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The detection of Ly$\alpha$ nebulae around $z\gtrsim 6$ quasars provides evidence for extended gas reservoirs around the first rapidly growing supermassive black holes. Observations of $z > 6$ quasars can be explained by cosmological models provided that the black holes by which they are powered evolve in rare, massive dark matter haloes. Whether these theoretical models also explain the observed extended Ly$\alpha$ emission remains an open question. We post-process a suite of cosmological, radiation-hydrodynamic simulations targeting a quasar host halo at $z>6$ with the Ly$\alpha$ radiative transfer code RASCAS. A combination of recombination radiation from photo-ionised hydrogen and emission from collisionally excited gas powers Ly$\alpha$ nebulae with a surface brightness profile in close agreement with observations. We also find that, even on its own, resonant scattering of the Ly$\alpha$ line associated to the quasar's broad line region can also generate Ly$\alpha$ emission on $\sim 100 \, \rm kpc$ scales, resulting in comparable agreement with observed surface brightness profiles. Even if powered by a broad quasar Ly$\alpha$ line, Ly$\alpha$ nebulae can have narrow line-widths $\lesssim 1000 \, \rm km \, s^{-1}$, consistent with observational constraints. Even if there is no quasar, we find that halo gas cooling produces a faint, extended Ly$\alpha$ glow. However, to light-up extended Ly$\alpha$ nebulae with properties in line with observations, our simulations unambiguously require quasar-powered outflows to clear out the galactic nucleus and allow the Ly$\alpha$ flux to escape and still remain resonant with halo gas. The close match between observations and simulations with quasar outflows suggests that AGN feedback already operates before $z \, = \, 6$ and confirms that high-$z$ quasars reside in massive haloes tracing overdensities., Comment: Submitted to MNRAS (21+3 pages, 11+3 figures)

Cosmic rays (CRs) are thought to play an important role in galaxy evolution. We study their effect when coupled to other important sources of feedback, namely supernovae (SNe) and stellar radiation, by including CR anisotropic diffusion and radiative losses but neglecting CR streaming. Using the ramses-rt code, we perform the first radiation-magnetohydrodynamics simulations of isolated disc galaxies with and without CRs. We study galaxies embedded in dark matter haloes of 1010, 1011, and $10^{12}\, \rm M_{\odot }$ with a maximum resolution of $9 \, \rm pc$. We find that CRs reduce the star formation (SF) rate in our two dwarf galaxies by a factor of 2, with decreasing efficiency with increasing galaxy mass. They increase significantly the outflow mass loading factor in all our galaxies and make the outflows colder. We study the impact of the CR diffusion coefficient, exploring values from κ = 1027 to $\rm 3\times 10^{29}\, cm^2\, s^{-1}$. With a lower κ, CRs remain confined for longer on small scales and are consequently efficient in suppressing SF, whereas a higher diffusion coefficient reduces the effect on SF and increases the generation of cold outflows. Finally, we compare CR feedback to a calibrated ’strong’ SN feedback model known to sufficiently regulate SF in high-redshift cosmological simulations. We find that CR feedback is not sufficiently strong to replace this strong SN feedback. As they tend to smooth out the ISM and fill it with denser gas, CRs also lower the escape fraction of Lyman continuum photons from galaxies.

The Ly$\alpha$ line is a powerful probe of distant galaxies, which contains information about inflowing/outflowing gas through which Ly$\alpha$ photons scatter. To develop our understanding of this probe, we post-process a zoom-in radiation-hydrodynamics simulation of a low-mass ($M_* \sim 10^9 M_\odot$) galaxy to construct 22500 mock spectra in 300 directions from $z = 3$ to 4. Remarkably, we show that one galaxy can reproduce the variety of a large sample of spectroscopically observed Ly$\alpha$ line profiles. While most mock spectra exhibit double-peak profiles with a dominant red peak, their shapes cover a large parameter space in terms of peak velocities, peak separation and flux ratio. This diversity originates from radiative transfer effects at ISM and CGM scales, and depends on galaxy inclination and evolutionary phase. Red-dominated lines preferentially arise in face-on directions during post-starburst outflows and are bright. Conversely, accretion phases usually yield symmetric double peaks in the edge-on direction and are fainter. While resonant scattering effects at $< 0.2\times R_{\rm vir}$ are responsible for the broadening and velocity shift of the red peak, the extended CGM acts as a screen and impacts the observed peak separation. The ability of simulations to reproduce observed Ly$\alpha$ profiles and link their properties with galaxy physical parameters offers new perspectives to use Ly$\alpha$ to constrain the mechanisms that regulate galaxy formation and evolution. Notably, our study implies that deeper Ly$\alpha$ surveys may unveil a new population of blue-dominated lines tracing inflowing gas., Comment: Accepted for publication in MNRAS

ALMA observations of z > 6 galaxies hav e rev ealed abnormally high [O III ] 88 μm /[C II ] 158 μm ratios and [C II ] 158 μm deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionization parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high- redshift phenomenon, we employ SPHINX 20 , a state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization which has been post-processed with CLOUDY to predict emission lines. We find that the observed z > 6 [O III ] 88 μm -SFR and [C II ] 158 μm -SFR relations can only be reproduced when the C/O abundance ratio is ∼8 ×lower than Solar and the total metal production is ∼4 ×higher than that of a Kroupa IMF. This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core-collapse supernovae with a more top-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [C II ] 158 μm -SFR and [C II ] 158 μm luminosity functions are predicted to converge to observed values at z ∼4.5. While we demonstrate that ionization parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [O III ] 88 μm /[C II ] 158 μm , observed values at z > 6 can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [C II ] 158 μm and [O III ] 88 μm can be used to predict the values of ionization parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed z > 6 galaxies. Finally, we demonstrate that [O I ] 63 μm can be used as a replacement for [C II ] 158 μm in high-redshift galaxies where [C II ] 158 μm is unobserved and argue that more observation time should be used to target [O I ] 63 μm at z > 6. Future simulations will be needed to self-consistently address the numerous uncertainties surrounding a varying IMF at high redshift and the associated metal returns.

The Ly$\alpha$ emission line is one of the most promising probes of cosmic reionisation but isolating the signature of a change in the ionisation state of the IGM is challenging because of intrinsic evolution and internal radiation transfer effects. We present the first study of the evolution of Ly$\alpha$ emitters (LAE) during the epoch of reionisation based on a full radiation-hydrodynamics cosmological simulation that is able to capture both the large-scale process of reionisation and the small-scale properties of galaxies. We predict the Ly$\alpha$ emission of galaxies in the $10^3$ cMpc$^3$ SPHINX simulation at $6\leq z\leq9$ by computing the full Ly$\alpha$ radiation transfer from ISM to IGM scales. SPHINX is able to reproduce many observational constraints such as the UV/Ly$\alpha$ luminosity functions and stellar mass functions at z $\geq$ 6 for the dynamical range probed by our simulation ($M_{\rm 1500}\gtrsim-18$, $L_{\rm Ly\alpha}\lesssim10^{42}$ erg/s, $M_{\star}\lesssim10^9$ M$_{\odot}$). As intrinsic Ly$\alpha$ emission and internal Ly$\alpha$ escape fractions barely evolve from $z=6$ to 9, the observed suppression of Ly$\alpha$ luminosities with increasing redshift is fully attributed to IGM absorption. For most observable galaxies ($M_{\rm 1500}\lesssim-16$), the Ly$\alpha$ line profiles are slightly shifted to the red due to internal radiative transfer effects which mitigates the effect of IGM absorption. Overall, the enhanced Ly$\alpha$ suppression during reionisation traces the IGM neutral fraction $x_{\rm HI}$ well but the predicted amplitude of this reduction is a strong function of the Ly$\alpha$ peak shift, which is set at ISM/CGM scales. We find that a large number of LAEs could be detectable in very deep surveys during reionisation when $x_{\rm HI}$ is still $\approx 50\%$., Comment: MNRAS accepted, 20 pages, 19 figures, plus appendices

