Not all peaks are created equal: the early growth of supermassive black holes

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ABSTRACT In this work, we use the constrained Gaussian realization technique to study the early growth of supermassive black holes (SMBHs) in cosmological hydrodynamic simulations, exploring its relationship with features of the initial density peaks on large scales, ∼ 1 h−1Mpc. Our constrained simulations of volume (20 h−1Mpc)3 successfully reconstruct the large-scale structure as well as the black hole growth for the hosts of the rare 109 M⊙ SMBHs found in the bluetides simulation at z ∼ 7. We run a set of simulations with constrained initial conditions by imposing a $5 \sigma _0(R_\mathrm{G})$ peak on the scale of $\\ R_\mathrm{ G} = 1$ h−1Mpc varying different peak features, such as the shape and compactness as well as the tidal field surrounding the peak. We find that initial density peaks with high compactness and low tidal field induce the most rapid BH growth at early epochs. This is because compact density peaks with a more spherical large-scale matter distribution lead to the formation of the highest gas inflows (mostly radial) in the centres of haloes which boost the early BH accretion. Moreover, such initially compact density peaks in low tidal field regions also lead to a more compact BH host galaxy morphology. Our findings can help explain the tight correlation between BH growth and host galaxy compactness seen in observations.