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([C 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([C II]) with Sigma(SFR). However, for Sigma(SFR) greater than or similar to 1M(circle dot) yr(-1) kpc(-2), Sigma([C II]) saturates. We conclude that underluminous [C II] systems can result from a combination of three factors: (a) large upward deviations from theKennicutt-Schmidt relation (kappa(s) >> 1), parametrized by the 'burstiness' parameter kappa(s); (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. [OIII]88 mu m, C III]1909 degrees, CO lines). Simple formulae are given to interpret available data for low- and high- z galaxies.

We study the formation and evolution of a sample of Lyman break galaxies in the epoch of reionization by using high-resolution (similar to 10 pc), cosmological zoom-in simulations part of the SERRA suite. In SERRA, we follow the interstellar medium thermochemical 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(star) similar or equal to 409 Myr, stellar mass M-star similar or equal to 4.2 x 10(9)M(circle dot), and a star formation rate (SFR), SFR similar or equal to 11.5M(circle dot) yr(-1), due to a recent burst. Freesia has two stellar components (A and B) separated by similar or equal to 2.5 kpc; other 11 galaxies are found within 56.9 +/- 21.6 kpc. The mean ISRF in the Habing band is G = 7.9G(0) and is spatially uniform; in contrast, the ionization parameter is U = 2(-2)(+20) x 10(-3), and has a patchy distribution peaked at the location of star-forming sites. The resulting ionizing escape fraction from Freesia is f(esc) similar or equal to 2 per cent. While [C II] emission is extended (radius 1.54 kpc), [O III] is concentrated in Freesia-Lambda (0.85 kpc), where the ratio Sigma([O III])/Sigma([C II]) similar or equal to 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) that disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [C II]-SFR deviations.

We present the first attempt to detect outflows from galaxies approaching the Epoch of Reionization (EoR) using a sample of nine star-forming (SFR = 31 +/- 20M(circle dot) yr(-1)) z similar to 5.5 galaxies for which the [C II] 158 mu m line has been previously obtained with Atacama Large Millimeter Array (ALMA). We first fit each line with a Gaussian function and compute the residuals by subtracting the best-fitting model from the data. We combine the residuals of all sample galaxies and find that the total signal is characterized by a flux excess of similar to 0.5 mJy extended over similar to 1000 km s(-1). Although we cannot exclude that part of this signal is due to emission from faint satellite galaxies, we show that the most probable explanation for the detected flux excess is the presence of broad wings in the [C II] lines, signatures of starburst-driven outflows. We infer an average outflow rate of. M(over dot) = 54 +/- 23M(circle dot) yr(-1), providing a loading factor eta = M(over dot)/SFR = 1.7 +/- 1.3 in agreement with observed local starbursts. Our interpretation is consistent with outcomes from zoomed hydrosimulations of Dahlia, a z similar to 6 galaxy (SFR similar to 100M(circle dot) yr(-1)), whose feedback-regulated star formation results into an outflow rate. M(over dot) similar to 30M(circle dot) yr(-1). The quality of the ALMA data is not sufficient for a detailed analysis of the [C II] line profile in individual galaxies. Nevertheless, our results suggest that starburst-driven outflows are in place in the EoR and provide useful indications for future ALMA campaigns. Deeper observations of the [C II] line in this sample are required to better characterize feedback at high-z and to understand the role of outflows in shaping early galaxy formation.

We study the CO line luminosity (L-CO), the shape of the CO spectral line energy distribution (SLED), and the value of the CO-to-H-2 conversion factor in galaxies in the Epoch of Reionization (EoR). For this aim, we construct a model that simultaneously takes into account the radiative transfer and the clumpy structure of giant molecular clouds (GMCs) where the CO lines are excited. We then use it to post-process state-of-the-art zoomed, high resolution (30 pc), cosmological simulation of a main-sequence (M-* approximate to 10(10) M-circle dot, SFR approximate to 100M(circle dot) yr(-1)) galaxy, 'Althaea', at z approximate to 6. We find that the CO emission traces the inner molecular disc (r approximate to 0.5 kpc) of Althaea with the peak of the CO surface brightness co-located with that of the [C-II] 158 mu m emission. Its LCO(1-0) = 10(4.85) L-circle dot is comparable to that observed in local galaxies with similar stellar mass. The high (Sigma(gas) approximate to 220M(circle dot) pc(-2)) gas surface density in Althaea, its large Mach number (M approximate to 30) and the warm kinetic temperature (T-k approximate to 45 K) of GMCs yield a CO SLED peaked at the CO(7-6) transition, i.e. at relatively high-J and a CO-to-H-2 conversion factor alpha(CO) approximate to 1.5M(circle dot) (K km s(-1) pc(2))(-1) lower than that of the Milky Way. The Atacama Large Millimeter/submillimeter Array observing time required to detect (resolve) at 5 sigma the CO(7-6) line from galaxies similar to Althaea is approximate to 13 h (approximate to 38 h).

