A supermassive black hole has been found in an ultracompact dwarf galaxy – the smallest galaxy known to contain such a massive black hole. This finding, published today in Nature, suggests that supermassive black holes might be much more common than previously thought.
Black holes are collapsed stars with such strong gravitational fields that not even light can escape from their vicinity. Supermassive black holes are black holes with masses more than 1 million times the mass of our sun and they are thought to exist at the centre of most massive galaxies.
The supermassive black hole at the centre of our Milky Way has a mass of about 4 million solar masses. This is only a tiny fraction of the Milky Way’s total mass of about 50 billion solar masses.
Ultracompact dwarf galaxies are among the densest stellar systems in the universe. They are only a few hundred light-years across but can contain up to 100 million solar masses.
So far astronomers’ opinions were divided about the nature of these systems. Some astronomers argued that they might be the remnants of once much larger galaxies that have been torn apart due to collisions with other galaxies.
Others suggested that ultracompact dwarf galaxies are simply large scale versions of so-called globular clusters, groups of a few hundred thousand stars that were all born together in a single star burst.
A supermassive discovery
We are part of an international team of astronomers lead by researchers at the University of Utah that have now discovered that an ultracompact dwarf galaxy known as M60-UCD1 harbours a supermassive black hole.
M60-UCD1 is about 54 million light-years from Earth and lies about 22,000 light-years from the centre of the massive elliptical galaxy M60, the third-largest galaxy in the Virgo cluster of galaxies.
Today’s study, that we co-authored, reports finding a black hole with a mass of 21 million times that of our sun in this tiny galaxy. That’s about 15% of the total mass of M60-UCD1.
That makes it the smallest and lightest object we know of that has a supermassive black hole, and it’s also makes this galaxy of the most black hole-dominated known galaxies.
Challenging observations with giant telescopes
Discovering this supermassive black hole was made possible by using two of the world’s most powerful telescopes: the Hubble Space Telescope, and the Gemini North telescope, located on the summit of Hawaii’s Mauna Kea.
Australia is part of an international consortium that built and runs the Gemini Observatory, giving Australian researchers dedicated access to this unique facility. Astronomers compete for this time, and only the best projects get the chance to make their observations.
Gemini’s light-collecting mirror is 8m in diameter, allowing the faint M60-UCD1 to be observed. But Earth’s turbulent atmosphere makes the images blurry so this team used a special technique called adaptive optics.
This required firing a powerful laser into the night sky, which makes part of the upper atmosphere light up, creating a fake star. This is then used to measure how the atmosphere has blurred the image. This allows the image to be corrected using tiny, rapidly moving mirrors to reverse the blurring effect.
Combining these observations with super-sharp images from Hubble Space telescope, the researchers were able to see the effects of the supermassive black hole, and rule out any other plausible explanations.
The observations showed that the stars in the centre of M60-UCD1 move with a velocity of about 360,000km/h, much faster than what would be expected based on the amount of starlight that M60-UCD1 emits alone.
In principle, a large number of very dim stars could explain such large velocities, but these stars would be spread throughout M60-UCD1 and would raise velocities everywhere. The new observations showed elevated velocities only in the centre however, leaving a central supermassive black hole as the only reasonable explanation.
The dwarfs were once much larger galaxies
The fact that M60-UCD1 contains a massive black hole strongly suggests that this galaxy is the stripped nucleus of a once much more massive galaxy which came too close to M60 and was ripped apart by the strong gravitational force of M60.
This suggests that many of the other ultracompact dwarf galaxies are also likely to be the remnants of more massive galaxies that merged with other galaxies many billions of years ago when our universe was much denser and encounters between galaxies were common.
Supermassive black holes might therefore be much more common than astronomers previously thought. They probably exist not only in the centres of massive galaxies but in many smaller galaxies as well, prompting astronomers to continue to look for them.