The surprisingly massive black holes of the early universe challenge cosmic theories

Surprisingly unspectacular: A black hole already weighed over a billion solar masses in the early universe, despite its average appetite.

Looking into the early stages of the 13.8 billion year old universe The James Webb Space Telescope he saw the galaxy as it existed only 700 million years after Big Bang. It is mysterious how Black hole it could already weigh a billion solar masses at its center when the universe was still in its infancy. James Webb’s observations were designed to take a closer look at the feeding mechanism, but found nothing unusual. Black holes apparently already grew in a similar way to today. But the result is all the more significant: it could show that astronomers know less about the formation of galaxies than they thought. And yet the measurements are by no means disappointing. On the contrary.

The mystery of early black holes

The first billion years of cosmic history present a challenge: The oldest known black holes at the centers of galaxies have surprisingly large masses. How did they get so massive, so fast? The new observations described here provide strong evidence against some proposed explanations, particularly an “ultra-efficient feeding regime” for the oldest black holes.

Growth limits of supermassive black holes

Stars and galaxies have changed enormously over the past 13.8 billion years, the lifetime of the universe. Galaxies grew larger and gained more mass, either by consuming the surrounding gas or (occasionally) merging with each other. Astronomers have long assumed that supermassive black holes at the centers of galaxies would gradually grow along with the galaxies themselves.

But the growth of black holes cannot be arbitrarily fast. Matter colliding with a black hole creates a swirling, hot, bright “accretion disk”. When this happens around a supermassive black hole, the result is an active galactic nucleus. The brightest such objects, known as quasars, are among the brightest astronomical objects in the entire universe. But that luminosity limits how much matter can fall onto a black hole: Light exerts a pressure that can prevent more matter from falling in.

How did black holes get so massive and so fast?

That’s why astronomers were surprised when, over the last twenty years, observations of distant quasars revealed very young black holes that nevertheless reached masses of up to 10 billion solar masses. Light takes time to travel from a distant object to us, so looking at distant objects means looking into the distant past. We see the most distant known quasars as they were in the era known as “Cosmic Dawn,” less than one billion years after the Big Bang, when the first stars and galaxies formed.

Explaining these early massive black holes is a significant challenge for current models of galaxy evolution. Could it be that early black holes were much more efficient at accreting gas than their modern counterparts? Or could the presence of dust affect quasar mass estimates in a way that caused researchers to overestimate the early masses of black holes? Currently, there are many proposed explanations, but none are widely accepted.

A closer look at the early growth of black holes

Deciding which, if any, of the explanations is correct requires a more complete picture of quasars than was previously available. With the arrival of the JWST space telescope, specifically the MIRI telescope’s mid-infrared instrument, astronomers’ ability to study distant quasars has taken a giant leap forward. For measuring the spectra of a distant quasar, MIRI is 4000 times more sensitive than any previous instrument.

Instruments like MIRI are produced by international consortia in which scientists, engineers and technicians work closely together. The consortium is naturally very interested in testing whether their tool works as well as planned. In return for building the instrument, consortia are usually granted some observation time. In 2019, years before the launch of JWST, the European MIRI consortium decided to use some of this time to observe the most distant quasar known at the time, an object designated J1120+0641.

Observation of one of the oldest black holes

The analysis of the observations was Dr. Sarah Bosman, postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) and member of the European MIRI consortium. MPIA’s contributions to MIRI include building a number of key internals. Bosman was asked to join the MIRI collaboration specifically to bring expertise on how best to use the instrument to study the early universe, particularly the first supermassive black holes.

The observations were made in January 2023, during JWST’s first observing cycle, and lasted about two and a half hours. They represent the first mid-infrared study of a quasar at the Cosmic Dawn, just 770 million years after the Big Bang (redshift z=7). The information comes not from the image, but from the spectrum: the iridescent decomposition of the object’s light into components of different wavelengths.

Tracking dust and fast moving gas

The overall shape of the mid-infrared spectrum (the “continuum”) encodes the properties of the large dust torus that surrounds the accretion disk in typical quasars. This torus helps guide matter into the accretion disk and “feed” the black hole. Bad news for those whose preferred solution to massive early black holes lies in alternative rapid growth modes: the torus, and thus the feeding mechanism, in this very early quasar appears to be the same as in its more modern counterparts. The only difference is one that no model of rapid early quasar growth predicted: a slightly higher dust temperature of about a hundred Kelvin hotter than the 1300 K found for the hottest dust in less distant quasars.

The shorter-wavelength portion of the spectrum, which is dominated by emission from the accretion disk itself, shows that for us as distant observers, the quasar’s light is not dampened more than usual by dust. Arguments that maybe we’re just overestimating the early masses of black holes because of extra dust aren’t a solution either.

“Shockingly Normal” Early Quasars

The broad-line region of the quasar, where clumps of gas orbit the black hole at speeds close to the speed of light—allowing inferences about the black hole’s mass and density and the ionization of the surrounding matter—also appears normal. In almost all properties that can be deduced from the spectrum, J1120+0641 does not differ from quasars at later times.

“Overall, these new observations only add to the mystery: the early quasars were surprisingly normal.” No matter what wavelengths we observe them at, quasars are almost identical in all epochs of the universe,” says Bosman. Not only the supermassive black holes themselves, but their feeding mechanisms were apparently already fully “mature” when the universe was only 5% of its current age. By ruling out a number of alternative solutions, the results strongly support the idea that supermassive black holes started out with significant masses from the very beginning, in astronomical jargon: that they are “primordial” or “massive”. Supermassive black holes did not form from the remnants of early stars, then grew in mass very quickly. They must have formed early with initial masses of at least a hundred thousand solar masses, probably by the collapse of massive early gas clouds.

Reference: “Mature Cosmic Dawn Quasar Revealed by Infrared Spectroscopy of the JWST Remnant” by Sarah EI Bosman, Javier Álvarez-Márquez, Luis Colina, Fabian Walter, Almudena Alonso-Herrero, Martin J. Ward, Göran Östlin, Thomas R. Greve, Gillian Wright , Arjan Bik, Leindert Boogaard, Karina Caputi, Luca Costantin, Andreas Eckart, Macarena García-Marín, Steven Gillman, Jens Hjorth, Edoardo Iani, Olivier Ilbert, Iris Jermann, Alvaro Labiano, Danial Langeroodi, Florian Peiig Rinaldi, Martin Topinka, Paul van der Werf, Manuel Güdel, Thomas Henning, Pierre-Olivier Lagage, Tom P. Ray, Ewine F. van Dishoeck and Bart Vandenbussche, 17 June 2024, Astronomy of nature.
DOI: 10.1038/s41550-024-02273-0