Astronomers have unveiled one of the largest known black holes, buried within a distant galaxy, five-and-a-half billion light-years away. Nestled at the heart of the "Cosmic Horseshoe" — a gravitational lensing system — this ultra-massive black hole (UMBH) weighs a staggering 36 billion solar masses, pushing the boundaries of current scientific understanding.
The discovery, led by Carlos Melo-Carneiro from Brazil's Universidade Federal do Rio Grande do Sul, was detailed in the study "Unveiling a 36 Billion Solar Mass Black Hole at the Centre of the Cosmic Horseshoe Gravitational Lens," available on arXiv.
A Cosmic Lens Reveals a Hidden Giant
First spotted in 2007, the Cosmic Horseshoe offers a rare cosmic alignment. The massive foreground galaxy, LRG 3-757, distorts and magnifies the light from a distant background galaxy, creating an Einstein Ring — a perfect circle of light caused by gravitational lensing.
LRG 3-757, a Luminous Red Galaxy (LRG), is about 100 times more massive than the Milky Way. Now, astronomers have confirmed that this colossal galaxy hosts one of the most massive black holes ever detected.
Supermassive black holes (SMBHs) typically sit at the centre of large galaxies, with their masses closely tied to their host galaxies. But this UMBH defies conventional expectations. The research highlights how LRG 3-757 deviates from the well-established MBH-sigmae relation — a correlation between a black hole’s mass and the speed of stars in its galaxy's core.
“Our findings place the Cosmic Horseshoe ~1.5 sigma above the MBH-sigmae relation,” the authors note, suggesting a steeper curve for the most massive black holes.
Challenging the Rules of Galaxy Evolution
So, why does LRG 3-757 host such a gigantic black hole? Astronomers suggest several possibilities.
One theory is that LRG 3-757 is part of a "fossil group" — ancient galaxy clusters where massive galaxies dominate, having absorbed their smaller neighbours. These groups are often quiet, with little star formation, earning them the nickname "red and dead."
Another explanation involves galaxy mergers. When two large galaxies collide, their central black holes can merge, while the surrounding stars get flung outward — a process known as "scouring." This could lower the measured star speeds without affecting the black hole's mass.
Active Galactic Nuclei (AGN) feedback is another possibility. As black holes consume gas and dust, they release powerful jets that can halt star formation and reshape the galaxy’s core.
Lastly, some astronomers think this UMBH might be the relic of a quasar — a bright, active galaxy core from the early universe that grew rapidly before settling into dormancy.
More Answers on the Horizon
This discovery raises fresh questions about galaxy evolution and the nature of ultra-massive black holes. But astronomers hope future missions will bring clarity.
The Euclid space telescope, launched to map the dark universe, is expected to identify hundreds of thousands of gravitational lenses over the next five years. Meanwhile, the upcoming Extremely Large Telescope (ELT) will offer detailed studies of galaxy dynamics, helping scientists decode the complex relationship between galaxies and their central black holes.
“This new era of discovery promises to deepen our understanding of galaxy evolution and the interplay between baryonic and dark matter,” the researchers conclude.
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