Smallest known galaxy with supermassive black hole discovered

Black holes may be more common than we thought
September 18, 2014

This Hubble Space telescope image shows the gargantuan galaxy M60 on the left and the ultracompact dwarf galaxy M60-UCD1 below it and to the right, and also enlarged as an inset. A new international study found that M60-UCD1 is the smallest known galaxy with a supermassive black hole at its center, suggesting that the dwarf galaxy originally was much larger but was stripped of its outer layers by gravity from galaxy M60 over billions of years. M60’s gravity also is pulling galaxy NGC4647, upper right, and the two will eventually collide. (Credit: NASA/Space Telescope Science Institute/European Space Agency)

Astronomers have discovered that an ultracompact dwarf galaxy harbors a supermassive black hole with a mass equal to 21 million suns — the smallest galaxy known to contain such a massive black hole. The finding suggests that huge black holes may be more common than previously believed.

“It is the smallest and lightest object that we know of that has a supermassive black hole,” says Anil Seth, lead author of an international study of the dwarf galaxy published in the Sept. 18 issue of the journal Nature. “It’s also one of the most black-hole-dominated galaxies known.”

The astronomers used the Gemini North 8-meter optical-and-infrared telescope on Hawaii’s Mauna Kea and photos taken by the Hubble Space Telescope to discover that a small galaxy named M60-UCD1 has a black hole

Other ultracompact dwarf galaxies likely

Their finding suggests that plenty of other ultracompact dwarf galaxies likely also contain supermassive black holes — and those dwarfs may be the stripped remnants of larger galaxies that were torn apart during collisions with yet other galaxies.

“We don’t know of any other way you could make a black hole so big in an object this small,” says Seth, an assistant professor of physics and astronomy at the University of Utah. “There are a lot of similar ultracompact dwarf galaxies, and together they may contain as many supermassive black holes as there are at the centers of normal galaxies.”

Black holes are collapsed stars and collections of stars with such strong gravity that even light is pulled into them, although material around them sometimes can spew jets of X-rays and other forms of radiation. Supermassive black holes — those with the mass of at least 1 million stars like our sun — are thought to be at the centers of many galaxies.

The central, supermassive black hole at the center of our Milky Way galaxy has the mass of 4 million suns, but as heavy as that is, it is less than 0.01 percent of the galaxy’s total mass, estimated at some 50 billion solar masses.

By comparison, the supermassive black hole at the center of ultracompact dwarf galaxy M60-UCD1 is five times larger than the Milky Way’s, with a mass of 21 million suns, and is a stunning 15 percent of the small galaxy’s total mass of 140 million suns.

“That is pretty amazing, given that the Milky Way is 500 times larger and more than 1,000 times heavier than the dwarf galaxy M60-UCD1,” Seth says.

The future of a black hole

“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, M60, and in that process all the stars and dark matter in the outer part of the galaxy got torn away and became part of M60,” he says. “That was maybe as much as 10 billion years ago. We don’t know.”

Seth says ultracompact dwarf galaxy M60-UCD1 may be doomed, although he cannot say when because the dwarf galaxy’s orbit around M60 isn’t known. M60 is among the largest galaxies in what astronomers refer to as “the local universe.”

“Eventually, this thing may merge with the center of M60, which has a monster black hole in it, with 4.5 billion solar masses — more than 1,000 times bigger than the supermassive black hole in our galaxy. When that happens, the black hole we found in M60-UCD1 will merge with that monster black hole.”

Galaxy M60 also is pulling in another galaxy, named NGC4647. M60 is about 25 times more massive than NGC4647.

Astronomers at the Max Planck Institute for Astronomy, European Southern Observatory, University of Queensland, Michigan State University; Smithsonian Astrophysical Observatory, Moscow State University; Macquarie University, University of California Santa Cruz, University of Heidelberg, Germany, and University of Texas Austin were also involved in the research.

Formation of Dwarf Galaxy M60-UCD1: University of Utah astronomer Anil Seth led an international research team that discovered the smallest galaxy known to harbor a supermassive black hole. The video simulation shows how this small galaxy, named M60-UCD1, was formed from a larger, normal galaxy. The video begins with a background image made by the Hubble Space Telescope, with the huge elliptical galaxy M60 in the center, galaxy NGC4647 in the upper right and MC60-UCD as a small whitish spot lower right. As the video begins, a normal galaxy (yellow and red) orbits M60. During an estimated 500 million years, M60’s gravity strips stars (red material) from the orbiting galaxy, leaving as a remnant the ultracompact dwarf galaxy now known as M60-UCD1. The end of the video zooms in on the Hubble Space Telescope close-up image of M60-UCD1, which today continues to orbit M60. Seth says that while this process took about 500 million years, astronomers don’t know when it happened, and that it could have been billions of years ago. This video was made by University of Queensland astronomer Holger Baumgardt, one of Seth’s co-authors on the new study published by Nature. (Credit: Holger Baumgardt, University of Queensland)