One of the findings they announced at the press conference is that the spin of the black hole is not aligned with the galactic plane but is tilted "towards us" so that it is viewed face on.
How unexpected is that?
How unexpected is that?
M87 and its CBH M87* are much more massive than the Milky Way and its respective CBH Sgr A*, but it M87 a giant elliptical galaxy that is almost circular (as far as we can tell from the highly random orbital motions of constituent parts like hydrogen and molecular gas clouds, resolvable globular and other star clusters, and so forth). So there is essentially no (bulk) galactic spin for M87* to couple to. M87*, the black hole, has significant spin however.
For a galaxy with strong axisymmetry to the point of a thin disc, there are still open questions about spin-spin coupling with a sufficiently massive CBH, under the assumption that the CBH spin and the galactic spin were identical in the early universe and that the CBH spin has not become perturbed (e.g. by black hole mergers, which may change the spin parameter, which translates to a CBH spin axis unaligned with the galactic spin axis, or a counter-rotating CBH, or some combination), and that the galactic spin has not become perturbed (by close-encounters or mergers with other galaxies). There are additional reasons why a jet and counter-jet may not trace out the extended spin axis of a CBH.
Finally, the spin-spin coupling is probably driven by the galactic spin imposing its "will" upon the CBH, rather than the other way around, because of the large mass-ratio and the distribution of mass at significant spatial remove from the CBH. However, there is a chicken/egg conundrum for very large CBHs, since we don't know if they become so huge (principally) primordially or by hierarchical mergers or by some other mechanism. The biggest CBHs may drive the initial angular momentum in the early protogalaxy, and then both the CBH and the later-time bulk galaxy will influence each others' spins. That is, the spin-spin can be correlated without any significant direct linking (in terms of forcing a drifting spin to realign), because the coupling can be arbitrarily weak.
So, we should not be surprised by central black holes spinning differently from their enclosing galaxies. But, we have a lot to learn about your question from further observations of CBHs!
J reflects the entire history of the black hole, including mergers and infalling matter. The entire history of the galaxy includes outflows driven by jets from the central black hole, and one expects J (and available inflows) to determine whether the jets increase or quench star formation. So the pecularities of the history of a large well-fed central black hole's J can shape the distribution and composition of stars around it. A forthcoming paper goes into this in detail : https://par.nsf.gov/biblio/10322445-which-agn-jets-quench-st...
The inverse is relevant too: what's the angular momentum of things falling onto a central black hole? In a spinning galaxy with significant \lambda, visible matter is entrained (via gravitational minimal coupling, and possible weak-scale interactions with halo dark matter) in such a way that most of what falls onto the central black hole has a correlated spin, so if J for a well-fed black hole drifts a little from correspondence with \lambda, infalling matter will tend to correct that. (central dark matter might also contribute weakly).
The mechanisms for correlations between the spins are ripe for even more study. Chandrasekhar dynamical friction is a probable component. There may be other components. Jets are probably relevant, and jet strength depends on black hole mass and spin, and the environment surrounding the black hole, but the action of the jet itself on the matter distribution immediately around the black hole is through electromagnetic interactions (so we may introduce Pauli coupling, and thus our spins may not be precisely "minimal"ly-coupled). We need to see more central black holes in more galaxies to answer fun question like: can the size and spin of M87* over time and the consequent strong jets, if allowed to tumble, have randomized the orbits of star-forming clouds (and thus M87's abundant globular clusters) in nonspinning parent galaxy M87? Or is it much more likely that galactic mergers drove out M87's bulk spin? If the latter, why is M87* still strongly spinning?
Indeed today's event <https://www.eso.org/public/news/eso2208-eht-mw/> involved many of the techniques mentioned on the pages above, with paper III (freshly un-embargoed, so I have not perused it) appears to be a good starting point if you feel technically inclined.
