GARCHING BEI MÜNCHEN.- The heart of our Milky Way holds one or two surprises – not least because this is where a giant black hole lurks. Now, a team led by the Max Planck Institute for Extraterrestrial Physics has peered deeper than ever before into the galactic centre and discovered, among other things, a star that orbits the mass giant on a narrow path. The researchers used the Gravity instrument on the Very Large Telescope of the European Southern Observatory in Chile. The images with extremely high detail resolution also made it possible to "weigh" the black hole as precisely as never before and to determine its distance to Earth exactly.

How do you investigate the black hole at the centre at the centre of our Milky Way? An object that – by definition – you cannot see? The path that led to the 2020 Physics Nobel Prize for Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) is to follow stars on close orbits around the supermassive black hole. His team’s latest results, which expand on their three-decade-long study of stars orbiting Sagittarius A*, were published in two papers in Astronomy & Astrophysics.

On a quest to find even more stars close to the black hole, the team developed a new analysis technique that has allowed them to obtain the deepest images yet of our Galactic Centre. “The VLTI gives us this incredible spatial resolution and with the new images we reach deeper than ever before. We are stunned by their amount of detail, and by the action and number of stars they reveal around the black hole,” explains Julia Stadler, a researcher at the Max Planck Institute for Astrophysics in Garching, who led the team’s imaging efforts during her time at MPE. Remarkably, they found a star, named S300, which had not been seen previously, showing how powerful this method is to spot very faint objects close to Sagittarius A*.

With the latest suite of observations, conducted between March and July 2021, the team focused on making precise measurements of the paths of stars as they approached the black hole. This includes the record-holder star S29, which made its nearest approach to the black hole in late May 2021 at a stunning speed of 8740 km/s, passing it at a distance of 13 billion kilometers, just 90 times the distance between the Sun and Earth. No other star has ever been observed to pass this close or travel this fast around the black hole.

“We now have the possibility to do precision astronomy in the Galactic Centre and use it as a laboratory to test the predictions from Einstein’s theory of General Relativity,” explains Stephan Gillessen, who has been observing the heart of our Milky Way with various techniques since the 1990s. “There are about 50 stars with known orbits close to Sagittarius A*, and we already saw the gravitational redshift and the Schwarzschild precession when the star S2 passed very close to the supermassive black hole in 2018. But there are still open questions such as: How massive is it exactly? Does it rotate?”

The new observations, combined with the team’s previous data, confirm that the stars follow paths exactly as predicted by General Relativity for objects moving around a black hole of mass 4.3 million times that of the Sun. This is the most precise estimate of the Milky Way’s central black hole mass to date, with a precision of about 0.25%.

The team’s measurements and images were made possible thanks to Gravity a unique instrument that the collaboration developed for ESO’s VLTI. Gravity combines the light of all four 8.2-metre telescopes of ESO’s Very Large Telescope (VLT) with a process called interferometry. The technique is complex, “but in the end you arrive at images 20 times sharper than those from the individual VLT telescopes alone, revealing the secrets of the Galactic Center”, says Frank Eisenhauer from MPE, principal investigator of Gravity."

To obtain the new detailed images, the astronomers used a state-of-the art machine learning technique, called Information Field Theory. They made a model of how the real sources may look, simulated how Gravity would see them, and compared this simulation with Gravity observations. This allowed them to find and track stars around Sagittarius A* with unparalleled depth and accuracy. In addition to the Gravity observations, the team also used data from NACO and SINFONI, two former VLT instruments, as well as measurements from the Keck Observatory and NOIRLab’s Gemini Telescope.

Gravity will be updated later this decade to Gravity+, pushing the sensitivity further to reveal fainter stars even closer to the black hole. The team aims to eventually find stars so close that their orbits would feel the gravitational effects caused by the black hole’s rotation. ESO’s upcoming Extremely Large Telescope (ELT), under construction in the Chilean Atacama Desert, will further allow the team to measure the line-of-sight velocity of these stars with very high precision. “With Gravity+’s and ELT’s powers combined, we will be able to find out how fast the black hole spins”, states Eisenhauer. “Nobody was able to do that so far.”