University of Colorado researchers work with an international team to uncover more about the mysterious objects detected by the James Webb Space Telescope

As the largest telescope in outer space, the James Webb Space Telescope (JWST) has been able to view celestial objects that are too dim or distant for its predecessors to detect. As a result, it has helped astronomers look deeper into topics like galaxy formation. However, the JWST can see only so far, and at the edge of its vision some of the most interesting recent astronomical observations have been made, in the form of strange, seemingly impossible objects.

They are small, red-tinted spots of light and were descriptively named little red dots (LRDs). Information on them is limited, though they are known to be extremely dense and to have existed twelve to thirteen billion years ago (for context, the Big Bang was slightly less than fourteen billion years ago). What can be seen of them now are afterimages, because looking so far into space also means looking back in time; even light takes a while to make it between galaxies. There are several theories about what LRDs are, but none of them can completely reconcile the evidence with established astronomical principles.

CU Boulder astrophysicist Erica Nelson and an international team of research colleagues found evidence that the little red dot dubbed Irony is a growing supermassive black hole, which suggests that at least some of the other little red dots are as well.

Erica Nelson, an assistant professor of astrophysics at the University of Colorado Boulder and one of the researchers who first discovered LRDs, recently published a study that focuses on a specific LRD dubbed Irony. The study was co-led by Francesco D’Eugenio at Cambridge University and included CU Boulder PhD student Vanessa Brown as well as an international team of scientists. They found evidence that Irony is a growing supermassive black hole, which suggests that at least some of the other LRDs are as well.

Little red dots

According to Nelson, there are two main interpretations of what little red dots are. “Either they are really massive galaxies, or they are growing supermassive black holes,” she says. The two can be difficult to distinguish because both are very luminous. Massive galaxies are luminous because they typically have more stars, but “contrary to what most people expect, supermassive black holes are incredibly luminous” too, Nelson continues, “especially when they’re growing.”

Either of these possibilities would have implications for our understanding of the history of the universe. If LRDs are massive galaxies, “it could mean that early galaxies grow much more rapidly than we think they should be able to,” Nelson explains. That could be because their stars formed in a different way than how scientists have observed stars to form previously.

If they are supermassive black holes, they could be a phase in the development of black holes long hypothesized by CU Boulder professor Mitch Begelman, though never observed. “For a long time, we have tried to understand how supermassive black holes can grow so fast,” Nelson says. If LRDs represent an early phase of supermassive black hole growth, it could help narrow down the possibilities for how they form, “which has been a mystery for a really, really long time.”

Regardless of what the answer is, if it falls into one of these interpretations, it will provide insight into a broader question: whether galaxies or supermassive black holes formed first. That matters because most large galaxies, including the Milky Way, seem to have supermassive black holes at their centers. So, even if LRDs are black holes, that fact will have implications for galaxy formation.

The Irony is…

Irony is the name of the LRD with the deepest medium-resolution JWST spectroscopy to date. Spectroscopy is a way of determining what elements objects are made of, along with other characteristics like density and heat, based on the light coming from them. Irony is an incredibly bright object, giving off more light than other LRDs, so the researchers were able to get more details about it using spectroscopy. Upon examination, these details reveal several oddities.

 

Images of little red dots captured by the James Webb Space Telescope. (Photo: NASA)

“One is that it was the first time we have detected forbidden iron lines in any distant object,” Nelson says. Spectroscopy uses lines in a spectrum to represent the types of light coming from an object, and this pattern of lines corresponds to iron. The reason they are considered forbidden is technical and not immediately relevant; their detection is significant because scientists do not expect to find iron in something as old as an LRD. “The universe began with just hydrogen and helium,” Nelson explains. “There was no carbon, no oxygen and no iron.”

Heavier elements like iron were produced in the cores of stars over several generations through nuclear fusion. When older generations of stars went supernova, they launched heavier elements than what they formed out of into space, to be picked up by newer generations of stars and fused into even heavier elements. “So, seeing a lot of iron at very early cosmic times means that there had to have been a lot of generations of star formation very rapidly,” Nelson says. Iron in particular is the heaviest element that a star can create during normal hydrogen fusion (the others are only made during supernovae), so it is strange to find iron in older objects.

Another oddity is the strength of Irony’s Balmer breaks, which are breaks in the spectrum of light coming from an object. “The thing we have started to find in some of these little red dots, and especially in Irony, is that the breaks are too strong and too smooth to be produced by stars,” Nelson explains. “No model we can generate produces a break like that, so we think, instead of the atmospheres of a bunch of old stars, it is actually this single atmosphere around a growing supermassive black hole.”

These features suggest that Irony is a supermassive black hole rather than a massive galaxy. Other LRDs may not be the same as Irony, but making this determination about Irony strengthens the argument that some LRDs are supermassive black holes.

Black hole sun

All of this raises a question: What does it mean for Irony and potentially other LRDs to be black holes if LRDs do not fit cleanly into the category of either galaxies or black holes? “The kind of supermassive black holes that these things might be, and that a subset of them likely are, is nothing like any supermassive black holes we’ve seen before,” Nelson answers. They could be a new class of object, called black hole stars or quasi-stars that have been hypothesized by CU Boulder professors Mitch Begelman and Jason Dexter, that in some ways look like incredibly large stars but function differently.

“Instead of being powered by nuclear fusion like our sun and all other stars are, they’re being powered by the energy that is radiated when matter falls into the supermassive black hole,” Nelson explains. This energy comes from the gravitational potential of the objects. Similar to how charging a battery allows it to release energy later, moving an object into a place like the edge of a cliff “charges” it with energy that will be released when it falls. This gravitational potential would be especially strong because of how much gravity black holes of this size exert.

Another telling detail is the mention of an atmosphere around the supermassive black hole, which is not part of the common image of a black hole. “Normally,” Nelson says, “you have the supermassive black hole, and then an accretion disk around it.” The accretion disk is the glowing ring and halo that has appeared in many depictions of black holes in popular culture. “The new theory of these black hole stars is that there is almost spherical accretion.” However, this is a more theoretical aspect of the research, and there are different opinions about the structure that this type of black hole would have.

More research is planned to help resolve these ambiguities, and several JWST proposals for next year are designed to help. Two major points that Nelson identifies are collecting data on more LRDs to understand the variations that exist between them and collecting new data to see if previously observed LRDs have changed since they were first documented.

“Maybe some of them are massive galaxies, maybe some of them are black hole stars, maybe some of them are something else entirely,” she says. “It also helps to have information at different times because things as compact as black holes should show variation on very short timescales, so that will tell us a lot about the nature of the object.

“It’s been a really cool time in extragalactic astrophysics,” Nelson continues, “because a big segment of our field is pitching in and collaborating to try to figure out a true mystery that the universe has shown us. It’s also a strange time, because a lot of funding has been cut from astrophysics in particular. But with support, it could be a golden era in astrophysics. A lot of new discoveries will be made with James Webb. We really are just at the beginning of the data that we’re getting.”

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