I'm not exactly sure what you mean by "non-quantum" -- in general making sense of extragalactic central black hole observables (and even obtaining them in the face of e.g. astronomical and atmospheric extinction) depends very sensitively on understanding various types of scattering (especially Compton and its inverse) and atomic electron transitions / (quantum-mechanical) spin-orbit interactions. This has to enter into matching the results of a simulation from data obtained by observatories.
I believe you are asking about how to solve the trajectories of electromagnetic radiation generated just outside these central black holes, since essentially that's what determines the images released to the public today.
I'm going to restrict this to the "lens" of the production of hard X-Rays and gammas around a black hole by inverse Compton scattering. <https://svs.gsfc.nasa.gov/vis/a010000/a011200/a011206/index....> has a pretty couple of visualizations. (It is not a coincidence that the swirls vaguely resemble some of the images that were revealed in today's ESO presentations.)
Pretty much nobody is using exact analytical solutions to the Einstein Field Equations of General Relativity to predict central black hole observables. Instead one uses a combination of numerical methods <https://en.wikipedia.org/wiki/Numerical_relativity> and approximations to the full Einstein Field Equations including linearized gravity, the effective one body formalism, and post-Newtonian expansions (the wikipedia article for which has a handy chart of the domain of applicability for these <https://en.wikipedia.org/wiki/Post-Newtonian_expansion>). One can do standard model physics set against any of these formalisms (or against several as things plunge inwards and/or climb outwards from the near-horizon) and get useful results.
If one sat down (as a theorist) and were to grind out an exact analytical solution (this would have to be for a very tiny sample of light-producing events to be tractable cf. [1]), one would find there is no need to make quantum corrections to the gravitational side of the Einstein Field Equations. The reason for this is that General Relativity guarantees a small patch of flat spacetime around every point everywhere. As long as the "small patch" is big enough to enclose an electron-gamma scattering event, there is no need for quantum corrections. This translates in practice to not having to introduce higher-order terms "correcting" the formalisms above for strong gravity, and in fact partially justifies each of those.
Where we worry theoretically is when spacetime curvature nearby is so strong that the "small patch" starts being smaller than a gamma ray. Smaller can be read as a combination of spatial extent vs wavelength or longer than the half-period of the frequency. When that happens, we have to mathematically stabilize the spacetime around the electron-gamma interaction in order to use the Standard Model's description of the scattering, and then we have to figure out how to undo the stabilization so the emitted photon has the right energy.
We would want to do this by adding in quantum corrections to whatever gravitational formalism we are using. These are easiest to see as additional higher-order terms added on to the Taylor-series-like post-Newtonian expansion.
It turns out that the strength of the local spacetime curvature (and thus the inverse of the extent of the "small patch" of flat spacetime: stronger curvature, smaller patch of flat space) outside even stellar-mass black holes is much larger than we need for pretty much any Standard Model physics to be feasible without -- or with only very gentle -- quantum corrections. For supermassive black holes, local spacetime curvature just outside the horizon is smaller than for stellar black holes, so the local patch of flat space everywhere near the black hole is much larger than that in any particle physics laboratory here on Earth. Since the tidal effects of Earth and the sun don't make much difference to physical experiments done at e.g. CERN, the even gentler tidal effects of Sgr A* and the weaker still tidal effects around M87* can basically be ignored.
Where do we start needing significant corrections, and start having to think about not using some of these formalisms instead of harder and harder work designing numerical methods based on the full theory of General Relativity? (For example, we might end up having to add many many many higher-order terms to our Taylor-series-like post-Newtonian expansion, each adjusting by something like a tiiiiiny 1/c^{ever larger number}). The answer: it's tractable until we are deeeeeep inside the event horizon, where we can't see the results of what's going on from outside. Very near the singularity the expansion approach starts requiring millions, billions, billions-to-the-power-of-billions of additional small correcting terms to retain accuracy, and it's a losing battle, even with mathematical tricks to shrink the number of terms and/or sizes of exponents ("renormalization", which is out of scope for this answer). At the singularity, this approach can only fail. Far from the singularity, but within the horizon of a large black hole, it works just fine. And in any event we only really care about what's outside the horizon, because we can't interact with anything inside: it just leaves no imprint for our telescopes to detect.