Limited constraints on the evolution of the Lyman Continuum (LyC) escape fraction represent one of the primary uncertainties in the theoretical determination of the reionization history. Due to the intervening intergalactic medium (IGM), the possibility of observing LyC photons directly in the epoch of reionization is highly unlikely. For this reason, multiple indirect probes of LyC escape have been identified, some of which are used to identify low-redshift LyC leakers (e.g. O32), while others are primarily useful at $z>6$ (e.g. [OIII]/[CII] far infrared emission). The flux ratio of the resonant MgII doublet emission at 2796$\dot{\rm A}$ and 2803$\dot{\rm A}$ as well as the MgII optical depth have recently been proposed as ideal diagnostics of LyC leakage that can be employed at $z>6$ with JWST. Using state-of-the-art cosmological radiation hydrodynamics simulations post-processed with CLOUDY and resonant-line radiative transfer, we test whether MgII is indeed a useful probe of LyC leakage. Our simulations indicate that the majority of bright, star-forming galaxies with high LyC escape fractions are expected to be MgII emitters rather than absorbers at $z=6$. However, we find that the MgII doublet flux ratio is a more sensitive indicator of dust rather than neutral hydrogen, limiting its use as a LyC leakage indicator to only galaxies in the optically thin regime. Given its resonant nature, we show that MgII will be an exciting probe of the complex kinematics in high-redshift galaxies in upcoming JWST observations., 25 Pages, 20 Figures, MNRAS Accepted

We measure escape fractions, $f_{\rm esc}$, of ionizing radiation from galaxies in the SPHINX suite of cosmological radiation-hydrodynamical simulations of reionization, resolving halos with $M_{\rm vir} \gtrapprox 7.5 \times 10^7 \ M_{\odot}$ with a minimum cell width of $\approx 10$ pc. Our new and largest $20$ co-moving Mpc wide volume contains tens of thousands of star-forming galaxies with halo masses up to a few times $10^{11} \ M_{\odot}$. The simulated galaxies agree well with observational constraints of the UV luminosity function in the Epoch of Reionization. The escape fraction fluctuates strongly in individual galaxies over timescales of a few Myrs, due to its regulation by supernova and radiation feedback, and at any given time a tiny fraction of star-forming galaxies emits a large fraction of the ionizing radiation escaping into the inter-galactic medium. Statistically, $f_{\rm esc}$ peaks in intermediate-mass, intermediate-brightness, and low-metallicity galaxies ($M_{*} \approx 10^7 \ M_{\odot}$, $M_{1500} \approx -17$, $Z\lesssim 5 \times 10^{-3} \ Z_{\odot}$), dropping strongly for lower and higher masses, brighter and dimmer galaxies, and more metal-rich galaxies. The escape fraction correlates positively with both the short-term and long-term specific star formation rate. According to SPHINX, galaxies too dim to be yet observed, with $M_{1500} \gtrapprox -17$, provide about $55$ percent of the photons contributing to reionization. The global averaged $f_{\rm esc}$ naturally decreases with decreasing redshift, as predicted by UV background models and low-redshift observations. This evolution is driven by decreasing specific star formation rates over cosmic time., Comment: 20 figures, 21 pages plus appendices. Accepted for publication in MNRAS

We use the SPHINX suite of high-resolution cosmological radiation hydrodynamics simulations to study how spatially and temporally inhomogeneous reionization impacts the baryonic content of dwarf galaxies and cosmic filaments. The SPHINX simulations simultaneously model an inhomogeneous reionization, follow the escape of ionising radiation from thousands of galaxies, and resolve haloes well below the atomic cooling threshold. This makes them an ideal tool for examining how reionization impacts star formation and the gas content of dwarf galaxies. We compare simulations with and without stellar radiation to isolate the effects of radiation feedback from that of supernova, cosmic expansion, and numerical resolution. We find that the gas content of cosmic filaments can be reduced by more than 80% following reionization. The gas inflow rates into haloes with $M_{vir}3$ at fixed mass. Finally, although the gas content of high-redshift dwarf galaxies is significantly impacted by reionization, we find that most haloes with $M_{vir}, 21 pages, 11 figures, MNRAS Accepted

Understanding the escape of Lyman continuum (LyC) and Lyman $\alpha$ (Ly$\alpha$) photons from giant molecular clouds (GMCs) is crucial if we are to study the reionization of the Universe and to interpret spectra of observed galaxies at high redshift. To this end, we perform high-resolution, radiation-magneto-hydrodynamic simulations of GMCs with self-consistent star formation and stellar feedback. We find that a significant fraction (15-70%) of ionizing radiation escapes from the simulated GMCs with different masses ($10^5$ and $10^6\,M_\odot$), as the clouds are dispersed within about $2$-$5\,{\rm Myr}$ from the onset of star formation. The fraction of LyC photons leaked is larger when the GMCs are less massive, metal-poor, less turbulent, and less dense. The most efficient leakage of LyC radiation occurs when the total star formation efficiency of a GMC is about 20%. The escape of Ly$\alpha$ shows a trend similar to that of LyC photons, except that the fraction of Ly$\alpha$ photons escaping from the GMCs is larger ($f_{\rm esc}^{\rm Ly\alpha}\approx f_{900}^{0.27}$) and that a GMC with strong turbulence shows larger $f_{\rm esc}^{\rm Ly\alpha}$. The simulated GMCs show a characteristic velocity separation of $\Delta v\approx 120 \,{\rm km\,s^{-1}}$ in the time-averaged emergent Ly$\alpha$ spectra, suggesting that Ly$\alpha$ could be useful to infer the kinematics of the interstellar and circumgalactic medium. We show that Ly$\alpha$ luminosities are a useful indicator of the LyC escape, provided the number of LyC photons can be deduced through stellar population modeling. Finally, we find that the correlations between the escape fractions of Ly$\alpha$, ultraviolet photons at 1500A, and the Balmer $\alpha$ line are weak., Comment: 32 pages, 20 figures, Accepted for publication in the ApJS

