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 ($\sim 10 \,{\rm pc}$),
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 ([CII], [NII], and [OIII]). At $z=8$, the most massive
galaxy, `Freesia', has an age $t_\star \simeq 409\,{\rm Myr}$, stellar mass
$M_{\star} \simeq 4.2\times 10^9 {\rm M}_{\odot}$, and a star formation rate
${\rm SFR} \simeq 11.5\,{\rm M}_{\odot}{\rm yr}^{-1}$, due to a recent burst.
Freesia has two stellar components (A and B) separated by $\simeq 2.5\, {\rm
kpc}$; other 11 galaxies are found within $56.9 \pm 21.6 \, {\rm kpc}$. The
mean ISRF in the Habing band is $G = 7.9\, G_0$ and is spatially uniform; in
contrast, the ionisation parameter is $U = 2^{+20}_{-2} \times 10^{-3}$, and
has a patchy distribution peaked at the location of star-forming sites. The
resulting ionising escape fraction from Freesia is $f_{\rm esc}\simeq 2\%$.
While [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in
Freesia-A (0.85 kpc), where the ratio $\Sigma_{\rm [OIII]}/\Sigma_{\rm [CII]}
\simeq 10$. As many high-$z$ 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.
