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Fast-moving stars reveal supermassive black hole inside nearby galaxy

By Will Dunham

WASHINGTON (Reuters) - The Large Magellanic Cloud is a dwarf galaxy residing near our Milky Way, visible to the naked eye as a luminous patch of light from Earth's southern hemisphere and named after Portuguese explorer Ferdinand Magellan, who observed it five centuries ago. New research is now providing a fuller understanding of the makeup of our galactic neighbor.

A study based on the trajectory of nine fast-moving stars observed at the fringes of the Milky Way provides strong evidence for the existence of a supermassive black hole inside the Large Magellanic Cloud. Most galaxies are thought to have such a black hole at their core, but this represents the first evidence for one within the Large Magellanic Cloud.

According to the researchers, data on the trajectory of these stars indicates they were flung out of the Large Magellanic Cloud after a violent close encounter with this black hole. Black holes are exceptionally dense objects with gravity so strong that not even light can escape.

The Large Magellanic Cloud is located about 160,000 light-years from Earth, making it among the closest galaxies to the Milky Way. That makes this the nearest supermassive black hole to us aside from the one called Sagittarius A*, or Sgr A*, situated at the heart of the Milky Way. Sgr A* is about 26,000 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

Just as the Milky Way is much more massive than the Large Magellanic Cloud, Sgr A* is much more massive than the newly identified black hole, which is among the least massive of any supermassive black holes known. Sgr A* has a mass roughly 4 million times greater than the sun's. This one has a mass about 600,000 times greater than the sun's.

Sgr A*, in turn, is dwarfed by some supermassive black holes detected in other large galaxies such as one with a mass 6.5 billion times greater than that of the sun in a galaxy called Messier 87. That one and Sgr A* are the only two black holes ever imaged by astronomers.

The new study focused on a class of stars called hypervelocity stars. They are produced when a binary star system - two stars gravitationally bound to each other - ventures too close to a supermassive black hole.

"The intense gravitational forces tear the pair apart. One star is captured into a tight orbit around the black hole, while the other is flung outward at extreme velocities - often exceeding thousands of kilometers per second - becoming a hypervelocity star," said Jesse Han, a doctoral student in astrophysics at Harvard University and lead author of the study being published in the Astrophysical Journal and made public on Thursday.

The sun travels through space at about 450,000 miles per hour (720,000 kph) while hypervelocity stars do so at several times that speed.

The researchers used data from the European Space Agency's Gaia space observatory that has tracked more than a billion stars in our galaxy with unprecedented precision.

There are 21 known hypervelocity stars in the Milky Way. Astronomers have confidently identified the origins of 16 of them, tracking seven of them back to Sgr A* at our galaxy's core and the other nine back to the Large Magellanic Cloud.

"The only plausible explanation is that the Large Magellanic Cloud harbors a supermassive black hole in its center as well, analogous to Sgr A* in our galaxy," Han said.

"The Large Magellanic Cloud, given its mass and structure, is totally expected to have a supermassive black hole of this mass. We just needed to find the evidence for it," Han said. "It's fun and exciting, but also something that really does make sense."

Until now, the closest known supermassive black hole from beyond the Milky Way was the one inside the Andromeda galaxy, about 2.5 million light-years from Earth. It is the nearest major galaxy to the Milky Way.

"The Large Magellanic Cloud is one of the best-studied galaxies, yet this supermassive black hole's existence was only inferred indirectly by tracing the origins of fast-moving stars. We have more work to do to actually pinpoint the location of the black hole," said Caltech astronomer and study co-author Kareem El-Badry.