The Event Horizon Telescope (EHT) has already provided groundbreaking images of black holes several times in recent years. Now the next step follows: The EHT collaboration has managed to shorten the wavelength of its radio telescope observations to 870 micrometers, thereby significantly increasing the resolution of its telescope network. The astronomers thus managed to make an astronomical observation with coupled radio telescopes at this higher frequency of 345 gigahertz for the first time. Thanks to this new technology, black holes or galactic nuclei can now be imaged with around 50 percent higher resolution than before, the team reports. This could provide valuable new insights into the behavior of the rotating gases around black holes, the formation of their high-energy jets and other cosmic phenomena.
The iconic images from the Event Horizon Telescope (EHT) network went around the world: The coupled radio telescopes showed for the first time the dark shadow of a supermassive black hole and the bright ring of light around the event horizon. These images confirmed important predictions by Albert Einstein about the size and shape of black holes and their event horizons. “With the EHT, we saw the first images of black holes by detecting radio waves at 230 gigahertz,” explains Alexander Raymond from the Harvard & Smithsonian Center for Astrophysics in the USA. “But the bright ring formed by light diffracted by gravity still looked blurry because we were at the absolute limit of what we could achieve in terms of sharpness.”
Shorter wavelength brings higher resolution
There are two ways to increase the resolution of a telescope network: Firstly, you can increase the distance between the telescopes, the so-called baseline. The further apart the outermost telescopes are, the larger the virtual “dish” that the network forms – and the higher the resolution. But the EHT radio telescopes, which are spread across almost all continents, are already at the maximum distance from each other – they form a receiver the size of our entire planet. The second option is to shorten the observation wavelength. As in microscopy, the same applies to telescopes: the shorter the wavelength at which you observe, the higher the possible resolution. The EHT’s images so far were taken at 230 gigahertz, which corresponds to a radio wavelength of 1300 micrometres.
In order to make the images sharper, astronomers have been working for decades to further develop the technology of radio telescope arrays coupled by interferometry (Very Long Baseline Interferometry, VLBI) so that observations at the shorter wavelength of 870 micrometers are also possible. But this is a major challenge for several reasons. For example, the water vapor in the atmosphere absorbs radio radiation at 870 micrometers wavelength much more strongly than at 1300 micrometers, which greatly weakens the detectable astronomical signals and increases the noise. At the same time, the radio waves at these higher frequencies are more sensitive to weather-related turbulence. But through improvements to the telescopes’ receivers, which are cooled to ultra-cold temperatures, the transmission technology for their precise coupling and the data processing, the EHT collaboration has now succeeded in making observations of cosmic objects at 870 micrometers wavelength for the first time.
Test shots with record sharpness
The astronomers carried out their test observations with two subsystems of the telescope network, which included the ALMA array and the Atacama Pathfinder Experiment (APEX) in Chile, as well as other telescopes in Spain, France, Greenland and Hawaii. They chose distant galaxies with active, supermassive black holes at their centres as their target. Despite partly unfavourable weather conditions at some European locations, the astronomers managed to capture the radio emission from these galaxies at 870 micrometres. Although the number of receivers used was not sufficient to create a sharp image from the data, the radio data achieved a new record resolution of 19 microarcseconds. This is the shortest wavelength ever recorded with a VLBI network of radio telescopes, as Raymond and his colleagues report. According to their calculations, the Event Horizon Telescope with its complete network could even resolve cosmic structures of 13 microarcseconds. For comparison: This corresponds to the image of a coin lying on the surface of the moon as seen from Earth.
This increase in resolution represents an important step for the observation of black holes and galactic nuclei, the team explains. This will enable the EHT to image black holes and the region of their ring of light in around 50 percent more detail and sharper than before. “To understand why this is such a breakthrough, compare it to the boost in extra detail that comes from switching from black-and-white to color photos,” says co-author Sheperd Doeleman of the Harvard & Smithsonian Center for Astrophysics. By expanding the frequency spectrum, it will be easier to study the effects of Einstein’s gravity, but also the interactions of the hot gases and magnetic fields at the black hole. In addition, the increased resolution will make it possible to image other, smaller or more distant black holes in the future. “These signal measurements with the VLBI at 870 micrometers are groundbreaking because they open a new observation window for the study of supermassive black holes,” explains co-author Thomas Krichbaum from the Max Planck Institute for Radio Astronomy in Bonn.
Source: Alexander Raymond (Harvard & Smithsonian Center for Astrophysics, Cambridge USA) et al., The Astronomical Journal, doi: 10.3847/1538-3881/ad5bdb