Highest-resolution observations yet from the surface of Earth

The EHT Collaboration released images of M87*, the supermassive black hole at the centre of the M87 galaxy, in 2019, and of Sgr A*, the black hole at the heart of our Milky Way galaxy, in 2022. These images were obtained by linking together multiple radio observatories across the planet, using a technique called very long baseline interferometry (VLBI), to form a single 'Earth-sized' virtual telescope.

To get higher-resolution images, astronomers typically rely on bigger telescopes -- or a larger separation between observatories working as part of an interferometer. But since the EHT was already the size of Earth, increasing the resolution of their ground-based observations called for a different approach. Another way to increase the resolution of a telescope is to observe light of a shorter wavelength -- and that's what the EHT Collaboration has now done.

"With the EHT, we saw the first images of black holes using the 1.3-mm wavelength observations, but the bright ring we saw, formed by light bending in the black hole's gravity, still looked blurry because we were at the absolute limits of how sharp we could make the images," said the study's co-lead Alexander Raymond, previously a postdoctoral scholar at the Center for Astrophysics | Harvard & Smithsonian (CfA), and now at the Jet Propulsion Laboratory, both in the United States. "At 0.87 mm, our images will be sharper and more detailed, which in turn will likely reveal new properties, both those that were previously predicted and maybe some that weren't."

To show that they could make detections at 0.87 mm, the Collaboration conducted test observations of distant, bright galaxies at this wavelength [2]. Rather than using the full EHT array, they employed two smaller subarrays, both of which included ALMA and the Atacama Pathfinder EXperiment (APEX) in the Atacama Desert in Chile. The European Southern Observatory (ESO) is a partner in ALMA and co-hosts and co-operates APEX. Other facilities used include the IRAM 30-meter telescope in Spain and the NOrthern Extended Millimeter Array (NOEMA) in France, as well as the Greenland Telescope and the Submillimeter Array in Hawai'i.

In this pilot experiment, the Collaboration achieved observations with detail as fine as 19 microarcseconds, meaning they observed at the highest-ever resolution from the surface of Earth. They have not been able to obtain images yet, though: while they made robust detections of light from several distant galaxies, not enough antennas were used to be able to accurately reconstruct an image from the data.

This technical test has opened up a new window to study black holes. With the full array, the EHT could see details as small as 13 microarcseconds, equivalent to seeing a bottle cap on the Moon from Earth. This means that, at 0.87 mm, they will be able to get images with a resolution about 50% higher than that of previously released M87* and SgrA* 1.3-mm images. In addition, there's potential to observe more distant, smaller and fainter black holes than the two the Collaboration has imaged thus far.

EHT Founding Director Sheperd "Shep" Doeleman, an astrophysicist at the CfA and study co-lead, says: "Looking at changes in the surrounding gas at different wavelengths will help us solve the mystery of how black holes attract and accrete matter, and how they can launch powerful jets that stream over galactic distances."

This is the first time that the VLBI technique has been successfully used at the 0.87 mm wavelength. While the ability to observe the night sky at 0.87 mm existed before the new detections, using the VLBI technique at this wavelength has always presented challenges that took time and technological advances to overcome. For example, water vapour in the atmosphere absorbs waves at 0.87 mm much more than it does at 1.3 mm, making it more difficult for radio telescopes to receive signals from black holes at the shorter wavelength. Combined with increasingly pronounced atmospheric turbulence and noise buildup at shorter wavelengths, and an inability to control global weather conditions during atmospherically sensitive observations, progress to shorter wavelengths for VLBI -- especially those that cross the barrier into the submillimetre regime -- has been slow. But with these new detections, that's all changed.

"These VLBI signal detections at 0.87 mm are groundbreaking since they open a new observing window for the study of supermassive black holes," states Thomas Krichbaum, a co-author of the study from the Max Planck Institute for Radio Astronomy in Germany, an institution that operates the APEX telescope together with ESO. He adds: "In the future, the combination of the IRAM telescopes in Spain (IRAM-30m) and France (NOEMA) with ALMA and APEX will enable imaging of even smaller and fainter emission than has been possible thus far at two wavelengths, 1.3 mm and 0.87 mm, simultaneously."

Notes

[1] There have been astronomical observations with higher resolution, but these were obtained by combining signals from telescopes on the ground with a telescope in space: https://www.mpifr-bonn.mpg.de/pressreleases/2022/2. The new observations released today are the highest-resolution ones ever obtained using only ground-based telescopes.

[2] To test their observations, the EHT Collaboration pointed the antennas to very distant 'active' galaxies, which are powered by supermassive black holes at their cores and are very bright. These types of sources help to calibrate the observations before pointing the EHT to fainter sources, like nearby black holes.