Free Astronomy Magazine November-December 2024

NOVEMBER-DECEMBER 2024 I n this world map, the yellow dots mark the location of the antennas and arrays that participated in a pilot experiment con- ducted by the Event Horizon Telescope (EHT) Collaboration. The experiment was the first time that the very long baseline interferometry technique, which connects telescopes hundreds or thousands of kilometers apart, was successfully used to observe light at a wavelength of 0.87 mm. By observing light at this lower wavelength, the EHT researchers were able to get higher resolution observations than they had before, without forming a bigger telescope. The detections made have the highest resolution ever obtained from the surface of Earth. [ESO/M. Kornmesser] reveal new properties, both those that were previously predicted and maybe some that weren’t.” To show that they could make de- tections at 0.87 mm, the Collabora- tion conducted test observations of distant, bright galaxies at this wave- length. 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 South- ern 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 Collab- oration achieved observations with detail as fine as 19 microarcseconds, meaning they observed at the high- est-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 microarc- seconds, equivalent to seeing a bot- tle 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 addi- tion, there’s potential to observe more distant, smaller and fainter black holes than the two the Collab- oration 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 sur- rounding gas at different wave- lengths 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 al- ways presented challenges that took time and technological ad- vances to overcome. For example, water vapour in the atmosphere ab- sorbs 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 pro- nounced atmospheric turbulence and noise buildup at shorter wave- lengths, and an inability to control global weather conditions during atmospherically sensitive observa- tions, progress to shorter wave- lengths for VLBI — especially those that cross the barrier into the sub- millimetre 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 Astron- omy in Germany, an institution that operates the APEX telescope to- gether 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 imag- ing of even smaller and fainter emis- sion than has been possible thus far at two wavelengths, 1.3 mm and 0.87 mm, simultaneously.” !