--- title: Sherwood-Relics Hydrodynamical Simulation Suite url: https://www.emergentmind.com/topics/sherwood-relics-hydrodynamical-simulation-suite type: topic --- # Sherwood-Relics Hydrodynamical Simulation Suite The Sherwood-Relics hydrodynamical simulation suite is a large suite of cosmological simulations designed to model the low-density intergalactic medium during and after hydrogen reionization, with particular emphasis on the high-redshift Lyman-\(\alpha\) forest, patchy reionization, pressure smoothing, and related inference problems in dark-matter and IGM physics. It generalizes the earlier Sherwood programme by retaining the P-GADGET-3 numerical backbone while extending the parameter space in reionization history, thermal history, cosmology, and warm dark matter, and by adding a lightweight hybrid radiative-transfer treatment that imprints spatially varying ionization and heating histories on the gas [2207.13098], [1605.03462]. ## 1. Origins, scope, and relation to the Sherwood programme Sherwood-Relics was introduced as a suite of **over 200 cosmological hydrodynamical simulations** aimed at modelling the intergalactic medium during and after the cosmic reionization of hydrogen [2207.13098]. Its immediate predecessor, the Sherwood simulation suite, had already established a high-dynamic-range SPH framework for the Lyman-\(\alpha\) forest over \(21000\) and temperature \(T<10^5\,\mathrm{K}\) is converted into collisionless star particles [2207.13098], [2406.19184], [2509.08613]. The rationale, stated repeatedly, is that the target observables arise in low-density gas rather than in the ISM. This expedient removes dense star-forming material cheaply while preserving the forest statistics of interest. In the mean-free-path analysis, the same approximation is noted to remove cold, dense ISM gas and most of the high-column DLA regime, while having only a modest effect on \(\lambda_{\rm mfp}\) at \(z\gtrsim 5\) [2406.19184]. The thermal state of the low-density IGM is described by the usual power-law equation of state, \[ T = T_0\,\Delta^{\gamma-1}, \] with \(\Delta=\rho/\bar{\rho}\) [2207.13098], [2411.17853], [2509.08613]. Sherwood-Relics is unusual in that it models not only the instantaneous parameters \(T_0\) and \(\gamma\), but also the integrated thermal history through pressure smoothing. In the patchy-reionization papers this is a dynamical consequence of spatially varying photoheating; in the homogeneous-grid work it is parameterized by the cumulative heat input per unit mass at mean density, \(u_0\) [2303.05167]. ## 3. Simulation inventory, representative runs, and parameter space The suite spans box sizes \[ L_{\rm box} = 5, 10, 20, 40, 80, 160~h^{-1}\,\mathrm{cMpc}, \] with particle loads from \(2\times512^3\) up to \(2\times2048^3\) [2207.13098]. Representative high-resolution and large-volume configurations recur across the literature. | Configuration | Size and resolution | Typical role | |---|---|---| | 40-2048 | \(40\,h^{-1}\,\mathrm{cMpc}\), \(2\times 2048^3\), \(M_{\rm dm}=5.37\times10^5\,h^{-1}M_\odot\), \(M_{\rm gas}=9.97\times10^4\,h^{-1}M_\odot\) | Fiducial high-resolution patchy-reionization and mean-free-path studies | | 160-2048 | \(160\,h^{-1}\,\mathrm{cMpc}\), \(2\times 2048^3\), \(M_{\rm dm}=3.44\times10^7\,h^{-1}M_\odot\), \(M_{\rm gas}=6.38\times10^6\,h^{-1}M_\odot\) | Large-volume late-reionization and halo-environment statistics | | 20 \(h^{-1}\,\mathrm{cMpc}\) WDM grid | \(20\,h^{-1}\,\mathrm{cMpc}\), \(1024^3\) dark matter + \(1024^3\) gas | \(4.1\le z\le 5.0\) density-field reconstruction and WDM inference | The 40-2048 configuration is the fiducial high-resolution Sherwood-Relics run in the late-reionization mean-free-path paper, with gravitational softening \(l_{\rm soft}=0.78\,h^{-1}\,\mathrm{ckpc}\) [2406.19184]. The 160-2048 run uses the same particle number in a \(64\times\) larger volume, giving \(64\times\) poorer mass resolution but much larger bubble statistics and halo counts [2406.19184]. In the Si III power-spectrum analysis, a single \(40\,h^{-1}\,\mathrm{cMpc}\) Sherwood-Relics run with \(2\times2048^3\) particles is used over \(2.2\le z\le 5.0\), explicitly because this combination resolves “the small-scale structure of the Ly\(\alpha\) forest” in the sense of Bolton & Becker (2009) [2509.08613]. Sherwood-Relics varies several classes of physics. The 2022 overview lists CDM and WDM runs with thermal relic masses \[ m_{\rm WDM}=1, 2, 3, 4, 8, 12~\mathrm{keV}, \] reionization histories with completion redshifts \(z_r\approx 5.3\)–7.4 and midpoints \(z_{\rm mid}\sim 7.1\)–9.1, and photoheating amplitudes obtained by multiplying H I and He I photoheating by factors \(0.5,1,2\), with corresponding He II factors \(0.66,1.0,1.5\) [2207.13098]. In the WDM machine-learning study, a specific Sherwood-Relics subset consists of the runs L20-ref, L20-ref-hot, and L20-ref-cold, each with the six WDM settings \[ m_{\mathrm{WDM}^{-1} \in \left\{ 0,\, \tfrac{1}{2},\, \tfrac{1}{3},\, \tfrac{1}{4},\, \tfrac{1}{8},\, \tfrac{1}{12} \right\} \,\mathrm{keV}^{-1}, \] corresponding to CDM and thermal relic masses \(m_{\rm WDM}=\{2,3,4,8,12\}\,\mathrm{keV}\), with \(T_0(z=4.6)=10{,}066\,\mathrm{K}\) (“ref”), \(13{,}957\,\mathrm{K}\) (“hot”), and \(6{,}598\,\mathrm{K}\) (“cold”) [2411.17853]. For late-reionization studies, the emissivity history is calibrated to observed Ly\(\alpha\) transmission. In Feron et al., the 40-2048 run is calibrated to Bosman et al. (2018) and Eilers et al. (2018), while 160-2048 is calibrated to the XQR-30 compilation of Bosman et al. (2022); the resulting midpoint redshifts are \(z_{\rm mid}=7.5\) and 7.2, with reionization completion \(z_r=5.7\) and 5.3, respectively [2406.19184]. The emissivity at \(z<6\) settles to \(\dot n\simeq 3\times 10^{50}\,\mathrm{s^{-1}\,cMpc^{-3}}\) in both models [2406.19184]. ## 4. Hybrid radiative transfer, patchy reionization, and data products The most distinctive Sherwood-Relics development is the **hybrid radiation–hydrodynamical** scheme. The workflow has three stages: a baseline hydrodynamical simulation with a homogeneous UVB, post-processing radiative transfer with **ATON** on snapshots separated by \(40\) Myr, and a second hydrodynamical simulation in which gas elements experience the spatially varying \(\Gamma_{\rm HI}(\mathbf{x},z)\) inferred from ATON [2207.13098]. ATON uses a moment-based RT solver with M1 closure and the full speed of light, on a fixed Eulerian grid with the same number of cells as gas particles in the patchy runs, such as \(2048^3\) cells for the \(40\,h^{-1}\,\mathrm{cMpc}\) realizations [2207.13098]. This scheme outputs both \(\Gamma_{\rm HI}(\mathbf{x},z)\) and a local reionization-redshift field \(z_{\rm reion}(\mathbf{x})\), defined as the redshift when the local H II fraction first exceeds \(3\%\) [2207.13098]. In the second hydrodynamical run, gas is unilluminated before local reionization, and after front passage it receives local photoionization and photoheating. The adopted monochromatic photon energy is \(E_\gamma=18.63\,\mathrm{eV}\), implying H I heating per ionization \[ \epsilon_{\rm HI}=\Gamma_{\rm HI}(E_\gamma-13.6\,\mathrm{eV})=\Gamma_{\rm HI}\times 5.03\,\mathrm{eV}, \] with \(\epsilon_{\rm HeI}=1.3\,\epsilon_{\rm HI}\), जबकि He II photoheating remains tied to the homogeneous UVB [2207.13098]. The hydrodynamical response to this inhomogeneous heating produces spatially varying pressure smoothing and long-lived temperature fluctuations. Sherwood-Relics also provides a large set of directly usable observables. Across several studies, standard products are 1D Ly\(\alpha\) skewers, transmitted-flux spectra, density and temperature fields, halo catalogues, and local reionization-redshift maps [2207.13098], [2502.02983]. The extraction conventions are consistent enough to support secondary analyses. The WDM density-field paper extracts \(5000\) random Ly\(\alpha\) sightlines per simulation box and redshift, each with \(2048\) pixels, together with \(\tau(v)\), \(\Delta\), and the optical-depth-weighted density field \[ \Delta_{\tau,i} = \frac{1}{\tau_i}\sum_j \Delta_j