A huge black hole has been found at the heart of the M60-UCD1 dwarf galaxy – one of the smallest and densest galaxies ever known.
The galaxy has a diameter of around 300 light years – that is just 1/500 the size of the Milky Way, yet its newly-discovered supermassive black hole is five times the mass of the one at the centre of our own galaxy.
Black holes are gravitationally collapsed, ultra-compact objects with a gravitational pull so strong that even light cannot escape. Supermassive black holes - those with the mass of at least one million stars like our sun - are thought to be at the centres of many galaxies.
By combining data from Nasa's Hubble Space Telescope and the Gemini North telescope, researchers at the University of Utah calculated the mass of the black hole and compared it to its surroundings.
The sharp Hubble images provided information about the galaxy’s diameter and stellar density, while Gemini measured the affect of the black hole's pull on the stars. The results are published in the journal Nature.
But how did such a massive black hole come to sit in such a tiny galaxy? The most likely explanation is that M60-UCD1 was once a large galaxy containing 10 billion stars, but when it passed very close to the centre of an even larger galaxy, M60, all the stars and dark matter in the outer part of the galaxy were torn away and became part of M60.
“We don’t know of any other way you could make a black hole so big in an object this small,” said Anil Seth, lead author of the study.
Both M60-UCD1 and M60 have their own black holes, and M60's is so enormous it weighs more than 1,000 times the black hole in our galaxy. It is thought that eventually the two galaxies will come together and their black holes will merge to create an even bigger one.
But that won't be for quite a while – M60-UCD1 and M60 are currently 50 million light-years apart.