Deep into the structure of the first galaxies: SERRA views

We study the formation and evolution of a sample of Lyman break galaxies in the epoch of reionization by using high-resolution (˜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 ˜eq 409 Myr, stellar mass M⋆ ≃ 4.2 × 109M⊙, and a star formation rate (SFR), SFR˜eq 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 ionization parameter is U = 2^{+20}_{-2} × 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˜eq 2{{ per cent}}. While [C II] emission is extended (radius 1.54 kpc), [O III] is concentrated in Freesia-A (0.85 kpc), where the ratio Σ _[O III]/Σ _[C II]≃ 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.