Essential physics of early galaxy formation

Abstract

We present a theoretical model embedding the essential physics of early galaxy formation (z ≃ 5–12) based on the single premise that any galaxy can form stars with a maximal limiting efficiency that provides enough energy to expel all the remaining gas, quenching further star formation. This simple idea is implemented into a merger-tree-based semi-analytical model that utilizes two mass and redshift-independent parameters to capture the key physics of supernova feedback in ejecting gas from low-mass haloes, and tracks the resulting impact on the subsequent growth of more massive systems via halo mergers and gas accretion. Our model shows that: (i) the smallest haloes (halo mass Mh ≤ 1010 M⊙) build up their gas mass by accretion from the intergalactic medium; (ii) the bulk of the gas powering star formation in larger haloes (Mh ≥ 1011.5 M⊙) is brought in by merging progenitors; (iii) the faint-end UV luminosity function slope evolves according to α = −1.75 log z − 0.52. In addition, (iv) the stellar mass-to-light ratio is well fitted by the functional form log M* = −0.38MUV − 0.13 z + 2.4, which we use to build the evolving stellar mass function to compare to observations. We end with a census of the cosmic stellar mass density (SMD) across galaxies with UV magnitudes over the range −23 ≤ MUV ≤ −11 spanning redshifts 5 < z < 12; (v) while currently detected LBGs contain ≈50 per cent (10 per cent) of the total SMD at z = 5 (8), the James Webb Space Telescope will detect up to 25 per cent of the SMD at z ≃ 9.5.