Astronomers exploring the distant universe with the James Webb Space Telescope, NASA's most powerful telescope, have discovered a class of galaxies that challenge even the most skilled mimics, like the mimic octopus. This creature can imitate other sea creatures to avoid predators. Need to be a flatfish? No problem. A sea serpent? Easy.
When astronomers analyzed the first Webb images of distant parts of the universe, they discovered a never-before-seen group of galaxies. These galaxies-numbering several hundred and called Little Red Dots-are very red and dense, and have only been visible for about 1 billion years of cosmic history. Like the mimic octopus, the Little Red Dots confuse astronomers because they resemble different astrophysical objects. They are either enormously massive galaxies or modestly sized galaxies, each with a supermassive black hole at its core.
One thing is for sure, though. The typical Little Red Dot is small, with a radius of only 2% of that of the Milky Way. 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?
The Imitation Contest
Astronomers analyze the light that our telescopes receive from distant galaxies to assess their physical properties, such as the number of stars they contain. We can use the properties of their light to study the Little Red Dots and figure out whether they are made of many stars or whether there is a black hole inside them.
Light reaching our telescopes ranges in wavelength from long radio waves to energetic gamma rays. Astronomers split the light into the 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 galaxy's X-ray emission can reveal the presence of a supermassive black hole.
The Little Red Dots are true masters of disguise, appearing as different astrophysical objects depending on whether astronomers study them using X-rays, emission lines, or something else.
The information astronomers have gathered so far from the spectra and emission lines of the Little Red Dots has led to two divergent models explaining their nature. These objects are either extremely dense galaxies with billions of stars, or they are home to a supermassive black hole.
The two hypotheses
In the stars-only hypothesis, the Little Red Dots contain huge numbers 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 yourself standing alone in a vast, empty room. This vast, silent space represents the region of the universe near our solar system where stars are sparsely distributed. Now imagine that same room filled with the entire population of China.
This crowded space 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 stellar systems can physically exist.
Then there is the black hole hypothesis. Most Little Red Dots show clear signs of the presence of a supermassive black hole at their center. Astronomers can determine if there is a black hole in the galaxy by looking for large emission lines in their spectra, created by gas around the black hole spinning at high speed.
Astronomers estimate that these black holes are too large compared to the size of the compact galaxies in which they live.
Black holes typically have masses of about 0.1% of the stellar mass of their host galaxies. But some of these Little Red Dots harbor a black hole nearly as massive as their entire galaxy. Astronomers call these supermassive black holes because their existence defies the conventional ratio typically observed in galaxies.
There is a catch, however. Unlike ordinary black holes, the black holes suspected of being in the Little Red Dots show no sign of X-ray emission. Even in the deepest, most energetic images available, where astronomers should be able to easily observe these black holes, there is no sign of them.
Few solutions and much hope
Are these astrophysical curiosities then enormous galaxies with far too many stars? Or do they harbour supermassive black holes at their centres that are too big and do not emit enough X-rays? What a mystery.
With more observations and theoretical models, astronomers are starting to come up with possible solutions. Perhaps the Little Red Dots are just stars, but these stars are so dense and compact that they mimic the emission lines you normally see from a black hole.
Or maybe supermassive - even overmassive - black holes lurk in the cores of these Little Red Dots. If so, two models could explain the lack of X-ray emission.
First, there could be huge amounts of gas floating around the black hole, which would block some of the high-energy radiation emitted from the center of the black hole. Second, the black hole could be accreting gas much faster than normal. This process would produce a different spectrum with less X-rays than astronomers normally see.
The fact that black holes are too big or too massive may not be a problem for our understanding of the universe, but rather the best indication of how the first black holes in the universe formed. If the first black holes ever formed were very massive - about 100,000 times the mass of the sun - theoretical models suggest that their ratio of black hole mass to host galaxy mass could remain high long after formation.
How can astronomers discover the true nature of these tiny points of light that shine at the beginning of time? As in the case of our master of disguise - the octopus - the secret lies in observing their behavior.
By using the Webb telescope and more powerful X-ray telescopes for additional observations, astronomers will eventually discover a feature that can be attributed to only one of the two scenarios.
For example, if astronomers could clearly detect X-rays, radio waves, or infrared light being emitted around the location where the black hole might be, they would know that the black hole hypothesis was correct.
Just as our sea friend pretends to be a starfish, he will eventually move his tentacles and reveal his true nature.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Fabio Pacucci, Smithsonian Institute Read more: Fabio Pacucci receives funding from NASA and SI and is a member of the AXIS Science Team.
