Dark matter-fuelled brown dwarfs could be lurking at the centres of galaxies. If astronomers manage to spot them, they could teach us about how dark matter interacts with regular matter.
Brown dwarfs are vast balls of gas, between 13 and 72 times as massive as Jupiter but smaller than stars and with too little matter to sustain the nuclear fusion of hydrogen in their cores. The threshold at which they start fusing hydrogen and become stars, known as the hydrogen burning limit, is around 0.075 times the mass of our sun.
at Durham University in the UK and her colleagues have now found that this limit might be higher if the brown dwarfs have dark matter annihilating inside them and releasing energy. According to calculations from Croon and her team, this extra energy would cause the brown dwarf to expand, making it cool slightly. “Therefore, you need a larger [brown dwarf] to actually still reach the temperatures that you need to start burning hydrogen,” she says.
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To calculate the properties of these objects, which the researchers call “dark dwarfs”, they came up with an equation that relates the dark dwarf’s internal pressure to how much energy it has. From this, they could then work out how key features of the objects, such as their size and temperature, change over time and compare them with regular brown dwarfs that don’t have the additional energy from dark matter.
Unlike regular brown dwarfs, which tend to cool over time and aren’t bright enough to easily spot, Croon and her team found that dark dwarfs should remain relatively bright for much longer periods of time. They may also be identifiable through elevated levels of lithium – because dark dwarfs are also slightly cooler than brown dwarfs, they wouldn’t be able to burn as much of the element as their dark matter-free cousins. If astronomers see a population of brown dwarfs that appear to have these characteristics, a statistical analysis could indicate they are being powered by dark matter, says Croon.
It is exciting to have specific observational signatures to look for these objects, says at the University of Pennsylvania. New instruments, such as the James Webb Space Telescope and the planned Nancy Grace Roman Space Telescope, might be able to detect signatures like elevated lithium, she says.
It isn’t clear exactly where we might find these brown dwarfs, but they require a relatively high concentration of dark matter, which is thought to only be a feature near the centre of galaxies, says Croon. If we do spot dark dwarfs, then it could tell us about how frequently dark matter interacts with normal matter, she says. However, it still won’t tell us anything about the mass of dark matter particles, as that doesn’t affect the equations that the researchers used.
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