Astronomers have recently discovered that a little galaxy has a big secret. Ultra compact dwarf galaxies (UCD) contain about 100 million stars spanning a couple hundred light years. As a comparison, our Milky Way has about 300 billion stars spread across 100,000 light years. Using the Hubble Space Telescope, an international team of astronomers led by Anil Seth of the University of Utah discovered that M60-UCD1 has a supermassive black hole, making it the smallest galaxy ever discovered to have one. The paper was published in Nature.
M60-UCD1 is located about 54 million light years away and is a satellite of the larger elliptical galaxy Messier 60 in the constellation Virgo. The tiny galaxy is only 300 light years across, but the team discovered in September 2013 that it is likely the densest known galaxy. This follow-up study has revealed that a supermassive black hole at the center might be the cause for that.
"We've known for some time that many UCDs are a bit overweight. They just appear to be too heavy for the luminosity of their stars," co-author Steffen Mieske said in a press release. "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. This is a very exciting result and we want to know how many more UCDs may harbor such extremely massive objects."
Supermassive black holes are the largest known type of black hole, which are over hundreds of thousands of times more massive than our Sun. The supermassive black hole at the center of M60-UCD1 is roughly equal to 20 million solar masses and makes up a whopping 15% of the galaxy’s total mass
“That is pretty amazing, given that the Milky Way is 500 times larger and more than 1000 times heavier than M60-UCD1," Seth explained. "In fact, even though the black hole at the center of our Milky Way galaxy has the mass of 4 million Suns it is still less than 0.01 percent of the Milky Way's total mass, which makes you realize how significant M60-UCD1's black hole really is."
As the black hole and galaxy have such extreme proportions, it became a wonder how they were ever able to form. The team suspects that the galaxy actually used to be much larger, balancing the mass ratios out a bit more. In the past, the galaxy might have been home to as many as 10 billion stars. When crossing near Messier 60, tidal forces from the larger galaxy might have stripped away the vast majority of M60-UCD1’s outer stars.
"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," Seth concluded. "We don't know of any other way you could make a black hole so big in an object this small."
If this is true for M60-UCD1, it could be possible that more dwarf galaxies also have massive black holes at the center as well. This could potentially double the amount of known black holes in the Universe.