Massive black hole seeds from low angular momentum material

Abstract

We present a cosmologically motivated model in which the seeds of supermassive black holes form out of the lowest angular momentum gas in protogalaxies at high redshift. We show that, under reasonable assumptions, this leads naturally to a correlation between black hole masses and spheroid properties, as observed today. We assume that the gas in early‐forming, rare‐peak haloes has a distribution of specific angular momentum similar to that derived for the dark matter in cosmological N‐body simulations. This distribution has a significant low angular momentum tail, which implies that every protogalaxy should contain gas that ends up in a high‐density disc. In haloes more massive than a critical threshold of ∼7 × 107 M⊙ at z∼ 15, the discs are gravitationally unstable, and experience an efficient Lin–Pringle viscosity that transfers angular momentum outwards and allows mass inflow. We assume that this process continues until the first massive stars disrupt the disc. The seed black holes created in this manner have a characteristic mass of ∼105 M⊙, roughly independent of the redshift of formation. This serves as a lower bound for black hole masses at galactic centres today. The comoving mass density in black hole seeds grows with time, tracking the continuous production of critical‐mass haloes, and saturates when cosmic reionization acts to prevent gas cooling in these low‐mass systems. By z∼ 15, the comoving mass density becomes comparable to that inferred from observations, with room for appropriate additional luminous growth during a later quasar accretion phase. The hierarchical merger process after z∼ 15 naturally leads to a linear correlation between black hole mass and stellar spheroid mass, with negligible black hole masses in disc‐dominated galaxies. The formation of massive seeds at high redshifts, and the relatively important role of mergers in the buildup of today's black holes, are key elements in the proposed scenario.