A multinational group of astronomers led by Dr Anil Seth of the University of Utah has discovered a supermassive black hole at the center of an ultra-compact dwarf galaxy called M60-UCD1, the smallest galaxy known to contain one.
This Hubble Space telescope image shows the giant elliptical galaxy Messier 60 in the center, and the ultra-compact dwarf galaxy M60-UCD1 below it and to the right, and also enlarged as an inset; Messier 60’s gravity also is pulling galaxy NGC4647, upper right, and the two eventually will collide. Image credit: NASA / Space Telescope Science Institute / European Space Agency.
The ultra-compact dwarf galaxy M60-UCD1 is located about 54 million light years away from our planet. But the galaxy is only 22,000 light years from the center of the elliptical galaxy Messier 60 (M60), which is closer than our Sun is to the center of the Milky Way.
The galaxy is only 300 light-years across, compared with our Milky Way’s 100,000-light-year diameter.
Ultra-compact dwarf galaxies are among the densest star systems in the Universe and M60-UCD1 is the most massive of these systems now known.
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 only 4,000 stars.
Artist’s view of a supermassive black hole at the center of M60-UCD1. Image credit: NASA / ESA.
“We believe this once was a very big galaxy with maybe 10 billion stars in it, but then it passed very close to the center of an even larger galaxy, Messier 60, and in that process all the stars and dark matter in the outer part of the galaxy got torn away and became part of Messier 60. That was maybe as much as 10 billion years ago,” Dr Seth said.
Using NASA’s Hubble Space Telescope and the Gemini North 8-m telescope on Hawaii’s Mauna Kea, Dr Seth’s group discovered that this galaxy harbors a black hole of some 21 million solar masses, which is five times more massive than the giant black hole at the Milky Way Galaxy’s center.
The astronomers estimated the mass of the M60-UCD1’s black hole by using data from the Gemini to measure the speed and motion of stars in orbit around it. The stars at the galaxy’s center move at about 370,000 kph – faster than stars would be expected to move without the black hole.
The finding suggests that other ultra-compact galaxies also may contain massive black holes. And that those galaxies may be the stripped remnants of larger galaxies that were torn apart during collisions with other galaxies.
“This means that the seeds of supermassive black holes are more likely to be something that occurred commonly in the early Universe,” concluded Dr Jay Strader of Michigan State University, who is a co-author of a paper published in the journal Nature.
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Anil C. Seth et al. 2014. A supermassive black hole in an ultra-compact dwarf galaxy. Nature 513, 398–400; doi: 10.1038/nature13762
