Distant 'little red dot' galaxies may contain baby black holes

A collection of “little red dots” spotted by the James Webb Space Telescope NASA, ESA, CSA, STScI, and D. Kocevski (Colby U.)/Space Telescope Science Institute Office of Public Outreach Impossibly bright galaxies discovered by the James Webb Space Telescope (JWST) may not be so bright after all. These galaxies once threatened to upend our understanding of the cosmos by suggesting it contained monstrous black holes or far more stars than we had anticipated, but astronomers now think the galaxies might actually contain “baby” black holes instead. In its first few years scanning the early universe, JWST unexpectedly discovered hundreds of very red and extremely bright galaxies, which astronomers called “little red dots” (LRDs). The amount of light coming from these galaxies suggested that they contained either a density of stars that was far greater than any galaxy we know of, and which would be difficult to square with the mass of the universe that we expect, or black holes that were far more massive than astronomers thought possible given the size of their host galaxies. Both of these scenarios would have required significant adjustments to our models of galaxy formation and black hole growth in the early universe. But these conclusions also rested on the assumption that the LRDs’ red colour was due to an abundance of dust, either around the black hole itself or surrounding the stars, because that is what is typically found in the very red galaxies of our local universe. Recently, that idea has been questioned, after researchers found a lack of evidence that LRDs contain dust after all. Free newsletter Sign up to Launchpad Bring the galaxy to your inbox every month, with the latest space news, launches and astronomical occurrences from New Scientist’s Leah Crane. Jenny Greene at Princeton University and her colleagues think that this finding means we need to reconceive what LRDs are. “We were sure that we could detect the dust emission, if indeed they were red because of dust, and then we did not find that emission at all,” says Greene. “That was the big clue that our assumption that they’re dusty is just wrong, that’s not why they’re red.” Previous observations had inferred the total brightness of LRDs by measuring a specific single frequency of light, associated with the element hydrogen, which can then be used to calculate the total brightness, based on typical models of how dust affects this light. In a new analysis, Greene and her team directly measured the total light emissions from two LRD galaxies by looking at many different frequencies of light, including X-rays and infrared. They found that for most frequencies, apart from visible light, there was much less light being emitted than for typical galaxies, suggesting that the LRDs were at least ten times dimmer than initial estimates suggested. This finding has implications for the black holes inside the LRDs. “If there’s actually not as much light there as we thought, the black hole masses are probably much more modest,” says Greene....

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