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.

We study the baryonic Tully-Fisher relation (BTFR) at z 0 using 153 galaxies from the SPARC sample. We consider different definitions of the characteristic velocity from H I and Hα rotation curves, as well as H I line-widths from single-dish observations. We reach the following results: (1) The tightest BTFR is given by the mean velocity along the flat part of the rotation curve. The orthogonal intrinsic scatter is extremely small (∼6%) and the best-fit slope is 3.85 ± 0.09, but systematic uncertainties may drive the slope from 3.5 to 4.0. Other velocity definitions lead to BTFRs with systematically higher scatters and shallower slopes. (2) We provide statistical relations to infer the flat rotation velocity from H I line-widths or less extended rotation curves (like Hα and CO data). These can be useful to study the BTFR from large H I surveys or the BTFR at high redshifts.(3) The BTFR is more fundamental than the relation between angular momentum and galaxy mass (the Fall relation). The Fall relation has about 7 times more scatter than the BTFR, which is merely driven by the scatter in the mass-size relation of galaxies. The BTFR is already the "fundamental plane" of galaxy discs: no value is added with a radial variable as a third parameter.

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.

We study how runaway stellar collisions in high-redshift, metal-poor star clusters form very massive stars (VMSs) that can directly collapse to intermediate-mass black holes (IMBHs). We follow the evolution of a pair of neighbouring high-redshift minihaloes with high-resolution, cosmological hydrodynamical zoom-in simulations using the adaptive mesh refinement code RAMSES combined with the non-equilibrium chemistry package KROME. The first collapsing mini-halo is assumed to enrich the central nuclear star cluster (NSC) of the other to a critical metallicity, sufficient for Population II (Pop. II) star formation at redshift z ≈ 27. Using the spatial configuration of the flattened, asymmetrical gas cloud forming in the core of the metal enriched halo, we set the initial conditions for simulations of an initially non-spherical star cluster with the direct summation code NBODY6 which are compared to about 2000 NBODY6 simulations of spherical star clusters for a wide range of star cluster parameters. The final mass of the VMS that forms depends strongly on the initial mass and initial central density of the NSC. For the initial central densities suggested by our RAMSES simulations, VMSs with mass > 400 M ⊙ can form in clusters with stellar masses of ≈ 10 4 M ⊙ , and this can increase to well over 1000 M ⊙ for more massive and denser clusters. The high probability we find for forming a VMS in these mini-haloes at such an early cosmic time makes collisional runaway of Pop. II star clusters a promising channel for producing large numbers of high-redshift IMBHs that may act as the seeds of supermassive black holes.

Infrared and nebular lines provide some of our best probes of the physics regulating the properties of the interstellar medium (ISM) at high-redshift. However, interpreting the physical conditions of high-redshift galaxies directly from emission lines remains complicated due to inhomogeneities in temperature, density, metallicity, ionisation parameter, and spectral hardness. We present a new suite of cosmological, radiation-hydrodynamics simulations, each centred on a massive Lyman-break galaxy that resolves such properties in an inhomogeneous ISM. Many of the simulated systems exhibit transient but well defined gaseous disks that appear as velocity gradients in [CII] 157.6µm emission. Spatial and spectral offsets between [CII] 157.6µm and [OIII] 88.33µm are common, but not ubiquitous, as each line probes a different phase of the ISM. These systems fall on the local [CII]-SFR relation, consistent with newer observations that question previously observed [CII] 157.6µm deficits. Our galaxies are consistent with the nebular line properties of observed z ∼ 2 − 3 galaxies and reproduce offsets on the BPT and mass-excitation diagrams compared to local galaxies due to higher star formation rate (SFR), excitation, and specific-SFR, as well as harder spectra from young, metal-poor binaries. We predict that local calibrations between Hα and [OII] 3727Å luminosity and galaxy SFR apply up to z > 10, as do the local relations between certain strong line diagnostics (R23 and [OIII] 5007Å/Hβ) and galaxy metallicity. Our new simulations are well suited to interpret the observations of line emission from current (ALMA and HST) and upcoming facilities (JWST and ngVLA).