tokens: which starts forming BHs in galaxies with . While modelling BH formation in low-mass galaxies is crucial to understand BH formation in the high-redshift Universe, as well as to understand the current populations of BHs in local dwarf galaxies, the regime of low-mass galaxies is barely resolved in such large-scale simulations of on a side (but see Habouzit et al. 2017 , and references therein for BH formation in low-mass galaxies from high redshift to low redshift). All the other simulations employ lower seeding masses than the TNG simulations. In Horizon-AGN fewer BHs of are present at z = 0 than at higher redshift. This is because BH formation stops at z = 1.5 by design in this simulation. In simulations, the initial mass of BHs was shown to affect the low-mass end of the M BH − M ⋆ diagram, and the overall normalization of the scaling relation (e.g. Bower et al. 2017 ). For higher stellar masses, the relations for different seeding masses converge to the same relation (whose normalization depends on the BH accretion efficiency parameter) as a result of self-regulation. The seed mass is important for the simulations using the Bondi accretion model (e.g. Illustris, TNG, Horizon-AGN, EAGLE), for which there is a degeneracy between the seeding mass and the boost factor to produce the same normalization of the M BH − M ⋆ mean relation. In simulations using a torque accretion model (SIMBA), the accretion rate on to the BHs does not strongly depend on their masses (, contrary to the Bondi model ). Therefore, the low-mass BHs can grow more efficiently than in the Bondi model, and they converge into the M BH − M ⋆ mean relation regardless of their seed mass (Anglés-Alcázar, Özel Davé 2013 ; Anglés-Alcázar et al. 2015 ). We point here again that the seeding in large-scale cosmological simulations is often not physically motivated (e.g. fixed BH mass, threshold in galaxy, or halo mass to form a BH), but instead parameters of the seeding models are chosen to reproduce the local scaling relation. 3.3.2 On the growth of BHs Getting the BHs on the main scaling relation depends both on the seeding BH mass and BH accretion rate. The parametrizations of these models are degenerate. Therefore, different choices allow the BH population to get on to the same empirical scaling relation. For example, the TNG model seeds BHs with a higher initial BH mass than the Illustris model (by one order of magnitude) but also does not include any boost factor in the accretion model (while Illustris does include a boost factor of 100). The colour code of Fig. 3 provides information on the accretion rates on to the BHs. At z ∼ 2, it is clear that the accretion properties are very different among the different simulations. For example, BHs of embedded in galaxies of have high bolometric luminosities of (limit commonly employed to define an AGN) in TNG, Horizon-AGN, and SIMBA, while a large fraction of these BHs in Illustris and EAGLE have much lower luminosities.
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