The Galactic Center under the magnifying glass

GRAVITY zooms in on stars around the supermassive black hole at the heart of our Milky Way

December 14, 2021

Using the GRAVITY instrument at the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) in Chile, astronomers have found a never-before-seen star close to our galaxy’s central black hole. The deep images at ultra-high resolution also allowed the team led by the Max Planck Institute for Extraterrestrial Physics to make the most precise measurement of the black hole’s mass yet. The new images zoom into the region around the supermassive black hole at the center of our galaxy – 20 times more than what was possible before interferometry. By tracking the orbits of stars in our Galactic Centre, the team may even be able to unveil how fast the black hole rotates.

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*, are published today in two papers in Astronomy & Astrophysics.

These annotated images, obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) between March and July 2021, show stars orbiting very close to Sgr A*, the supermassive black hole at the heart of the Milky Way. One of these stars, named S29, was observed as it was making its closest approach to the black hole at 13 billion kilometres, just 90 times the distance between the Sun and Earth. Another star, named S300, was detected for the first time in the new VLTI observations.

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 researchers also managed to determine the distance to Sagittarius A* more accurate than ever before, finding it to be 27000 light-years away.

The next-generation interferometer GRAVITY was installed at the VLTI in 2015, and has been in operation since 2016.

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.”

Watch the stars move around the Milky Way’s central black hole

This animation shows the orbits of the stars S29 and S55 as they move close to Sgr A* (centre), the supermassive black hole at the heart of the Milky Way. As we follow the stars along in their orbits, we see real images of the region obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) in March, May, June and July 2021. In addition to S29 and S55, the images also show two fainter stars, S62 and S300, the latter having only been discovered following these new VLTI observations.

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