Deep into the structure of the first galaxies: SERRA views

Abstract

We study the formation and evolution of a sample of Lyman Break Galaxies in the Epoch of Reionization by using high-resolution ($\sim 10\mathrm{pc}$), 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 $t_{\star }\simeq 409\mathrm{Myr}$, stellar mass $M_{\star }\simeq 4.2\times 10^9{\mathrm{M}}_{\odot }$, and a star formation rate $\mathrm{SFR}\simeq 11.5{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}$, due to a recent burst. Freesia has two stellar components (A and B) separated by $\simeq 2.5\mathrm{kpc}$; other 11 galaxies are found within $56.9\pm 21.6\mathrm{kpc}$. 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 $f_{\mathrm{esc}}\simeq 2\%$. While [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in Freesia-A (0.85 kpc), where the ratio $\hat{I}{\pounds }_{[\mathrm{OIII}]}/\hat{I}{\pounds }_{[\mathrm{CII}]}\simeq 10$. 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.

22 pages, 14 figures, accepted by MNRAS