Our Milky Way galaxy is thought to be about 100,000 light-years wide. The black hole at the center of our galaxy is thought to be as massive as 4.1 million of our suns. Now imagine a black hole five times the mass of our Milky Way’s central black hole, inside a galaxy only 300 light-years across. That’s the situation inside the dwarf galaxy M60-UCD1, whose diameter is only 1/500th that of our Milky Way. NASA said:
If you lived inside this dwarf galaxy, the night sky would dazzle with at least 1 million stars visible to the naked eye. Our nighttime sky as seen from Earth’s surface shows 4,000 stars.
We know now that most galaxies contain supermassive black holes at their centers. The finding of such a massive black hole in such a tiny galaxy suggests that many other compact galaxies also have supermassive black holes at their cores.
How did they get there? Astronomers are saying that dwarf galaxies containing supermassive black holes may be the stripped remnants of larger galaxies. The larger galaxies may have been torn apart during collisions with other galaxies, and the dwarf galaxies – with their supermassive black holes – may have been left behind. University of Utah astronomer Anil Seth, lead author of an international study of the dwarf galaxy published in the September 18, 2014 issue of the journal Nature, said:
We don’t know of any other way you could make a black hole so big in an object this small.
Seth’s team of astronomers used the Hubble Space Telescope and the Gemini North 8-meter optical and infrared telescope on Hawaii’s Mauna Kea to observe M60-UCD1 and measure the black hole’s mass. The Hubble images provide information about the galaxy’s diameter and stellar density. Gemini measures the stellar motions as affected by the black hole’s pull. These data are used to calculate the mass of the black hole.
Bottom line: The dwarf galaxy M60-UCD1, whose 300-light-year diameter is only 1/500th that of our Milky Way, has a central supermassive black hole with five times the mass of the Milky Way’s central black hole.