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We present the Sherwood–Relics simulations, a new suite of large cosmological hydrodynamical simulations aimed at modelling the intergalactic medium (IGM) during and after the cosmic reionization of hydrogen. The suite consists of over 200 simulations that cover a wide range of astrophysical and cosmological parameters. It also includes simulations that use a new lightweight hybrid scheme for treating radiative transfer effects. This scheme follows the spatial variations in the ionizing radiation field, as well as the associated fluctuations in IGM temperature and pressure smoothing. It is computationally much cheaper than full radiation hydrodynamics simulations, and circumvents the difficult task of calibrating a galaxy formation model to observational constraints on cosmic reionization. Using this hybrid technique, we study the spatial fluctuations in IGM properties that are seeded by patchy cosmic reionization. We investigate the relevant physical processes and assess their impact on the z > 4 Lyman-α forest. Our main findings are: (i) consistent with previous studies patchy reionization causes large-scale temperature fluctuations that persist well after the end of reionization, (ii) these increase the Lyman-α forest flux power spectrum on large scales, and (iii) result in a spatially varying pressure smoothing that correlates well with the local reionization redshift. (iv) Structures evaporated or puffed up by photoheating cause notable features in the Lyman-α forest, such as flat-bottom or double-dip absorption profiles. [ABSTRACT FROM AUTHOR]

Patchy cosmic reionization resulted in the ionizing UV background asynchronous rise across the Universe. The latter might have left imprints visible in present-day observations. Several numerical simulation-based studies show correlations between the reionization time and overdensities and object masses today. To remove the mass from the study, as it may not be the sole important parameter, this paper focuses solely on the properties of paired haloes within the same mass range as the Milky Way. For this purpose, it uses CoDaII, a fully coupled radiation hydrodynamics reionization simulation of the local Universe. This simulation holds a halo pair representing the Local Group, in addition to other pairs, sharing similar mass, mass ratio, distance separation, and isolation criteria but in other environments, alongside isolated haloes within the same mass range. Investigations of the paired halo reionization histories reveal a wide diversity although always inside-out, given our reionization model. Within this model, haloes in a close pair tend to be reionized at the same time but being in a pair does not bring to an earlier time their mean reionization. The only significant trend is found between the total energy at z  = 0 of the pairs and their mean reionization time: Pairs with the smallest total energy (bound) are reionized up to 50 Myr earlier than others (unbound). Above all, this study reveals the variety of reionization histories undergone by halo pairs similar to the Local Group, that of the Local Group being far from an average one. In our model, its reionization time is ∼625 Myr against 660 ± 4 Myr (z ∼ 8.25 against 7.87 ± 0.02) on average. [ABSTRACT FROM AUTHOR]

We present an analysis of the evolution of the Lyman-series forest into the epoch of reionization using cosmological radiative transfer simulations in a scenario where reionization ends late. We explore models with different mid-points of reionization and gas temperatures. We find that once the simulations have been calibrated to match the mean flux of the observed Lyman- α (Ly  α) forest at 4 < z < 6, they also naturally reproduce the distribution of effective optical depths of the Lyman- β (Ly  β) forest in this redshift range. We note that the tail of the largest optical depths that is most challenging to match corresponds to the long absorption trough of ULAS J0148+0600, which we have previously shown to be rare in our simulations. We consider the evolution of the Lyman-series forest out to higher redshifts, and show that future observations of the Ly  β forest at z > 6 will discriminate between different reionization histories. The evolution of the Ly  α and Ly  γ forests are less promising as a tool for pushing studies of reionization to higher redshifts due to the stronger saturation and foreground contamination, respectively. [ABSTRACT FROM AUTHOR]

Cosmic Dawn II (CoDa II) is a new, fully coupled radiation-hydrodynamics simulation of cosmic reionization and galaxy formation and their mutual impact, to redshift z < 6. With 40963 particles and cells in a 94 Mpc box, it is large enough to model global reionization and its feedback on galaxy formation while resolving all haloes above 108 M⊙. Using the same hybrid CPU–GPU code RAMSES–CUDATON as CoDa I in Ocvirk et al. (2016), CoDa II modified and re-calibrated the subgrid star formation algorithm, making reionization end earlier, at z ≳ 6, thereby better matching the observations of intergalactic Lyman α opacity from quasar spectra and electron-scattering optical depth from cosmic microwave background fluctuations. CoDa II predicts a UV continuum luminosity function in good agreement with observations of high- z galaxies, especially at z  = 6. As in CoDa I, reionization feedback suppresses star formation in haloes below ∼2 × 109 M⊙, though suppression here is less severe, a possible consequence of modifying the star formation algorithm. Suppression is environment dependent, occurring earlier (later) in overdense (underdense) regions, in response to their local reionization times. Using a constrained realization of lambda cold dark matter constructed from galaxy survey data to reproduce the large-scale structure and major objects of the present-day Local Universe, CoDa II serves to model both global and local reionization. In CoDa II, the Milky Way and M31 appear as individual islands of reionization, i.e. they were not reionized by the progenitor of the Virgo cluster, or by nearby groups, or by each other. [ABSTRACT FROM AUTHOR]

