Abstract
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 (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 $t_409\,Myr$, stellar mass
$M_\star 4.210^9 M_ødot$, and a star formation rate
$SFR 11.5\,M_ødotyr^-1$, due to a recent burst.
Freesia has two stellar components (A and B) separated by $2.5\, \rm
kpc$; 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 $f_esc2\%$.
While CII emission is extended (radius 1.54 kpc), OIII is concentrated in
Freesia-A (0.85 kpc), where the ratio $\Sigma_OIII/\Sigma_CII
10$. 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.
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