We present the first results from SPHINX-MHD, a suite of cosmological radiation-magnetohydrodynamics simulations designed to study the impact of primordial magnetic fields (PMFs) on galaxy formation and the evolution of the intergalactic medium during the epoch of reionization. The simulations are among the first to employ multi-frequency, on-the-fly radiation transfer and constrained transport ideal MHD in a cosmological context to simultaneously model the inhomogeneous process of reionization as well as the growth of PMFs. We run a series of $(5\,\text{cMpc})^3$ cosmological volumes, varying both the strength of the seed magnetic field ($B_0$) and its spectral index ($n_B$). We find that PMFs that have $n_B > -0.562\log_{10}\left(\frac{B_0}{1{\rm n}G}\right) - 3.35$ produce electron optical depths ($\tau_e$) that are inconsistent with CMB constraints due to the unrealistically early collapse of low-mass dwarf galaxies. For $n_B\geq-2.9$, our constraints are considerably tighter than the $\sim{\rm n}G$ constraints from Planck. PMFs that do not satisfy our constraints have little impact on the reionization history or the shape of the UV luminosity function. Likewise, detecting changes in the Lya forest due to PMFs will be challenging because photoionisation and photoheating efficiently smooth the density field. However, we find that the first absorption feature in the global 21cm signal is a sensitive indicator of the properties of the PMFs, even for those that satisfy our $\tau_e$ constraint. Furthermore, strong PMFs can marginally increase the escape of LyC photons by up to 25\% and shrink the effective radii of galaxies by $\sim44\%$ which could increase the completeness fraction of galaxy surveys. Finally, our simulations show that surveys with a magnitude limit of ${\rm M_{UV,1500}=-13}$ can probe the sources that provide the majority of photons for reionization out to $z=12$., Comment: 29 pages, 27 figures, accepted MNRAS

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]~158.6$\mu$m emission. Spatial and spectral offsets between [CII]~158.6$\mu$m and [OIII]~88.33$\mu$m 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]~158.6$\mu$m deficits. Our galaxies are consistent with the nebular line properties of observed $z\sim2-3$ 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 H$\alpha$ and [OII]~3727$\AA$ luminosity and galaxy SFR apply up to $z>10$, as do the local relations between certain strong line diagnostics (R23 and [OIII]~5007$\AA$/H$\beta$) 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)., Comment: 21 pages, 16 figures, MNRAS accepted

Aims. The primary difficulty in understanding the sources and processes that powered cosmic reionization is that it is not possible to directly probe the ionizing Lyman-continuum (LyC) radiation at that epoch as those photons have been absorbed by the intervening neutral hydrogen. It is therefore imperative to build a model to accurately predict LyC emission using other properties of galaxies in the reionization era. Methods. In recent years, studies have shown that the LyC emission from galaxies may be correlated to their Lyman-alpha (Lyα) emission. In this paper we study this correlation by analyzing thousands of simulated galaxies at high redshift in the SPHINX cosmological simulation. We post-process these galaxies with the Lyα radiative transfer code RASCAS and analyze the Lyα – LyC connection. Results. We find that the Lyα and LyC luminosities are strongly correlated with each other, although with dispersion. There is a positive correlation between the escape fractions of Lyα and LyC radiations in the brightest Lyman-alpha emitters (LAEs; escaping Lyα luminosity LescLyα > 1041 erg s−1), similar to that reported by recent observational studies. However, when we also include fainter LAEs, the correlation disappears, which suggests that the observed relation may be driven by selection effects. We also find that the brighter LAEs are dominant contributors to reionization, with LescLyα > 1040 erg s−1 galaxies accounting for > 90% of the total amount of LyC radiation escaping into the intergalactic medium in the simulation. Finally, we build predictive models using multivariate linear regression, where we use the physical and Lyα properties of simulated reionization era galaxies to predict their LyC emission. We build a set of models using different sets of galaxy properties as input parameters and predict their intrinsic and escaping LyC luminosity with a high degree of accuracy (the adjusted R2 of these predictions in our fiducial model are 0.89 and 0.85, respectively, where R2 is a measure of how much of the response variance is explained by the model). We find that the most important galaxy properties for predicting the escaping LyC luminosity of a galaxy are its LescLyα, gas mass, gas metallicity, and star formation rate. Conclusions. These results and the predictive models can be useful for predicting the LyC emission from galaxies using their physical and Lyα properties and can thus help us identify the sources of reionization.

We investigate the effect of the Biermann battery during the Epoch of Reionization (EoR) using cosmological Adaptive Mesh Refinement simulations within the framework of the SPHINX project. We develop a novel numerical technique to solve for the Biermann battery term in the Constrained Transport method, preserving both the zero divergence of the magnetic field and the absence of Biermann battery for isothermal flows. The structure-preserving nature of our numerical method turns out to be very important to minimise numerical errors during validation tests of the propagation of a Str\"omgren sphere and of a Sedov blast wave. We then use this new method to model the evolution of a 2.5 and 5 co-moving Mpc cosmological box with a state-of-the-art galaxy formation model within the RAMSES code. Contrary to previous findings, we show that three different Biermann battery channels emerge: the first one is associated with linear perturbations before the EoR, the second one is the classical Biermann battery associated with reionization fronts during the EoR, and the third one is associated with strong, supernova-driven outflows. While the two former channels generate spontaneously volume-filling magnetic fields with a strength on the order or below $10^{-20}$ G, the latter, owing to the higher plasma temperature and a marginally-resolved turbulent dynamo, reaches a field strength as high as $10^{-18}$ G in the intergalactic medium around massive haloes., Comment: 14 pages, 18 figures, accepted in MNRAS

(Abridged) Context. Massive stars form in magnetized and turbulent environments, and are often located in stellar clusters. Their accretion mechanism, as well as the origin of their system's stellar multiplicity are poorly understood. Aims. We study the influence of both magnetic fields and turbulence on the accretion mechanism of massive protostars and their multiplicity. Methods. We present a series of four Radiation-MHD simulations of the collapse of a massive magnetized, turbulent core of 100 $M_\odot$ with the AMR code Ramses, including a hybrid radiative transfer method for stellar irradiation and ambipolar diffusion. We vary the Mach and Alfvenic Mach numbers to probe sub- and superalfvenic turbulence as well as sub- and supersonic turbulence regimes. Results. Subalfvenic turbulence leads to single stellar systems while superalfvenic turbulence leads to binary formation from disk fragmentation following spiral arm collision, with mass ratios of 1.1-1.6. In those runs, infalling gas reaches the individual disks via a transient circumbinary structure. Magnetically-regulated, thermally-dominated (plasma beta $\beta>1$), Keplerian disks form in all runs, with sizes 100-200 AU and masses 1-8 $M_\odot$. The disks around primary and secondary sink particles share similar properties. We observe higher accretion rates onto the secondary stars than onto their primary star companion. The primary disk orientation is found to be set by the initial angular momentum carried by turbulence. Conclusions. Small (300 AU) massive protostellar disks as those frequently observed nowadays can only be reproduced so far in the presence of (moderate) magnetic fields with ambipolar diffusion, even in a turbulent medium. The interplay between magnetic fields and turbulence sets the multiplicity of stellar clusters. A plasma beta $\beta>1$ is a good indicator of streamers and disks., Comment: Accepted in A&A. Main body: 20 pages, 17 figures

The neutral intergalactic medium above redshift 6 is opaque to ionizing radiation, therefore one needs indirect measurements of the escape fraction of ionizing photons from galaxies of this epoch. Low-ionization state absorption lines are a common feature in the spectrum of galaxies, showing a diversity of strengths and shapes. Since these lines indicate the presence of neutral gas in front of the stars, they have been proposed to carry information on the escape of ionizing radiation from galaxies. We study which processes are responsible for the shape of the absorption lines, to better understand their origin. We then explore whether the absorption lines can be used to predict the escape fractions. Using a radiation-hydrodynamical zoom-in simulation and the radiative transfer code RASCAS, we generate mock CII 1334 and LyB lines of a virtual galaxy at redshift 3 as seen from many directions of observation. We also compute the escape fraction of ionizing photons in those directions and look for correlations between the lines and the escape fractions. We find that the resulting mock absorption lines are comparable to observations and that the lines and the escape fractions vary strongly depending on the direction of observation. Gas velocity and dust always affect the absorption profile significantly. We find no strong correlations between observable LyB or CII 1334 and the escape fraction. After correcting the continuum for attenuation by dust to recover the intrinsic continuum, the residual flux of CII 1334 correlates well with the escape fraction for directions with a dust corrected residual flux larger than 30%. For other directions, the relations have a strong dispersion, and the residual flux overestimates the escape fraction for most cases. Concerning LyB, the residual flux after dust correction does not correlate with the escape fraction but can be used as a lower limit. (abridged), 25 pages, 22 figures, accepted for publication in Astronomy and Astrophysics. New title, shorter abstract, corrected typos and url for Krome website

