On April 10, 2019, the world was presented with the first image of a black hole courtesy of the Event Horizon Telescope (EHT). Specifically, the image showed the supermassive black hole (SMBH) at the center of the super-giant elliptical galaxy known as M87 (also known as Virgo A). These powerful natural forces reside at the centers of most massive galaxies, which include the Milky Way (where the SMBH known as Sagittarius A * is located).
Using a technique known as Very Long Baseline Interferometry (VLBI), this image signaled the birth of a new era for astronomers to finally conduct detailed studies of these powerful forces of nature. Thanks to research by the EHT collaboration team during a six-hour observation period in 2017, astronomers are now receiving images of the core region of Centaurus A and the radio jet emanating from it.
The study describing their results, which was recently published in Nature Astronomy, was carried out by the EHT collaboration, which includes more than 300 researchers from Africa, Asia, Europe, North and South America. There were also researchers from the Max Planck Institute for Radio Astronomy, the Black Hole Initiative (BHI), the Yale Center for Astronomy and Astrophysics, the Princeton Center for Theoretical Science, the Flatiron Institute and several universities and research institutes.
Image of the Centaurus A galaxy combining optical, X-ray, and infrared data. Photo credit: Roentgen: NASA / CXC / SAO; Optical: Rolf Olsen; Infrared: NASA / JPL-Caltech
For decades, astronomers have known that SMBHs are at the heart of most massive galaxies, surrounded by massive rings of dust and gas. These rings are caused by the tremendous pull of the SMBHs, which accelerates the dust and gas to relativistic speeds (a fraction of the speed of light) and releases large amounts of electromagnetic energy (including radio waves).
This process causes galactic nuclei to become “active” – also known as. an active galactic core (AGN) or quasar – where the core region outshines the galactic disk many times over. While matter at the edge of the black hole is accreted on its surface, part of the surrounding matter escapes shortly before it is captured in the form of relativistic jets – one of the most energetic properties in the known universe.
As they indicate in their study, data from the 2017 EHT observation campaign enabled the team to capture images that were ten times higher in frequency and sixteen times sharper in resolution. This was made possible by the resolving power of the EHTs, which results from eight radio observatories, which together result in a virtual telescope with an earth-sized aperture.
Centaurus A is over 13 million light years from the Milky Way and is the closest radio galaxy to us and (when recorded in radio wavelengths) one of the largest and brightest objects in the night sky. Using the same interferometry technique that was used to capture images of M87, Team Centaurus A was able to observe with incredibly sharp resolution at a wavelength of 1.3 mm.
Close-ups of the relativistic jet emanating from Centaurus A. Credit: M. Janssen, H. Falcke, M. Kadler, E. Ros, M. Wielgus et al.
Study co-author Heino Falcke, an EHT board member and professor of astrophysics at Radboud University, said in a NOVA press release:
“This enables us for the first time to see and study an extragalactic radio jet on smaller scales than light travels in a day. We see up close and personal how a monstrous gigantic jet shot down from a supermassive black hole is born …
“These data come from the same observation campaign that provided the famous image of the black hole in M87. The new results show that the EHT offers a treasure trove of data on the rich diversity of black holes and more to come. “
Previously, the task of monitoring Centaurus A at radio wavelengths was overseen by Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI), a multi-wavelength program comprised of nine radio telescopes on four continents. Since the mid-2000s, TANAMI has been investigating the core region of Centaurus A with VLBI at centimeter wavelengths as well as other relativistic jets and active galactic nuclei (AGN) in the southern sky.
Not only was the new image much higher in terms of resolution, it also showed features of Centaurus A that had never been seen before. For example, the EHT team found that Centaurus A is brighter around the edges than in the center, a phenomenon that has been seen on other jets but has never been as pronounced. These observations will feed into the attempts of astrophysicists to model the behavior of matter in the presence of SMBHs, which is still unclear.
Astrophysicists in particular are still trying to figure out how relativistic jets are launched, or how they can stretch over light years without spreading. “We found it difficult to explain using the same models we used for the M87,” said Sera Markoff, vice chair of the EHT Science Council and co-author of the study. “Something else has to happen, like spiral magnetic fields that give us new clues as to how to” squeeze “the jets.”
Thanks to the new EHT observations of the Centaurus A jet, it is believed that the launch point of the jets coincides with the likely location of the SMBH. On this basis, the research team predicts that future observations at even shorter wavelengths and resolutions could photograph the SMBH in the center of Centaurus A – similar to 2019 with the M87.
This will likely require space-based observations that will allow for more accurate baseline interferometry (which is free from atmospheric distortion). Ongoing study of this phenomenon, made possible thanks to arrays like the EHT, also allows astronomers to observe how the laws of physics work in the most extreme environments in the universe.
Further reading: NOVA, Natur