“Millimeter-VLBI image of the jet and the black hole shadow in Messier 87, obtained with the GMVA array including ALMA, the Greenland Telescope and the telescope at Metsähovi.
An international team of scientists, including Aalto University researchers, has used new millimetre-wavelength observations to produce an image that shows, for the first time, both the ring-like structure that reveals the matter falling into the central black hole and the powerful relativistic jet in the prominent radio galaxy Messier 87. The image underlines for the first time the connection between the accretion flow near the central supermassive black hole and the origin of the jet. The new observations were obtained with the Global Millimetre VLBI Array (GMVA), complemented by the phased Atacama Large Millimetre/submillimetre Array (ALMA) and the Greenland Telescope (GLT). The addition of these two observatories has greatly enhanced the imaging capabilities of the GMVA. The results are published in the current issue of Nature.
‘Previously, we had seen both the black hole and the jet in separate images, but now we have taken a panoramic picture of the black hole together with its jet at a new wavelength,’ says Ru-Sen Lu of the Shanghai Astronomical Observatory, who also leads a Max Planck Research Group at the Chinese Academy of Sciences.
The surrounding material is thought to fall into the black hole in a process known as accretion, but no one has ever imaged it directly. ‘The ring that we have seen before is becoming larger and thicker at 3.5 mm observing wavelength. This shows that the material falling into the black hole produces additional emission that is now observed in the new image. This gives us a more complete view of the physical processes acting near the black hole,’ he added.
The participation of ALMA and GLT in the GMVA observations and the resulting increase in resolution and sensitivity of this intercontinental network of telescopes has made it possible to image the ring-like structure in M87 for the first time at a wavelength of 3.5 mm. The diameter of the ring measured by the GMVA is 64 microarcseconds, which corresponds to the size of a small (5-inch/13-cm) selfie ring light as seen by an astronaut on the Moon looking back at Earth. This diameter is 50 percent larger than what was seen in observations by the Event Horizon Telescope at 1.3 mm, in accordance with the expectations for the emission from relativistic plasma in this region.
‘With the greatly improved imaging capabilities by adding ALMA and GLT into GMVA observations, we have gained a new perspective. We do indeed see the triple-ridged jet that we knew about from earlier VLBI observations,’ says Thomas Krichbaum from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. ‘But now we can see how the jet emerges from the emission ring around the central supermassive black hole and we can measure the ring diameter also at another (longer) wavelength.’
The 14-metre radio telescope of the Aalto University Metsähovi Radio Observatory was one of the stations that collected data for the new image.
Tuomas Savolainen, a senior scientist at Aalto University and a co-author of the paper, says that the Metsähovi Radio Observatory has participated in the GMVA measurement campaigns for well over a decade and in VLBI observations at 3.5 mm in general since the mid-1990s.
‘Our radio telescope at Metsähovi was one of only five stations in Europe that participated in these observations in 2018. There are not so many antennas capable of doing measurements at 3.5 mm wavelength, which makes the data gathered at Metsähovi valuable,’ he says.
‘The Event Horizon Telescope image shows the black hole shadow in M87, but those observations were not able to detect the weaker and more extended jet because of the small number of telescopes that participated in them. There are even fewer telescopes capable of observing at 1.3 mm wavelength than there are telescopes observing at 3.5 mm,’ Savolainen says.
The light from M87 is produced by the interplay between highly energetic electrons and magnetic fields, a phenomenon called synchrotron radiation. The new observations, at a wavelength of 3.5 mm, reveal more details about the location and energy of these electrons.
They also tell us something about the nature of the black hole itself: it is not very hungry. It consumes matter at a low rate, converting only a small fraction of it into radiation. Keiichi Asada of the Academia Sinica Institute of Astronomy and Astrophysics explains: ‘To understand the physical origin of the bigger and thicker ring, we had to use computer simulations to test different scenarios. As a result, we concluded that the larger extent of the ring is associated with the accretion flow.’
Kazuhiro Hada from the National Astronomical Observatory of Japan adds: ‘We also find something surprising in our data: the radiation from the inner region close to the black hole is broader than we expected. This could mean that there is more than just gas falling in. There could also be a wind blowing out, causing turbulence and chaos around the black hole.’
The quest to learn more about Messier 87 is not over, as further observations and a fleet of powerful telescopes continue to unlock its secrets. ‘Future observations at millimetre wavelengths will study the time evolution of the M87 black hole and provide a poly-chromatic view of the black hole with multiple colour images in radio light,’ says Jongho Park of the Korea Astronomy and Space Science Institute.
Some of these new observations are taking place this spring, and the Metsähovi Radio Observatory is again taking part in them.
‘3.5 mm is the shortest wavelength at which we operate at the moment, and those observations require good, dry weather conditions. Luckily, the weather in April is often good here. In a couple of years, we will get a new receiver for our telescope that will allow making observations simultaneously over a wide range of wavelengths. Then we will be able to better correct for the distortions in data caused by the atmosphere and obtain even higher quality images,’ says Petri Kirves, Metsähovi Radio Observatory Operations Engineer.
R.-S. Lu et al., A ring-like accretion structure in M87 connecting its black hole and jet, Nature, April 27, 2023: https://doi.org/10.1038/s41586-023-05843-w”