A black hole in a near-pristine galaxy 700 million years after the Big Bang

TL;DR

This study reports the discovery of an extremely low-metallicity massive black hole at high redshift, challenging existing models of early black hole formation and growth.

Contribution

It provides the first metallicity measurement of a high-redshift black hole in a low-mass galaxy, highlighting difficulties for current formation theories.

Findings

The black hole has about 0.4% of solar metallicity.

The host galaxy has very low dynamical mass.

Standard models struggle to explain such low metallicity with a massive black hole.

Abstract

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$\beta$ emission indicates extremely low metallicity, about $4\times 10^{-3}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.