Overmassive Black holes live in compact galaxies in the early Universe

TL;DR

This study shows that early universe quasars with overmassive black holes reside in unusually compact host galaxies, linking galaxy size and black hole growth during cosmic dawn.

Contribution

It provides the first statistical evidence that high-redshift quasars with massive black holes are hosted by compact galaxies, suggesting a connection between galaxy compactness and black hole formation.

Findings

Quasar host galaxies at z~6 are more compact than typical star-forming galaxies.

Compact quasar hosts have less cold gas and sizes similar to massive quiescent galaxies at z~4-5.

These compact hosts may evolve into the first quiescent galaxy population.

Abstract

A significant population of quasars have been found to exist within the first Gyr of cosmic time. Most of them have high black hole (BH) masses ($M_{\rm BH} \sim 10^{8-10} M_{\odot}$) with an elevated BH-to-stellar mass ratio compared to typical local galaxies, posing challenges to our understanding of the formation of supermassive BHs and their coevolution with host galaxies. Here, based on size measurements of [CII] 158$\mu$m emission for a statistical sample of $z \sim 6$ quasars, we find that their host galaxies are systematically more compact (with half-light radius $R_{\rm e} \sim 1.6$ kpc) than typical star-forming galaxies at the same redshifts. Specifically, the sizes of the most compact quasar hosts, which also tend to contain less cold gas than their more extended counterparts, are comparable to that of massive quiescent galaxies at $z \sim 4-5$. These findings reveal an intimate connection between the formation of massive BHs and compactness of their host galaxies in the early universe. These compact quasar hosts are promising progenitors of the first population of quiescent galaxies.