Black Hole May Have Formed by Direct Collapse

Astronomers may have spotted a supermassive black hole in the early universe that formed when a gargantuan gas cloud imploded.

The black hole’s host galaxy, UHZ1, was spotted in James Webb Space Telescope (JWST) observations of galaxies in the early universe. These distant galaxies’ light has been bent and magnified by the intervening galaxy cluster Abell 2744, bringing them into view.

Ákos Bogdán (Center for Astrophysics, Harvard & Smithsonian) and others used the Chandra X-ray Observatory to take a second look at 11 of the lensed galaxies. Based on which wavelengths the galaxies are detectable at, each of the 11 appeared to lie at a redshift greater than 9, which means they’re shining at us from the universe’s first 500 million years. The team picked up X-rays from just one galaxy, the most magnified of the bunch.

The X-ray source is a dead ringer for a gigantic black hole shrouded in gas. Based on the data, the astronomers think the object has a mass of roughly 40 million Suns, or 10 times larger than the black hole at the center of the Milky Way today. Subsequent work with JWST confirms that UHZ1 has a redshift of 10, appearing to us as it was a mere 470 million years into cosmic history.

Meanwhile, the galaxy itself appears to be fairly normal compared with its peers at that time, with a mass of some 100 million Suns, similar to the Small Magellanic Cloud near the Milky Way. But that puts the galaxy on a level with the black hole it holds — in stark contrast with the modern universe, where black holes are usually 0.1% as massive as their host galaxies. 

This too-big-for-its-galactic-britches black hole has the team excited — in fact, they set out hoping to find exactly this kind of object.

How to Make A Big Black Hole

Astronomers debate how the first supermassive black holes formed. In one scenario, big stars collapsed, then grew at a breakneck pace by scarfing down gas and whamming into each other. In another, big, pristine clouds of gas collapsed directly into black holes of tens of thousands of solar masses, maybe or maybe not without forming a star-ish object first. They then kept growing from there.

The more pedestrian star-turned-black-hole scenario struggles to explain rare leviathans found madly guzzling gas within 1 billion years of the Big Bang. These black holes pack in billions of Suns’ worth of mass — hard to explain if they started less than a billion years earlier as 100-solar-mass objects. (Black holes can only eat so fast.)

On the other hand, the direct-collapse scenario suffers from a fragile convergence of factors. The gas can’t be contaminated by heavy elements made by stars, yet it has to be heated by nearby stars so that it doesn’t collapse too early. One solution is that the black hole forms in a satellite next to a protogalaxy brimming with young suns.

Priyamvada Natarajan (Yale), Bhaskar Agarwal (then at the Max Planck Institute for Extraterrestrial Physics, Germany), and others previously predicted that the satellite cloud holding the direct-collapse black hole would merge with the larger, burgeoning galaxy soon after the black hole’s formation. This mingling would create an over-massive black-hole galaxy, or OBG, in which the black hole’s mass roughly matched that of the new host galaxy. These objects should exist between 350 and 550 million years after the Big Bang.

Smack-dab when UHZ1 is.

This galaxy could be the first OBG found, Bogdán, Natarajan, and others argue in three papers posted to the arXiv preprint server and currently undergoing peer review. UHZ1’s appearance at different wavelengths matches the predictions, and the galaxy is a bit distended, as though it recently merged with something.

Xiaohui Fan (University of Arizona), who was not involved in the trio of papers, says he thinks this result and others related to early black holes are among the most exciting finds from JWST. If follow-up X-ray observations confirm this black hole’s mass, then the direct-collapse scenario would be a “very natural explanation.”

UHZ1’s black hole did not necessarily keep growing to become one of the mysterious behemoths seen a few hundred million years later, the authors caution. A black hole’s growth depends on its environment, and simulations indicate that the largest black holes don’t always stay the largest over time.

Still, this is the first chance astronomers have to explore the earliest black holes’ formation with real data instead of theory. As observers build up a census of these black holes, they’ll be able to say more about how the objects form.

References:

A. Bogdán et al. “Detection of an X-ray Quasar in a Gravitationally-Lensed z = 10.3 Galaxy Suggests that Early Supermassive Black Holes Originate from Heavy Seeds.” Posted to arXiv.org May 24, 2023.

P. Natarajan et al. “First Detection of an Over-Massive Black Hole Galaxy: UHZ1—Evidence for Heavy Black Hole Seeds from Direct Collapse?” Posted to arXiv.org August 4, 2023.

A. D. Goulding et al. “UNCOVER: The Growth of the First Massive Black Holes from JWST/NIRSpec – Spectroscopic Confirmation of an X-ray Luminous AGN at z=10.1.” Posted to arXiv.org August 8, 2023.