§3 Discussion: "The estimated mass is also too large for one neutron stars or two neutron stars in a close orbit, so we are left with the possibility of: 1) a single BH; 2) an inner binary containing two BHs; or 3) a BH and another compact object. Although the single BH is the simplest explanation, the hypothesis of an inner binary of two BHs cannot be excluded [...] radial-velocity [...] perturbations are too small to be detected in the Gaia RVS [radial-velocity] data[.] For the purposes of the subsequent discussion we have adopted the single BH hypothesis as the most likely explanation."

Someone with more patience (or more troll-energy) than I have right now might try to introduce this into the handful of threads here containing various complaints that (clearly assuming a single BH), "size" is being punned as mass in the newspaper article linked at the top. If, for example, the ~33 M_sun ("mass of the sun") mass is evenly divided between two central BHs with the giant branch star companion orbiting them (cf. Alpha Centauri AB + wide companion Alpha Centauri C and similar triples), what's the most reasonable understanding of "size"? Or what if the dark component has a mass-ratio of ~ 30:2 split between a BH and a satellite NS? Total mass remains ~33 M_sun, how has "size" changed?

(Answer: "it hasn't". Generally I agree with the comments that read "size" as meaning mass and disagree that it should mean the Schwarzschild radius, and not just because the dark component might not be very Schwarzschild-like at all. Perhaps someone might do an ELI5 of dimensional analysis g = T^-2 L -> M = T^-2 L^3 or Gauss's law for gravity. (Serious ambition would be ELI12 of the argument around the "scale separation" towards the end of the excellent <https://physicscourses.colorado.edu/phys2210/phys2210_fa20/l...>, where we'd distinguish the scale of a possible inner-binary from the scale of the giant star's orbit around it.))

So the current working hypothesis for massive black holes in galaxies is that they formed directly from the gravitational collapse of "vast clouds of gas and dust" with such high gravitational pressure that the core couldn't keep up, and the entire mass of gas and dust collapsed into a massive black hole. There is no supernova involvement; merely a direct transition into a black hole.

Of course, take my laic simplification into account.

The one at the center of the galaxy is along for the ride! A good video: https://www.youtube.com/watch?v=e-P5IFTqB98