A long (110 cMpc  h−1) and deep absorption trough in the Ly α forest has been observed extending down to redshift 5.5 in the spectrum of ULAS J0148+0600. Although no Ly α transmission is detected, Ly β spikes are present which has led to claims that the gas along this trough must be ionized. Using high-resolution cosmological radiative transfer simulations in large volumes, we show that in a scenario where reionization ends late (z ∼ 5.2), our simulations can reproduce troughs as long as observed. In this model, we find that the troughs are caused by islands of neutral hydrogen. Small ionized holes within the neutral islands allow for the transmission of Ly β. We have also modelled the Ly α emitter population around the simulated troughs, and show that there is a deficit of Ly α emitters close to the trough as is observed. [ABSTRACT FROM AUTHOR]

We present a deep learning model trained to emulate the radiative transfer during the epoch of cosmological reionization. CRADLE (Cosmological Reionization And Deep LEarning) is an auto-encoder convolutional neural network that uses 2D maps of the star number density and the gas density field at z  = 6 as inputs and that predicts 3D maps of the times of reionization t reion as outputs. These predicted single fields are sufficient to describe the global reionization history of the intergalactic medium in a given simulation. We trained the model on a given simulation and tested the predictions on another simulation with the same parameters but with different initial conditions. The model is successful at predicting t reion maps that are in good agreement with the test simulation. We used the power spectrum of the t reion field as an indicator to validate our model. We show that the network predicts large scales almost perfectly but is somewhat less accurate at smaller scales. While the current model is already well suited to get average estimates about the reionization history, we expect it can be further improved with larger samples for the training, better data pre-processing and finer tuning of hyper-parameters. Emulators of this kind could be systematically used to rapidly obtain the evolving H  ii regions associated with hydro-only simulations and could be seen as precursors of fully emulated physics solvers for future generations of simulations. [ABSTRACT FROM AUTHOR]

We study the inhomogeneity of the reionization process by comparing the reionization times of z = 0 galaxies as a function of their mass. For this purpose, we combine the results of the CODA-I AMR radiative hydrodynamics simulation of the Reionization with the halo merger trees of a pure dark matter tree-code z = 0 simulation evolved from the same set of initial conditions. We find that galaxies with M (z = 0) > 1011 M ⊙ are reionized earlier than the whole Universe, with e.g. MW-like haloes reionized between 100 and 300 million years before the diffuse IGM. Lighter galaxies reionized as late as the global volume, probably from external radiation. [ABSTRACT FROM AUTHOR]

Cosmic reionization by starlight from early galaxies affected their evolution, thereby impacting reionization itself. Star formation suppression, for example, may explain the observed underabundance of Local Group dwarfs relative to N-body predictions for cold dark matter. Reionization modelling requires simulating volumes large enough [~ (100 Mpc)³] to sample reionization 'patchiness', while resolving millions of galaxy sources above ~108 M⊚ combining gravitational and gas dynamics with radiative transfer. Modelling the Local Group requires initial cosmological density fluctuations pre-selected to form the well-known structures of the Local Universe today. Cosmic Dawn ('CoDa') is the first such fully coupled, radiation-hydrodynamics simulation of reionization of the Local Universe. Our new hybrid CPU-GPU code, RAMSES-CUDATON, performs hundreds of radiative transfer and ionization ratesolver timesteps on the GPUs for each hydro-gravity timestep on the CPUs. CoDa simulated (91Mpc)³ with 4096³ particles and cells, to redshift 4.23, on ORNL supercomputer Titan, utilizing 8192 cores and 8192 GPUs. Global reionization ended slightly later than observed. However, a simple temporal rescaling which brings the evolution of ionized fraction into agreement with observations also reconciles ionizing flux density, cosmic star formation history, CMB electron scattering optical depth and galaxy UV luminosity function with their observed values. Photoionization heating suppressed the star formation of haloes below ~2 × 109 M⊚, decreasing the abundance of faint galaxies around MAB1600 = [-10, -12]. For most of reionization, star formation was dominated by haloes between 1010-1011 M⊚, so low-mass halo suppression was not reflected by a distinct feature in the global star formation history. Intergalactic filaments display sheathed structures, with hot envelopes surrounding cooler cores, but do not self-shield, unlike regions denser than 100 <ρ>. [ABSTRACT FROM AUTHOR]

