Like tiny photobombers, cosmic anomalies resembling small, brilliant red factors present up in nearly each snapshot taken by the strongest area telescope ever made. Astronomers now name them little red dots, or LRDs, however there isn’t a settlement but on what precisely they are.
Since NASA’s James Webb Space Telescope began peering into the universe 4 years in the past, lots of of the puzzling objects have appeared in its photos. Their unknown origins successfully launched a scientific case that lots of of research have tried to crack.
“This is the first time in my career that I have studied an object where we truly do not understand why it looks the way it does,” mentioned Jenny Greene, a professor of astrophysical sciences at Princeton University. “I think it’s fair to call them a mystery.”
One factor was clear from the starting — these unusual objects had been frequent. “Every deep pointing you did with James Webb, you were finding a few,” mentioned Greene, referring to the motion of focusing the telescope on the identical patch of sky for an prolonged time to gather extraordinarily faint gentle.
Initially, some astronomers recommended the dots could possibly be large galaxies from the early universe, or black holes surrounded by mud. However, these preliminary assumptions had been later upended by additional observations, paving the approach for a number of new hypotheses, a lot of them nonetheless involving black holes.
“I certainly think they’re powered by growing black holes, but there are other, more exotic suggestions, like some kind of very massive star dying,” Greene mentioned. An skilled in supermassive black holes and galaxy evolution, she defined that she believes a black gap as the fundamental element of LRDs matches the largest variety of the observations made from the objects to date.
However, she added, somebody might make a completely new statement that overturns each assumption about what LRDs are. “So far, that’s what’s happened. We’ve had an expectation, it’s been wrong. We’ve had another expectation, it’s been wrong. So I would leave that possibility open still.”
Whether these curious dots in the end affirm older theories or characterize a novel discovery, scientists are set to realize a brand new understanding of the universe.
The title little red dots first appeared in a 2024 study, nearly two years after scientists had begun learning the objects. The moniker was coined by Jorryt Matthee, head of the analysis group on the astrophysics of galaxies at the Institute of Science and Technology Austria, who selected it as a result of it was easier and catchier than the extra scientifically correct time period: “broad-line H-alpha emitters.”
The motive astronomers solely noticed LRDs after Webb got here on-line is that different telescopes in operation at the time, like Hubble, didn’t have sufficient decision or lacked the sensitivity in the longer infrared wavelengths, past the threshold of seen gentle, to see them. But the Webb telescope, with its 21.6-foot-wide (6.5-meter-wide) main mirror, has revealed objects that had been beforehand hidden.
The dots seem red as a result of they are so distant, and as the universe expands, gentle from extraordinarily distant objects will get stretched into the infrared because it travels to achieve Earth — a phenomenon astronomers name “redshift.”
But the dots are additionally inherently red, though the precise motive why is one among the trickiest components of the puzzle.
“The main interpretation in our 2024 study was that these are growing black holes, and that they are red because they are surrounded by dust particles,” Matthee mentioned. “I would say that was the consensus after our paper for at least one or two years, but now the consensus has actually changed a bit. We still think they are growing black holes, but we now think they are not red because there’s dust, but because there’s hydrogen gas.”
Much of the uncertainty round the objects stems from their distance. Even although astronomers have detected about 1,000 of them, Matthee famous they are nearly all extremely distant.
“LRDs are widespread in the early universe — primarily the first billion years of cosmic time, with the current age being 13.8 billion years — but they are extremely rare in the more nearby, or later, universe,” he defined, referring to the indisputable fact that taking a look at a distant object in area basically means wanting again in time. That’s as a result of the farther away one thing is, the longer it takes for its gentle to achieve us.
Last 12 months, a crew of researchers discovered three LRDs a lot nearer to Earth for the first time, and research are underway to investigate them. But primarily based on that discovering, Matthee mentioned, native LRDs could possibly be 100,000 occasions rarer than these discovered farther away in the early universe.
However, if extra native LRDs are discovered, they may reveal extra of their secrets and techniques, as a result of it’s simpler to check an object that’s nearer.
“In terms of how LRDs could change our understanding of black holes, I think they might turn out to be some kind of missing link,” Matthee mentioned. “We know that galaxies, like our own Milky Way, have supermassive black holes in their center, and while this is very common, it’s basically a mystery how these supermassive black holes formed. The LRDs may actually be the birth phase, or the baby phase, of this formation, and we might be observing that for the first time.”
