A supermassive black hole (SMBH) has been found lurking in an unexpected location – at the heart of an ultra-compact dwarf galaxy – according to new observations made by an international team of astronomers. Although SMBHs are thought to reside at the centre of most large galaxies including our own Milky Way, this is the smallest galaxy known to host a black hole. The team’s findings suggest that many other such ultra-compact dwarf galaxies may house black holes, meaning that there may be many more SMBHs in our galactic neighbourhood than previously thought.

SMBHs are the largest type of black hole, and can have masses that are 105–109 times that of the Sun. On the other hand, ultra-compact dwarf galaxies are small galaxies that are also among the densest star systems in the universe. They are less than a few hundred light-years across as compared with our Milky Way’s 100,000 light-year diameter. However, astronomers have been puzzled by the very large estimated masses of these small galaxies, which seemed to suggest the unexpected presence of SMBHs.

Black hole inside

This theory now seems to be confirmed by observations, made by Anil Seth from the University of Utah in the US and colleagues, of a supermassive black hole inside the brightest-known ultra-compact dwarf galaxy M60-UCD1.

"We've known for some time that many ultra-compact dwarf galaxies are a bit overweight. They just appear to be too heavy for the luminosity of their stars," says team member Steffen Mieske from the European Southern Observatory in Chile. "We had already published a study that suggested this additional weight could come from the presence of supermassive black holes, but it was only a theory. Now, by studying the movement of the stars within M60-UCD1, we have detected the effects of such a black hole at its centre."

The team’s observations have also highlighted that there may be many black holes that have gone unnoticed to date. Indeed, there may be as many as double the known number of black holes in what astronomers refer to as our "local universe".

Lying about 50 million light-years away from Earth, M60-UCD1 is a tiny galaxy with a diameter of 300 light-years across. However, despite its modest size, it contains some 140 million stars. While this is a characteristic of an ultra-compact dwarf galaxy, M60-UCD1 happens to be the densest ever seen. The black hole itself has a mass of nearly 21 million Suns, which accounts for almost 15% of M60-UCD1’s total mass.

Small but dense

"That is pretty amazing, given that the Milky Way is 500 times larger and more than 1000 times heavier than M60-UCD1," says Seth. "In fact, even though the black hole at the centre of our Milky Way galaxy has the mass of four million Suns, it is still less than 0.01% of the Milky Way's total mass, which makes you realize how significant M60-UCD1's black hole really is."

The team made its discoveries using both the NASA/ESA Hubble Space Telescope and the Gemini North 8-metre optical and infrared telescope in Hawaii. The sharp Hubble images provided information about the galaxy's diameter and stellar density, while Gemini was used to measure the movement of stars in the galaxy as they were affected by the black hole's gravitational pull. These data were then used to calculate the mass of the unseen gravitational behemoth.

Stellar struggle

The team's findings also have an impact on current theories of how ultra-compact dwarf galaxies themselves are formed. "This finding suggests that dwarf galaxies may actually be the stripped remnants of larger galaxies that were torn apart during collisions with other galaxies, rather than small islands of stars born in isolation," explains Seth. "We don't know of any other way you could make a black hole so big in an object this small."

Seth and colleagues suggest that M60-UCD1 was, at one time, a much larger galaxy made up of 10 billion stars and hosted an appropriately sized SMBH. This ancient galaxy may have then passed too close to the centre of its much larger neighbouring galaxy, M60, thereby losing its outer part to its larger companion, leaving behind the small, compact galaxy we observe today. (M60 is also pulling in another galaxy, named NGC4647, which is 25 times less massive than it.)

The team says that, in the future, M60-UDC1 may merge with M60 – which harbours its own humongous black hole that is 4.5 billion solar masses and 1000 times bigger than our galaxy’s black hole – to form a single galaxy. A merger between the two galaxies would also cause the black holes to merge, creating an even more monstrous black hole.

The research was published in Nature.