The physical origin of low escape fractions of ionizing radiation derived from massive star-forming galaxies at $z\sim3$-$4$ is not well understood. We perform idealised disc galaxy simulations to understand how galactic properties such as metallicity and gas mass affect the escape of Lyman Continuum (LyC) photons using radiation-hydrodynamic simulations with strong stellar feedback. We find that the luminosity-weighted escape fraction from a metal-poor ($Z=0.002$) galaxy embedded in a halo of mass $M_h\simeq10^{11}\,M_\odot$ is $\left\simeq 10\,\%$. Roughly half of the LyC photons are absorbed within scales of 100 pc, and the other half is absorbed in the ISM ($\lesssim 2\, {\rm kpc}$). When the metallicity of the gas is increased to $Z=0.02$, the escape fraction is significantly reduced to $\left\simeq1\%$ because young stars are enshrouded by their birth clouds for a longer time. In contrast, increasing the gas mass by a factor of 5 leads to $\left\simeq 5\, \%$ because LyC photons are only moderately absorbed by the thicker disc. Our experiments suggest that high metallicity is likely more responsible for the low escape fractions observed in massive star-forming galaxies, supporting the scenario in which the escape fraction is decreasing with increasing halo mass. Finally, negligible correlation is observed between the escape fraction and surface density of star formation or galactic outflow rates., Comment: 19 pages, 13 figures, accepted for the publication of MNRAS

With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible to detect spatially extended Lyman alpha emission from individual faint (M_UV ~ -18) galaxies at redshifts, 3 < z < 6, tracing gas out to circum-galactic scales comparable to the dark matter halo virial radius. To explore the implications of such observations, we present a cosmological radiation hydrodynamics simulation of a single galaxy, chosen to be typical of the Lyman alpha-emitting galaxies detected by MUSE in deep fields. We use this simulation to study the origin and dynamics of the high-redshift circum-galactic medium (CGM). We find that the majority of the mass in the diffuse CGM is comprised of material infalling for the first time towards the halo center, but with the inner CGM also containing a comparable amount of mass that has moved past first-pericentric passage, and is in the process of settling into a rotationally supported configuration. Making the connection to Lyman alpha emission, we find that the observed extended surface brightness profile is due to a combination of three components: scattering of galactic Lyman alpha emission in the CGM, in-situ emission of CGM gas (mostly infalling), and Lyman alpha emission from small satellite galaxies. The weight of these contributions vary with distance from the galaxy such that (1) scattering dominates the inner regions (r < 7 kpc), at surface brightness larger than a few 10^-19 cgs, (2) all components contribute equally around r ~ 10 kpc (or SB ~10^-19), and (3) the contribution of small satellite galaxies takes over at large distances (or SB ~10^-20). Our simulation fails to reproduce the characteristic observed Lyman alpha spectral morphology that is red-shifted with respect to the systemic velocity, with the implication that the simulation is missing an important component of neutral outflowing gas., MNRAS accepted, 20 pages, 10 figures

We present the Obelisk project, a cosmological radiation-hydrodynamics simulation following the assembly and reionization of a protocluster progenitor during the first two billions of years from the big bang, down to $z = 3.5$. The simulation resolves haloes down to the atomic cooling limit, and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The Obelisk project is designed specifically to study the coevolution of high redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to high redshift protoclusters. We find that hydrogen reionization happens inside-out and is completed by $z \sim 6$ in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at $z \sim 4$., 24+2 pages, 18+1 figures, published in Astronomy & Astrophysics

We introduce the "Engineering Dwarfs at Galaxy formation's Edge" (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low mass halo ($M_{\rm halo}=10^{9}~M_\odot$), simulated to redshift $z=0$ at a mass and spatial resolution of $\sim 20~M_\odot$ and $\sim 3$ pc. We consider different star formation prescriptions, supernova feedback strengths and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm ($\sim 10^4$ K) even before cosmic reionisation. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, $V$-band magnitudes and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass-metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass-metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics., Comment: 17 pages, 6 figures, Minor changes to mach published MNRAS version

Winds from young massive stars contribute a large amount of energy to their host molecular clouds. This has consequences for the dynamics and observable structure of star-forming clouds. In this paper, we present radiative magnetohydrodynamic simulations of turbulent molecular clouds that form individual stars of 30, 60 and 120 solar masses emitting winds and ultraviolet radiation following realistic stellar evolution tracks. We find that winds contribute to the total radial momentum carried by the expanding nebula around the star at 10 % of the level of photoionisation feedback, and have only a small effect on the radial expansion of the nebula. Radiation pressure is largely negligible in the systems studied here. The 3D geometry and evolution of wind bubbles is highly aspherical and chaotic, characterised by fast-moving "chimneys" and thermally-driven "plumes". These plumes can sometimes become disconnected from the stellar source due to dense gas flows in the cloud. Our results compare favourably with the findings of relevant simulations, analytic models and observations in the literature while demonstrating the need for full 3D simulations including stellar winds. However, more targeted simulations are needed to better understand results from observational studies., Comment: 18 pages, 11 figures, accepted to MNRAS

We study how star formation is regulated in low-mass field dwarf galaxies ($10^5 \leq M_{\star} \leq 10^6 \, \text{M}_{\odot}$), using cosmological high-resolution ($3 \, \text{pc}$) hydrodynamical simulations. Cosmic reionization quenches star formation in all our simulated dwarfs, but three galaxies with final dynamical masses of $3 \times 10^{9} \,\text{M}_{\odot}$ are subsequently able to replenish their interstellar medium by slowly accreting gas. Two of these galaxies re-ignite and sustain star formation until the present day at an average rate of $10^{-5} \, \text{M}_{\odot} \, \text{yr}^{-1}$, highly reminiscent of observed low-mass star-forming dwarf irregulars such as Leo T. The resumption of star formation is delayed by several billion years due to residual feedback from stellar winds and Type Ia supernovae; even at $z=0$, the third galaxy remains in a temporary equilibrium with a large gas content but without any ongoing star formation. Using the "genetic modification'' approach, we create an alternative mass growth history for this gas-rich quiescent dwarf and show how a small $(0.2\,\mathrm{dex})$ increase in dynamical mass can overcome residual stellar feedback, re-igniting star formation. The interaction between feedback and mass build-up produces a diversity in the stellar ages and gas content of low-mass dwarfs, which will be probed by combining next-generation HI and imaging surveys., Comment: Minor revisions, matching published version in MNRAS. Results and conclusions unchanged

