Once an object fall through the event horizon of a black hole, they’re lost forever. “It’s an exit door from our universe," said Shep Doeleman, assistant director at the MIT Haystack Observatory and research associate at the Smithsonian Astrophysical Observatory. "You walk through that door, you’re not coming back.” Supermassive black holes are the most extreme objects predicted by Albert Einstein’s theory of gravity — where, according to Doeleman, “gravity completely goes haywire and crushes an enormous mass into an incredibly close space.”
The scientists linked together radio dishes in Hawaii, Arizona and California to create a telescope array called the “Event Horizon Telescope” (EHT) that can see details 2,000 times finer than what’s visible to the Hubble Space Telescope. These radio dishes were trained on M87, a galaxy some 50 million light years from the Milky Way in the Virgo Cluster (image above). M87 harbors a black hole 6 billion times more massive than our sun; using this array, the team observed the glow of matter near the edge of this black hole — a region known as the “event horizon.”
At the edge of a black hole, the gravitational force is so strong that it pulls in everything from its surroundings. However, not everything can cross the event horizon to squeeze into a black hole. The result is a “cosmic traffic jam” in which gas and dust build up, creating a flat pancake of matter known as an accretion disk. This disk of matter orbits the black hole at nearly the speed of light, feeding the black hole a steady diet of superheated material. Over time, this disk can cause the black hole to spin in the same direction as the orbiting material.
Caught up in this spiraling flow are magnetic fields, which accelerate hot material along powerful beams above the accretion disk The resulting high-speed jet, launched by the black hole and the disk, shoots out across the galaxy, extending for hundreds of thousands of light-years. These jets can influence many galactic processes, including how fast stars form.
A jet’s trajectory may help scientists understand the dynamics of black holes in the region where their gravity is the dominant force. Doeleman says such an extreme environment is perfect for confirming Einstein’s theory of general relativity — today’s definitive description of gravitation.
“Einstein’s theories have been verified in low-gravitational field cases, like on Earth or in the solar system,” Doeleman says. “But they have not been verified precisely in the only place in the universe where Einstein’s theories might break down — which is right at the edge of a black hole."
According to Einstein’s theory, a black hole’s mass and its spin determine how closely material can orbit before becoming unstable and falling in toward the event horizon. Because M87’s jet is magnetically launched from this smallest orbit, astronomers can estimate the black hole’s spin through careful measurement of the jet’s size as it leaves the black hole.
Until now, no telescope has had the magnifying power required for this kind of observation.* “We are now in a position to ask the question, ‘Is Einstein right?’” Doeleman says. “We can identify features and signatures predicted by his theories, in this very strong gravitational field.”
The team used a technique called Very Long Baseline Interferometry, or VLBI, which links data from radio dishes located thousands of miles apart. Signals from the various dishes, taken together, create a “virtual telescope” with the resolving power of a single telescope as big as the space between the disparate dishes. The technique enables scientists to view extremely precise details in faraway galaxies.
Using the technique, Doeleman and his team measured the innermost orbit of the accretion disk to be only 5.5 times the size of the black hole event horizon. According to the laws of physics, this size suggests that the accretion disk is spinning in the same direction as the black hole — the first direct observation to confirm theories of how black holes power jets from the centers of galaxies.
The team plans to expand its telescope array, adding radio dishes in Chile, Europe, Mexico, Greenland and Antarctica, in order to obtain even more detailed pictures of black holes in the future.
Christopher Reynolds, a professor of astronomy at the University of Maryland, says the group’s results provide the first observational data that will help scientists understand how a black hole’s jets behave. “The basic nature of jets is still mysterious,” Reynolds says. “Many astrophysicists suspect that jets are powered by black hole spin … but right now, these ideas are still entirely in the realm of theory. This measurement is the first step in putting these ideas on a firm observational basis.”
The image at the top of the page shows X-rays from emitted Sgr A**, the super massive black hole at the center of our Milky Way galaxy. The image contains X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* only in X-rays, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards. This hot gas originates from winds produced by a disk-shaped distribution of young massive stars observed in infrared observations (mouse over the image for the distribution of these massive stars).
These findings are the result of one of the biggest observing campaigns ever performed by Chandra. During 2012, Chandra collected about five weeks worth of observations to capture unprecedented X-ray images and energy signatures of multi-million degree gas swirling around Sgr A*, a black hole with about 4 million times the mass of the Sun. At just 26,000 light years from Earth, Sgr A* is one of very few black holes in the Universe where we can actually witness the flow of matter nearby.
The authors infer that less than 1% of the material initially within the black hole's gravitational influence reaches the event horizon, or point of no return, because much of it is ejected. Consequently, the X-ray emission from material near Sgr A* is remarkably faint, like that of most of the giant black holes in galaxies in the nearby Universe.
The captured material needs to lose heat and angular momentum before being able to plunge into the black hole. The ejection of matter allows this loss to occur.
This work should impact efforts using radio telescopes to observe and understand the "shadow" cast by the event horizon of Sgr A* against the background of surrounding, glowing matter. It will also be useful for understanding the impact that orbiting stars and gas clouds might make with the matter flowing towards and away from the black hole.
Source: Sheperd S. Doeleman et al., Jet-Launching Structure Resolved Near the Supermassive Black Hole in M87, Science, 2012, DOI: 10.1126/science.1224768
The image at the top of the page shows X-rays from emitted Sgr A**, the super massive black hole at the center of our Milky Way galaxy.
The Daily Galaxy via Science, The Perimeter Institute, and University of Waterloo
Image credits: Courtesy Q. Daniel Wang, UMass Amherst and Avery E. Broderick, Perimeter Institute & University of Waterloo