Last May, but only now approaching ToughSF's notice, we got enough data from other galaxies to evaluate the spin-rate of this one. Our galaxy's "super-massive" point mass in Sagittarius is more like a kilomassive. Galaxies like ours - starting with the one in Andromeda - have bigger black holes. Like fifty times bigger, for a total stellar field of not even twice bigger.
Over time this galaxy has gobbled some smaller galaxies; "Kraken" at 11 Bya (so 2 billion years after the Milky Way formed) being the most relatively-massive.
The question here is whether one of those galactic mergers led to a black-hole merger. These raise up gravitational tsunamis which, perhaps, roll black holes out of a galaxy entirely. Then some other point-mass might take its place and gather up its own additional mass. But it will have started from a much lower baseline, thus leading to the lightweight we see today.
UPDATE 1/10/23 Although we lost the original, Atacama is reporting a binary of black holes, from a past merger; and they're looking at mergers elsewhere. Also JWST can see all sorts of two-billion-year-old galaxies, eleven billion light years distant.
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