Zooming on the internal structure of z≃6 galaxies

We present zoom-in, adaptive mesh refinement, high-resolution (≃30 pc) simulations of high-redshift (z ≃ 6) galaxies with the aim of characterizing their internal properties and interstellar medium. Among other features, we adopt a star formation model based on a physically sound molecular hydrogen prescription, and introduce a novel scheme for supernova feedback, stellar winds and dust-mediated radiation pressure. In the zoom-in simulation, the target halo hosts ‘Dahlia’, a galaxy with a stellar mass M⋆ = 1.6 × 1010 M⊙, representative of a typical z ∼ 6 Lyman-break galaxy. Dahlia has a total H2 mass of 108.5 M⊙ that is mainly concentrated in a disc-like structure of effective radius ≃0.6 kpc and scale height ≃200 pc. Frequent mergers drive fresh gas towards the centre of the disc, sustaining a star formation rate per unit area of ≃15 M⊙ yr−1 kpc−2. The disc is composed of dense (n ≳ 25 cm−3), metal-rich (Z ≃ 0.5 Z⊙) gas that is pressure supported by radiation. We compute the 158 μm [C ii] emission arising from Dahlia, and find that ≃95 per cent of the total [C ii] luminosity (⁠L[CII]≃107.5L⊙⁠) arises from the H2 disc. Although 30 per cent of the C ii mass is transported out of the disc by outflows, such gas negligibly contributes to [C ii] emission, due to its low density (n ≲ 10 cm−3) and metallicity (Z ≲ 10−1 Z⊙). Dahlia is underluminous with respect to the local [C ii]–SFR relation; however, its luminosity is consistent with upper limits derived for most z ∼ 6 galaxies.