The recent discovery of dusty galaxies well into the Epoch of Reionization (redshift z > 6) poses challenging questions about the properties of the interstellar medium in these pristine systems. By combining state-of-the-art hydrodynamic and dust radiative transfer simulations, we address these questions focusing on the recently discovered dusty galaxy A2744_YD4 (z = 8.38, Laporte et al.). We show that we can reproduce the observed spectral energy distribution (SED) only using different physical values with respect to the inferred ones by Laporte et al., i.e. a star formation rate of SFR = 78 M(circle dot)yr(-1), a factor approximate to 4 higher than deduced from simple SED fitting. In this case, we find: (i) dust attenuation (corresponding to tau(v) = 1.4) is consistent with a Milky Way (MW) extinction curve; (ii) the dust-to-metal ratio is low, f(d) similar to 0.08, implying that early dust formation is rather inefficient;(iii) the luminosity-weighted dust temperature is high, T-d = 91 23 K, as a result of the intense (approximate to 100 x MW) interstellar radiation field; and (iv) due to the high T-d, the Atacama Large Millimeter/submillimeter Array Band 7 detection can be explained by a limited dust mass, M-d = 1.6 x 10(6) M-circle dot. Finally, the high dust temperatures might solve the puzzling low infrared excess (IRX) recently deduced for high-z galaxies from the IRX-beta relation.

We study the photoevaporation of molecular clumps exposed to a UV radiation field including hydrogen-ionizing photons (h nu > 13.6 eV) produced by massive stars or quasars. We follow the propagation and collision of shockwaves inside clumps and take into account self-shielding effects, determining the evolution of clump size and density with time. The structure of the ionization-photodissociation region is obtained for different initial clump masses (M = 0.01- 10(4)M(circle dot)) and impinging fluxes (G(0) = 10(2)-10(5) in units of the Habing flux). The cases of molecular clumps engulfed in the HII region of an OB star and clumps carried within quasar outflows are treated separately. We find that the clump undergoes in both cases an initial shock-contraction phase and a following expansion phase, which lets the radiation penetrate in until the clump is completely evaporated. Typical evaporation time-scales are similar or equal to 0.01 Myr in the stellar case and 0.1 Myr in the quasar case, where the clump mass is 0.1 M-circle dot and 10(3)M(circle dot) , respectively. We find that clump lifetimes in quasar outflows are compatible with their observed extension, suggesting that photoevaporation is the main mechanism regulating the size of molecular outflows.

To improve our understanding of high-z galaxies, we study the impact of H-2 chemistry on their evolution, morphology and observed properties. We compare two zoom-in high-resolution (30 pc) simulations of prototypical M-star similar to 1010M(circle dot) galaxies at z = 6. The first, 'Dahlia', adopts an equilibrium model for H-2 formation, while the second, 'Alth ae a', features an improved non-equilibrium chemistry network. The star formation rate (SFR) of the two galaxies is similar (within 50 per cent), and increases with time reaching values close to 100M(circle dot) yr(-1) at z = 6. They both have SFR-stellar mass relation consistent with observations, and a specific SFR of similar or equal to 5Gyr(-1). The main differences arise in the gas properties. The non-equilibrium chemistry determines the H -> H-2 transition to occur at densities > 300 cm(-3), i.e. about 10 times larger than predicted by the equilibrium model used for Dahlia. As a result, Alth ae a features a more clumpy and fragmented morphology, in turn makingSNfeedback more effective. Also, because of the lower density and weaker feedback, Dahlia sits 3 sigma away from the Schmidt-Kennicutt relation; Alth ae a, instead nicely agrees with observations. The different gas properties result in widely different observables. Alth ae a outshines Dahlia by a factor of 7 (15) in [C (II)] 157.74 mu m (H(2)17.03 mu m) line emission. Yet, Alth ae a is underluminous with respect to the locally observed [C (II)]-SFR relation. Whether this relation does not apply at high-z or the line luminosity is reduced by cosmic microwave background and metallicity effects remain as an open question.