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| The first image of a black hole - Image of the black hole at the center of the Messier 87 galaxy shows the effect of the accretion disc as well as the black hole's "shadow" in the center. (Courtesy: Akiyama et al and ApJL) |
The first direct visual evidence of a black hole and its
“shadow” has been revealed today by astronomers working on the Event Horizon
Telescope (EHT). The image is of the supermassive black hole that lies at the
center of the huge Messier 87 galaxy, in the Virgo galaxy cluster. Located 55
million light-years from Earth, the black hole has been determined to have a
mass 6.5-billion times that of the Sun, with an uncertainty of 0.7 billion
solar masses. Although black holes are inherently invisible because of their
extreme density and gravitational field, the researchers have managed to obtain
images near the point where matter and energy can no longer escape – the
so-called event horizon.
“We are giving humanity its first view of a black hole — a
one-way door out of our universe,” says Sheperd Doeleman of the Haystack
Observatory at the Massachusetts Institute of Technology (MIT) who is the EHT’s
lead astronomer. “This is a landmark in astronomy, an unprecedented scientific
feat accomplished by a team of more than 200 researchers.” Doeleman says that
the result would have “presumed to be impossible just a generation ago”, adding
that breakthroughs in technology and the completion of new radio telescopes
over the past decade have allowed researchers to now “see the unseeable”. The results, announced today at multiple
press conferences around the world, have been published in six papers in a
special issue of Astrophysical Journal Letters, which is published by the
Institute of Physics on behalf of the American Astronomical Society.
Flashing Disk
Supermassive black holes are thought to lie at the centers
of most galaxies in the universe, and astronomers are keen to decipher their
key properties – such as how their extreme gravity affects the space-time
around them, and how some of them fuel the massive jets of material that spew
out from the galaxies that host them. A key feature of a black hole is its
event horizon – the boundary at which even light cannot escape its
gravitational pull, as the velocity required to do so would be greater than the
speed of light, which is forbidden by Einstein’s general theory of relativity.
And while that theory has passed many tests, researchers want to see how well
it holds up at the “ultimate proving ground” – a black hole’s edge.
Despite their name, black holes are not, however, all dark.
The gas and dust trapped around them in an accretion disc are so compact that
it is often heated to billions of degrees even before the matter eventually
succumbs to the black hole, making them glow brightly. Indeed, general
relativity also predicts that a black hole will have a “shadow” around it,
measuring around three times larger than the event horizon. The shadow is of
great interest as its size and shape depend mainly on the mass and – to a
lesser extent – on any possible spin of the black hole, thereby revealing its
inherent properties.
“If immersed in a bright region, like a disc of glowing gas,
we expect a black hole to create a dark region similar to a shadow — something
predicted by Einstein’s general relativity that we’ve never seen before,” says
Heino Falcke from Radboud University in the Netherlands, who chair the EHT’s
science council. “This shadow, caused by the gravitational bending and capture
of light by the event horizon, reveals a lot about the nature of these fascinating
objects.”
An orange
on the black hole Colour
To directly observe the black hole at the center of Messier
87 – dubbed M87* – astronomers require a telescope with an angular resolution
comparable to the event horizon, which is on the order of tens of
micro-arcseconds across. But to achieve that resolution with an ordinary
telescope – which is like spotting an orange on the surface of the Moon – would
require a dish the size of our planet, which is clearly impractical.
EHT astronomers instead use the radio-astronomy technique of
very-long-baseline interferometry (VLBI). It involves picking up radio signals
from an astronomical source by a network of individual radio telescopes and
telescopic arrays scattered across the globe. The EHT, which first turned on in
2007, consists of eight radio dishes in six different locations across the
globe all operating at a wavelength of 1.3 mm. These telescopes include the
Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the South Pole
Telescope (SPT) in Antarctica, and the IRAM 30-meter telescope in Spain (see
image left). The distance between individual EHT telescopes – known as the
“baseline” – ranges from 160 m to 10 700 km.
Schematic
of the Event Horizon Telescope
Global Networking: Event Horizon Telescope incorporates
signals from eight radio telescope observatories, including the Atacama Large
Millimeter / Submilimeter Array (ALMA) in Chile and the South Pole Telescope
(SPT) in Antarctica. Solid lines represent the telescope involved with an
overview of M87 *, while the dashed lines are used for calibration run.
(Courtesy: Akima et al and APJL)
The signals received at each individual telescope dish in
the network are precisely tagged with a very accurate time stamp, normally
using an atomic clock at each location. Each telescope produces roughly 350
terabytes per day, which is stored on high-performance helium-filled hard
drives. The data is then later correlated and used to build up a complete image
by supercomputers that are located at the Max Planck Institute for Radio
Astronomy in Bonn, Germany, and the MIT Haystack Observatory in the US. This
process makes the EHT the highest-resolution instrument on Earth, capable of
taking images up to 2000 times better resolution than the Hubble Space
Telescope and able to resolve features as small as 20 micro-arcseconds.
An astronomy "milestone"
As a black hole’s size is proportional to its mass, the more
massive a black hole, the larger its shadow. Thanks to its enormous mass and
relative proximity, M87* was predicted to be one of the largest viewable from
Earth — making it a perfect target for the EHT. Astronomers observed M87* on 5,
6, 10 and 11 April 2017 with the telescope taking a series of scans of three to
seven minutes in duration each day.
These multiple independent EHT observations have now
resulted in the first image of a black hole including its shadow, revealing a
ring-like structure with a dark central region. The diameter of the ring is 42
micro-arcseconds with a width less than 20 micro-arcseconds. By comparing the
image with theoretical models such as general relativistic magnetohydrodynamic
(GRMHD) simulations, the observed image is consistent with expectations for the
shadow of a Kerr black hole – one that is uncharged and rotates about a central
axis – as predicted by the general relativity.
The researchers were able to deduce the mass of the M87* at
6.5-billion times that of the Sun. Previous estimates — based on models as well
as spectroscopic observations of the galaxy by the Hubble Space Telescope —
range between 3.5-7.7 billion solar masses. EHT scientists also deduced the radius
of the event horizon as 3.8 micro-arcseconds. They also found that the rotation
of the black hole is in a clockwise direction and that it is spin points away
from us. The brightness in the lower part of the image is due to the
relativistic movement of material in a clockwise direction as seen by us so
that it is moving towards us.
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| Image of black hole and simulations Shadowlands of black hole : From left to right, EHT observations of M87* taken on 6 April 2017; a simulation of M87*; simulation convoluted to the resolution of the Event Horizon Telescope. (Courtesy: Akiyama et al and ApJL) |
