A Galaxy Sized Black Hole?

6a00d8341bf7f753ef01b7c7d33576970b.jpg

 

Black holes at the heart of galaxies could swell to 50 billion times the mass of the sun before losing the discs of gas they rely on to sustain themselves, according to research at the University of Leicester. In a study titled 'How Big Can a Black Hole Grow?', Professor Andrew King from the University of Leicester's Department of Physics and Astronomy explores supermassive black holes at the center of galaxies, around which are regions of space where gas settles into an orbiting disc. This gas can lose energy and fall inwards, feeding the black hole. But these discs are known to be unstable and prone to crumbling into stars.


Professor King calculated how big a black hole would have to be for its outer edge to keep a disc from forming, coming up with the figure of 50 billion solar masses.

The study suggests that without a disc, the black hole would stop growing, meaning 50 billion suns would roughly be the upper limit. The only way it could get larger is if a star happened to fall straight in or another black hole merged with it.

"The significance of this discovery is that astronomers have found black holes of almost the maximum mass, by observing the huge amount of radiation given off by the gas disc as it falls in," said King. "The mass limit means that this procedure should not turn up any masses much bigger than those we know, because there would not be a luminous disc.

"Bigger black hole masses are in principle possible — for example, a hole near the maximum mass could merge with another black hole, and the result would be bigger still. But no light would be produced in this merger, and the bigger merged black hole could not have a disc of gas that would make light.

"One might nevertheless detect it in other ways, for example as it bent light rays passing very close to it (gravitational lensing) or perhaps in future from the gravitational waves that Einstein's General Theory of Relativity predicts would be emitted as it merged."

In 2014, astronomers using data from NASA’s Hubble Space Telescope and ground observation found an unlikely object in an improbable place — a monster black hole lurking inside one of the tiniest galaxies ever known. “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,” said Anil Seth, a University of Utah astronomer.

The finding implies there are many other compact galaxies in the universe that contain supermassive black holes. The observation also suggests 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.

“We don’t know of any other way you could make a black hole so big in an object this small,” said University of Utah astronomer Seth, lead author of an international study of the dwarf galaxy published in Thursday’s issue of the journal Nature.

The black hole is five times the mass of the one at the center of our Milky Way galaxy. It is inside one of the densest galaxies known to date — the M60-UCD1 dwarf galaxy that crams 140 million stars within a diameter of about 300 light-years, which is only 1/500th of our galaxy’s diameter.

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.

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 sharp 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.

Black holes are gravitationally collapsed, ultra-compact objects that have 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 centers of many galaxies.

The black hole at the center of our Milky Way galaxy has the mass of four million suns. As heavy as that is, it is less than 0.01 percent of the Milky Way’s total mass. By comparison, the supermassive black hole at the center of M60-UCD1, which has the mass of 21 million suns, is a stunning 15 percent of the small galaxy’s total mass.

One explanation is that M60-UCD1 was once a large galaxy containing 10 billion stars, but then it passed very close to the center of giant elliptical galaxy, M60, and in that process all the stars and dark matter in the outer part of the galaxy were torn away and became part of M60.

 

6a00d8341bf7f753ef01a73e18a2a1970d.jpg

 

In 2003, Texas astronomer Karl Gebhardt estimated the mass of M60's black hole at about 2 billion times by the mass of the Sun by measuring the motions of stars near the center of the galaxy. The stars are accelerated by the black hole's powerful gravity, so motions of their orbits reveals the black hole's mass.

The team believes that M60-UCD1 may eventually be pulled to fully merge with M60, which has its own monster black hole that weighs a whopping 4.5 billion solar masses, or more than 1,000 times bigger than the black hole in our galaxy. When that happens, the black holes in both galaxies also likely will merge. Both galaxies are 50 million light-years away.

The paper 'How Big Can a Black Hole Grow?' is available at: http://arxiv.org/abs/1511.08502

The Daily Galaxy via University of Leicester and http://www.nasa.gov/hubble

"The Galaxy" in Your Inbox, Free, Daily