Frequency-dependent/hybrid approaches for stellar irradiation are of primary importance in numerical simulations of massive star formation. We seek to compare outflow and accretion mechanisms in star formation simulations. We investigate the accuracy of a hybrid radiative transfer method using the gray M1 closure relation for proto-stellar irradiation and gray flux-limited diffusion (FLD) for photons emitted everywhere else. We have coupled the FLD module of the adaptive-mesh refinement code Ramses with Ramses-RT, which is based on the M1 closure relation. Our hybrid (M1+FLD) method takes an average opacity at the stellar temperature for the M1 module, instead of the local environmental radiation field. We have tested this approach in radiative transfer tests of disks irradiated by a star for three levels of optical thickness and compared the temperature structure with RADMC-3D and MCFOST. We applied it to a radiation-hydrodynamical simulation of massive star formation. Our tests validate our hybrid approach for determining the temperature structure of an irradiated disk in the optically-thin and moderately optically-thick regimes and the most optically-thick test shows the limitation of our approach. The optically-thick setups highlight the ability of the hybrid method to partially capture the self-shielding in the disk while the FLD cannot. The radiative acceleration is 100 times greater with the hybrid method. It consistently leads to about +50% more extended and wider-angle radiative outflows in the massive star formation simulation. We obtain a $17.6 M_\odot$ at $t{\simeq}0.7 \tau_\mathrm{ff}$, while the accretion phase is ongoing. Finally, despite the use of refinement to resolve the radiative cavities, no Rayleigh-Taylor instability appears in our simulations, and we justify their absence by physical arguments based on the entropy gradient. (abridged), Comment: 17 pages, 17 figures, accepted for publication in A&A

We investigate the impact of ram pressure stripping due to the intracluster medium (ICM) on star-forming disk galaxies with a multi-phase interstellar medium (ISM) maintained by strong stellar feedback. We carry out radiation-hydrodynamics simulations of an isolated disk galaxy embedded in a 10^11 Msun dark matter halo with various ICM winds mimicking the cluster outskirts (moderate) and the central environment (strong). We find that both star formation quenching and triggering occur in ram pressure-stripped galaxies, depending on the strength of the winds. HI and H$_2$ in the outer galactic disk are significantly stripped in the presence of the moderate winds, whereas turbulent pressure provides support against ram pressure in the central region where star formation is active. Moderate ICM winds facilitate gas collapsing, increasing the total star formation rates by ~40% when the wind is oriented face-on or ~80% when it is edge-on. In contrast, strong winds rapidly blow away neutral and molecular hydrogen gas from the galaxy, suppressing the star formation by a factor of two within ~200 Myr. Dense gas clumps with N_H > 10 Msun pc^-2 are easily identified in extraplanar regions, but no significant young stellar populations are found in such clumps. In our attempts to enhance radiative cooling by adopting a colder ICM of T=10^6K, only a few additional stars are formed in the tail region, even if the amount of newly cooled gas increases by an order of magnitude., 26 pages, 17 figures, accepted for publication in ApJ

Molecular clouds are turbulent structures whose star formation efficiency (SFE) is strongly affected by internal stellar feedback processes. In this paper we determine how sensitive the SFE of molecular clouds is to randomised inputs in the star formation feedback loop, and to what extent relationships between emergent cloud properties and the SFE can be recovered. We introduce the yule suite of 26 radiative magnetohydrodynamic (RMHD) simulations of a 10,000 solar mass cloud similar to those in the solar neighbourhood. We use the same initial global properties in every simulation but vary the initial mass function (IMF) sampling and initial cloud velocity structure. The final SFE lies between 6 and 23 percent when either of these parameters are changed. We use Bayesian mixed-effects models to uncover trends in the SFE. The number of photons emitted early in the cluster's life and the length of the cloud provide are the strongest predictors of the SFE. The HII regions evolve following an analytic model of expansion into a roughly isothermal density field. The more efficient feedback is at evaporating the cloud, the less the star cluster is dispersed. We argue that this is because if the gas is evaporated slowly, the stars are dragged outwards towards surviving gas clumps due to the gravitational attraction between the stars and gas. While star formation and feedback efficiencies are dependent on nonlinear processes, statistical models describing cloud-scale processes can be constructed., 24 pages, 16 figures, 6 tables. Accepted to MNRAS, version updated with published title

We present cosmological radiation-hydrodynamic simulations, performed with the code Ramses-RT, of radiatively-driven outflows in a massive quasar host halo at $z = 6$. Our simulations include both single- and multi-scattered radiation pressure on dust from a quasar and are compared against simulations performed with thermal feedback. For radiation pressure-driving, we show that there is a critical quasar luminosity above which a galactic outflow is launched, set by the equilibrium of gravitational and radiation forces. While this critical luminosity is unrealistically high in the single-scattering limit for plausible black hole masses, it is in line with a $\approx 3 \times 10^9 \, \rm M_\odot$ black hole accreting at its Eddington limit, if infrared (IR) multi-scattering radiation pressure is included. The outflows are fast ($v \, \gtrsim \, 1000 \, \rm km \, s^{-1}$) and strongly mass-loaded with peak mass outflow rates $\approx 10^3 - 10^4 \, \rm M_\odot \, yr^{-1}$, but short-lived ($< 10 \, \rm Myr$). Outflowing material is multi-phase, though predominantly composed of cool gas, forming via a thermal instability in the shocked swept-up component. Radiation pressure- and thermally-driven outflows both affect their host galaxies significantly, but in different, complementary ways. Thermally-driven outflows couple more efficiently to diffuse halo gas, generating more powerful, hotter and more volume-filling outflows. IR radiation, through its ability to penetrate dense gas via diffusion, is more efficient at ejecting gas from the bulge. The combination of gas ejection through outflows with internal pressurisation by trapped IR radiation leads to a complete shut down of star formation in the bulge. We hence argue that radiation pressure-driven feedback may be an important ingredient in regulating star formation in compact starbursts, especially during the quasar's `obscured' phase., 34 pages (26 main paper + 8 pages of references and appendices). Accepted by MNRAS

Most stars in the Galaxy are believed to be formed within star clusters from collapsing molecular clouds. However, the complete process of star formation, from the parent cloud to a gas-free star cluster, is still poorly understood. We perform radiation-hydrodynamical simulations of the collapse of a turbulent molecular cloud using the RAMSES-RT code. Stars are modelled using sink particles, from which we self-consistently follow the propagation of the ionising radiation. We study how different feedback models affect the gas expulsion from the cloud and how they shape the final properties of the emerging star cluster. We find that the star formation efficiency is lower for stronger feedback models. Feedback also changes the high mass end of the stellar mass function. Stronger feedback also allows the establishment of a lower density star cluster, which can maintain a virial or sub-virial state. In the absence of feedback, the star formation efficiency is very high, as well as the final stellar density. As a result, high energy close encounters make the cluster evaporate quickly. Other indicators, such as mass segregation, statistics of multiple systems and escaping stars confirm this picture. Observations of young star clusters are in best agreement with our strong feedback simulation., Comment: 20 pages, 14 figures. Accepted by MNRAS

We present a multi-scale analysis of molecular hydrogen in a Milky Way-like simulated galaxy. Our census covers the gas content of the entire disc, to radial profiles and the Kennicutt-Schmidt relation, to a study of its molecular clouds, and finally down to a cell-by-cell analysis of the gas phases. Where observations are available we find agreement. A significant fraction of the H2 gas is in low-density regions mixed with atomic hydrogen and would therefore be difficult to observe. We use the molecular addition to ramses-rt, an adaptive mesh refinement grid code with the hydrodynamics coupled to moment-based radiative transfer. Three resolutions of the same galaxy detail the effects it has on H2 formation, with grid cells sized 97, 24, and 6.1 pc. Only the highest resolution yields gas densities high enough to host significant H2 fractions, and resolution is therefore key to simulating H2. Apart our pieces of galactic analysis are disparate, but assembled they provide a cohesive portrait of H2 in the interstellar medium. H2 chemistry on the atomic scale is sufficient to generate its dynamics throughout an entire galaxy.

