Astronomers exploring the distant universe with NASA’s most powerful telescope, the James Webb Space Telescope, have found a class of galaxies that challenge even the most talented creatures at mimicking—like the mimic octopus. This creature can imitate other marine animals to avoid predators. Need to be a flatfish? No problem. Need to be a sea snake? Easy.
When astronomers analyzed the first Webb images of the distant reaches of the universe, they detected a group of galaxies that had never been seen before. These galaxies—a few hundred in all, called the Little Red Spots—are very red and compact and have only been visible for about 1 billion years of cosmic history. Like the mimic octopus, the Little Red Spots have puzzled astronomers because they resemble different astrophysical objects. They are either very massive galaxies or medium-sized galaxies, each with a supermassive black hole at their center.
But one thing is certain. The typical Little Red Spot is small, only about 2% of the radius of the Milky Way galaxy. Some are even smaller.
As an astrophysicist who studies distant galaxies and black holes, I am interested in understanding the nature of these small galaxies. What powers their light and what are they really like?
Imitation competition
Astronomers analyze the light our telescopes receive from distant galaxies to assess their physical properties, such as the number of stars they contain. Using the properties of their light, we can study Little Red Spots and figure out whether they are made up of many stars or if they contain a black hole.
The light reaching our telescopes ranges in wavelength from long radio waves to energetic gamma rays. Astronomers separate the light into different frequencies and visualize them with a graph called a spectrum.
Sometimes the spectrum contains emission lines, which are frequency ranges where more intense light emission occurs. In this case, we can use the shape of the spectrum to predict whether the galaxy hosts a supermassive black hole and estimate its mass.
Similarly, studying the X-ray emissions from the galaxy could reveal the presence of a supermassive black hole.
Masters of disguise, Little Red Dots appear as different astrophysical objects depending on whether astronomers choose to study them using X-rays, emission lines, or something else.
The information astronomers have gathered so far from the spectra and emission lines of Little Red Spots has led to two different models explaining their nature: These objects are either extremely dense galaxies containing billions of stars, or they host a supermassive black hole.
Two hypotheses
In the star-only hypothesis, the Little Red Spots contain an enormous number of stars—up to 100 billion stars. That’s about the same number of stars as in the Milky Way—a much larger galaxy.
Imagine standing alone in a large, empty room. This vast, quiet space represents the region of the universe near our solar system where the stars are sparsely scattered. Now imagine that same room, but filled with the entire population of China.
This packed room is what the core of the densest Little Red Dots would feel like. These astrophysical objects may be the densest stellar environments in the entire universe. Astronomers aren’t even sure if such star systems physically exist.
Then there is the black hole hypothesis. Most Little Red Spots show clear signs of a supermassive black hole at their center. Astronomers can tell if a black hole is present in a galaxy by looking at the large emission lines in their spectra, which are created by the high-speed spinning of gas around the black hole.
Astronomers actually estimate that these black holes are very large compared to the size of the compact galaxies they contain.
Black holes typically have a mass of about 0.1% of the stellar mass of their host galaxy. However, some of these Little Red Spots host a black hole with a mass nearly as massive as their entire galaxy. Astronomers call these supermassive black holes because their existence defies the conventional ratio typically observed in galaxies.
But there’s another problem. Unlike regular black holes, the ones found in Little Red Spots probably show no signs of X-ray emission. Even in the deepest, high-energy images that should allow astronomers to easily observe these black holes, there’s no sign of them.
Little solution and much hope
But are these astrophysical curiosities simply giant galaxies with lots of stars? Or do they have supermassive black holes at their centers that are so big they don’t emit enough X-rays? What a puzzle.
With more observations and theoretical modeling, astronomers are starting to come up with some possible solutions. Perhaps the Little Red Spots are just stars, but these stars are so dense and compact that they mimic the emission lines typically seen from a black hole.
Or maybe supermassive—or even ultramassive—black holes lurk at the cores of these Little Red Dots. If so, two models could explain the lack of X-ray emissions.
First, there could be a large amount of gas floating around the black hole, blocking some of the high-energy radiation emitted from the black hole’s center. Second, the black hole could be pulling in gas much faster than normal. This process could produce a different spectrum, with fewer X-rays than astronomers usually see.
The fact that black holes are very large or extremely massive may not be a problem for our understanding of the universe, but it may be the best indicator of how the first black holes in the universe were born. In fact, if the first black holes to form were very large — about 100,000 times the mass of the Sun — theoretical models suggest that the ratio of the black hole mass to the mass of the parent galaxy could remain high for a long time after formation.
So how can astronomers discover the true nature of these tiny points of light that shone at the dawn of time? As with our master of disguise, the octopus, the secret lies in observing their behavior.
Additional observations using the Webb telescope and more powerful X-ray telescopes will eventually reveal a feature that astronomers can only attribute to one of two scenarios.
For example, if astronomers could clearly detect X-ray or radio emission or infrared light emanating from the vicinity of the black hole, they would know that the black hole hypothesis was correct.
Just as our marine friend may pretend to be a starfish, it too will eventually move its tentacles and reveal its true nature.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. By Fabio Pacucci Smithsonian Institution
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Fabio Pacucci receives funding from NASA and SI and is on the AXIS Science Team.