustic-method study targeting CATARSIS, a non-iterative filling-factor treatment gives median \(M_{200,\rm caustic}/M_{200,\rm true}=1.16\) or 1.12, depending on the assumed anisotropy model, whereas the iterative approaches reduce the median to 0.94 or 0.93 and lower the scatter to 0.18–0.20 [2606.03342]. CATARSIS itself is described as targeting 16 clusters at \(0.14<z<0.27\), aiming for 500–1000 secure redshifts per cluster within \(2\times R_{200,c}\), with forecast per-cluster \(M_{200}\) biases \(\lesssim5\%\) and scatter \(\sim20\%\) [2606.03342]. In this survey-facing mode, The Three Hundred Project serves as a controlled environment for turning dynamical, thermodynamical, and photometric mock data into mass-calibration algorithms and observational strategy tests.

Source: https://www.emergentmind.com/topics/the-three-hundred-project