N_{\mathrm{HI},j} \,\phi_\alpha(\Delta v_{ij}), \qquad \tau_i = \sum_j N_{\mathrm{HI},j}\,\phi_\alpha(\Delta v_{ij}), \] which is then used as a supervised-learning target [2411.17853]. The Si III study similarly extracts \(5000\) periodic sightlines per redshift snapshot from a \(40\,h^{-1}\,\mathrm{cMpc}\) box, including \(\Delta\), \(n_{\rm H}\), \(T\), \(v_{\rm pec}\), and \(x_{\rm HI}\), and then computes Ly\(\alpha\) and Si III optical depths in post-processing [2509.08613]. The suite is explicitly presented as a public community resource. Both the 2022 overview and later application papers note data access through the Nottingham Sherwood-Relics site: \[ \texttt{https://www.nottingham.ac.uk/astronomy/sherwood-relics/}. \] This institutionalized release model is part of the suite’s role as common infrastructure rather than a one-off simulation campaign [2207.13098], [2509.08613]. ## 5. Principal scientific uses and derived results Sherwood-Relics has been used to establish several recurrent results about reionization-era IGM structure. The 2022 overview argues that patchy reionization causes large-scale temperature fluctuations that persist well after the end of reionization, that these fluctuations increase the Lyman-\(\alpha\) forest flux power spectrum on large scales, that pressure smoothing varies spatially and correlates with the local reionization redshift, and that photoheated structures can generate flat-bottom or double-dip absorption features in Ly\(\alpha\) spectra [2207.13098]. In a later power-spectrum analysis, Molaro et al. use Sherwood-Relics to compare homogeneous-UVB and patchy-reionization templates against new data at \(4.0\le z\le 4.6\), finding a **\(2.7\sigma\)** preference for a large-scale enhancement in the 1D Ly\(\alpha\) forest power spectrum relative to a spatially uniform UVB, while also emphasizing that the statistical precision is not yet sufficient for a robust detection [2303.05167]. In late-reionization mean-free-path studies, the suite is used to measure the free path of H I-ionizing photons through the inhomogeneous IGM. Feron et al. find that Sherwood-Relics models calibrated to a late end to reionization at \(z<6\) are consistent with recent mean-free-path measurements at \(z<5.9\), while lying \(1.2\sigma\) and \(1.8\sigma\) above the highest-redshift point at \(z=5.93\) for the 40-2048 and 160-2048 realizations, respectively [2406.19184]. A central physical conclusion is that, near the end of reionization, the majority of the Lyman-limit opacity is attributable to highly ionized Ly\(\alpha\) forest absorbers with \[ N_{\rm HI}=10^{16}\text{--}10^{17}\,\mathrm{cm}^{-2}, \] rather than only to classically self-shielded neutral systems [2406.19184]. The same work shows a strong environmental dependence: after reionization, overdense gas reduces the mean free path by up to \(20\%\) around haloes with \(M_h\sim10^9\text{--}10^{11}\,h^{-1}M_\odot\), whereas during reionization ionized bubbles can boost the local mean free path around haloes by up to an order of magnitude when the IGM is as much as \(90\%\) neutral by volume [2406.19184]. The suite has also been used to connect galaxies to Ly\(\alpha\) transmission. In the galaxy–forest cross-correlation paper, Sherwood-Relics predicts that the shape of the galaxy–Ly\(\alpha\) transmission cross-correlation is sensitive to both the halo mass of the ionizing sources and the volume-averaged neutral fraction \(\bar{x}_{\rm HI}\) [2502.02983]. The reported excess transmission on scales \(r\sim10\) cMpc at \(\langle z\rangle\approx5.2\), measured using C IV absorbers as galaxy proxies, is quantitatively reproduced by Sherwood-Relics at \(z=6\) if the ionizing galaxies occupy haloes with \[ M_h\ge 10^{10}\,h^{-1}M_\odot, \] but the same paper argues that this redshift mismatch is equivalent to requiring \(\bar{x}_{\rm HI}\sim 0.1\) at \(z\simeq 5.2\), in tension with the observed Ly\(\alpha\) effective-optical-depth distribution [2502.02983]. After reionization, the same study shows that relic IGM temperature fluctuations