Ultracompact: The Black Hole at the center of our Milky Way

October 26, 2023

Every now and then, luminous gas is seen swirling around Sagittarius A*, the black hole at the center of the Milky Way. Now, astronomers at the Max Planck Institute for Extraterrestrial Physics (MPE) have succeeded in measuring the black hole mass from this motion – and it perfectly matches the measurement honoured with the Nobel Prize in Physics in 2020, which has been refined ever since. The conclusion: the 4.3 million solar masses are contained within an orbit smaller than that of Venus around the Sun. A truly astounding mass concentration!

At the center of our Milky Way there is a black hole with a mass of 4.3 million solar masses – several teams have established this beyond any reasonable doubt over the past four decades. In 2020, this finding was even honoured by the Nobel Prize in Physics for MPE director Reinhard Genzel. Since then, the research has focused on using the galactic centre as a laboratory to test the theory of general relativity in the very strong gravitational field close to this black hole – and to pin down its properties with high precision.

The team at MPE has now used GRAVITY, the near-infrared interferometer at ESO’s Very Large Telescope Interferometer (VLTI) to closely monitor the emission from the region around the black hole and probe for extremely bright states: flares. Such flares occur once or twice per day, and the emission becomes bright enough that it is possible to trace the motion of surrounding gas. The team analysed flares observed during 2018, 2021 and 2022, for which GRAVITY simultaneously delivered position and polarisation measurements.

This combined data set allowed the team to determine the mass of the black hole with high accuracy to be 4.297 million solar masses, a strong and independent constraint to previous measurements. The new data also show that the mass has to be enclosed inside the flares’ radius of around nine gravitational radii, which is smaller than the orbital radius of the planet Venus around the Sun.

“The mass we derived now from the flares at just a few gravitational radii is compatible with the value measured from the orbits of stars at several thousand gravitational radii,” emphasizes Diogo Ribeiro, who was responsible for the theoretical modelling at MPE. “This strengthens the case for a single black hole at the center of the Milky Way.”

Studying the motion of this orbiting gas can also shed light on the formation history of the structures at the Galactic Center. The orientation of the flare orbits is close to that of a stellar disk observed at 100,000 gravitational radii, suggesting a physical connection. “It is great to see the repeated, similar behaviour of the flares,” points out Antonia Drescher, who analysed the polarimetric measurements. “All of them show a clockwise looped motion on the sky; all have a similar radius and a similar orbital period. This is really beautiful to see.”

Strong winds from the stars farther out probably fuel the accretion flow of gas, which carries the initial angular momentum down to scales close to the event horizon. “The amount of information from the polarization was extremely fruitful and we learn a lot about the physics in the Galactic Center region from the joint data set,” adds Ribeiro. The dynamics of the flares may even carry information on the spin of the black hole – an open question today.

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