A 30-metre eye in the Sierra Nevada

The IRAM antenna is part of the Event Horizon Telescope

For the first time, researchers have observed the shadow of a black hole. It is located in the Messier 87 galaxy and is being examined by the Event Horizon Telescope. A total of eight observatories all over the world, from Europe to Chile to Hawaii to the South Pole, are linked to form a giant telescope. The 30-metre telescope in the Spanish Sierra Nevada belonging to the Institut de Radioastronomie Millimétrique (IRAM) is one of the facilities that receives support from the Max-Planck-Gesellschaft. It was the only station in Europe to participate in the observation campaign.

Antenna into space: The 30-metre IRAM dish is one of the most sensitive radio telescopes in the Event Horizon Telescope consortium, which spans the globe.

The great distances involved mean that the shadow of the black hole in the supergiant M 87 galaxy appears on Earth at a very small angle. To overcome this obstacle, researchers use interferometry. The principle of this technology is that, instead of using one giant telescope, researchers combine several observatories so that they function as small parts of a single gigantic antenna. In this manner, scientists can simulate a telescope that, optimally, has a diameter equal to that of the Earth. This is important because the larger the telescope, the finer the details that can be captured; the so-called angular resolution increases.

The EHT project applies this process and works in interferometry mode at a frequency of 230 Gigahertz, which corresponds to a wavelength of 1.3 millimetres. This sort of measurement at the limits of what is observable can be made only under optimal conditions – in dry locations at high altitudes. Those conditions are on offer at the IRAM observatory, which boasts a 30-metre antenna on a 2,800-metre mountain (Pico Veleta in the Spanish Sierra Nevada) and receives funding from the Max-Planck-Gesellschaft. The telescope is the most sensitive singular telescope in the EHT collaboration. “The surface of our antenna is adjusted with a precision that corresponds to the thickness of a human hair,” says IRAM astronomer Pablo Torne.

In April 2017, researchers oriented the 30-metre mirror, synchronized with the other EHT stations across the globe, towards M 87 and its black hole. “We could not have wished for better weather conditions for the season, and we were especially pleased with the way the technology in the observatory worked, from the high-precision atomic clock to the receiving system and the data recorders,” says Torne. In all, more than 500 terabytes of data were recorded during these observations by the IRAM telescope alone.

But EHT is not the only area in which IRAM has done pioneering work. For instance, the first high-resolution radio observations of the heart of our own Milky Way galaxy and its black hole, named Sagittarius A*, were made in 1995 – with a combination of the 30-metre antenna and the NOEMA array. The latter is currently the best and most powerful radio telescope of its kind in the Northern Hemisphere. Its 15-metre antennas are located on a high plateau in the French Alps.

Since the autumn of 2018, the observatory has also been part of EHT. “The expansion of NOEMA also means the start of a new era for EHT. NOEMA will give this unique network an unprecedented spatial resolution and sensitivity,” says Karl Schuster, IRAM Director and member of the EHT Management Board.

HOR/ZA

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