General Relativity and its approximations work perfectly well outside Sgr A* for known particle physics (and even some higher-energy extensions to the Standard Model).
Today's results fail to support several alternatives to General Relativity that correspond to a need for quantum gravity corrections just outside the horizon of Sgr A*. Among them are theories which predict "bouncing" or "reflecting" surfaces, and radiating compact stars (e.g. quark stars, boson stars -- things that are even more compact than neutron stars, but held up from collapse by an as yet undiscovered degeneracy pressure as in <https://www.einstein-online.info/en/explandict/degeneracy-pr...> for electrons).
So, in other words, there is no need for a theory of quantum gravity for the findings made public today. (The findings do cause possibly fatal trouble for alternative theories of gravity that expect quantum effects just at the horizon of Sgr A*.)
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[1] a discussion of how this works, and a neat simulator, for one photon around a Kerr black hole: <https://duetosymmetry.com/tool/kerr-circular-photon-orbits/>
The links are really helpful btw. Where do you get your daily news feed? If you have something like 'hacker news for astronomers', or some very active blog, do let know please.
We only look down on its north pole approximately. Extending the spin axis of Sgr A* could miss our solar system by thousands of light years. A polar jet, moreover, can be slightly unaligned from the extended spin axis, and over a 27 kilolightyear distance, that can be even more significant. Finally, we don't know whether the spin axis of the black hole will keep pointing roughly towards us, or whether it sweeps through (up or down) or around the midplane of the galaxy (or on what time scale such "precession", if any, occurs: for all we know, in a few years we might have an image that evidences an Earth-based view almost perpendicular to the spin axis).
Active in the sense of (from <https://en.wikipedia.org/wiki/Active_galactic_nucleus>) "much higher than normal ... excess non-stellar emission" from the region very close to the central black hole. Our central parsec is simply dim in practically every wavelength compared to its enclosing central bulge. An active galactic nucleus in our galaxy would be very noticeable to the naked human eye: it would "light up" (with far ultraviolet to gamma radiation) lots of interstellar material that, having been heated, would glow very brightly in the reds and oranges.
The luminosity of AGNs and quasars (especially luminous AGNs) is mostly from matter-matter collisions between gas and dust on intersecting geodesics (a fancy sort of friction that becomes enormous in the material nearest the black hole), with a contribution from daughter products of those collisions.
There isn't much matter circulating close to Sgr A*. It's "starving". In the panel they tried to put it in terms of a human diet: if you were eating like Sgr A* is, scaled down to human size, you'd be eating a grain of rice every few million years. Good luck keeping your body heat up on that diet. :-)
In the past -- tens of thousands to many millions of years ago -- there might have been a lot of matter circulating around Sgr A*, being swept up into luminous jets, which generated the Fermi bubbles <https://en.wikipedia.org/wiki/Galactic_Center#Gamma-_and_X-r...>. That's an area of current research: do central black holes eventually go radio-quiet in general? Is there a active-quiet-active-quiet cycle in central black holes in general? Or is this all an ultraviolet herring with the Fermi bubbles being produced by some mechanism that doesn't involve (or only very weakly depends upon) Sgr A*?
ETA: I forgot to re-read my own (grandparent) comment. The "luminous structures nearby" are not just an accretion disc. Jets count, too. Also anything that they strike (molecular gas clouds, for instance) will tend to become luminous in some part of the spectrum (sometimes this results in "frustrated lobes", sometimes "light echoes" or ionization echoes from flares).
The beams come from the accretion disk. There are two opposing beams because the disk has two opposing faces. The profile of the disk is narrow, so it stands to reason that very little radiation would leak out around the edge. On the other hand, there is a black hole in the middle of the disk and that should be smearing the beam out quite a lot. Somehow the magnetic fields and currents within the disk must conspire to keep the beam fairly narrow.
Is it at all possible that the glow is more of a spherical cloud and the black spot would be visible from any angle you look at it?