From Science News: “The M87 black hole image showed the best way to measure black hole masses”

From Science News

April 22, 2019
Lisa Grossman

Its diameter suggests the black hole is 6.5 billion times the mass of the sun.

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SUPERMASSIVE SOURCE The gases and stars in galaxy Messier 87, shown in this composite image from the Chandra X-ray telescope and the Very Large Array, gave different numbers for the mass of the galaxy’s supermassive black hole.

NASA/Chandra X-ray Telescope

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

The measure of a black hole is what it does with its stars.

That’s one lesson astronomers are taking from the first-ever picture of a black hole, released on April 10 by an international telescope team (SN Online: 4/10/19).

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SHADOW SIZE The Event Horizon Telescope captured the first image of M87’s black hole. That image showed that the black hole’s mass is about 6.5 billion times the mass of the sun, close to what astronomers expected based on the galaxy’s stars.

That image confirmed that the mass of the supermassive black hole in the center of galaxy Messier 87 is close to what astronomers expected from how nearby stars orbit — solving a long-standing debate over how best to measure a black hole’s mass.

The black hole in Messier 87, which is located about 55 million light-years from Earth, is the first black hole whose mass has been calculated by three precise methods: measuring the motion of stars, the swirl of surrounding gases and now, thanks to the Event Horizon Telescope imaging project, the diameter of the black hole’s shadow.

EHT map

In 1978, the first mass estimates to track the motions of stars whipping around the great gravitational center found that the stars must be orbiting something containing about 5 billion times the mass of the sun. A more precise estimate in 2011 using a similar stellar technique bumped its heft up to 6.6 billion times the mass of the sun.

Star S0-2 Andrea Ghez Keck/UCLA Galactic Center Group at SGR A*, the supermassive black hole at the center of the milky way

Meanwhile, astronomers in 1994 made another estimate by tracing how gases closer to the black hole than the stars swirl around the behemoth. That technique suggested that the black hole was 2.4 billion solar masses, which was revised in 2013 to 3.5 billion solar masses.

For years, it wasn’t clear which technique got closer to the truth.

Now the EHT picture showing a glowing orange ring of gases and dust around the black hole has solved the conflict. According to Einstein’s general theory of relativity, the diameter of the dark space in the center of the image — the black hole’s shadow — is directly related to its mass.

“Bigger black holes cast bigger shadows,” EHT team member Michael Johnson, an astrophysicist at the Harvard Smithsonian Center for Astrophysics, said April 12 at a talk at MIT. “Easy check, we can see whether one or the other of these [mass measuring methods] is correct.” The shadow of M87’s black hole yielded a diameter of 38 billion kilometers, which let astronomers calculate a mass of 6.5 billion suns [The Astrophysical Journal Letters]— very close to the mass suggested by the motion of stars.

The size of the shadow also negated the idea that the black hole is a wormhole, a theoretical bridge between distant points in spacetime (SN: 5/31/14, p. 16). If M87’s black hole had been a wormhole, theory predicts it should look smaller than it does. “It’s a stunning confirmation” of general relativity, Johnson said. “We instantly rule out all these exotic possibilities.”

The mass confirmation may boost confidence in current simulations for how black holes develop, says Priyamvada Natarajan, a Yale University astrophysicist who was not involved with the EHT project. Most black hole mass estimates already use the stellar technique, in part because it’s easier to track a galaxy’s stars from farther away.

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STARS AND STREAKS Astrophysicists have used both stars and gases to weigh in on the mass of the black hole in the galaxy NGC 4258, shown in this composite image. P.Ogle et al/Caltech/CXC/NASA, R.Gendler, STScI/NASA, Caltech-JPL/NASA, VLA/NRAO/NSF

SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

Two other black holes whose masses have been measured in multiple ways, the Milky Way’s Sagittarius A* [Astronomy and Astrophysics] and the galaxy NGC 4258’s black hole, also suggest the star method works better. “These three cases now offer renewed faith in our current method,” Natarajan says.

That faith won’t solve the most pressing black hole problems, such as how black holes grew so big so fast in the early universe — at least not right away (SN Online: 3/16/18). The gas versus star measurement of the M87 black hole mass differed by only a factor of two, which is not enough to explain how it got so massive in the first place. A black hole could double its mass in about a million years, at most.

“What we don’t know is how we get supermassive black holes within a billion years,” says Hannalore Gerling-Dunsmore, a former Caltech physicist who is joining the University of Colorado Boulder later this year. She was not on the EHT team. “Once you’re already that big, what’s a million years between friends?”

See the full article here .


NSF press conference on the EHT Messier 87 Black Hole project


European Research Council press conference on the EHT Messier 87 Black Hole project


Katie Bouman on the EHT Messier 87 Black Hole project at Caltech


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