In summer 2022, just weeks after the James Webb Space Telescope (JWST) began operating, astronomers noticed something unexpected rather strange, tiny red objects scattered across the sky that had never been seen before. These "little red dots" appeared extremely compact and red, emitting light primarily in the mid-infrared, at these wavelengths the Hubble Space Telescope couldn't detect them but JWST was perfectly designed for the purpose.
The objects turned out to be incredibly distant, with light that had traveled 12 billion years to reach us, meaning we see them as they existed just 1.8 billion years after the Big Bang. But here's where things got really strange, if these were galaxies, they were impossibly massive for such an early time in the history of the universe.
The general consensus of the scientific community was that they were star rich galaxies containing hundreds of billions of solar masses worth of stars, packed into tiny volumes. To put this in perspective, if our Solar System sat in a cube one light year across, it would contain just our Sun. These proposed galaxies would pack several hundred thousand stars into the same space!
"The night sky of such a galaxy would be dazzlingly bright. If this interpretation holds, it implies that stars formed through extraordinary processes which have never been observed before.” - Bingjie Wang from Penn State University.
Such massive early galaxies challenged our current model of the universe and how it has evolved, raising questions about whether theories could even explain their existence. The breakthrough came from an object discovered in July 2024 by the RUBIES survey, led by Anna de Graaff at the Max Planck Institute for Astronomy. They dubbed it "The Cliff" after the most prominent feature in its spectrum, appearing as a steep rise in ultraviolet light that had been stretched into the near infrared by the expansion of space.
The Cliff appeared to be an extreme example of the little red dots, making it a perfect test case. But when researchers tried to fit existing models, be it a massive star forming galaxy or a dust shrouded black hole, the data seemed to contradict all models.
This led to their "black hole star" model, where a supermassive black hole is surrounded by an accretion disk, but also embedded within a thick, turbulent envelope of hydrogen gas. Unlike a real star, there's no nuclear fusion at the centre. Instead, the black hole's activity heats the gas envelope, creating an appearance remarkably similar to a genuine star.
The model isn't perfect and more work is required but it describes the data far better than any alternative. For The Cliff, the central black hole star would dominate the object's brightness, while other little red dots might show a mixture of light from both the black hole star and surrounding material.
The discovery represents a major step forward in being able to explain The Cliff's unusual spectrum. However, many questions still remain… How do such objects form? How is the gas envelope sustained when the black hole constantly consumes surrounding material? How do other spectral features arise? While promising, this research is still new and requires further work to validate. De Graaff's team already has approval for follow up JWST observations of little red dots scheduled for next year, which should help determine whether black hole stars are indeed the explanation for these mysterious objects.
Source : “Black Hole Stars” could solve JWST riddle of overly massive early galaxies