Understanding the escape of Lyman continuum (LyC) and Lyman alpha (Lya) photons from molecular clouds is one of the keys to constraining the reionization history of the Universe and the evolution of galaxies at high redshift. Using a set of radiation-hydrodynamic simulations with adaptive mesh refinement, we investigate how photons propagate and escape from turbulent clouds with different masses, star formation efficiencies (SFEs), and metallicities, as well as with different models of stellar spectra and supernova feedback. We find that the escape fractions in both LyC and Lya are generally increasing with time if the cloud is efficiently dispersed by radiation and supernova feedback. When the total SFE is low (1% of the cloud mass), 0.1-5% of LyC photons leave the metal-poor cloud, whereas the fractions increase to 20-70% in clouds with a 10% SFE. LyC photons escape more efficiently if gas metallicity is lower, if the upper mass limit in the stellar initial mass function is higher, if binary interactions are allowed in the evolution of stars, or if additional strong radiation pressure, such as Lya pressure, is present. As a result, the number of escaping LyC photons can easily vary by a factor of $\sim4$ on cloud scales. The escape fractions of Lya photons are systemically higher (60-80%) than those of LyC photons despite large optical depths at line centre ($\tau_0\sim10^6-10^9$). Scattering of Lya photons is already significant on cloud scales, leading to double-peaked profiles with peak separations of $v_{\rm sep}\sim400\,{\rm km\,s^{-1}}$ during the initial stage of the cloud evolution, while it becomes narrower than $v_{\rm sep} \le 150 \, {\rm km\,s^{-1}}$ in the LyC bright phase. Comparisons with observations of low-redshift galaxies suggest that Lya photons require further interactions with neutral hydrogen to reproduce their velocity offset for a given LyC escape fraction., Comment: 23 pages, 20 figures, accepted for publication in MNRAS

We use the photon flux and absorption tracer algorithm presented in Katz et al.~2018, to characterise the contribution of haloes of different mass and stars of different age and metallicity to the reionization of the Universe. We employ a suite of cosmological multifrequency radiation hydrodynamics AMR simulations that are carefully calibrated to reproduce a realistic reionization history and galaxy properties at $z \geq 6$. In our simulations, haloes with mass $10^9{\rm M_{\odot}}h^{-1}, Comment: 13 pages, 8 figures, Accepted to MNRAS

We present a new, on-the-fly photon flux and absorption tracer algorithm designed to directly measure the contribution of different source populations to the metagalactic UV background and to the ionisation fraction of gas in the Universe. We use a suite of multifrequency radiation hydrodynamics simulations that are carefully calibrated to reproduce a realistic reionization history and galaxy properties at $z \ge 6$, to disentangle the contribution of photons emitted by different mass haloes and by stars with different metallicities and ages to the UV background during reionization. While at very early cosmic times low mass, metal poor haloes provide most of the LyC photons, their contribution decreases steadily with time. At $z = 6$ it is the photons emitted by massive systems (${\rm M_{halo}}/{\rm M_\odot} > 10^{10} \, {\rm h ^{-1}}$) and by the metal enriched stars ($10^{-3} < Z/Z_{\rm \odot} < 10^{-1.5}$) that provide the largest contribution to the ionising UV background. We demonstrate that there are large variations in the escape fraction depending on the source, with the escape fraction being highest ($\sim 45-60\%$) for photons emitted by the oldest stars that penetrate into the IGM via low opacity channels carved by the ionising photons and supernova from younger stars. Before HII regions begin to overlap, the photoionisation rate strongly fluctuates between different, isolated HII bubbles, depending on the embedded ionising source, which we suggest may result in spatial variations in the properties of dwarf galaxies., 19 pages, 14 figures, Accepted MNRAS

Understanding the origin of strong galactic outflows and the suppression of star formation in dwarf galaxies is a key problem in galaxy formation. Using a set of radiation-hydrodynamic simulations of an isolated dwarf galaxy embedded in a $10^{10}\,M_\odot$ halo, we show that the momentum transferred from resonantly scattered Lyman-$\alpha$ (Lya) photons is an important source of stellar feedback which can shape the evolution of galaxies. We find that Lya feedback suppresses star formation by a factor of two in metal-poor galaxies by regulating the dynamics of star-forming clouds before the onset of supernova explosions (SNe). This is possible because each Lya photon resonantly scatters and imparts 10-300 times greater momentum than in the single scattering limit. Consequently, the number of star clusters predicted in the simulations is reduced by a factor of $\sim 5$, compared to the model without the early feedback. More importantly, we find that galactic outflows become weaker in the presence of strong Lya radiation feedback, as star formation and associated SNe become less bursty. We also examine a model in which radiation field is arbitrarily enhanced by a factor of up to 10, and reach the same conclusion. The typical mass loading factors in our metal-poor dwarf system are estimated to be $\sim5-10$ near the mid plane, while it is reduced to $\sim1$ at larger radii. Finally, we find that the escape of ionizing radiation and hence the reionization history of the Universe is unlikely to be strongly affected by Lya feedback., Comment: 19 pages, 16 figures, Accepted for publication in MNRAS

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 $\approx10$ 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 $z\approx 7$, 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 $\approx 7-10$ percent, about $3.5$ 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., Comment: 18 pages, 15 figures. Accepted for publication in MNRAS

We analyse the properties of circumgalactic gas around simulated galaxies in the redshift range z >= 3, utilising a new sample of cosmological zoom simulations. These simulations are intended to be representative of the observed samples of Lyman-alpha emitters recently obtained with the MUSE instrument (halo masses ~10^10-10^11 solar masses). We show that supernova feedback has a significant impact on both the inflowing and outflowing circumgalactic medium by driving outflows, reducing diffuse inflow rates, and by increasing the neutral fraction of inflowing gas. By temporally stacking simulation outputs we find that significant net mass exchange occurs between inflowing and outflowing phases: none of the phases are mass-conserving. In particular, we find that the mass in neutral outflowing hydrogen declines exponentially with radius as gas flows outwards from the halo centre. This is likely caused by a combination of both fountain-like cycling processes and gradual photo/collisional ionization of outflowing gas. Our simulations do not predict the presence of fast-moving neutral outflows in the CGM. Neutral outflows instead move with modest radial velocities (~ 50 kms^-1), and the majority of the kinetic energy is associated with tangential rather than radial motion., 24 pages, 17 figures, submitted to MNRAS after responding to a first referee report

We introduce non-equilibrium molecular hydrogen chemistry into the radiation hydrodynamics code Ramses-RT. This is an adaptive mesh refinement grid code with radiation hydrodynamics that couples the thermal chemistry of hydrogen and helium to moment-based radiative transfer with the Eddington tensor closure model. The H2 physics that we include are formation on dust grains, gas phase formation, formation by three-body collisions, collisional destruction, photodissociation, photoionization, cosmic ray ionization, and self-shielding. In particular, we implement the first model for H2 self-shielding that is tied locally to moment-based radiative transfer by enhancing photodestruction. This self-shielding from Lyman-Werner line overlap is critical to H2 formation and gas cooling. We can now track the non-equilibrium evolution of molecular, atomic, and ionized hydrogen species with their corresponding dissociating and ionizing photon groups. Over a series of tests we show that our model works well compared to specialized photodissociation region codes. We successfully reproduce the transition depth between molecular and atomic hydrogen, molecular cooling of the gas, and a realistic Stromgren sphere embedded in a molecular medium. In this paper we focus on test cases to demonstrate the validity of our model on small scales. Our ultimate goal is to implement this in large-scale galactic simulations., Comment: 21 pages, 12 figures, printed in MNRAS