continue to influence the cross-correlation on scales of a few comoving Mpc at \(4\le z\le 5\) [2502.02983]. Two later methodological applications illustrate the suite’s broader role as inference infrastructure. First, the WDM density-field study trains a Bayesian neural network on Sherwood-Relics flux–density pairs over \(4.1\le z\le 5.0\), achieving an in-simulation validation rate in which about \(85\%\) of pixels lie within the predicted \(1\sigma\) interval and \(97\%\) within \(2\sigma\), and then uses reconstructed \(\Delta_\tau\) PDFs to derive lower bounds \(m_{\rm WDM}\gtrsim 3.8\,\mathrm{keV}\) and \(m_{\rm WDM}\gtrsim 2.2\,\mathrm{keV}\) at \(2\sigma\) from small UVES and GHOST samples [2411.17853]. Second, the Si III contamination paper uses Sherwood-Relics plus Cloudy post-processing to derive an improved analytic model for correlated Si III absorption, showing that distinct Ly\(\alpha\) and Si III line profiles and a variable coeval optical-depth ratio produce additional small-scale power relative to the McDonald (2006) ansatz; the resulting fitting function is validated for \(2.2\le z\le5.0\) and \(k<0.2\,\mathrm{s\,km^{-1}}\) [2509.08613]. ## 6. Limitations, tensions, and methodological significance The suite’s major approximations are explicit in the source papers. The hybrid RT scheme is **one-way coupled**: ATON sees the density field of the baseline hydrodynamical run rather than the final density field of the patchy simulation, so hydrodynamic back-reaction does not modify the radiative-transfer solution itself [2207.13098]. The radiation transport is **monochromatic**, which simplifies spectral hardening and prevents an inhomogeneous treatment of He II reionization [2207.13098], [2406.19184]. The quick-Ly\(\alpha\) conversion suppresses detailed modelling of dense halo gas and the highest-\(N_{\rm HI}\) population, which is acceptable for the low-density IGM but limits fidelity for the LLS/DLA tail and CGM-scale gas [2406.19184], [2502.02983]. In the Si III work, metals are not tracked self-consistently but are painted in post-processing from an observationally motivated abundance model and Cloudy ionization fractions, which is sufficient for low-density IGM Si III but not for a self-consistent CGM enrichment history [2509.08613]. There are also substantive astrophysical tensions. The mean-free-path paper notes that the emissivity calibration required by the Ly\(\alpha\) forest produces a photon-starved end stage of reionization, and explicitly connects this to the broader “photon budget crisis” discussed in the literature [2406.19184]. The galaxy–Ly\(\alpha\) cross-correlation paper finds a tension between reproducing the Meyer et al. signal with \(\bar{x}_{\rm HI}\sim0.1\) at \(z\simeq5.2\) and the observed \(\tau_{\rm eff}\) distribution, suggesting instead that the host halo masses of high-\(z\) C IV absorbers may have been underestimated [2502.02983]. In the WDM-density-field study, the authors note that their Sherwood-Relics training set does not vary \(\gamma\), even though additional runs varying \(\gamma\) exist elsewhere; the paper interprets mild discrepancies in the low-density tail as likely reflecting limitations of the thermal parameter coverage rather than a failure of the WDM modelling [2411.17853]. Taken together, these properties make Sherwood-Relics a highly structured simulation suite rather than a monolithic single model. It combines a Sherwood-style homogeneous-UVB backbone, an efficient patchy-reionization augmentation, and a parameter grid broad enough to support forward modelling, emulation, and field-level inference. A plausible implication is that its greatest methodological importance lies not in any one flagship result, but in providing a common, internally consistent basis for moving between flux statistics, halo-environment diagnostics, reconstructed density fields, and contamination models within the same high-redshift IGM framework [2207.13098], [2411.17853]. Source: https://www.emergentmind.com/topics/sherwood-relics-hydrodynamical-simulation-suite