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 Reionisation 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 ([C II], [N II], and [O III]). At z = 8, the most massive galaxy, "Freesia", has an age t 409 Myr, stellar mass M 4.2 × 10 9 M , and a star formation rate SFR 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 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 esc 2%. While [C II] emission is extended (radius 1.54 kpc), [O III] is concentrated in Freesia-A (0.85 kpc), where the ratio Σ [OIII] /Σ [CII] 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) which disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [C II]-SFR deviations.

A physical model for [C ii] line emission from galaxies

“…6; the corresponding model parameters are n = 500 cm −3 , k s = 1 and a metallicity Z = 0.003, a value close to the metallicity of the intergalactic medium at z = 6 (D'Odorico et al 2013). Additionally, simulations of highz galaxies (Pallottini et al 2019) shows that the presence of low metallicity regions plays a sub-dominant role in determining the galaxy deviation from the De Looze relation. Hence, we consider this explanation unlikely, albeit we point out that at least some degeneracy exists between density and metallicity in determining the observed [CII] surface brightness.…”

Weak evolution of the mass–metallicity relation at cosmic dawn in the FirstLight simulations

“…Therefore, it is now the best time to make theoretical predictions about the evolution of the MZR at cosmic dawn. Current cosmological simulations have become very advanced tools to study galaxy formation around cosmic dawn (Ceverino et al 2017;Barrow et al 2017;Rosdahl et al 2018;Pallottini et al 2019;Katz et al 2019;Ma et al 2019). However, most of these works focus on the formation of a few galaxies, by using the zoom-in technique.…”

Shaping the structure of a GMC with radiation and winds

“…The thermochemistry module of RAMSES-RT has been coupled with the package KROME (Grassi et al 2014) in order to include a complex chemical network accounting for nine species (H, H + , H − , H 2 , H + 2 , He, He + , He ++ , and free electrons). This coupling between RAMSES-RT and KROME has already been tested and used in previous works (Decataldo et al 2019;Pallottini et al 2019).…”

“…An analysis of the radiation field in molecular clouds is particularly relevant for cosmological simulations of galaxy formation, where generally it is possible only to marginally resolve the internal structure of molecular clouds (Leung et al 2019). With a typical resolution of tens of parsecs (Rosdahl et al 2018;Pallottini et al 2019), a cell represents a whole GMC and therefore its properties are averaged, so local inhomogeneities cannot be taken into account. GMC emission properties -in particular far-infrared lines, as [C II], [N II], and [O III] and CO -has been shown to be sensitive to their internal density structure (Vallini et al 2017(Vallini et al , 2018, while the radiation field is typically assumed to be constant.…”