We present radiation-hydrodynamic simulations of radiatively-driven gas shells launched by bright active galactic nuclei (AGN) in isolated dark matter haloes. Our goals are (1) to investigate the ability of AGN radiation pressure on dust to launch galactic outflows and (2) to constrain the efficiency of infrared (IR) multi-scattering in boosting outflow acceleration. Our simulations are performed with the radiation-hydrodynamic code RAMSES-RT and include both single- and multi-scattered radiation pressure from an AGN, radiative cooling and self-gravity. Since outflowing shells always eventually become transparent to the incident radiation field, outflows that sweep up all intervening gas are likely to remain gravitationally bound to their halo even at high AGN luminosities. The expansion of outflowing shells is well described by simple analytic models as long as the shells are mildly optically thick to IR radiation. In this case, an enhancement in the acceleration of shells through IR multi-scattering occurs as predicted, i.e. a force dP/dt = tau_IR L/c is exerted on the gas. For high optical depths tau_IR > 50, however, momentum transfer between outflowing optically thick gas and IR radiation is rapidly suppressed, even if the radiation is efficiently confined. At high tau_IR, the characteristic flow time becomes shorter than the required trapping time of IR radiation such that the momentum flux dP/dt << tau_IR L/c. We argue that while unlikely to unbind massive galactic gaseous haloes, AGN radiation pressure on dust could play an important role in regulating star formation and black hole accretion in the nuclei of massive compact galaxies at high redshift., 25 pages, 12 figures, 3 tables (accepted by MNRAS)

Radiation feedback is typically implemented using subgrid recipes in hydrodynamical simulations of galaxies. Very little work has so far been performed using radiation-hydrodynamics (RHD), and there is no consensus on the importance of radiation feedback in galaxy evolution. We present RHD simulations of isolated galaxy disks of different masses with a resolution of 18 pc. Besides accounting for supernova feedback, our simulations are the first galaxy-scale simulations to include RHD treatments of photo-ionisation heating and radiation pressure, from both direct optical/UV radiation and multi-scattered, re-processed infrared (IR) radiation. Photo-heating smooths and thickens the disks and suppresses star formation about as much as the inclusion of ("thermal dump") supernova feedback does. These effects decrease with galaxy mass and are mainly due to the prevention of the formation of dense clouds, as opposed to their destruction. Radiation pressure, whether from direct or IR radiation, has little effect, but for the IR radiation we show that its impact is limited by our inability to resolve the high optical depths for which multi-scattering becomes important. While artificially boosting the IR optical depths does reduce the star formation, it does so by smoothing the gas rather than by generating stronger outflows. We conclude that although higher-resolution simulations, and potentially also different supernova implementations, are needed for confirmation, our findings suggest that radiation feedback is more gentle and less effective than is often assumed in subgrid prescriptions., Comment: 28 pages, 26 figures, accepted for publication in MNRAS. Revised to match published version

We compare 5 sub-grid models for supernova (SN) feedback in adaptive mesh refinement (AMR) simulations of isolated dwarf and L-star disk galaxies with 20-40 pc resolution. The models are thermal dump, stochastic thermal, 'mechanical' (injecting energy or momentum depending on the resolution), kinetic, and delayed cooling feedback. We focus on the ability of each model to suppress star formation and generate outflows. Our highest-resolution runs marginally resolve the adiabatic phase of the feedback events, which correspond to 40 SN explosions, and the first three models yield nearly identical results, possibly indicating that kinetic and delayed cooling feedback converge to wrong results. At lower resolution all models differ, with thermal dump feedback becoming inefficient. Thermal dump, stochastic, and mechanical feedback generate multiphase outflows with mass loading factors $\beta \ll 1$, which is much lower than observed. For the case of stochastic feedback we compare to published SPH simulations, and find much lower outflow rates. Kinetic feedback yields fast, hot outflows with $\beta\sim 1$, but only if the wind is in effect hydrodynamically decoupled from the disk by using a large bubble radius. Delayed cooling generates cold, dense and slow winds with $\beta> 1$, but large amounts of gas occupy regions of temperature-density space with short cooling times. We conclude that either our resolution is too low to warrant physically motivated models for SN feedback, that feedback mechanisms other than SNe are important, or that other aspects of galaxy evolution, such as star formation, require better treatment., Comment: 22 pages, 15 figures. Accepted for publication in MNRAS with minor revisions

The quasar mode of Active Galactic Nuclei (AGN) in the high-redshift Universe is routinely observed in gas-rich galaxies together with large-scale AGN-driven winds. It is crucial to understand how photons emitted by the central AGN source couple to the ambient interstellar-medium to trigger large-scale outflows. By means of radiation-hydrodynamical simulations of idealised galactic discs, we study the coupling of photons with the multiphase galactic gas, and how it varies with gas cloud sizes, and the radiation bands included in the simulations, which are ultraviolet (UV), optical, and infrared (IR). We show how a quasar with a luminosity of $10^{46}$ erg/s can drive large-scale winds with velocities of $10^2-10^3$ km/s and mass outflow rates around $10^3$ M$_\odot$/yr for times of order a few million years. Infrared radiation is necessary to efficiently transfer momentum to the gas via multi-scattering on dust in dense clouds. However, IR multi-scattering, despite being extremely important at early times, quickly declines as the central gas cloud expands and breaks up, allowing the radiation to escape through low gas density channels. The typical number of multi-scattering events for an IR photon is only about a quarter of the mean optical depth from the center of the cloud. Our models account for the observed outflow rates of $\sim$500-1000 M$_\odot$/yr and high velocities of $\sim 10^3$ km/s, favouring winds that are energy-driven via extremely fast nuclear outflows, interpreted here as being IR-radiatively-driven winds., Comment: submitted to MNRAS

Low mass galaxies are thought to provide the bulk of the ionizing radiation necessary to reionize the Universe. The amount of photons escaping the galaxies is poorly constrained theoretically, and difficult to measure observationally. Yet it is an essential parameter of reionization models. We study in detail how ionizing radiation can leak from high redshift galaxies. For this purpose, we use a series of high resolution radiation hydrodynamics simulations, zooming on three dwarf galaxies in a cosmological context. We find that the energy and momentum input from the supernova explosions has a pivotal role in regulating the escape fraction, by disrupting dense star forming clumps, and clearing sight lines in the halo. In the absence of supernovae, photons are absorbed very locally, within the birth clouds of massive stars. We follow the time evolution of the escape fraction, and find that it can vary by more than six orders of magnitude. This explains the large scatter in the value of the escape fraction found by previous studies. This fast variability also impacts the observability of the sources of reionization: a survey even as deep as $M_{\rm UV} = -14$ would miss about half of the underlying population of Lyman-continuum emitters., Comment: 18 pages, 17 figures. Accepted for publication in MNRAS

