Black Holes in Early Universe Are Too Big

Astronomers studying the early universe have found black holes there are “too big” for their galaxies. The find potentially paves the way toward better understanding how supermassive black holes form and how they and their galaxies evolve.

Nearly all large galaxies in the nearby universe have a supermassive black hole in their heart. These black holes’ masses range from millions to billions of Suns. The masses are usually wedded to the properties of the host galaxy — things like the galaxy’s total mass in stars, the range of speeds at which stars whip around the galaxy’s center, and so forth.

Astronomers still debate why these relations exist. It could be a lockstep process of coevolution, for example, or it could be that one of the partners pulls the other’s strings.

One way to answer the question is to see if the patterns change when we look back in cosmic time to the early universe. Astronomers have long suspected that they do change, and indeed observations have hinted that early black holes may be bigger than expected based on the relations with their galaxies that we see in today’s universe.

But this work is challenging. When we peer back to the first couple of billion years, it becomes far more difficult to detect massive black holes. The glow from their accretion disks can be swamped out by the light of their host galaxies or hidden behind the dusty gas clouds that surround them. Bigger black holes madly gobbling gas are the easiest to spot, potentially skewing our assessments.

Fabio Pacucci (Center for Astrophysics, Harvard & Smithsonian) and others have now tackled this question using spectroscopic data from the James Webb Space Telescope. Spectroscopy can pick up black holes that other methods don’t. Such detections give us a better peek at black holes at these early times and a better ability to compare their properties with those of their host galaxies.

The researchers gathered 21 supermassive black holes previously detected by a combination of teams. Each object has a mass estimate based on the speed of the hydrogen gas zipping around it, which reveals the object’s gravitational pull. The glow of these black holes also doesn’t overwhelm that of their galaxies, enabling astronomers to estimate how big the host galaxies are.

In galaxies in the local universe, the total mass of the galaxy’s stars is 1,000 times greater than the mass of the central black hole. But the team found that this set of faraway black holes is “overmassive”: They’re 10 or even 100 times bigger than we’d expect, based on the galaxies’ stellar masses, Pacucci explained during a press conference January 9th at the winter meeting of the American Astronomical Society in New Orleans.

Statistically, the team can’t explain these black holes’ masses as outliers of the normal relation, Pacucci says. And astronomers have more recently found additional, even larger black holes in the early universe that also seem to follow this new trend.

The work is a robust analysis of JWST’s cutting-edge data, says Jan-Torge Schindler (Hamburg Observatory, Germany), who wasn’t involved with the paper. But being “cutting edge” comes with caveats, he adds. Interpreting the data can be challenging, especially if the black holes lie close to the detection limit. Furthermore, astronomers are still grappling with the nature of these black holes at early times. Nevertheless, this work strongly supports the emerging picture that supermassive black holes bulk up faster than the stars in their host galaxies do.

In the case of these 21 galaxies, the black holes might have kept their galaxies small for a while, thanks to the energy pouring forth as they try to swallow copious amounts of gas, Pacucci suggests. When that lighthouse blaze shuts off, the gas can cool and form stars, building up the galaxies’ stellar mass and nudging things toward the relation we see today.

It may be that these black holes were born big, created when huge clouds of gas collapsed directly to become black holes instead of being built up from smaller seeds created from the cores of stars that explode as supernovae. Notably, several JWST results hint at this direct-collapse scenario — perhaps surprising, given the confluence of conditions that must exist in order to enable the scenario to unfold. But the data don’t rule out star-made black holes.

JWST spectra should reveal many black holes of somewhat lower, more “normal” masses at these early times, if they’re out there, Pacucci says. Within a few years, we should be able to say with more confidence whether most early black holes really were unexpectedly big for their galaxies.

References:

Fabio Pacucci et al. “JWST CEERS & JADES Active Galaxies at z = 4 – 7 Violate the Local M•–M⋆ Relation at >3σ: Implications for Low-mass Black Holes and Seeding Models.” Astrophysical Journal Letters. November 1, 2023.

Marta Volonteri et al. “What if young z > 9 JWST galaxies hosted massive black holes?” Monthly Notices of the Royal Astronomical Society. May 2023.