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What is this all about?: At 12 May 2022 at the ESO official announcement live streaming event the EHT representatives claimed that the Sgr A* BH accretion disc spin axis is sort of facing Earth's position thus BH spin axis possible not perpendicular to the Milky way galaxy accretion disc?!

https://www.youtube.com/watch?v=rIQLA6lo6R0&t=2365s (watch at specific time stamp)

Also here the explanation given: https://www.youtube.com/watch?v=rIQLA6lo6R0&t=6980s

This if true, is an unexpected result and raised a lot of eye brows!!

I am not satisfied from the given explanation.

Are there any alternative explanations?

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  • $\begingroup$ Can this be actually a relative elevation perspective problem as illustrated here?: tinyurl.com/bdezh9m3 $\endgroup$
    – Markoul11
    May 13 at 19:28
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    $\begingroup$ A non-video link about the claimed orientation of the axis would be helpful. $\endgroup$
    – rob
    May 13 at 19:43
  • $\begingroup$ I'm fairly certain we would take into account our perspective from the position of the earth and the black hole. What specifically did you find that was lacking from the explanation provided? It seems reasonable to believe. $\endgroup$
    – Triatticus
    May 13 at 19:51
  • $\begingroup$ @rob Here are the first six papers released by the EHT team I to VI: iopscience.iop.org/journal/2041-8205/page/… $\endgroup$
    – Markoul11
    May 13 at 19:53
  • $\begingroup$ @Triatticus Assuming initially the BH spin axis was aligned vertical with galaxy accretion disc and normal evolution I find it difficult any local influence to throw off the BH spin axis since the Sgr* BH is the most massive object at that region and also because the conservation of angular momentum. $\endgroup$
    – Markoul11
    May 13 at 19:57

2 Answers 2

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I think what you mean is that the spin axis of the black hole is not aligned with spin axis of the Milky Way? NB: That's about all you can say - the conclusion of the actual science paper (Akiyama et al. 2022, ApJL, 930, L12 is:

Our model comparisons disfavor scenarios where the black hole is viewed at high inclination ($i > 50^{\circ}$),

not that it is pointing towards us (which would be $i=0$ and highly improbable even in a statistical sense). The current analysis also fails to identify any preferred position angle of the rotation axis.

I have not watched the video to see whether they really claimed the spin axis points towards the Earth. I doubt it, and the published papers don't support that conclusion (though it is possible). Previous work on Sgr A* using infrared interferometry to monitor gas orbital motions within a few Schwarzschild radii of the black hole had already suggested that it had a low inclination (Gravity Collaboration 2018).

There isn't really any reason that the black hole spin should be aligned with the Milky Way angular momentum. When you look at the dynamics of things near the centre of the Milky Way they are not distributed in a uniform disk-like way. The stars that are immediately orbiting the black hole (which have now been studied for decades) have what looks to me like a random orientation in space$^1$. e.g. (from https://www.eso.org/public/images/eso1825d/).

Orbits of stars around Sgr A*

Murchikova et al. (2019) show (reproduced below) the orientation of various other complex structures in the central $\sim 2$ pc of the Milky Way. They have a variety of orientations and none really lie in the galactic plane (perpendicular to the screen with the yellow line marking the position angle).

Central 2 pc of the Galactic centre

Indeed in a more recent paper (Murchikova et al. 2022), an accretion flow fed by multiple stellar sources is found to describe the sub-mm emission best. The final conclusion of that paper is important to the discussion here:

Thus, a key property of our Sgr A* wind-fed models (and thus by inference the Galactic Center accretion flow itself) is that the gas does not circularize at large radii as discussed in Ressler et al. (2020). If the parcels of plasma in the inner tens of rg do indeed have the broad distribution of angular momenta (i.e., inclinations and eccentricities) that results from stochastic feeding by multiple wind sources rather than the narrow distribution provided by a torus, then certain properties of the accretion flow may never match models that consider only torus initializations.

Since the black hole at the centre may be the product of many, many accretion events with essentially random angular momentum vectors, or mergers between black holes with differing spin vectors, potentially from accreted satellite galaxies, any initial angular momentum direction could have been scrambled. In just the same way- the spins of stars in the galaxy, or the spins of binary systems, do not align with the galaxy's angular momentum vector.

It is probably worth noting that a $M_{\rm BH}\simeq 4\times 10^6 M_\odot$ black hole doesn't really have that much angular momentum. A maximally spinning black hole of this mass has $J \sim 10^{55}$ kg m$^2$ s$^{-1}$. An object of mass $m$ at the innermost stable circular orbit, just prior to accreting into the black hole has an angular momentum of $\sim \sqrt{12}GM_{\rm BH}m/c$, which is $10^{49} (m/M_\odot)$ kg m$^2$ s$^{-1}$. Thus the black hole angular momentum could be significantly changed by accreting $\sim 10$% of its current mass.

$^1$ In fact Ali et al. (2020) have shown that the galactic centre stars currently orbiting the black hole tend to fall into two groups. Both have close-to-edge-on orbits, but their position angles are at right angles to each other and are at 45 degrees to the galactic plane. i.e. Their orbital planes are also not aligned with the Milky Way disk.

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  • $\begingroup$ By black hole standards it's a pretty feeble accretion disk, suggesting that it doesn't have access to a large scale flow from the galaxy at large. $\endgroup$
    – John Doty
    May 14 at 19:28
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    $\begingroup$ Small comment: The shadow of a Kerr black hole is always approximately circular, even if the spin is high. I don't think the EHT could see a difference in the shadow size, especially if the BH is not seen edge-on. $\endgroup$
    – Javier
    May 14 at 22:07
  • $\begingroup$ @Javier I agree. I confused this with the displacement of the shadow, which could be significant, but I guess cannot be accurately measured? $\endgroup$
    – ProfRob
    May 14 at 23:21
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This recent Nature publication seems to contradict the EHT claim in their presentation about the orientation of the Sgr*A BH accretion disc plane relative to the Galactic plane.

Seems to me that gas accretion disc of BH is in the same plane with the galactic plane (see fig.2 red circle on yellow line):

https://www.nature.com/articles/s41586-019-1242-z

Arxiv: https://arxiv.org/abs/1906.08289

The Authors in the Nature paper consider also cooler gas circulation on the accretion disc of the BH extending it therefore to a radius of 26 arcsec compared to the 52 micro arcsec of EHT image but I cannot imagine why the near field part of the accretion disc should spontaneously change orientation and misalign itself from the rest?

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  • $\begingroup$ "This implies that the disk is either close to face-on (inclined at an angle ∼ 15 degrees) as was suggested to be the configuration at a few Schwarzschild radii (Rs) [21]". $\endgroup$
    – ProfRob
    May 14 at 18:49
  • $\begingroup$ There are several statements in the paper that contradict your ideas. e.g."cloudlets are embedded in a hot accretion flow or magnetocentrifugal wind [17, 18, 8, 9, 10]. Note, that rotation axis of the accretion flow can vary with the radius for many reasons [22]. The rotation of the flow at ∼ 104Rs , which is reported here, may differ from the orientation of the flow at a few Rs suggested by GRAVITY observations [21]. $\endgroup$
    – ProfRob
    May 14 at 18:51

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