Astronomers from the University of Portsmouth in the UK, working with researchers at the Universidade Federal do Rio Grande do Sul in Brazil, have found what could be the most massive black hole ever measured. It sits at the centre of a "Cosmic Horseshoe" galaxy, which is so large that it bends spacetime and warps the light of a more distant galaxy into a horseshoe-shaped ring called the Einstein Ring.
The black hole is estimated to have a mass of 36 billion Suns, making it about 10,000 times heavier than the one at the centre of the Milky Way. This puts it close to the upper limit of how big black holes are thought to get in the universe. For comparison, the Milky Way’s central black hole has a mass of about 4 million Suns.
Scientists know that supermassive black holes (SMBHs) exist at the centre of nearly every large galaxy. Their growth is linked to the growth of their host galaxies over billions of years. In massive elliptical galaxies, the mass of the black hole is usually tied to how fast stars move around in the galaxy’s centre, a property called stellar velocity dispersion. Until now, most measurements of black hole mass have been made in nearby galaxies, because it is very hard to study the central regions of galaxies that are farther away.
The Cosmic Horseshoe gave researchers a rare chance to push beyond that limit. They combined two methods: stellar dynamics, which looks at how stars move, and gravitational lensing, which studies how the galaxy bends light. Using Multi Unit Spectroscopic Explorer (MUSE) integral-field spectroscopy and sharp images from the Hubble Space Telescope, they modelled both the galaxy’s radial arc, which is sensitive to the inner mass, and the motion of stars in the host galaxy. This allowed them to pin down the central mass distribution and the black hole’s size.
The results were checked using Bayesian model comparison, which confirmed the presence of an ultramassive black hole. The outcome was consistent across different tests, giving the team confidence in the measurement. The Cosmic Horseshoe black hole was found to sit above the expected relation between black hole mass and stellar velocity dispersion. This matches a trend seen in the most massive galaxies, where the relationship appears steeper than in smaller systems. It suggests that at the very highest masses, black holes and galaxies may grow together in a different way.
The black hole’s presence was detected in two clear ways. It bent the path of light passing nearby, and it caused stars in the inner regions of its galaxy to move at nearly 400 kilometres per second. Together, these effects gave a reliable measurement of its mass.
The Cosmic Horseshoe is about 5 billion light-years from Earth. Scientists think that mergers of galaxies over time can lead to the growth of ultramassive black holes, and such a process is expected when the Milky Way and Andromeda galaxies collide in about 4.5 billion years.
The discovery came while researchers were studying the galaxy’s dark matter. Now that the method has been shown to work, they hope to use data from the European Space Agency’s Euclid space telescope to find more supermassive black holes across a wider range of distances and sizes. These results could help explain how black holes shape the growth of galaxies throughout cosmic history.
Source: University of Portsmouth, Royal Astronomical Society | Image via Depositphotos
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