The closest factor to a census of the little red dots got here in 2023, after a crew of researchers led by Anna de Graaff, a Clay Fellow at the Harvard–Smithsonian Center for Astrophysics, began a program known as RUBIES, or Red Unknowns: Bright Infrared Extragalactic Survey. The program spent a big quantity of Webb telescope time — 60 hours — analyzing hundreds of red and brilliant objects.
“It was really the first program to go after these red sources systematically, observing all sorts of strange objects — not just little red dots — but among them, also 40 or so LRDs,” de Graaff mentioned.
The largest shock, de Graaff added, is an object she calls “The Cliff,” the options of which seem to disprove early hypotheses for what LRDs could possibly be. “This source is really the first one where we could say unambiguously, this is neither a normal galaxy nor a dust-shrouded black hole — it has to be something else,” she mentioned. “It was a bit of a breakthrough moment.”
The Cliff received its title as a result of its gentle spectrum has a really steep transition — from weak ultraviolet to intense red. “A feature that can only be caused by very dense hydrogen gas that is somewhat warm in temperature,” de Graaff mentioned. “This is surprising, because it means that LRDs are not red because they have old stars or because they have dust, but they are red because the light is being absorbed by a very dense gas surrounding a central engine, which we think is a black hole. And that is something that has never been observed before,” de Graaff mentioned, underscoring the indisputable fact that The Cliff suggests the existence of a brand new kind of cosmic object.
In some papers, de Graaff refers to such objects as “black hole stars,” a reputation she describes as barely clickbaity, however not totally fallacious.
“We do think that there is a black hole there that’s powering it, and the light from this black hole is illuminating the gas around it, in a way that is a little bit similar to what we see in stars,” she mentioned. Black holes themselves don’t emit gentle, however the superheated materials that falls into them intensely glows, so rising black holes are amongst the brightest objects in the universe.
The Cliff additionally shares similarities with theoretical objects known as quasi-stars, which had been predicted in 2006 — properly earlier than little red dots had been found — by Mitch Begelman, a professor in the division of astrophysical and planetary sciences at the University of Colorado Boulder, with colleagues Marta Volonteri and Martin Rees.
They described a quasi-star as a star that’s powered not by nuclear fusion however by a black gap, which is surrounded by an enormous cloud of fuel that makes it shine like a star. Unlike de Graaff’s black gap star, a looser time period for a star powered by a black gap of unknown origin, a quasi-star is an outlined theoretical mannequin, in which the black gap is the results of the collapse of an enormous protostar.
“I realized that we had predicted the existence of black holes with enormous envelopes of matter. I don’t think we necessarily have the smoking gun that this is the explanation for LRDs, but so far, I haven’t seen any evidence that poses an insurmountable problem for that picture,” Begelman mentioned.
A wierd hybrid between a star and a black gap can be a brand new kind of cosmic object, so there may be understandably some warning from researchers on declaring quasi-stars the winners of the little red dot debate.
“It could well be that LRDs are quasi-stars, but in my view we have not yet fully ruled out other scenarios,” Matthee mentioned. “I would definitely love this to be true, as it would imply we discovered a new type of astrophysical phenomena that bridges stars and supermassive black holes, but it’s too early to tell, in my view.”
For de Graaff, the fundamental concern with quasi-stars is that they are a selected kind of object, and we merely don’t know sufficient about LRDs simply but. “It’s very hard to prove that there is a black hole in LRDs, the evidence is nonexistent at the moment,” she mentioned. “The only reason we think that there are black holes in them is because they are so luminous and because there are so many of them. That’s our scientific gut feeling, but actually proving that is difficult.”
It’s laborious to pinpoint at what stage of the little red dot debate the scientific neighborhood may be proper now, however most researchers suppose they are not even near a decision. However, that’s what makes the objects so fascinating.
“I think they are the biggest surprise from James Webb, and it’s the sort of surprise that you’d hope for,” de Graaff mentioned.
“James Webb is a $10 billion space mission, and you hope to find things that are truly unknown,” she added. “I think it has delivered. It’s really given us a new puzzle, something that looks a bit like a galaxy, a bit like a black hole and a bit like a star — experts from all these communities are now trying to chip in and put forward their pet theory or their insights. And I think that’s really unique.”
Sign up for NCS’s Wonder Theory science newsletter. Explore the universe with information on fascinating discoveries, scientific developments and extra.