The Race to See Our Supermassive Black Hole

Using the power of interferometry, two astronomical projects are, for the first time, close to directly observing the black hole in the center of the Milky Way.

There's a monster living in the center of the galaxy.

We know the supermassive black hole is there by tracking the motions of stars and gas clouds that orbit an invisible point. That point exerts an overwhelming tidal influence on all objects that get trapped in its gravitational domain and this force can be measured through stellar orbits to calculate its mass.

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It certainly isn't the biggest black hole in the universe, but it isn't the smallest either, it "weighs in" at an incredible 4 million times the mass of our sun.

But this black hole behemoth, called Sagittarius A*, is over 20,000 light-years from Earth making direct observations, before now, nigh-on impossible. Despite its huge mass, the black hole is minuscule when seen from Earth; a telescope with an unprecedented angular resolution is needed.

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Though we already know a lot about Sagittarius A* from indirect observations, seeing is believing and there's an international race, using the world's most powerful observatories and sophisticated astronomical techniques, to zoom-in on the Milky Way's black hole. This won't only prove it's really there, but it will reveal a region where space-time is so warped that we will be able to make direct tests of general relativity in the strongest gravity environment known to exist in the universe.

The Event Horizon Telescope and GRAVITY

A huge global effort is currently under way to link a network of global radio telescopes to create a virtual telescope that will span the width of our planet. Using the incredible power of interferometry, astronomers can combine the light from many distant radio antennae and collect it at one point, to mimic one large radio antenna spanning the globe.

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This effort is known as the Event Horizon Telescope (EHT) and it is hoped the project will be able to attain the angular resolution and spatial definition required to soon produce its first radio observations of the bright ring just beyond Sagittarius A*'s event horizon -- the point surrounding a black hole where nothing, not even light, can escape.

However, another project has the same goal in mind, but it's not going to observe in radio wavelengths, it's going to stare deep into the galactic core to seek out optical and infrared light coming from Sagittarius A* and it just needs one observatory to make this goal a reality.

The GRAVITY instrument is currently undergoing commissioning at the ESO's Very Large Telescope at Paranal Observatory high in the Atacama Desert in Chile (at an altitude of over 2,600 meters or 8,300 ft) and it will also use the power of interferometry to resolve our supermassive black hole. But rather than connecting global observatories like the EHT, GRAVITY will combine the light of the four 8 meter telescopes of the VLT Interferometer (collectively known as the VLTI) to create a "virtual" telescope measuring the distance between each individual telescope.

"By doing this you can reach the same resolution and precision that you would get from a telescope that has a size, in this case, of roughly a hundred meters, simply because these eight meter-class telescopes are separated by roughly one hundred meters," astronomer Oliver Pfuhl, of Max Planck Institute for Extraterrestrial Physics, Germany, told DNews. "If you combine the light from those you reach the same resolution as a virtual telescope of a hundred meters would have."

Strong Gravity Environment

When GRAVITY is online it will be used to track features just outside Sagittarius A*'s event horizon.

"For about ten years, we've known that this black hole is actually not black. Once in awhile it flares, so we see it brightening and darkening," he said. This flaring is matter falling into the event horizon, generating a powerful flash of energy. The nature of these flares are poorly understood, but the instrument should be able to track this flaring material as it rapidly orbits the event horizon and fades away. These flares will also act as tracers, helping us see the structure of space-time immediately surrounding a black hole for the first time.

"Our goal is to measure these motions. We think that what we see as this flaring is actually gas which spirals into the black hole. This brightening and darkening is essentially the gas, when it comes too close to the black hole, the strong tidal forces make it heat up," said Pfuhl.

"If we can study these motions which happen so close to the black hole, we have a direct probe of the space time close to the black hole. In this way we have a direct test of general relativity in one of the most extreme environments which you can find in the universe."

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While GRAVITY will be able to track these flaring events very close to the black hole, the Event Horizon Telescope will see the shadow, or silhouette, of the dark event horizon surrounded by radio wave emissions. Both projects will be able to measure different components of the region directly surrounding the event horizon, so combined observations in optical and radio wavelengths will complement one other.

It just so happens that the Atacama Large Millimeter/submillimeter Array (ALMA), the largest radio observatory on the planet -- also located in the Atacama Desert -- will also be added to the EHT.

"The Event Horizon Telescope will combine ALMA with telescopes around the world like Hawaii and other locations, and with that power you can look at really fine details especially in the black hole in the center of our galaxy and perhaps in some really nearby other galaxies that also have black holes in their centers," ESO astronomer Linda Watson told DNews.

ALMA itself is an interferometer combining the collecting power of 66 radio antennae located atop Chajnantor plateau some 5,000 meters (16,400 ft) in altitude. Watson uses ALMA data to study the cold dust in interstellar space, but when added to the EHT, its radio-collecting power will help us understand the dynamics of the environment surrounding Sagittarius A*.

"ALMA's an interferometer with 66 antennas, (the EHT) will treat ALMA as just one telescope and will combine it with other telescopes around the world to be another interferometer," she added.

Black Hole Mysteries

Many black holes are thought to possess an accretion disk of swirling gas and dust. ALMA, when combined with the EHT, will be able to measure this disk's structure, speed and direction of motion. Lacking direct observations, many of these characteristics have only been modeled by computer simulations or inferred from indirect observations. We're about to enter an era when we can truly get to answer some of the biggest mysteries surrounding black hole dynamics.

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"The first thing we want to see is we want to understand how accretion works close to the black hole," said Pfuhl. "This is also true for the Event Horizon Telescope. Another thing we want to learn is does our black hole have spin? That means, does it rotate?"

Though the EHT and GRAVITY are working at different wavelengths, observing phenomena around Sagittarius A* will reveal different things about the closest supermassive black hole to Earth. By extension it is hoped that we may observe smaller black holes in our galaxy and other supermassive black holes in neighboring galaxies.

But as we patiently wait for the first direct observations of the black hole monster lurking in the center of our galaxy, an event that some scientists say will be as historic as the "Pale Blue Dot" photo of Earth as captured by Voyager 1 in 1990, it's hard not to wonder which project will get there first.

"I think it's a very tight race," said Pfuhl. "Let's see."

DNews visited the European Southern Observatory facilities in Chile during the inaugural #MeetESO social media event earlier this month, touring Paranal Observatory and ALMA. We would like to thank the ESO and ALMA staff for being so accommodating and the #MeetESO participants for making the trip an unforgettable experience.