Early Galaxy Hosts Black Hole with the Mass of 50 Million Suns

New James Webb Space Telescope observations of a supermassive black hole near the edge of the observable universe have the potential to uncover how such behemoths came to be.

Supermassive black holes — singularities surrounded by vast event horizons, boundaries from which nothing, not even light, can return — are surprisingly common in our universe. Almost every large galaxy near us has one. Some of them lurk quietly at galactic centers (like the one in our Milky Way), while others blaze across the electromagnetic spectrum as they feed on gas buffets.

Yet we don’t understand their origins. Now, a new study posted on the astronomy arXiv preprint server provides a glimpse at the earliest years of one of these monstrous black holes.

How to “Weigh” a Black Hole

In imaging the galaxy, Webb had some help from nature: The distant galaxy, known as QSO1, is gravitationally lensed by the foreground galaxy cluster Abell 2744, whose gravity has distorted and magnified the object’s light into three identical images. Combining nature’s optics with that of the Webb telescope, PhD candidate Ignas Juodžbalis (University of Cambridge, UK) and colleagues zoomed in on the galaxy’s center, picking apart the motions of gas swirling within mere light-years of the black hole.

That the team could resolve gas moving on such small scales is incredible, given the sheer distance of the object — the light that gas is emitting traveled 13 billion years before it arrived at Earth. Yet those gas motions are key to directly measuring the black hole’s mass.

Juodžbalis’ team used Webb’s Integral Field Spectrograph, which imaged the far-away galaxy in such a way that every pixel contains its own spectrum. Using those spectra (specifically, shifts in the hydrogen-alpha emission line), the team mapped the motions of the gas at different points in the galaxy.  

But even with nature’s help, Webb struggles to resolve the galaxy’s inner regions. So Juodžbalis also used a newer technique called spectroastrometry. Rather than starting from the image itself, with this technique the team starts with the spectra. By making images of gas that’s moving at different velocities, the team can map gas motions much more precisely.

The result is the best mass measurement ever obtained for a black hole so far away. The behemoth has the heft of roughly 50 million Suns. That’s 10 times the mass of the Milky Way’s own supermassive black hole, yet it’s not a record-breaker by modern-day standards. Still, accumulating so much mass in just 800 million years is hard to explain — especially because, despite the swirling gas we see, the black hole only appears to be feeding on dribs and drabs.

It gets even weirder: The black hole is so massive that it accounts for almost the whole mass of the galaxy, with little for anything else. The galaxy is nearly devoid of stars, leading the team to call the object “the most ‘naked’ black hole ever found.”

Black Hole Seeds

There are two main ways to grow such an object, both of them requiring a massive “seed” — but neither one is a perfect match.

In the first scenario, a pristine gas cloud collapses directly into a black hole, skipping the intermediary of star formation. In the second, the black hole is primordial, coming into being just after the Big Bang among clouds of annihilating electrons and positrons.

Both of these scenarios have their drawbacks. The galaxy’s mass is so low compared to the black hole that it makes the direct-collapse scenario seem less likely. Yet the primordial scenario doesn’t make a black hole quite massive enough — it would have had to feed quickly or merge with lots of other primordial black holes to grow to its current size.

“The authors are right to suggest that it could be evidence in support of the ‘direct collapse black hole’ scenario or even the idea that this could be a remnant of a ‘primordial’ black hole from right after the Big Bang,” says Kevin Hainline (University of Arizona), who is not a coauthor of the paper. “I think the issue is that it’s difficult to find the ‘smoking gun’ of either scenario.”

The result provides exactly the sort of fodder that theorists thrive on, he adds.

At the same time, this black hole is only one of a new galaxy type discovered in Webb data, called “Little Red Dots” for their red color and compact appearance. Exactly what these galaxies are is still an open question, but it’s becoming clear that a significant fraction of them host growing supermassive black holes. The kinds of mass measurements done for QSO1 will need to be replicated in the hundreds of other Little Red Dots, too.

As Webb continues to turn up more of these galaxies, they’re providing astronomers with their best view yet of black hole seeds in the universe’s early years.