The second image of the first black hole ever to be pictured by humanity shows that its shadow persists a year on.
The newly released image of the supermassive black hole at the heart of the galaxy Messier 87 (M87) was captured by the Event Horizon Telescope (EHT) on April 21, 2018, a year and 10 days after it was first pictured.
Just like in the April 2017 image, this second picture of the supermassive black hole, known as M87*, shows a glowing golden ring representing matter swirling around the black hole being heated to extreme temperatures. Still at the heart of this ring is a dark shadow, as predicted by Einstein's 1915 theory of gravity, known as general relativity.
"A fundamental requirement of science is to be able to reproduce results," Academia Sinica Institute for Astronomy and Astrophysics associate research fellow Keiichi Asada said in a statement. "Confirmation of the ring in a completely new data set is a huge milestone for our collaboration and a strong indication that we are looking at a black hole shadow and the material orbiting around it."
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The new image of this supermassive black hole confirms the accuracy of this theory of gravity, which predicts that M87*'s width should remain the same as long as its mass does not change significantly, thus confirming that the radius of a black hole is indeed connected to its mass.
The image also confirms that some changes in the brightness of the disk have occurred, which are connected to the turbulence in the matter around the black hole and being gradually fed to it.
M87* in 2017 and 2018: What changed and what stayed the same
Located 55 million light-years from Earth at the heart of the galaxy M87, the supermassive black hole M87* has a mass equivalent to around 6.5 billion suns.
M87* powers the bright, active galactic nucleus (AGN) heart of the elliptical galaxy as it gradually feeds on surrounding matter, heating what it does not consume with powerful magnetic fields funneling material to its poles before blasting it out at nearly the speed of light.
M87* made history when it was first imaged by the EHT on April 11, 2017. Further data analysis of the M87* EHT image showed how the light was polarized around the black hole, giving hints into the structure of jet-launching magnetic fields and the nature of the heated gas, or plasma, surrounding the supermassive black hole.
The 2017 and 2018 images of M87* are remarkably similar, with the bright rings around the supermassive black hole remaining the same size.
This is an important observation as it shows that because the mass of this supermassive black hole hasn't changed significantly, neither has the diameter of its outer layer, the light-trapping surface called the event horizon which acts as the outer boundary of the black hole. This helps confirm the suggestion derived from general relativity that the diameter of a black hole is dependent on its mass.
"One of the remarkable properties of a black hole is that its radius is strongly dependent on only one quantity: Its mass," NASA Jet Propulsion Laboratory scientist Nitika Yadlapalli Yurk said. "Since M87* is not accreting material (which would increase its mass) at a rapid rate, general relativity tells us that its radius will remain fairly unchanged over human history. It's pretty exciting to see that our data confirm this prediction."
Scientists expect that the black hole M87* isn't accreting matter fast enough for its growth to become noticeable over the duration of a human lifetime, and this new image also helps confirm this is likely the case.
However, that doesn't mean nothing has changed for M87* between its two EHT close-ups. In the new image, the brightest peak of the ring around the black hole has shifted by 30 degrees counterclockwise. This is something the EHT team was expecting to see and confirms the variability of the turbulent matter around the black hole.
"The biggest change that the brightness peak shifted around the ring is actually something we predicted when we published the first results in 2019," Academia Sinica Institute for Astronomy postdoctoral fellow Britt Jeter said. "While general relativity says the ring size should stay pretty fixed, the emission from the turbulent, messy accretion disk around the black hole will cause the brightest part of the ring to wobble around a common center.
"The amount of wobble we see over time is something we can use to test our theories for the magnetic field and plasma environment around the black hole."
What next for the supermassive black hole M87*?
The first image of M87* and the in-depth analysis of the data used to build it kickstarted a new era of black hole investigation and also gave scientists a new laboratory in which to test general relativity.
The next step in these investigations was collecting repeated observations of this supermassive black hole, with this new image representing the first use of data collected from M87* after 2017 by the EHT.
The EHT was given a helping hand to collect new and improved images of M87* in 2018 when, five months after its construction was completed in the Arctic Circle, the Greenland Telescope joined the other antennas in the array that comprise this Earth-sized telescope. This improved the image fidelity of the EHT and its coverage of the sky, especially directed from the north to the south.
The repeated observations of M87* have also allowed the EHT to be used to test cutting-edge developments in an astronomical technique called high-frequency radio interferometry and independent imaging and modeling techniques.
"The inclusion of the Greenland Telescope in our array filled critical gaps in our Earth-sized telescope," Instituto de Astrofísica de Andalucía PhD candidate Rohan Dahale said. "The inclusion of the Greenland Telescope in our array filled critical gaps in our Earth-sized telescope."
The EHT continued to monitor M87* after 2018, with further observations conducted in 2021 and 2022, and with the next observation of M87* by the EHT planned for the first half of this year.
One thing astronomers will be hoping to see in post-2018 observations is the jet of material emerging from M87*, something the EHT array wasn't advanced enough to see 6 years ago.
"The 2021, 2022, and the forthcoming 2024 observations witness improvements to the array, fueling our enthusiasm to push the frontiers of black hole astrophysics, Dahale concluded.
The team's research and the new images of M87* are published in the journal Astronomy & Astrophysics.
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