The point of no return: In astronomy, it’s known as a black hole – a region in space where the pull of gravity is so strong that nothing, not even light, can escape.
At the heart of most galaxies may reside black holes that can be billions of times more massive than our sun. Such supermassive black holes are so powerful that activity at their boundary can ripple throughout their host galaxies.
Now, an international team including University of Arizona researchers has for the first time measured the edge of a black hole at the center of a distant galaxy. Also called the event horizon, this edge is the closest distance at which matter can approach before being irretrievably pulled into the black hole.
The Arizona Radio Observatory's
Sub-millimeter Telescope, or SMT, and its detector system, one of the most sensitive instruments of its kind in the world, were linked together with radio dishes in Hawaii and California to create a virtual telescope called the Event Horizon Telescope
, or EHT – a telescope array that can see details 2,000 times finer than the Hubble Space Telescope.
Radio dishes in these locations were trained on M87, a relatively close, neighboring galaxy 50 million light years from the Milky Way. M87 harbors a black hole 6 billion times more massive than our sun.
“Once objects fall through the event horizon, they’re lost forever,” said project lead Shep Doeleman, assistant director of the Massachusetts Institute of Technology’s Haystack Observatory
and a research associate at the Harvard Smithsonian Center for Astrophysics. “It’s an exit door from our universe. You walk through that door, you’re not coming back.”
The results of the study are published
in the journal Science.
Jets at the edge of a black hole
Supermassive black holes are the most extreme objects predicted by Einstein’s theory of gravity – where, according to Doeleman, “gravity completely goes haywire and crushes an enormous mass into an incredibly close space.” 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 near light-speeds, feeding the black hole a steady diet of super-hot material.
Caught up in the in-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 processes in the galaxy, including how fast stars form.
Is Einstein right?
Where the jet is launched may help scientists understand the dynamics of black holes in the region where gravity is the dominant force. Doeleman said such an extreme environment is perfect for testing Albert Einstein’s theory of general relativity – today’s definitive description of gravitation.
According to Einstein’s theory, a black hole’s mass and its spin determine how close 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 how big the jet is as it leaves the black hole. Until now, no telescope yet has had the magnifying power required for this kind of observation.
“The accretion disk can either spin in the same direction as the black hole, or against its rotation,” explained Dan Marrone, an assistant professor at the UA Steward Observatory and a member of the EHT team. “We see clearly that the disk around M87’s black hole must spin in the same direction.”
Marrone added that discoveries like this help scientists understand the nature of black holes, including the one that sits at the center of our own galaxy, the Milky Way.
“Right now, we have the crudest possible image of the black hole at the center of the Milky Way,” Marrone said. “As we add more sites to the EHT, we will refine this picture and fill in the gaps. The future is to go from having a rough idea to actually taking a picture of our black hole.”
The team used a technique called Very Long Baseline Interferometry, or VLBI, that 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, sharp details in faraway galaxies.
“Ideally, you’d like a telescope as big as the solar system to see the finest details,” said Lucy Ziurys
, professor of astronomy and chemistry and director of the Arizona Radio Observatory, or ARO. “But obviously that’s not possible so we do the best we can here on Earth by stretching our telescope across the globe. The EHT allows us to observe the extreme environment around black holes and answer questions like ‘Could the black hole in our Milky Way one day throw out a jet?’”
Using the VLBI technique, the team made the first direct observations to confirm theories of how black holes power jets from the centers of galaxies. M87’s event horizon spans about six times the distance from Earth to Pluto – a tiny speck on a galactic scale.
A 'funny' way of observing
Accurate timing of the observations obtained at the different sites is key to success, explained Robert Freund, principal engineer at ARO and Steward Observatory.
“The radio signals coming from this object 50 million light years away are extremely weak and buried in the noise generated by the electronics in our detectors. The noise is different at each telescope, but the signal will stay the same and we can dig it out of the noise by comparing what we see at each location.”
Since timing is crucial to ensure the observed data are completely in sync, each site is equipped with a super-accurate atomic clock.
The ARO team at the UA put together a complex set of electronics specifically for the purpose of time-stamping the observations.
“A special supercomputer at MIT goes through the data from each telescope and searches for the faint signal,” he said. “We won’t know until months after the data have been gathered whether everything was working properly.”
“It’s a funny way of observing,” Marrone added. “Everybody goes to these telescopes around the world and records noise onto big hard drives at precisely the same time. We then mail them off to be compared, which is months of work. You try to test everything as best as you can in advance, but one little mistake can wipe out your observations for the year.”
Christopher Reynolds, professor of astronomy at the University of Maryland, said 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 said. “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 team plans to expand its telescope array, adding radio dishes in Chile, Europe, Mexico, Greenland and the South Pole – for which Marrone is currently building the detector – in order to obtain even more detailed pictures of black holes in the future.
This research was supported by the National Science Foundation.