We study the formation and evolution of a sample of Lyman Break Galaxies in the Epoch of Reionisation by using high-resolution (∼ 10 pc), cosmological zoom-in simulations part of the serra suite. In serra, we follow the interstellar medium (ISM) thermo-chemical non-equilibrium evolution, and perform on-the-fly radiative transfer of the interstellar radiation field (ISRF). The simulation outputs are post-processed to compute the emission of far infrared lines ([C II], [N II], and [O III]). At z = 8, the most massive galaxy, "Freesia", has an age t 409 Myr, stellar mass M 4.2 × 10 9 M , and a star formation rate SFR 11.5 M yr −1 , due to a recent burst. Freesia has two stellar components (A and B) separated by 2.5 kpc; other 11 galaxies are found within 56.9 ± 21.6 kpc. The mean ISRF in the Habing band is G = 7.9 G 0 and is spatially uniform; in contrast, the ionisation parameter is U = 2 +20 −2 × 10 −3 , and has a patchy distribution peaked at the location of star-forming sites. The resulting ionising escape fraction from Freesia is f esc 2%. While [C II] emission is extended (radius 1.54 kpc), [O III] is concentrated in Freesia-A (0.85 kpc), where the ratio Σ [OIII] /Σ [CII] 10. As many high-z galaxies, Freesia lies below the local [C II]-SFR relation. We show that this is the general consequence of a starburst phase (pushing the galaxy above the Kennicutt-Schmidt relation) which disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [C II]-SFR deviations.

A tight relation between the [C ii] 158 $\mu$m line luminosity and star formation rate is measured in local galaxies. At high redshift (z > 5), though, a much larger scatter is observed, with a considerable (15–20 per cent) fraction of the outliers being [C ii]-deficient. Moreover, the [C ii] surface brightness ($\Sigma_{\rm [C\, \small {II}]}$) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [C ii]-deficiency, we have developed an analytical model that fits local [C ii] data and has been validated against radiative transfer simulations performed with cloudy. The model predicts an overall increase of $\Sigma_{\rm [C\, \small {II}]}$ with ΣSFR. However, for ΣSFR ${\gtrsim} 1 \, \mathrm{M}_\odot \,{\rm yr}^{-1}\,{\rm kpc}^{-2}$, $\Sigma_{\rm [C\, \small {II}]}$ saturates. We conclude that underluminous [C ii] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt–Schmidt relation (κs ≫ 1), parametrized by the ‘burstiness’ parameter κs; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [C ii] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [O iii]88 $\mu$m, C iii]1909 Å, CO lines). Simple formulae are given to interpret available data for low- and high-z galaxies.

Abstract ALMA observations have revealed the presence of dust in galaxies in the Epoch of Reionization (redshift z > 6). However, the dust temperature, Td, remains unconstrained, and this introduces large uncertainties, particularly in the dust mass determinations. Using an analytical and physically-motivated model, we show that dust in high-z, star-forming giant molecular clouds (GMC), largely dominating the observed far-infrared luminosity, is warmer (Td ≳ 60 K) than locally. This is due to the more compact GMC structure induced by the higher gas pressure and turbulence characterising early galaxies. The compactness also delays GMC dispersal by stellar feedback, thus $\sim 40\%$ of the total UV radiation emitted by newly born stars remains obscured. A higher Td has additional implications: it (a) reduces the tension between local and high-z IRX-β relation, (b) alleviates the problem of the uncomfortably large dust masses deduced from observations of some EoR galaxies.

Recent stacked ALMA observations have revealed that normal, star-forming galaxies at z ≈ 6 are surrounded by extended (≈10 kpc) [C ii]-emitting haloes, which are not predicted by the most advanced, zoom-in simulations. We present a model in which these haloes are the result of supernova-driven cooling outflows. Our model contains two free parameters, the outflow mass loading factor, η, and the parent galaxy dark matter halo circular velocity, vc. The outflow model successfully matches the observed [C ii] surface brightness profile if η = 3.20 ± 0.10 and $v_{\rm c} = 170 \pm 10 \, \rm km\, s^{-1}$, corresponding to a dynamical mass of ${\approx }10^{11}\, {\rm M}_{\odot }$. The predicted outflow rate and velocity range are $128 \pm 5\, {\rm M}_{\odot }\, {\rm yr}^{-1}$ and 300–500 $\, \rm km\, s^{-1}$, respectively. We conclude that (a) extended haloes can be produced by cooling outflows; (b) the large η value is marginally consistent with starburst-driven outflows, but it might indicate additional energy input from active galactic nuclei; and (c) the presence of [C ii] haloes requires an ionizing photon escape fraction from galaxies fesc ≪ 1. The model can be readily applied also to individual high-z galaxies, as those observed, e.g. by the ALMA ALPINE survey now becoming available.

We study the photoevaporation of Jeans-unstable molecular clumps by isotropic FUV (6 eV < hν < 13.6 eV) radiation, through 3D radiative transfer hydrodynamical simulations implementing a non-equilibrium chemical network that includes the formation and dissociation of H 2 . We run a set of simulations considering different clump masses (M = 10 − 200 M ) and impinging fluxes (G 0 = 2 × 10 3 − 8 × 10 4 in Habing units).In the initial phase, the radiation sweeps the clump as an R-type dissociation front, reducing the H 2 mass by a factor 40 − 90%. Then, a weak (M 2) shock develops and travels towards the centre of the clump, which collapses while loosing mass from its surface. All considered clumps remain gravitationally unstable even if radiation rips off most of the clump mass, showing that external FUV radiation is not able to stop clump collapse. However, the FUV intensity regulates the final H 2 mass available for star formation: for example, for G 0 < 10 4 more than 10% of the initial clump mass survives. Finally, for massive clumps ( 100 M ) the H 2 mass increases by 25 − 50% during the collapse, mostly because of the rapid density growth that implies a more efficient H 2 self-shielding.1 The Habing flux (1.6 × 10 −3 erg s −1 cm −2 ) is the average interstellar radiation field of our Galaxy in the range [6 eV, 13.6 eV] (Habing 1968)