The Event Horizon Telescope (EHT) Collaboration has conducted test observations, using the Atacama Large Millimeter/submillimeter Array (ALMA) and other facilities, that achieved the best resolution ever obtained from the surface of Earth [1]. They managed this feat by detecting light from distant galaxies at a frequency of around 345 GHz, comparable to a wavelength of 0.87 mm. The Collaboration estimates that in future they may have the ability to make black hole images which might be 50% more detailed than was possible before, bringing the region immediately outside the boundary of nearby supermassive black holes into sharper focus. They will even have the ability to image more black holes than they’ve done thus far. The brand new detections, a part of a pilot experiment, were published today in The Astronomical Journal.
The EHT Collaboration released images of M87*, the supermassive black hole on the centre of the M87 galaxy, in 2019, and of Sgr A*, the black hole at the center of our Milky Way galaxy, in 2022. These images were obtained by linking together multiple radio observatories across the planet, using a way called very long baseline interferometry (VLBI), to form a single ‘Earth-sized’ virtual telescope.
To get higher-resolution images, astronomers typically depend on larger telescopes — or a bigger separation between observatories working as a part of an interferometer. But for the reason that EHT was already the scale of Earth, increasing the resolution of their ground-based observations called for a special approach. One other approach to increase the resolution of a telescope is to look at light of a shorter wavelength — and that is what the EHT Collaboration has now done.
“With the EHT, we saw the primary images of black holes using the 1.3-mm wavelength observations, but the intense ring we saw, formed by light bending within the black hole’s gravity, still looked blurry because we were at absolutely the limits of how sharp we could make the photographs,” said the study’s co-lead Alexander Raymond, previously a postdoctoral scholar on the Center for Astrophysics | Harvard & Smithsonian (CfA), and now on the Jet Propulsion Laboratory, each in the US. “At 0.87 mm, our images can be sharper and more detailed, which in turn will likely reveal recent properties, each those who were previously predicted and possibly some that weren’t.”
To indicate that they may make detections at 0.87 mm, the Collaboration conducted test observations of distant, brilliant galaxies at this wavelength [2]. Reasonably than using the complete EHT array, they employed two smaller subarrays, each of which included ALMA and the Atacama Pathfinder EXperiment (APEX) within 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 Prolonged Millimeter Array (NOEMA) in France, in addition to the Greenland Telescope and the Submillimeter Array in Hawai’i.
On this pilot experiment, the Collaboration achieved observations with detail as wonderful as 19 microarcseconds, meaning they observed on the highest-ever resolution from the surface of Earth. They’ve not been in a position to obtain images yet, though: while they made robust detections of sunshine from several distant galaxies, not enough antennas were used to have the ability to accurately reconstruct a picture from the information.
This technical test has opened up a brand new window to check black holes. With the complete array, the EHT could see details as small as 13 microarcseconds, comparable to seeing a bottle cap on the Moon from Earth. Because of this, at 0.87 mm, they may have the ability to get images with a resolution about 50% higher than that of previously released M87* and SgrA* 1.3-mm images. As well as, there’s potential to look at more distant, smaller and fainter black holes than the 2 the Collaboration has imaged up to now.
EHT Founding Director Sheperd “Shep” Doeleman, an astrophysicist on the CfA and study co-lead, says: “Taking a look at changes in the encircling gas at different wavelengths will help us solve the mystery of how black holes attract and accrete matter, and the way they will launch powerful jets that stream over galactic distances.”
That is the primary time that the VLBI technique has been successfully used on the 0.87 mm wavelength. While the flexibility to look at the night sky at 0.87 mm existed before the brand new detections, using the VLBI technique at this wavelength has at all times presented challenges that took time and technological advances to beat. For instance, water vapour within the atmosphere absorbs waves at 0.87 mm rather more than it does at 1.3 mm, making it tougher for radio telescopes to receive signals from black holes on the shorter wavelength. Combined with increasingly pronounced atmospheric turbulence and noise buildup at shorter wavelengths, and an inability to regulate global weather conditions during atmospherically sensitive observations, progress to shorter wavelengths for VLBI — especially those who cross the barrier into the submillimetre regime — has been slow. But with these recent detections, that is all modified.
“These VLBI signal detections at 0.87 mm are groundbreaking since they open a brand 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 establishment that operates the APEX telescope along with ESO. He adds: “In the longer term, the mixture 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 up to now at two wavelengths, 1.3 mm and 0.87 mm, concurrently.”
Notes
[1] There have been astronomical observations with higher resolution, but these were obtained by combining signals from telescopes on the bottom with a telescope in space: https://www.mpifr-bonn.mpg.de/pressreleases/2022/2. The brand new observations released today are the highest-resolution ones ever obtained using only ground-based telescopes.
[2] To check their observations, the EHT Collaboration pointed the antennas to very distant ‘energetic’ galaxies, that are powered by supermassive black holes at their cores and are very brilliant. These kinds of sources help to calibrate the observations before pointing the EHT to fainter sources, like nearby black holes.
