Event Horizon Telescope detects black holes at high resolution from Earth

The Event Horizon Telescope (EHT) collaboration has performed test observations reaching the highest resolution ever obtained at the Earth’s surface, detecting light coming from the centers of distant galaxies at a frequency of about 345 GHz.

Combined with existing images of supermassive black holes at the heart of M87 and Sgr A at the lower frequency of 230 GHz, these new results will not only make black hole photographs 50% sharper, but will also produce multi-colored views of the region immediately outside the boundary of these cosmic beasts.

The new detections, led by scientists at the Harvard & Smithsonian Center for Astrophysics (CfA) which includes the Smithsonian Astrophysical Observatory (SAO), are published in The Astronomical Journal.

“With the EHT, we saw the first images of black holes by detecting radio waves at 230 GHz, but the bright ring we saw, formed by the bending of light under the black hole’s gravity, still looked blurry because we were at the absolute limits of how sharp we could get the images,” said study co-leader Alexander Raymond, a former postdoctoral researcher at CfA and now at NASA’s Jet Propulsion Laboratory (NASA-JPL). “At 345 GHz, our images will be sharper and more detailed, likely revealing new properties, both those that had been predicted before and perhaps some that had not been predicted before.”

The EHT creates a virtual Earth-sized telescope by linking together multiple dishes spread across the planet, using a technique called very long baseline interferometry (VLBI). To obtain higher-resolution images, astronomers have two options: increase the distance between the dishes or observe at a higher frequency. Since the EHT was already the size of our planet, increasing the resolution of ground-based observations required extending its frequency range, and the EHT collaboration has now done just that.

“To understand why this is a major breakthrough, consider how much more detail you get by going from black-and-white to color photos,” says study co-leader Sheperd “Shep” Doeleman, an astrophysicist at the CfA and SAO, and founding director of the EHT. “This new ‘color vision’ allows us to distinguish the effects of Einstein’s gravity from the hot gas and magnetic fields that power black holes and launch powerful jets that flow across galactic distances.”

A prism splits white light into a rainbow of colors because the different wavelengths of light travel at different speeds through the glass. But gravity bends all light the same way, which is why Einstein predicted that the rings observed by the EHT should be similar in size at 230 GHz and 345 GHz, while the hot gas swirling around black holes will look different at those two frequencies.

This is the first time that VLBI has been successfully used at a frequency of 345 GHz. Although the ability to observe the night sky with a single telescope at 345 GHz has existed before, using VLBI at this frequency has long posed problems that have required time and technological advances to overcome.

Water vapor in the atmosphere absorbs waves at 345 GHz much more than at 230 GHz, weakening the black hole signals at the higher frequency. The key was to improve the EHT’s sensitivity, which the researchers did by increasing the bandwidth of the instrumentation and waiting for favorable weather at all sites.

The new experiment used two small EHT subarrays, consisting of the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX) in Chile, the IRAM 30-meter telescope in Spain, the Northern Extended Millimeter Array (NOEMA) in France, the Submillimeter Array (SMA) on Maunakea in Hawaii, and the Greenland Telescope, to make measurements with resolution as fine as 19 microarcseconds.

“The most powerful observing sites on Earth are at high altitudes, where atmospheric transparency and stability are optimal, but weather can be more dramatic,” said Nimesh Patel, an astrophysicist at CfA and SAO and a project engineer at SMA, adding that at SMA, the new observations required braving icy roads on Maunakea to open the array in stable weather after a snowstorm with just minutes to spare.

“Today, with high-bandwidth systems that process and capture broader swaths of the radio spectrum, we are beginning to overcome basic sensitivity issues, such as weather. The time is ripe, as the new detections demonstrate, to move forward to 345 GHz,” Patel added.

This achievement also represents a new step in creating high-fidelity movies of the event horizon environment surrounding black holes, which will rely on upgrades to the existing global network. The planned Next Generation EHT (ngEHT) project will add new antennas to the EHT in optimized geographic locations and enhance existing stations by upgrading them to operate at multiple frequencies between 100 GHz and 345 GHz at the same time.

With these and other improvements, the global network is expected to increase the amount of crisp, clear data available to the EHT for imaging by a factor of 10, allowing scientists to produce not only more detailed and sensitive images, but also movies featuring these violent cosmic beasts.

“The successful EHT observation at 345 GHz is a major scientific milestone,” said Lisa Kewley, director of the CfA and SAO. “By pushing the boundaries of resolution, we are achieving the unprecedented clarity in black hole imaging that we promised from the beginning, and we are setting new, higher standards for ground-based astrophysics research capabilities.”