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If the galactic mass is rotating around a central supermassive black hole, should their spin axis not be the same, just as we would obtain for the rotation of a star and its planets ?

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    $\begingroup$ One difference is, in a planet-ring system or a star-planets system, the mass and gravity of the central body completely dominates the system (99%+ of the mass). For a black hole-galaxy this is not the case, even with the baryonic matter, but even more so if we include the dark matter. $\endgroup$
    – RC_23
    May 27 at 2:37
  • $\begingroup$ Eg, our SMBH, Sagittarius A*, only contains roughly 4 millionths of the total Milky Way mass. $\endgroup$
    – PM 2Ring
    May 27 at 7:23
  • $\begingroup$ Just because they may have started out with the same spin alignment does not mean they they still have the same alignment billions of years later. Specifically the supermassive black hole presumably has been capturing and absorbing other stars which can alter it's spin alignment. $\endgroup$ May 27 at 12:03

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The nearest supermassive black hole has a spin that isn't aligned with the angular momentum vector of its galaxy. So the answer must be no.

The black hole at the centre of our Galaxy has a spin axis that is probably inclined by less than 50 degrees to our line of sight Akiyama et al. 2022, as revealed by the recent Event Horizon Telescope results. Given that the Sun is close to the Galactic plane but 25,000 light years from the Galactic centre, this means the black hole spin is nowhere near parallel to the angular momentum of the disk of the Milky Way.

Whether the spin is parallel or not to that of its host galaxy would depend very much on how "old" the black hole is, whether it formed from the merger of two or more black holes and the angular momentum of any material that is being fed to it.

For example, in our Galaxy, the black hole only dominates the dynamics of material within a few parsecs of the centre. It is likely being fed material (and angular momentum) from the winds of a cluster of massive stars that orbit it. These stars are not arranged in a disk and the structures around the central black hole have a variety of orientations, none of which seem to align closely with the Galactic plane (Murchikova et al. 2019).

In other galaxies with supermassive black holes, it has often been observed that the relative orientations of the galaxy disk, the accretion disk around the black hole and the jets emerging from the central regions are essentially random (Schmitt & Kinney 2002). On the other hand, if most of the black hole mass is built up by accretion of gas fed to it from gas circulating in a similar way to the galactic plane, then alignment would be expected.

The observational evidence isn't very decisive. The orientations of the spins of supermassive black holes in other galaxies have not been measured. Indirect evidence comes from the measurements of spin magnitudes via X-ray observations of accreting gas. Reynolds (2021) reviews this evidence and concludes that the low spin rates seen in many of the more massive black holes ($>3\times 10^7$ solar masses) argues in favour of multiple mergers and incoherent accretion. These would favour a fairly random level of alignment between galaxy and black hole spins.

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There is no particular reason they need to. A planet does not necessarily have its axis aligned with the solar system or the galaxy. A star does not necessarily have its axis aligned with its stellar system or the galaxy. Our own star's axis is about 7 degrees out of alignment with the plane of the ecliptic.

If a black hole were aligned with the galaxy, and a large mass (say a star) impacted the BH at some weird angle, the result would not still be aligned. There is no particular reason that accretion has to proceed symmetrically. So the evolution of the BH could pass through a phase where it is aligned, but is unlikely to stay there.

Probably it won't be massively far off, because the average of accretion is probably going to be round-about aligned with the galaxy. But it is unlikely to be perfectly aligned.

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