Feast your eyes on the first image of the black hole at the center of our galaxy

At the heart of our Milky Way galaxy lies a supermassive black hole, more than four million times the mass of our sun. Scientists with the international Event Horizon Telescope (EHT) collaboration have now captured the first-ever image of that supermassive black hole. The partnership made the announcement during a live-streamed press conference this morning from the European Southern Observatory’s headquarters in Munich, Germany, as well as numerous other simultaneous press conferences around the world. Six articles on the research have been published in The Astronomical Journal Letters.

In 1933, physicist Karl Jansky noticed a radio signal emanating from somewhere in the constellation Sagittarius, near the center of our Milky Way galaxy, which he named Sagittarius A. in 1974) was particularly bright and compact. It was named Sagittarius A* (pronounced A-star). It’s so named because (according to co-discoverer Robert Brown) the radio source was “exciting,” and in physics, the excited states of atoms are denoted by an asterisk. Since the 1980s, physicists have been convinced that the central component of Sagittarius A* — and the source of all those radio emissions — was probably a supermassive black hole, similar to the one believed to be at the center of most spirals. and elliptical galaxies.

The only way to “see” a black hole is to image the shadow created by light as it bends in response to the object’s powerful gravitational field. As John Timmer of Ars reported in 2019, the EHT is not a telescope in the traditional sense. Instead, it is a collection of telescopes scattered around the world. The EHT is created by interferometry, which uses light in the microwave regime of the electromagnetic spectrum, captured at different locations. These captured images are combined and processed to build an image with a resolution comparable to that of a telescope the size of the most remote locations. Interferometry has been used for facilities such as ALMA (the Atacama Large Millimeter/submillimeter Array), where telescopes can be spread across 10 miles of desert.

In theory, there is no upper limit to the size of the array, but to determine which photons originated simultaneously at the source, you need very accurate location and timing information at each of the sites. And you still have to collect enough photons to see anything. So, atomic clocks were installed in many of the sites, and precise GPS measurements were built up over time. For the EHT, ALMA’s large collecting region, combined with choosing a wavelength where supermassive black holes are very bright, ensured enough photons. The net result is a telescope that can do the equivalent of reading the year on a Los Angeles coin from New York City — assuming the coin glowed at radio wavelengths.

The EHT announced the first-ever direct image of a black hole at the center of an elliptical galaxy in 2019, located in the constellation Virgo some 55 million light-years away: Messier 87 (M87). This view would have been impossible just a generation ago, and it was made possible by technological breakthroughs, innovative new algorithms and (of course) connecting several of the world’s best radio observatories. The image confirmed that the object at the center of M87 is indeed a black hole.

Last year, the EHT collaboration released a new image of M87 showing what the black hole looks like in polarized light — a signature of the magnetic fields at the object’s edge — providing new insight into how black holes gobble up matter and create powerful ejecting jets from their nuclei. A few months later, the EHT was back with images of the “dark heart” of a radio galaxy known as Centaurus A, allowing the collaboration to pinpoint the supermassive black hole’s location at the center of the galaxy.

After successful imaging of M87, scientists hoped that the EHT would soon produce a similar image of Sagittarius A*, which is much smaller, but also much closer, than M87. However, as theoretical physicist Matt Strassler recently wrote:

[T]The measurements of the Milky Way’s black hole turned out to be a bit more challenging, precisely because it is smaller. EHT takes about a day to collect the information needed for an image. M87’s black hole is so large that it takes days and weeks for it to change materially — even light takes many days to move from one side of the accretion disk to the other — so the image of EHT is like a short-exposure photograph. and the picture of M87 is relatively clear. But the Milky Way’s black hole can change on the time scale of minutes and hours, so EHT takes a long exposure image, similar to taking a 1 second exposure of a tree on a windy day. Things get blurrier and it can be difficult to determine the true shape of what is captured in the image.

Physicists have other means of determining the mass of Sagittarius A*. For example, UCLA astronomer Andrea Ghez shared the 2020 Nobel Prize in Physics for her work (building on that of fellow Nobelist Reinhard Genzel of the University of California, Berkeley) to map the orbits of stars closest to the center. of our galaxy. This provided an indirect way to determine that the object in the center is indeed a supermassive black hole. (No other known object could be so massive and densely packed.)

But according to Strassler, we still don’t know how the Milky Way’s supermassive black hole is oriented or how fast it is spinning. Those are the kinds of questions that the EHT collaboration hopes to find answers to. The EHT observations will allow physicists to directly study the gravitational effects near a black hole and the accretion and outflow dynamics of matter orbiting the object. It should also provide new tests for general relativity and perhaps answer some nagging fundamental questions about the existence of the black hole’s point of no return: the event horizon.

The new image shows that Sagittarius A* is remarkably similar to M87*, even though our galaxy’s black hole is more than a thousand times smaller and less massive. “This tells us that general relativity controls these objects up close, and any differences we see further away must be due to differences in the material surrounding the black holes,” said Sera Markoff, co-chair of the EHT Science Council and a physicist at the University of Amsterdam, the Netherlands.

“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada of the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images of 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 much further in testing how gravity behaves in these extreme environments than ever before.”††

This story is breaking and will be updated.