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 ($\sim 10\mathrm{p}\mathrm{c}$), 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 ([CII], [NII], and [OIII]). At $z=8$, the most massive galaxy, `Freesia’, has an age $t_{\star }\simeq 409\mathrm{M}\mathrm{y}\mathrm{r}$, stellar mass $M_{\star }\simeq 4.2\times {10}^9{\mathrm{M}}_{\odot }$, and a star formation rate $\mathrm{S}\mathrm{F}\mathrm{R}\simeq 11.5{\mathrm{M}}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$, due to a recent burst. Freesia has two stellar components (A and B) separated by $\simeq 2.5, {\rm kpc}$;other11galaxiesarefoundwithin$56.9 \pm 21.6 , {\rm kpc}$. The mean ISRF in the Habing band is $G=7.9G_0$ and is spatially uniform; in contrast, the ionisation parameter is $U=2_{-2}^{+20}\times {10}^{-3}$, and has a patchy distribution peaked at the location of star-forming sites. The resulting ionising escape fraction from Freesia is $f_{\mathrm{e}\mathrm{s}\mathrm{c}}\simeq 2\mathrm{\%}$. While [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in Freesia-A (0.85 kpc), where the ratio $\Sigma_{\rm [OIII]}/\Sigma_{\rm [CII]} \simeq 10$\mathrm{.}Asmanyhigh-$z$ galaxies, Freesia lies below the local [CII]-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 [CII]-SFR deviations.