A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2 < z < 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 × 10 10 M ⊙ . Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.

The fraction of hydrogen ionizing photons escaping from galaxies into the intergalactic medium is a critical ingredient in the theory of reionization. We use two zoomed-in, high-resolution (4 pc), cosmological radiation hydrodynamic simulations with adaptive mesh refinement to investigate the impact of two physical mechanisms (supernova feedback and runaway OB stars) on the escape fraction ( f esc ) at the epoch of reionization (z ≥ 7). We implement a new, physically motivated supernova feedback model that can approximate the Sedov solutions at all (from the free expansion to snowplow) stages. We find that there is a significant time delay of about ten million years between the peak of star formation and that of escape fraction, due to the time required for the build-up and subsequent destruction of the star-forming cloud by supernova feedback. Consequently, the photon number-weighted mean escape fraction for dwarf galaxies in halos of mass 10 8 − 10 10.5 M ⊙ is found to be f esc ∼ 11%, although instantaneous values of f esc > 20% are common when star formation is strongly modulated by the supernova explosions. We find that the inclusion of runaway OB stars increases the mean escape fraction by 22% to f esc ∼ 14%. As supernovae resulting from runaway OB stars tend to occur in less dense environments, the feedback effect is enhanced and star formation is further suppressed in halos with M vir > ∼ 10 9 M ⊙ in the simulation with runaway OB stars compared with the model without them. While both our models produce enough ionizing photons to maintain a fully ionized universe at z ≤ 7 as observed, a still higher amount of ionizing photons at z ≥ 9 appears necessary to accommodate the high observed electron optical depth inferred from cosmic microwave background observations.

We present the SPHINX suite of cosmological adaptive mesh refinement simulations, the first radiation-hydrodynamical simulations to simultaneously capture large-scale reionization and the escape of ionizing radiation from thousands of resolved galaxies. Our 5 and 10 co-moving Mpc volumes resolve haloes down to the atomic cooling limit and model the inter-stellar medium with better than ≈ 10 pc resolution. The project has numerous goals in improving our understanding of reionization and making predictions for future observations. In this first paper we study how the inclusion of binary stars in computing stellar luminosities impacts reionization, compared to a model that includes only single stars. Owing to the suppression of galaxy growth via strong feedback, our galaxies are in good agreement with observational estimates of the galaxy luminosity function. We find that binaries have a significant impact on the timing of reionization: with binaries, our boxes are 99.9 percent ionized by volume at z ≈ 7, while without them our volumes fail to reionize by z = 6. These results are robust to changes in volume size, resolution, and feedback efficiency. The escape of ionizing radiation from individual galaxies varies strongly and frequently. On average, binaries lead to escape fractions of ≈ 7 − 10 percent, about 3 times higher than with single stars only. The higher escape fraction is a result of a shallower decline in ionizing luminosity with age, and is the primary reason for earlier reionization, although the higher integrated luminosity with binaries also plays a sub-dominant role.

State‐of‐the‐art hydrodynamical simulations show that gas inflow through the virial sphere of dark matter haloes is focused (i.e. has a preferred inflow direction), consistent (i.e. its orientation is steady in time) and amplified (i.e. the amplitude of its advected specific angular momentum increases with time). We explain this to be a consequence of the dynamics of the cosmic web within the neighbourhood of the halo, which produces steady, angular momentum rich, filamentary inflow of cold gas. On large scales, the dynamics within neighbouring patches drives matter out of the surrounding voids, into walls and filaments before it finally gets accreted on to virialized dark matter haloes. As these walls/filaments constitute the boundaries of asymmetric voids, they acquire a net transverse motion, which explains the angular momentum rich nature of the later infall which comes from further away. We conjecture that this large‐scale driven consistency explains why cold flows are so efficient at building up high‐redshift thin discs inside out.

Reionisation in the early Universe is likely driven by dwarf galaxies. Using cosmological radiation-hydrodynamic simulations, we study star formation and the escape of Lyman continuum (LyC) photons from mini-haloes with M halo < ∼ 10 8 M . Our simulations include a new thermo-turbulent star formation model, non-equilibrium chemistry, and relevant stellar feedback processes (photoionisation by young massive stars, radiation pressure, and mechanical supernova explosions). We find that feedback reduces star formation very efficiently in minihaloes, resulting in the stellar mass consistent with the slope and normalisation reported in Kimm & Cen (2014) and the empirical stellar mass-to-halo mass relation derived in the local Universe. Because star formation is stochastic and dominated by a few gas clumps, the escape fraction in mini-haloes is generally determined by radiation feedback (heating due to photoionisation), rather than supernova explosions. We also find that the photon number-weighted mean escape fraction in mini-haloes is higher (∼ 20-40%) than that in atomic-cooling haloes, although the instantaneous fraction in individual haloes varies significantly. The escape fraction from Pop III stars is found to be significant ( > ∼ 10%) only when the mass is greater than ∼100 M . Based on simple analytic calculations, we show that LyC photons from mini-haloes are, despite their high escape fractions, of minor importance for reionisation due to inefficient star formation. We confirm previous claims that stars in atomic-cooling haloes with masses 10 8 M < ∼ M halo < ∼ 10 11 M are likely to be the most important source of reionisation.