The Formation of Supermassive Black Holes from Low-Mass Pop III Seeds

TL;DR

This paper investigates the potential of low-mass Pop III stars as seeds for supermassive black holes and finds that natal kicks prevent their growth into supermassive black holes, unlike more massive Pop III stars.

Contribution

It demonstrates that natal kicks eject low-mass Pop III black holes from their halos, making them unlikely seeds for supermassive black holes, challenging previous assumptions.

Findings

Low-mass Pop III BHs are ejected from halos due to natal kicks.

Supermassive black holes are unlikely to originate from low-mass Pop III stars.

More massive Pop III stars are more plausible seeds for supermassive black holes.

Abstract

The existence of 10$^9$ M$_{\odot}$ black holes (BH) in massive galaxies by $z \ sim$ 7 is one of the great unsolved mysteries in cosmological structure formation. One theory argues that they originate from the black holes of Pop III stars at $z \sim$ 20 and then accrete at the Eddington limit down to the epoch of reionization, which requires that they have constant access to rich supplies of fuel. Because early numerical simulations suggested that Pop III stars were $\gtrsim$ 100 M$_{\odot}$, the supermassive black hole seeds considered up to now were 100 - 300 M$_{\odot}$. However, there is a growing numerical and observational consensus that some Pop III stars were tens of solar masses, not hundreds, and that 20 - 40 M$_{\odot}$ black holes may have been much more plentiful at high redshift. However, we find that natal kicks imparted to 20 - 40 M$_{\odot}$ Pop III BHs during formation eject them from their halos and hence their fuel supply, precluding them from Eddington-limit growth. Consequently, supermassive black holes are far less likely to form from low-mass Pop III stars than from very massive ones.