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Making Sense of a Massive Black Hole in the Early Universe

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Artist's impression of an active galactic nucleus surrounded by a dusty accretion disk
Artist's impression of an active galactic nucleus surrounded by a dusty accretion disk. [NASA/SOFIA/Lynette Cook]

JWST has uncovered a population of point-like red objects in the early universe that may be active black holes shrouded in dust. Recent follow-up observations of one of these objects suggest that it’s an active supermassive black hole that’s disproportionately large compared to the galaxy it inhabits.

Supermassive Black Holes in the Making

illustration of a gas cloud collapsing to form a black hole

The collapse of a primordial gas cloud into a black hole is a possible way to make supermassive black hole seeds in the 104–105-solar-mass range. [NASA/CXC/M. Weiss; CC BY 4.0]

Most galaxies have a central supermassive black hole, and even galaxies in the first billion years of our universe’s history appear to host black holes millions of times as massive as the Sun. How these black holes get so massive in such a small amount of time is an open question; could the small black holes left behind by dying stars accrete enough gas to go from stellar-mass to supermassive in a few hundred million years, or must the seeds of black holes be far larger?

Both scenarios have challenges: it’s not clear how stellar-mass black holes could sustain the accretion rate necessary to transform into supermassive black holes, and we don’t yet have direct evidence for the formation of massive black hole seeds. To learn more about how supermassive black holes come to be, researchers need to amass observations of these objects in the early universe.

Red Source Revealed

Vasily Kokorev (Kapteyn Astronomical Institute) and collaborators followed up on observations of a reddish point source made through the JWST Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) program. Spectra of the source, named UNCOVER ID 20466, reveal it to be at a redshift of z = 8.5, placing it about 600 million years after the Big Bang.

spectrum of the red point source investigated in this work

Two-dimensional spectrum (top) and one-dimensional extracted spectrum (bottom) of the source investigated in this study. Click to enlarge. [Adapted from Kokorev et al. 2023]

The source’s reddish color and point-like nature suggest that it’s a galaxy hosting an active galactic nucleus — a supermassive black hole accreting gas from its surroundings — that is cocooned in dusty gas. This assessment was backed up by its spectrum, which showed a mixture of broad and narrow emission lines. The broad emission lines come from high-density, high-velocity gas near the central black hole, while the narrow lines come from less dense gas farther from the nucleus. This is the most distant active galactic nucleus for which researchers have clearly detected broad emission lines.

How Large Is the Seed?

Plot of observed black hole masses and modeled black hole masses over cosmic time.

Observed black hole masses and ages (symbols) and the black hole masses achievable as a function of time for accretion at the Eddington rate onto a 100-solar-mass seed (red line) and a 104-solar-mass seed (gray line). Click to enlarge. [Kokorev et al. 2023]

The black hole at the center of this distant galaxy has a mass of more than 100 million solar masses, which amounts to at least 30% of the galaxy’s mass being contained in the black hole. This is a much larger proportion than we see for supermassive black holes in the local universe or for typical black holes in the early universe. How did this black hole get so large so quickly, and why is it so large compared to its home galaxy?

Kokorev’s team favors a scenario in which the black hole was seeded by the collapse of a primordial gas cloud into a 104-solar-mass black hole. In this scenario, the black hole can reach its observed mass in the allotted time without surpassing the Eddington limit, the theoretical maximum accretion rate. This also tends to create black holes that are large compared to their host galaxy.

It’s also possible for the black hole’s seed to be smaller, 100 solar masses or so, if it can accrete at super-Eddington rates for hundreds of millions of years — but exactly how that might happen is unclear, especially without the galaxy’s stellar mass growing in tandem. As JWST reveals more of the high-redshift universe, we’ll gain a better understanding of how black hole seeds are sown and sprouted.

Citation

“UNCOVER: A NIRSpec Identification of a Broad-Line AGN at z = 8.50,” Vasily Kokorev et al 2023 ApJL 957 L7. doi:10.3847/2041-8213/ad037a