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 $M_{\rm halo} \le 10^8\,M_\odot$. 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 mini-haloes, resulting in the stellar mass consistent with the slope and normalisation reported in Kimm \& Cen 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 photo-ionisation), rather than supernova explosions. We also find that the photon number-weighted mean escape fraction in mini-haloes is higher ($\sim20$-$40\%$) 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 ($\ge10\%$) only when the mass is greater than $\sim$100\,\msun. 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 $10^8\,M_\odot\le M_{\rm halo} \le 10^{11}\,M_\odot$ are likely to be the most important source of reionisation., 22 pages, 15 figures, Accepted for the publication in MNRAS

Molecular cloud structure is regulated by stellar feedback in various forms. Two of the most important feedback processes are UV photoionisation and supernovae from massive stars. However, the precise response of the cloud to these processes, and the interaction between them, remains an open question. In particular, we wish to know under which conditions the cloud can be dispersed by feedback, which in turn can give us hints as to how feedback regulates the star formation inside the cloud. We perform a suite of radiative magnetohydrodynamic simulations of a 10^5 solar mass cloud with embedded sources of ionising radiation and supernovae, including multiple supernovae and a hypernova model. A UV source corresponding to 10% of the mass of the cloud is required to disperse the cloud, suggesting that the star formation efficiency should be on the order of 10%. A single supernova is unable to significantly affect the evolution of the cloud. However, energetic hypernovae and multiple supernovae are able to add significant quantities of momentum to the cloud, approximately 10^{43} g cm/s of momentum per 10^{51} ergs of supernova energy. This is on the lower range of estimates in other works, since dense gas clumps that remain embedded inside the HII region cause rapid cooling in the supernova blast. We argue that supernovae alone are unable to regulate star formation in molecular clouds, and that strong pre-supernova feedback is required to allow supernova blastwaves to propagate efficiently into the interstellar medium, 15 pages, 10 figures, submitted to MNRAS

StarBench is a project focused on benchmarking and validating different star-formation and stellar feedback codes. In this first StarBench paper we perform a comparison study of the D-type expansion of an HII region. The aim of this work is to understand the differences observed between the twelve participating numerical codes against the various analytical expressions examining the D-type phase of HII region expansion. To do this, we propose two well-defined tests which are tackled by 1D and 3D grid- and SPH- based codes. The first test examines the `early phase' D-type scenario during which the mechanical pressure driving the expansion is significantly larger than the thermal pressure of the neutral medium. The second test examines the `late phase' D-type scenario during which the system relaxes to pressure equilibrium with the external medium. Although they are mutually in excellent agreement, all twelve participating codes follow a modified expansion law that deviates significantly from the classical Spitzer solution in both scenarios. We present a semi-empirical formula combining the two different solutions appropriate to both early and late phases that agrees with high-resolution simulations to $\lesssim2\%$. This formula provides a much better benchmark solution for code validation than the Spitzer solution. The present comparison has validated the participating codes and through this project we provide a dataset for calibrating the treatment of ionizing radiation hydrodynamics codes., Comment: 20 pages, 12 Figures, 4 Tables. Accepted for publication in MNRAS. Comments are welcome. Participation in future StarBench tests is also welcome

We present numerical simulations of a 15 solar mass star in a suite of idealised environments in order to quantify the amount of energy transmitted to the interstellar medium (ISM). We include models of stellar winds, UV photoionisation and the subsequent supernova based on theoretical models and observations of stellar evolution. The system is simulated in 3D using RAMSES-RT, an Adaptive Mesh Refinement Radiation Hydrodynamics code. We find that stellar winds have a negligible impact on the system owing to their relatively low luminosity compared to the other processes. The main impact of photoionisation is to reduce the density of the medium into which the supernova explodes, reducing the rate of radiative cooling of the subsequent supernova. Finally, we present a grid of models quantifying the energy and momentum of the system that can be used to motivate simulations of feedback in the ISM unable to fully resolve the processes discussed in this work., 19 pages, 12 figures, accepted by MNRAS

We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with $M_{\rm halo} \sim4\times10^7 M_\odot$ at $z>10$ using cosmological radiation-hydrodynamics simulations. We find that very compact ($\lesssim$ 1 pc) and massive ($\sim6\times10^5 M_\odot$) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient Ly$\alpha$ emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short ($\ll 1\,{\rm Myr}$), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs., Comment: 7 pages, 5 figures, Accepted for publication in ApJ

We describe and test an updated version of radiation-hydrodynamics (RHD) in the RAMSES code, that includes three new features: i) radiation pressure on gas, ii) accurate treatment of radiation diffusion in an unresolved optically thick medium, and iii) relativistic corrections that account for Doppler effects and work done by the radiation to first order in v/c. We validate the implementation in a series of tests, which include a morphological assessment of the M1 closure for the Eddington tensor in an astronomically relevant setting, dust absorption in a optically semi-thick medium, direct pressure on gas from ionising radiation, convergence of our radiation diffusion scheme towards resolved optical depths, correct diffusion of a radiation flash and a constant luminosity radiation, and finally, an experiment from Davis et al. of the competition between gravity and radiation pressure in a dusty atmosphere, and the formation of radiative Rayleigh-Taylor instabilities. With the new features, RAMSES-RT can be used for state-of-the-art simulations of radiation feedback from first principles, on galactic and cosmological scales, including not only direct radiation pressure from ionising photons, but also indirect pressure via dust from multi-scattered IR photons reprocessed from higher-energy radiation, both in the optically thin and thick limits., Comment: 25 pages, 13 figures, accepted for publication in MNRAS. Revised to match published version

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., Comment: 48 pages, 37 figures, accepted for publication in MNRAS

Lyman-$\alpha$ nebulae are usually found in massive environments at high redshift ($z > 2$). The origin of their Lyman-$\alpha$ (Lya) emission remains debated. Recent polarimetric observations showed that at least some Lya sources are polarized. This is often interpreted as a proof that the photons are centrally produced, and opposed to the scenario in which the Lya emission is the cooling radiation emitted by gas heated during the accretion onto the halo. We suggest that this scenario is not incompatible with the polarimetric observations. In order to test this idea, we post-process a radiative hydrodynamics simulation of a blob with the MCLya Monte Carlo transfer code. We compute radial profiles for the surface brightness and the degree of polarization and compare them to existing observations. We find that both are consistent with a significant contribution of the extragalactic gas to the Lya emission. Most of the photons are centrally emitted and scattered inside the filament afterwards, producing the observed high level of polarization. We argue that the contribution of the extragalactic gas to the Lya emission does not prevent polarization to arise. On the contrary, we find that pure galactic emission causes the polarization profile to be too steep to be consistent with observations., Comment: 14 pages, 9 figures (+4 in appendices). Accepted for publication in Astronomy & Astrophysics

To use a 3D numerical MHD experiment representing magnetic flux emerging into an open field region as a background field for tracing charged particles. The interaction between the two flux systems generates a localised current sheet where MHD reconnection takes place. We investigate how efficiently the reconnection region accelerates charged particles and what kind of energy distribution they acquire. The particle tracing is done numerically using the Guiding Center Approximation on individual data sets from the numerical MHD experiment. We derive particle and implied photon distribution functions having power law forms, and look at the impact patterns of particles hitting the photosphere. We find that particles reach energies far in excess of those seen in observations of solar flares. However the structure of the impact region in the photosphere gives a good representation of the topological structure of the magnetic field., Comment: 9 pages, 7 figures, accepted for publication in A&A