Strangely bright galaxies spotted by the James Webb Space Telescope (JWST), called 鈥渓ittle red dots鈥�, may have more stars packed into them than any other galaxies we know of. The density appears so high that it鈥檚 unclear how the stars even survive without crashing into their neighbours, challenging astronomers鈥� best ideas of how galaxies grow.
Shortly after JWST started searching the extremely distant universe in 2022, astronomers started to see extremely bright and red, but apparently tiny, galaxies, which they called little red dots (LRDs). Their brightness had two obvious, yet problematic, explanations. Either it came from supermassive black holes that were so large they would have had to form incredibly quickly, close to the beginning of the universe, or it came from the stars themselves, which would require masses of stars that were also difficult to explain.
There were further puzzles for astronomers. When they analysed the light, it had some features of what supermassive black holes look like, such as a broad spectrum of different frequencies, but it was missing other key features seen with black holes, such as X-rays and radio waves. There were also other features, such as a peak in the light鈥檚 brightness at certain frequencies, that suggested it came from stars. It was difficult to distinguish between the two scenarios for these faraway galaxies, in part because astronomers lacked accurate properties for them, like their size.
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Now at Yale University and her colleagues have used JWST data to calculate the properties of three LRD galaxies, including their size. The results show that these galaxies are so small that if the light they produce is coming from stars, those stars must be impossibly tightly packed.
鈥淭hey鈥檙e so compact, it was kind of a shock the first time we measured these sizes,鈥� says Baggen. 鈥淭hey鈥檙e about 100 times smaller than a typical galaxy that we see today, like the Milky Way, but it鈥檚 possible that they have the same stellar mass. That鈥檚 a big issue,鈥� says Baggen.
They also found further evidence that suggests this light was coming, at least in part, from stars. Using the galaxies鈥� size and mass, Baggen and her team then calculated how fast these stars were moving and found that, on average, they were travelling around 10 times faster than stars in regular galaxies. This speed might be enough to account for the broad-frequency radiation that astronomers have observed from the LRDs but that is normally associated with black holes, says Baggen.
鈥淭his is now a very self-consistent story,鈥� says Baggen. 鈥淲e don鈥檛 see X-ray; we don鈥檛 see radio; the broad lines are simply explained by the size and the stellar mass. Everything works, but the densities are just so high that we still need to understand how that happened. I like this story a lot, but it requires more evidence.鈥�
This is an intriguing hypothesis, says at the University of Oxford, and there is a way to test it. Another instrument on JWST, which measures a different, less red part of the light spectrum, can measure these star velocities more directly, which could tell us whether we really are seeing stars, he says.
In a separate study, at the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues looked at the densities for a much larger sample of around 500 LRD galaxies, though with less detailed data on each one than Baggen and her team, and also found impossibly high stellar densities. 鈥淚t鈥檚 possible that we are overestimating the stellar masses, but if they are real, it鈥檚 a serious problem for astronomy,鈥� says Pacucci.
The fate of these galaxies is a further conundrum. It is unclear whether star densities this high are even possible without stars colliding and eventually forming a black hole, says Pacucci. But if they do form such large black holes, like quasars, then it doesn鈥檛 match how many quasars we see in the early universe.
If the stars can avoid coalescing into a black hole and instead evolve into a regular galaxy, then they should become less dense, says Baggen, but there aren鈥檛 good physical explanations for how this might happen, or what they might look like. 鈥淣obody really understands what they are evolving into. It鈥檚 such a weird population,鈥� says Pacucci.
Another thing to consider is that while the LRD galaxies appear similar in being bright, red and small, they could actually be made up of very different galaxy types, such as a mixture of black hole-dominated and star-dominated galaxies, says at the University of Cambridge. This could make their apparently extreme properties easier to explain for astronomers because it would better match the diversity of galaxies we see in our local universe, which would be like the LRD鈥檚 descendants.
arXiv
Research Notes of the AAS
