A first: Astronomers measure radius of supermassive black hole

It took an array of four radio dishes positioned around the globe and an international team of astronomers to do it, but it was well worth the effort: Astronomers have measured the radius of a supermassive black hole in the M87 galaxy that is 50 million light years away and 6 billion times more massive than our sun.

Update: As Erin Bow explains in the comments, the image above isn't actually new — it's from 2000. What's new is the study of the black hole, which doesn't yet have an image associated with it. Read on for more details about the study.


The new data was compiled by a research team led by astronomers at MIT's Haystack Observatory. To do so, they linked together radio dishes located in Hawaii, Arizona, and California, to create the "Event Horizon Telescope" that can see details 2,000 times finer than the Hubble Space Telescope. And by using the EHT, the astronomers were able to measure — for the first time — the radius of a black hole at the center of a distant galaxy.

Cosmological traffic jams and magnetically accelerated high-speed jets

The EHT allowed the team to catch a vivid glimpse of the glowing accretion disk and the massive plume that's emanating outwards.


As gravitationally bound matter makes its way closer and closer to the black hole's event horizon, its spin causes the black hole to spin itself. At the same time, the black hole collects so much matter that it can't swallow it all, thus resulting in a kind of cosmological traffic jam. It's this super-dense and super-fast collection of spinning debris that results in the shining light that appears just outside the event horizon.


The telescope also allowed the astronomers to determine that the particle jet shooting outwards from the heart of the galaxy was launched from a region very close to the black hole — one that's a mere 5.5 times larger than the estimated size of the event horizon (or radius) of the cosmological singularity.

And in fact, the presence of the jet proves that there's a black hole involved. The super-dense object that's causing this effect must occupy a small volume of space — one that's smaller than the jet's source region. In other words, a supermassive black hole.


The plume itself is caused by strong magnetic fields. They accelerate hot material (tight streams of electrons and other sub-atomic particles) along powerful beams above the accretion disk, resulting in a high-speed jet (launched by the black hole) that shoots out across the galaxy for hundreds of thousands of light-years and at nearly the speed of light (just for perspective, the Milky Way itself is about 120,000 light years in diameter). It's thought that these jets are a major influence on galactic processes, including the speed of star formation.

The smallest orbit

By studying the jet's trajectory, scientists are hoping to better understand the dynamics of black holes in a region where gravity is the dominant force. And in fact, the EHT will allow the scientists to confirm Einsteinian theories of gravitation. Specifically, astronomers can now estimate rate of the black hole's spin by measuring the size of the jet as it leaves the black hole. They're now able to do so because, for the first time ever, they can measure the smallest orbit just outside the event horizon.


In terms of next steps, the astronomers are going to expand the telescope array by adding radio dishes in Chile, Europe, Mexico, Greenland and Antarctica.

You can read the entire study at Science.

Images: Perimeter Institute for Theoretical Physics. Other sources: Nature.


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