Scientists Refine Mass of Black Hole M87 Based on Stellar Movements
The mass of the black hole M87, whose shadow was photographed, has been refined based on the movement of surrounding stars. It turns out to be about a billion solar masses less than previously estimated. New measurements will help clarify which details of future images are determined by the black hole itself and which are influenced by the properties of the hot gas around it.
At the center of the elliptical galaxy Messier 87, located about 55 million light-years from Earth, lies the supermassive black hole M87*. In 2019, astronomers presented its famous image: a bright ring of emitting material surrounding a dark area associated with the shadow of this cosmic “monster.”
Since the black hole itself cannot be seen — light cannot escape its event horizon — scientists estimated the mass of the object to be about 6.5 billion solar masses based on the size of the ring. A later analysis of observations from 2017 and 2018 yielded approximately 6.16 billion.
Fortunately, the invisible object can also be weighed differently. Gravity affects the orbits of stars: the closer they are to the center of the galaxy, the more noticeable the influence of the black hole. By measuring the speeds of numerous stars and constructing a model of their motion, scientists can calculate the mass needed for such a gravitational field.
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This is what a team led by Emily Liepold from the University of California did. The researchers combined data from the James Webb Space Telescope and the Keck Observatory. The former allowed them to study stars located almost right next to the center of M87 — at distances of about 33 light-years. The latter showed the movement of stars at distances of up to 32,600 light-years. Thus, astronomers tracked stellar movements across scales differing by thousands of times.
However, periodic distortions arose in the spectra from Webb, hindering the extraction of the stellar signal. Therefore, scientists developed a special data-cleaning algorithm. It turned out that closer to the center, the speed dispersion reaches about 500 kilometers per second.
Then astronomers built computer models of the galaxy, taking into account gravity, the mass of stars, dark matter, and the complex three-dimensional shape of M87*. For comparison with observations, they used nearly five thousand measurements of stellar movements in 618 regions of the galaxy. By adjusting parameters, the researchers found a model that best reproduces the observed picture.
The new mass estimate for M87* is 5.38 billion solar masses, with a statistical error of plus or minus 130 million solar masses. Additional uncertainty related to the distance to the galaxy is 220 million. This result is consistent with the previous estimate based on stellar movements, but the possible measurement inaccuracies have been reduced two to three times.
However, the new data do not contradict previous estimates. Although the figures differ, both measurement methods yield similar results when accounting for possible errors. The refined mass will help astronomers better study the rotation of the black hole and understand how the hot gas around it behaves.
The results of the scientific work have been published on the arXiv.org preprint server.
