An Over-Massive Black Hole in a Typical Star-Forming Galaxy, 2 Billion Years After the Big Bang

TL;DR

This paper reports the discovery of a galaxy at high redshift with an unusually massive black hole, challenging existing models of black hole and galaxy coevolution by showing the black hole's rapid growth relative to its host.

Contribution

It provides observational evidence of a supermassive black hole in a typical star-forming galaxy that is disproportionately massive compared to its host, at an early cosmic epoch.

Findings

Black hole mass is about 10% of the host galaxy mass.

The galaxy is actively forming stars despite black hole-driven outflows.

The black hole has grown more efficiently than the galaxy, contrary to coevolution models.

Abstract

Supermassive black holes (SMBHs) and their host galaxies are generally thought to coevolve, so that the SMBH achieves up to about 0.2 to 0.5% of the host galaxy mass in the present day. The radiation emitted from the growing SMBH is expected to affect star formation throughout the host galaxy. The relevance of this scenario at early cosmic epochs is not yet established. We present spectroscopic observations of a galaxy at redshift z = 3.328, which hosts an actively accreting, extremely massive BH, in its final stages of growth. The SMBH mass is roughly one-tenth the mass of the entire host galaxy, suggesting that it has grown much more efficiently than the host, contrary to models of synchronized coevolution. The host galaxy is forming stars at an intense rate, despite the presence of a SMBH-driven gas outflow.