On April 10th, 2019, the international consortium known as the Event Horizon Telescope (EHT) announced the first-ever image of a supermassive black hole (SMBH). The image showed the bright disk surrounding the black hole at the center of the M87 galaxy (aka. Virgo A). In 2021, they followed up on this by acquiring an image of the core region of the Centaurus A galaxy and the radio jet emanating from it. Earlier this month, the European Southern Observatory (ESO) announced that the EHT would be sharing the results from its latest campaign – observations of Sagittarius A*!

This supermassive black hole resides at the center of the Milky Way Galaxy, roughly 27,000 light-years from Earth, 44 million km (27.34 million mi) in diameter, and has a mass of 4.31 million Suns. The campaign’s results were shared in an ESO press release and a series of live-streamed press conferences worldwide, including the ESO Headquarters in Munich, Germany. The team’s results (which were shared in six papers) were also published today in a special issue of The Astrophysical Journal Letters.

Since the 1970s, astronomers have speculated about the existence of SMBHs, which were believed to be why massive galaxies had such energetic core regions. Also known as Active Galactic Nuclei (AGNs), or “quasars,” these regions are known to temporarily outshine all of the stars contained in their disks. In some cases, jets of superheated material (relativistic jets) have also been found emanating from them at a fraction of the speed of light.

The study of SMBHs has led to new theories about how galaxies formed and evolved in our Universe and has allowed astronomers to test the laws of physics under the most extreme conditions (i.e., Einstein’s Theory of General Relativity). Until 2019 the study of these massive black holes was confined to observing their effect on the surrounding environment. In particular, astronomers have noted how the gravitational forces of SMBHs cause gas and dust to fall in around their outer edges (the Event Horizon).

This matter is then accelerated to relativistic speed and slowly be accreted onto the faces of the black hole, releasing the tremendous amounts of energy that allow AGNs to outshine their galactic disks. However, visualizing these massive objects in telescopes is extremely difficult because they are nestled within the tightly packed group of stars at the galaxy’s center (aka. the “galactic bulge”), which produces a tremendous amount of light interference.

But thanks to a technique known as Very Long Baseline Interferometry (VLBI), where telescopes worldwide combine light to achieve high-resolution imaging, astronomers at the EHT Collaboration created a virtual telescope with an aperture equivalent in size to the Earth. When concentrated on an object that is difficult to resolve, this telescope can gather light over time and reconstruct an image of what it looks like (similar to a long exposure time with a camera).

This allowed the EHT team to image the bright Event Horizon around the M87 supermassive black hole (M87*) for the first time and has led to new insights on several other SMBHs. For example, the EHT also observed Sgr A* on multiple nights in 2017 and collected data for several hours straight. The latest image shows the Event Horizon of Sagittarius A* (Sgr A* for short) and is the first definitive evidence of this SMBHs existence.

The Atacama Large Millimeter/submillimeter Array (ALMA) looking up at the Milky Way and the location of Sagittarius A*, the supermassive black hole at our galactic center. Credit: ESO

Geoffrey Bower, an EHT Project Scientist from the Academia Sinica’s Institute of Astronomy and Astrophysics, spoke of the results in an ESO press release. “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,” he said. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”

As noted, the EHT Collaboration is an international effort that includes facilities worldwide, which
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