In 2019, the Event Horizon Telescope (EHT) Collaboration shared the first-ever photo of a black hole using data collected in 2017 by a planet-scale array of eight ground-based telescopes. The following year, a ninth telescope, the Greenland Telescope, was added to the array, introducing vital new data that shows the M87* supermassive black hole in unprecedented detail.
As the EHT Collaboration explains in a new press release, M87*, the “beating heart of the giant elliptical galaxy Messier 87,” is about 55 million light-years from Earth. The original 2019 image revealed a bright circular ring, which was brighter in the southern portion. A polarized-light view of M87* released in 2021 showed more about how black holes behave.
Last September, thanks to data collected by more than 20 telescopes, an international team of scientists determined that M87* is spinning.
Unsurprisingly, given that the new images released today are built using data from 2018, it takes an extremely long time to process data concerning M87*. And of course, the EHT is “under continuous development.”
“The analysis of the 2018 data features eight independent imaging and modeling techniques, including methods used in the previous 2017 analysis of M87* and new ones developed from the collaboration’s experience analyzing Sgr A*,” the EHT explains.
The Greenland Telescope’s 2018 contribution is vital. Thanks to this new construction far above the Arctic Circle, the EHT array’s imaging power significantly increased, and improved the coverage of M87*, especially in the North-South direction. Further, in 2018, the Large Millimeter Telescope also participated for the first time using its full 50-meter surface, which further improved image quality. The EHT array also observed in four frequency bands around 230 GHz in 2018, up from just two in 2017.
These factors combined deliver a familiar, bright ring of emission of the same size as was observed in 2017. However, the position of the brightest area, which surrounds a dark central shadow, shifted about 30 degrees relative to its position in 2017. Less sensitive telescope arrays have made similar observations in the following years.
This shifting of the brightest region was predicted by the EHT Collaboration when it released its initial image of M87* in 2019.
“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,” explains Dr. Britt Jeter, a postdoctoral fellow at the Academic Sinica Institute for Astronomy and Astrophysics in Taiwan.
Calling the 2018 image “remarkably similar” to what was seen in 2017, the EHT Collaboration explains that the ring is about the same size, with a dark central region and an asymmetric brightness. The mass and distance of M87* will not “appreciably increase” during a human lifetime.
The theory of general relativity suggests that the ring’s diameter should be stable from year to year, and this is supported by the 2018 data.
“A fundamental requirement of science is to be able to reproduce results. 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,” explains Dr. Keiichi Asada, an associate research fellow at Academia Sinica Institute for Astronomy and Astrophysics in Taiwan.
“One of the remarkable properties of a black hole is that its radius is strongly dependent on only one quantity: its mass,” says Dr. Nitika Yadlapalli Yurk, a former graduate student at the California Institute of Technology (Caltech) and a current postdoctoral fellow at the Jet Propulsion Laboratory in California. “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.”
“Repeated observations with an improved array are essential to demonstrate the robustness of our findings and strengthen our confidence in our results. In addition to the groundbreaking science, the EHT also serves as a technology testbed for cutting-edge developments in high-frequency radio interferometry,” adds the EHT Collaboration.
The EHT conducted additional successful observations in 2021 and 2022 and is slated to perform another set of observations in the first half of this year, so while the new images of M87* are the sharpest ever, they may not hold that title for too long.
“Each year, the EHT array has improved in some way, either through the addition of new telescopes, better hardware, or additional observing frequencies. Within the collaboration, we are working very hard to analyze all this data and are excited to show you more results in the future,” the EHT explains.
The Event Horizon Telescope Collaboration comprises more than 300 researchers from Africa, Asia, Europe, and North and South America. The individual telescopes involved in the project are ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope, the Large Millimeter Telescope, the Submillimeter Array, the Submillimeter Telescope, the South Pole Telescope, the Kitt Peak Telescope, and the Greenland Telescope. Data were correlated at the Max-Planck-Institut für Radioastronomie and MIT Haystack Observatory. The postprocessing was performed within the collaboration by an international team at various institutions.
Image credits: EHT Collaboration