The black hole in the nearby galaxy M87 weighs in at 6.6 billion suns, making it the local universe's heavyweight champ. It's big enough to swallow our solar system in one gulp.
"This is the biggest black hole in the nearby universe," said astronomer Karl Gebhardt of the University of Texas at Austin in a press conference today here at the American Astronomical Society meeting.
The behemoth's bulk, plus the fact that it lives just 50 million light-years away, makes M87 the best candidate for future efforts to take a direct image of a black hole's event horizon for the first time.
"In terms of the largest galaxies, it really is in our backyard," Gebhardt said. "Being so close to such a massive black hole allows us a remarkable chance to study what happens around a black hole."
At nearly six trillion times the mass of the sun, M87 is the most massive galaxy in the Milky Way's cosmic neighbourhood. Astronomers expected it to host a correspondingly huge black hole, but the most commonly accepted estimates - based on measurements from the Hubble Space Telescope - found the black hole weighed just 3 billion solar masses, give or take a billion.
But although Hubble "has taken the lead in terms of black hole measures, it can't do the biggest ones," Gebhardt said.
To pin down the monstrous black hole's mass, Gebhardt and his colleagues used the Gemini North telescope atop Mauna Kea in Hawaii to measure the speeds of stars zipping past the galactic centre.
Using a technique called adaptive optics, in which astronomers shine a laser on the sky and use that point of light to subtract out the stars' twinkling, Gebhardt's team was able to measure the velocities of stars within about two light-years of M87's centre using the Gemini telescope. The scientists also took data with a telescope at the McDonald Observatory in Texas.
The closer stars got to the centre of the galaxy, the faster they moved, indicating that a huge hunk of mass lurks at the centre to speed stars up. Gebhardt and colleagues used supercomputer models to calculate the black hole's true heft: 6.6 billion suns, give or take 0.25 billion. For comparison, the black hole at the centre of the Milky Way is a mere 4 million solar masses.
Most of that mass probably came from gas and stars the black hole has devoured over the millennia. But the trajectories of the stars orbiting the black hole suggest that the solo monster that exists today is the product of two smaller black holes merging into one.
It probably took a few hundred such mergers to build the beast in M87, said Caltech astronomer George Djorgovski, who was not involved in the new work. In the same press conference, Djorgovski announced 16 new black hole pairs that will probably merge in the next few million years.
Big black holes also have big event horizons, the point at which a black hole's gravity is so great even light can't escape. The black hole in M87's event horizon is about 12 billion miles across, three times the size of Pluto's orbit.
"This black hole could swallow our solar system whole," Gebhardt said.
That extensive event horizon would cast a dark shadow on the galactic dust behind it. Future observations with a worldwide network of telescopes looking at wavelengths of light smaller than a millimeter could potentially take a picture of that shadow, proving once and for all that black holes exist.
"We don't know whether black holes are black holes," Gebhardt said. "To actually determine whether an object is a black hole, you need to have some type of proof of the event horizon. That doesn't exist yet. To have an object where we might be able to image it, it's remarkably important."
Image: An artist's rendition of what the black hole's shadow might look like. Credit: Gemini Observatory/AURA illustration by Lynette Cook