EMMA is a cosmological simulation code aimed at investigating the reionization epoch. It handles simultaneously collisionless and gas dynamics, as well as radiative transfer physics using a moment-based description with the M1 approximation. Field quantities are stored and computed on an adaptive three-dimensional mesh and the spatial resolution can be dynamically modified based on physically motivated criteria. Physical processes can be coupled at all spatial and temporal scales. We also introduce a new and optional approximation to handle radiation: the light is transported at the resolution of the non-refined grid and only once the dynamics has been fully updated, whereas thermo-chemical processes are still tracked on the refined elements. Such an approximation reduces the overheads induced by the treatment of radiation physics. A suite of standard tests are presented and passed by EMMA, providing a validation for its future use in studies of the reionization epoch. The code is parallel and is able to use graphics processing units (GPUs) to accelerate hydrodynamics and radiative transfer calculations. Depending on the optimizations and the compilers used to generate the CPU reference, global GPU acceleration factors between ×3.9 and ×16.9 can be obtained. Vectorization and transfer operations currently prevent better GPU performance and we expect that future optimizations and hardware evolution will lead to greater accelerations. [ABSTRACT FROM AUTHOR]

We calibrate here cosmological radiative transfer simulations with aton/ramses with a range of measurements of the Ly α opacity from Quasi-Stellar Objects (QSO) absorption spectra. We find the Ly α opacity to be very sensitive to the exact timing of hydrogen reionization. Models reproducing the measured evolution of the mean photoionization rate and average mean free path reach overlap at z ~ 7 and predict an accelerated evolution of the Ly α opacity at z > 6 consistent with the rapidly evolving luminosity function of Ly α emitters in this redshift range. Similar to 'optically thin' simulations our full radiative transfer simulations fail, however, to reproduce the high-opacity tail of the Ly α opacity PDF (probability distribution function) at z > 5. We argue that this is due to spatial UV fluctuations in the post-overlap phase of reionization on substantially larger scales than predicted by our source model, where the ionizing emissivity is dominated by large numbers of sub-L* galaxies. We further argue that this suggests a significant contribution to the ionizing UV background by much rarer bright sources at high redshift. [ABSTRACT FROM AUTHOR]

A radiative transfer scheme is presented, based on a moment description of the equation of radiative transfer and the so-called ‘M1 closure model’ for the Eddington tensor. This model features a strictly hyperbolic transport step for radiation: it has been implemented using standard Godunov-like techniques in a new code calledaton. Coupled to simple models of ionization chemistry and photoheating,aton is able to reproduce the results of other schemes on a various set of standard tests such as the expansion of a H ii region, the shielding of the radiation by dense clumps and cosmological ionization by multiple sources. Being simple yet robust, such a scheme is intended to be naturally and easily included in grid-based cosmological fluid solvers. [ABSTRACT FROM AUTHOR]

We present a numerical technique to compute the gravitational lensing induced by simulated haloes. It relies on a 2D-Tree domain decomposition in the lens plane combined with a description of N-body particles as extended clouds with a non-singular density. This technique is made fully adaptive by the use of a density-dependent smoothing which allows one to probe the lensing properties of haloes from the densest regions in the centre or in substructures to the low-density regions in the outskirts. ‘Smooth Particle Lensing’ (SPL) has some promising features. First, the deflection potential, the deflection angles, the convergence and the shear are direct and separate end-products of the SPL calculation and can be computed at an arbitrary distribution of points on the lens plane. Secondly, this flexibility avoids the use of interpolation or a finite differentiation procedure on a grid, does not require padding the region with zeros and focuses the computing power on relevant regions. The SPL algorithm is tested by populating isothermal spheres and ellipsoids with particles and then comparing the lensing calculations to the classical fast Fourier transform based technique and analytic solutions. We assess issues related to the resolution of the lensing code and the limitations set by the simulations themselves. We conclude by discussing how SPL can be used to predict the impact of substructures on strong lensing and how it can be generalized to weak-lensing and cosmic shear simulations. [ABSTRACT FROM AUTHOR]