Astronomers using the James Webb Space Telescope have re-evaluated the mysterious 'little red dots,' distant galaxies that initially appeared impossibly bright. New analysis suggests these galaxies host modest 'baby' black holes rather than massive ones or excessive stars. This finding resolves tensions in models of early universe galaxy formation.
The James Webb Space Telescope (JWST), launched in 2021, has revealed hundreds of very red and bright galaxies in the early universe, nicknamed 'little red dots' (LRDs). These observations initially puzzled scientists, as the galaxies' light output implied either an unprecedented density of stars or black holes far larger than expected for their size, challenging theories of cosmic evolution.
Early interpretations assumed the red hue came from dust obscuring the light, similar to local galaxies. However, Jenny Greene at Princeton University and her team found no evidence of such dust emission. '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,' Greene said. 'That was the big clue that our assumption that they’re dusty is just wrong, that’s not why they’re red.'
In a fresh study, the researchers measured total light from two LRDs across multiple wavelengths, including X-rays and infrared. They discovered the galaxies emit far less light than previously estimated— at least ten times dimmer in most frequencies except visible light. This adjustment points to smaller black holes inside. 'If there’s actually not as much light there as we thought, the black hole masses are probably much more modest,' Greene explained. 'Then they don’t have to be that over-massive, and we don’t have to have too much mass in black holes at early times, so it really alleviates a lot of the tension that was perplexing us.'
Rohan Naidu at the Massachusetts Institute of Technology described these as 'baby black holes,' possibly 'black hole stars' surrounded by gas, where visible light represents most of the energy output. 'The little red dots we now understand should really be thought of as these puffed-up black hole stars,' Naidu said. 'What you see is what you get.'
Yet, Roberto Maiolino at the University of Cambridge cautioned that emitted light reveals growth rates, not total mass, leaving some uncertainty. Greene countered that fewer photons imply a downward shift in mass scales. The research appears in the Astrophysical Journal (DOI: 10.3847/1538-4357/ae1836).