Astronomers have reached an unprecedented level of detail in observations taken from Earth. They accomplished this by capturing light from far-off galaxies at a frequency close to 345 GHz, which corresponds to a wavelength of 0.87 mm. Looking ahead, they anticipate being able to produce images of black holes that are 50% more detailed than before. This enhancement will allow for a clearer view of the area just outside the boundaries of nearby supermassive black holes, and broaden the number of black holes they can image compared to previous efforts. These findings stem from a pilot experiment.
The Event Horizon Telescope (EHT) Collaboration has completed observational tests using the Atacama Large Millimeter/submillimeter Array (ALMA) and other facilities, achieving the highest resolution recorded from Earth’s surface [1]. They succeeded in detecting light from remote galaxies at a frequency of approximately 345 GHz, equal to a wavelength of 0.87 mm. The Collaboration believes that, in the future, they will be able to generate black hole images that are 50% more detailed, enhancing the visibility of regions immediately surrounding nearby supermassive black holes. They will also expand their ability to image additional black holes compared to past efforts. Their new detections, part of a pilot project, were published today in The Astronomical Journal.
In 2019, the EHT Collaboration unveiled images of M87*, the supermassive black hole at the center of the M87 galaxy, and followed up with images of Sgr A*, the black hole located in our Milky Way galaxy, in 2022. These images were created by linking various radio observatories worldwide through a method known as very long baseline interferometry (VLBI), effectively forming an ‘Earth-sized’ virtual telescope.
To acquire images with greater resolution, astronomers usually depend on larger telescopes or wider separations between observatories involved in interferometry. However, since the EHT’s size already equals that of Earth, enhancing ground-based observations required an alternative approach. An effective way to boost telescope resolution is to capture light at shorter wavelengths, which is exactly what the EHT Collaboration has accomplished.
“Using the EHT, we captured the first images of black holes at a 1.3-mm wavelength; however, the bright ring produced by light bending under the black hole’s gravitational influence appeared somewhat unclear as we had reached the limits of our imaging capabilities,” said study co-lead Alexander Raymond, a former postdoctoral scholar at the Center for Astrophysics | Harvard & Smithsonian (CfA), now at the Jet Propulsion Laboratory in the United States. “At 0.87 mm, our images will be sharper and more refined, potentially uncovering new features, some of which might not have been anticipated.”
To demonstrate their ability to detect at 0.87 mm, the Collaboration performed test observations on distant, luminous galaxies at this wavelength [2]. Rather than employing the full EHT array, they used two smaller subarrays that included ALMA and the Atacama Pathfinder EXperiment (APEX) located in the Atacama Desert in Chile. The European Southern Observatory (ESO) is a partner in ALMA and collaborates on APEX. Additional telescopes involved were the IRAM 30-meter telescope in Spain and the NOrthern Extended Millimeter Array (NOEMA) in France, along with the Greenland Telescope and the Submillimeter Array in Hawai’i.
In this pilot test, the Collaboration reached observations with a precision of 19 microarcseconds, marking the highest resolution ever recorded from Earth’s surface. However, they were unable to produce images yet, as while they reliably detected light from several distant galaxies, not enough antennas were available to accurately reconstruct an image from the data.
This technical trial has unveiled a new opportunity to investigate black holes. With the complete array, the EHT can observe details as fine as 13 microarcseconds, comparable to detecting a bottle cap on the Moon from Earth. This indicates that at 0.87 mm, they will achieve images with 50% greater resolution than the previously released 1.3-mm images of M87* and Sgr A*. Furthermore, there is potential to detect smaller, fainter, and more distant black holes beyond the two that have been previously imaged.
Sheperd “Shep” Doeleman, the EHT Founding Director and an astrophysicist at the CfA, stated: “Observing changes in the surrounding gas at varying wavelengths will assist in resolving the enigma of how black holes attract and accumulate matter and how they can launch powerful jets that extend across galactic scales.”
This marks the first successful application of the VLBI technique at the 0.87 mm wavelength. Although observing the night sky at this wavelength existed before the new detections, employing VLBI at this frequency posed challenges that required time and technological advancements to overcome. For instance, water vapor in the atmosphere absorbs 0.87 mm waves significantly more than 1.3 mm waves, complicating the reception of signals from black holes at shorter wavelengths. Combined with marked atmospheric turbulence and noise amplification at shorter wavelengths as well as difficulties in regulating global weather conditions during sensitive observations, progress in shorter wavelength VLBI—particularly those entering the submillimeter range—has been gradual. However, these new detections mark a turning point.
“These VLBI signal detections at 0.87 mm are revolutionary as they open a fresh observational avenue for the investigation of supermassive black holes,” remarked Thomas Krichbaum, a co-author of the study from the Max Planck Institute for Radio Astronomy in Germany, which operates the APEX telescope in collaboration with ESO. He added: “In the future, combining the IRAM telescopes in Spain (IRAM-30m) and France (NOEMA) with ALMA and APEX will facilitate imaging of even smaller and fainter emissions than previously achievable at the two wavelengths of 1.3 mm and 0.87 mm, used simultaneously.”
Notes
[1] While higher resolution astronomical observations have been made, those were achieved by merging signals from both ground-based and space telescopes: https://www.mpifr-bonn.mpg.de/pressreleases/2022/2. The latest observations released today represent the highest resolution achieved solely using ground-based telescopes.
[2] For their observational tests, the EHT Collaboration directed antennas towards very distant ‘active’ galaxies, which are energized by supermassive black holes at their centers and are significantly bright. Such sources aid in calibrating observations before the EHT targets dimmer sources, like nearby black holes.
