First direct visual evidence – ring-like structure like M87* - Theoretical Physicists of Goethe University Frankfurt instrumental in interpreting the data
Astronomers have unveiled the first image of the supermassive black hole at the centre of our own Milky Way galaxy. This result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the workings of such giants, which are thought to reside at the centre of most galaxies. The image was produced by a global research team called the Event Horizon Telescope (EHT) Collaboration, using observations from a worldwide network of radio telescopes. Theoretical Physicists from Goethe University Frankfurt were instrumental in interpreting the data.
FRANKFURT. The
image is a long-anticipated look at the massive object that sits at the very
centre of our galaxy. Scientists had previously seen stars orbiting around
something invisible, compact, and very massive at the centre of the Milky Way.
This strongly suggested that this object — known as Sagittarius A* (Sgr A*,
pronounced "sadge-ay-star") — is a black hole, and today's image
provides the first direct visual evidence of it.
Although we cannot see the black hole
itself, because it is completely dark, glowing gas around it reveals a
tell-tale signature: a dark central region (called a “shadow") surrounded by a
bright ring-like structure. The new view captures light bent by the powerful
gravity of the black hole, which is four million times more massive than our
Sun.
“We were stunned by how well the size of
the ring agreed with predictions from Einstein's
theory of general relativity," says EHT
Project Scientist Geoffrey Bower from the Institute of Astronomy and
Astrophysics, Academia Sinica, Taipei. “These unprecedented observations have
greatly improved our understanding of what happens at the very centre of our
galaxy and offer new insights on how these giant black holes interact with
their surroundings."
Because the black hole is about 27,000
light-years away from Earth, it appears to us to have about the same size in
the sky as a donut on the Moon. To image it, the team created the powerful EHT,
which linked together eight existing radio observatories across the planet to
form a single “Earth-sized" virtual telescope [1]. The EHT observed Sgr A* on
multiple nights, collecting data for many hours in a row, similar to using a
long exposure time on a camera.
The enormous amount of observational data
collected had to be interpreted theoretically. For this, a research team led by
theoretical astrophysicist Luciano Rezzolla from Goethe University Frankfurt
used supercomputers to simulate how a black hole could look like when observed
by the EHT – based on what had already been known about Sgr A*. In this way,
the scientists created a library of millions of images. Then, they compared
this image library with the thousands of
different images of the EHT to deduce the properties of Sgr A*.
The breakthrough follows the EHT
Collaboration's 2019 release of the first image of a black hole, called M87*,
at the centre of the more distant Messier 87 galaxy.
The two black holes look remarkably
similar, even though our galaxy's black hole is more than a thousand times
smaller and less massive than M87* [2]. “We have two completely different types
of galaxies and two very different black hole masses, but close to the edge of these
black holes they look amazingly similar," says Sera Markoff, Vice Chair of the
EHT Science Council and a professor of theoretical astrophysics at the
University of Amsterdam, the Netherlands. “This tells us that general
relativity governs these objects up close, and any differences we see further
away must be due to differences in the material that surrounds the black
holes."
This achievement was considerably more
difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist
Chi-kwan ('CK') Chan, from Steward Observatory, the Department of Astronomy and
the Data Science Institute at the University of Arizona, US, explains: “The gas
in the vicinity of the black holes moves at the same speed — nearly as fast as
light — around both Sgr A* and M87*. But where gas takes days to weeks to orbit
the larger M87*, in the much smaller Sgr A* it completes an orbit in mere
minutes. This means the brightness and pattern of the gas around Sgr A* was
changing rapidly as the EHT Collaboration was observing it — a bit like trying
to take a clear picture of a puppy quickly chasing its tail."
The researchers had to develop
sophisticated new tools that accounted for the gas movement around Sgr A*.
While M87* was an easier, steadier target, with nearly all images looking the
same, that was not the case for Sgr A*. The image of the Sgr A* black hole is
an average of the different images the team extracted, finally revealing the
giant lurking at the centre of our galaxy for the first time.
The effort was made possible through the
ingenuity of more than 300 researchers from 80 institutes around the world that
together make up the EHT Collaboration. In addition to developing complex tools
to overcome the challenges of imaging Sgr A*, the team worked rigorously for
five years, using supercomputers to combine and analyse their data, all while
compiling an unprecedented library of simulated black holes to compare with the
observations.
Luciano Rezzolla, professor of Theoretical
Astrophysics at Goethe University Frankfurt, explains: “The mass and distance
of the object were known very precisely before our observations. We thus used
these tight constraints on the size of the shadow to rule out other compact
objects – such as boson stars or wormholes – and conclude that: 'What we're
seeing definitely looks like a black hole!'"
