We present a new implementation of radiation hydrodynamics (RHD) in the adaptive mesh refinement (AMR) code RAMSES. The multi-group radiative transfer (RT) is performed on the AMR grid with a first-order Godunov method using the M1 closure for the Eddington tensor, and is coupled to the hydrodynamics via non-equilibrium thermochemistry of hydrogen and helium. This moment-based approach has the large advantage that the computational cost is independent of the number of radiative sources -it can even deal with continuous regions of emission such as bound-free emission from gas. As it is built directly into RAMSES, the RT takes natural advantage of the refinement and parallelization strategies already in place. Since we use an explicit advection solver for the radiative transport, the time step is restricted by the speed of light -a severe limitation that can be alleviated using the so-called "reduced speed of light" approximation. We propose a rigorous framework to assess the validity of this approximation in various conditions encountered in cosmology and galaxy formation. We finally perform with our newly developed code a complete suite of RHD tests, comparing our results to other RHD codes. The tests demonstrate that our code performs very well and is ideally suited for exploring the effect of radiation on current scenarios of structure and galaxy formation.It is highly desirable to follow self-consistently, with RHD simulations, the time-evolution and morphology of large-scale intergalactic medium (IGM) reionization and at the same time the smaller scale formation of the presumed sources of reionization; how galaxy formation is regulated by the ionizing radiation being released, how much of the radiation escapes from the galaxies to ionize the IGM, how first generation stars are formed in a metal-free environment and how radiative and supernovae feedback from those stars affect the inter-galactic medium. The galaxies and the IGM are indeed inter-connected via the ionizing radiation: the photons released from the galaxies affect the state of the surrounding gas via ionization and heating and may even prevent it from falling in or condensing into external gravitational potentials, especially small ones (e.g. Wise & Abel 2008; Ocvirk & Aubert 2011), which can then in turn significantly alter the ionization history. c 0000 RAS arXiv:1304.7126v2 [astro-ph.CO]

High-redshift quasi-stellar object (QSO) spectra show large spatial fluctuations in the Ly α opacity of the intergalactic medium on surprisingly large scales at $z$ ≳ 5.5. We present a radiative transfer simulation of cosmic reionization driven by galaxies that reproduces this large scatter and the rapid evolution of the Ly α opacity distribution at 5 < $z$ < 6. The simulation also reproduces the low Thomson scattering optical depth reported by the latest cosmic microwave background (CMB) measurement and is consistent with the observed short near-zones and strong red damping wings in the highest redshift QSOs. It also matches the rapid disappearance of observed Ly α emission by galaxies at $z$ ≳ 6. Reionization is complete at $z$ = 5.3 in our model, and 50 per cent of the volume of the Universe is ionized at $z$ = 7. Agreement with the Ly α forest data in such a late reionization model requires a rapid evolution of the ionizing emissivity of galaxies that peaks at $z$ ∼ 6.8. The late end of reionization results in a large scatter in the photoionization rate and the neutral hydrogen fraction at redshifts as low as $z$ ≲ 5.5 with large residual neutral ‘islands’ that can produce very long Gunn–Peterson troughs resembling those seen in the data.

We measure the anisotropy of dark matter flows on small scales (∼500 kpc) in the near environment of haloes using a large set of simulations. We rely on two different approaches to quantify the anisotropy of the cosmic infall: we measure the flows at the virial radius of the haloes while describing the infalling matter via fluxes through a spherical shell; and we measure the spatial and kinematical distributions of satellites and substructures around haloes detected by the subclump finder adaptahop described for the first time in the appendix. The two methods are found to be in agreement both qualitatively and quantitatively via one‐ and two‐point statistics. The peripheral and advected momenta are correlated with the spin of the embedded halo at levels of 30 and 50 per cent. The infall takes place preferentially in the plane perpendicular to the direction defined by the spin of the halo. We computed the excess of equatorial accretion both through rings and via a harmonic expansion of the infall. The level of anisotropy of infalling matter is found to be ∼15 per cent. The substructures have their spin orthogonal to their velocity vector in the rest frame of the halo at a level of about 5 per cent, suggestive of an image of a flow along filamentary structures, which provides an explanation for the measured anisotropy. Using a ‘synthetic’ stacked halo, it is shown that the positions and orientations of satellites relative to the direction of spin of the halo are not random even in projection. The average ellipticity of stacked haloes is 10 per cent, while the alignment excess in projection reaches 2 per cent. All measured correlations are fitted by a simple three‐parameter model. We conclude that a halo does not see its environment as an isotropic perturbation, we investigate how the anisotropy is propagated inwards using perturbation theory, and we discuss briefly the implications for weak lensing, warps and the thickness of galactic discs.

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 ∼ 10 8 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 4096 3 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 × 10 9 M , decreasing the abundance of faint galaxies around M AB1600 = [−10, −12]. For most of reionization, star formation was dominated by haloes between 10 10 − 10 11 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 .