Astronomers determined the spin of the supermassive black hole in the Milky Way

The Spin of the Supermassive Black Hole in our Galaxy.


The Milky Way galaxy hosts a supermassive black hole (SMBH) at its center, Sagittarius A, with about four million solar masses.

The Galactic Center SMBH is surrounded by a cluster of stars and clumps of faintly glowing material. In recent years astronomers have been able to push tests of General Relativity to new limits by measuring and modeling the motions of these clumps as they swing around the SMBH.

The spin of the massive black hole (BH) at the center of the Milky Way, SgrA*, has been poorly constrained so far.

In a new study, CFA astronomers realized that the spatial distribution of one group of cluster objects, the so-called S-stars, could be used to probe the spin.

Almost 40 known S-stars lie in two nearly edge-on disks, with the stars in each disc rotating around the black hole. These stars orbit the SMBH in as little as 9.9 years.

According to astronomers, this unusual geometry could allow them to estimate the measurement of the spin.

One of the more curious and non-intuitive predictions of relativity is that space is not only warped by the gravity of a massive body, it is also warped (though to a lesser degree) by the spinning of a body. This is known as the “frame-dragging effect,” a small and hard-to-measure phenomenon.

In the case of SgrA, this phenomenon will have an appreciable effect on the S-stars’ orbits in these disks. By assuming that the S-stars orbital planes are stable over time, astronomers have shown that the SMBH’s spin in the Milky Way must be less than about 0.1.

Journal Reference:
  1. Giacomo Fragione et al. An Upper Limit on the Spin of SgrA* Based on Stellar Orbits in Its Vicinity, The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/abb9b4
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