Using advanced numerical codes, theorists
in Frankfurt have performed extensive calculations on the properties of the
plasma accreting onto the black hole. Rezzolla: “We managed to calculate three
million synthetic images varying the accretion and radiation emission models,
and considering the variations seen by observers at different inclinations with
respect to the black hole."
This last operation was necessary because
the image of a black hole can be radically different when seen by observers at
different inclinations. “Indeed, a reason why our images of Sgr A* and M87* are
rather similar is because we're seeing the two black holes from an almost
identical angle," Rezzolla explains.
“To understand how the EHT has produced an
image of Sgr A* one can think of producing a picture of a mountain peak based
on a time-lapse video. While most of the time the peak will be visible in the
time-lapse video, there are times when it is not because it is obscured by
clouds. On average, however, the peak is clearly there. Something similar is
true also for Sgr A*, whose observations lead to thousands of images which have
been collected in four classes and then averaged according to their properties.
The end result is a clear first image of the black hole at the centre of the
Milky Way." Rezzolla concludes.
Scientists are particularly excited to
finally have images of two black holes of very different sizes, which offers
the opportunity to understand how they compare and contrast. They have also
begun to use the new data to test theories and models of how gas behaves around
supermassive black holes. This process is not yet fully understood but is
thought to play a key role in shaping the formation and evolution of galaxies.
“Now we can study the differences between
these two supermassive black holes to gain valuable new clues about how this
important process works," says EHT scientist Keiichi Asada from the Institute
of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two
black holes — one at the large end and one at the small end of supermassive
black holes in the Universe — so we can go a lot further in testing how gravity
behaves in these extreme environments than ever before."
Progress on the EHT continues: a major
observation campaign in March 2022 included more telescopes than ever before.
The ongoing expansion of the EHT network and significant technological upgrades
will allow scientists to share even more impressive images as well as videos of
black holes in the near future.
To Goethe University are associated a
number of scientists in the EHT Collaboration. Together with Professor Luciano
Rezzolla, Dr Alejandro Cruz Orsorio, Dr Prashant Kocherlakota and Kotaro
Moriyama, also Prof Mariafelicia De Laurentis (University of Naples), Dr
Christian Fromm (University of Würzburg), Prof Roman Gold (University of
Southern Denmark), Dr Antonios Nathanail (University of Athens), and Dr Ziri
Younsi (University College London) have provided essential contributions to the
theoretical research in the EHT Collaboration.
This work has been supported by the
European Research Council.
Notes:
[1] The individual telescopes involved in
the EHT in April 2017, when the observations were conducted, were: the Atacama
Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment
(APEX), the IRAM 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT),
the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array
(SMA), the UArizona Submillimeter Telescope (SMT), the South Pole Telescope
(SPT). Since then, the EHT has added the Greenland Telescope (GLT), the NOrthern
Extended Millimeter Array (NOEMA) and the UArizona 12-meter Telescope on Kitt
Peak to its network.
ALMA is a partnership of the European
Southern Observatory (ESO; Europe, representing its member states), the U.S.
National Science Foundation (NSF) and the National Institutes of Natural
Sciences (NINS) of Japan, together with the National Research Council (Canada),
the Ministry of Science and Technology (MOST; Taiwan), Academia Sinica
Institute of Astronomy and Astrophysics (ASIAA; Taiwan) and Korea Astronomy and
Space Science Institute (KASI; Republic of Korea), in cooperation with the
Republic of Chile. The Joint ALMA Observatory is operated by ESO, the
Associated Universities, Inc./National Radio Astronomy Observatory (AUI/NRAO)
and the National Astronomical Observatory of Japan (NAOJ). APEX, a
collaboration between the Max Planck Institute for Radio Astronomy (Germany),
the Onsala Space Observatory (Sweden) and ESO, is operated by ESO. The 30-meter
telescope is operated by IRAM (the IRAM partner organizations are MPG
(Germany), CNRS (France) and IGN (Spain)). The JCMT is operated by the East
Asian Observatory on behalf of the Center for Astronomical Mega-Science of the
Chinese Academy of Sciences, NAOJ, ASIAA, KASI, the National Astronomical
Research Institute of Thailand and organizations in the United Kingdom and
Canada. The LMT is operated by INAOE and UMass, the SMA is operated by Center
for Astrophysics | Harvard & Smithsonian and ASIAA, and the UArizona SMT is
operated by the University of Arizona. The SPT is operated by the University of
Chicago with specialized EHT instrumentation provided by the University of
Arizona.
The Greenland Telescope (GLT) is operated by ASIAA and the Smithsonian Astrophysical Observatory (SAO). The GLT is part of the ALMA-Taiwan project, and is supported in part by the Academia Sinica (AS) and MOST. NOEMA is operated by IRAM and the UArizona 12-meter telescope at Kitt Peak is operated by the University of Arizona.
[2] Black holes are the only objects we
know of where mass scales with size. A black hole a thousand times smaller than
another is also a thousand times less massive.
Publications:
First Sagittarius A* Event Horizon
Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center
of the Milky Way: 10.3847/2041-8213/ac6674 and https://iopscience.iop.org/article/10.3847/2041-8213/ac6674
All 10 publications in Astrophysical
Journal Letters:
https://iopscience.iop.org/journal/2041-8205/page/Focus_on_First_Sgr_A_Results
Pictures
for Download:
https://www.uni-frankfurt.de/119021712
1) EHT_PR_Main_Image_Original.tiff
First
image of the black hole at the centre of the Milky Way
This is the first image of Sagittarius A*
(Sgr A*), the supermassive black hole at the centre of our galaxy, captured by the
Event Horizon Telescope (EHT). It is the first direct visual evidence of the
presence of this black hole. The telescope is named after the “event horizon",
the boundary of the black hole beyond which no light can escape.
Although we cannot see the event horizon itself, because it cannot emit light,
glowing gas orbiting around the black hole reveals a tell-tale signature: a
dark central region (called a “shadow") surrounded by a bright ring-like
structure. The new view captures light bent by the powerful gravity of the
black hole, which is four million times more massive than our Sun. The image of
the Sgr A* black hole is an average of the different images the EHT
Collaboration has extracted from its 2017 observations.
Image credit: EHT Collaboration
2) Simulation_AccretionDisk_SgrAStar.png
Simulation
of the Accretion Disk around the Black Hole Sgr A*
Example of a simulation of how the gas orbits the black hole in the center of
our Milky Way and emits radio waves at 1.3 mm. Credit: Younsi, Fromm, Mizuno
& Rezzolla (University College London, Goethe University Frankfurt)
3) EHT_PR_Secondary_Image.tiff
Making of the image of the black hole at the
centre of the Milky Way (image)
The Event
Horizon Telescope (EHT) Collaboration has created a single image (top frame) of
the supermassive black hole at the centre of our galaxy, called Sagittarius A*
(or Sgr A* for short), by combining images extracted from the EHT observations.
The main image was produced by averaging together thousands of images created
using different computational methods — all of which accurately fit the EHT
data. This averaged image retains features more commonly seen in the varied
images and suppresses features that appear infrequently.
The images can also be clustered into four groups based on similar features. An
averaged, representative image for each of the four clusters is shown in the
bottom row. Three of the clusters show a ring structure, but with differently
distributed brightness around the ring. The fourth cluster contains images that
also fit the data but do not appear ring-like.
The bar graphs show the relative number of images belonging to each cluster.
Thousands of images fell into each of the first three clusters, while the
fourth and smallest cluster contains only hundreds of images. The heights of
the bars indicate the relative “weights", or contributions, of each cluster to
the averaged image at top.
Image credit: EHT Collaboration
4) Rezzolla_Luciano_2019_Credit_JuergenLecher.jpg
Luciano
Rezzolla
Luciano
Rezzolla, Professor für Theoretische Astrophysik, Goethe-Universität Frankfurt.
Credit: Juergen Lecher for Goethe University Frankfurt
Youtube-Playlist
Black Hole
Find
further animations on how the picture of the black hole in the center of our
galaxy was made on the Goethe
University's playlist „Black Hole“
https://youtube.com/playlist?list=PLn5gYfEKIag8nps1GKLqUW35AOgQY7aM2
Further
pictures and video clips provided by EHT Collaboration:
https://eventhorizontelescope.teamwork.com/#notebooks/240600
(Animationen)
https://eventhorizontelescope.teamwork.com/#notebooks/240540
(Bilder)
Websites
https://eventhorizontelescope.org/ EHT Website
https://blackholecam.org/ Black Hole Cam-Project
Contact:
Professor Luciano Rezzolla
Institute for Theoretical Physics
Goethe University Frankfurt, Germany
Phone: +49 (69) 798-47871
rezzolla@itp.uni-frankfurt.de
https://astro.uni-frankfurt.de/rezzolla/