Why don't they make an image of the black hole in the M33 galaxy?

  • Since we see the Triangulum galaxy M33 from a quite vertical position (contrary to our Milky Way and a bit the Andromeda galaxy) it should be easy to image the black hole in the center of it, shouldn't it? Why did they prefer to first image the black hole in a galaxy which is about 20 times farther and thus harder to photograph the black hole in its center?


  • M33 does not appear to contain a supermassive black hole: in fact there's no evidence that it contains a central black hole at all. The upper limit on the mass of a central black hole based on the dynamics of the core region is a few thousand solar masses.



    Merritt et al. (2001) "No Supermassive Black Hole in M33?" derive an upper limit of 3000 solar masses on a central compact object in M33, noting that this is still consistent with the M-σ relationship between the mass of a supermassive black hole and the velocity dispersion in the stellar bulge, using which they obtain a predicted mass of 2600–26300 solar masses.



    Gebhardt et al. (2001) "M33: A Galaxy with No Supermassive Black Hole" obtain an even smaller upper limit on the mass of a black hole of only 1500 solar masses (their best fit mass is zero, i.e. no central black hole), which they state is significantly lower than the predicted mass from the M-σ relationship.


    Umm, what? If there's no black hole in the center of M33, what else? Since it is a spiral galaxy, there must be something like a central star around which the other stars revolve.

    @user30007 Most stars in a galaxy don't actually orbit around the central black hole of a galaxy. Despite it's enormous mass, it's only a tiny fraction of the mass of the galaxy. The orbit is more within the entire galaxy's mass, so a central black hole is probably not necessary for orbits. Interestingly, for M33, the stars near the center seem to orbit or rotate around the galaxy more slowly, the opposite of what's expected. https://science.sciencemag.org/content/293/5532/1116

    So stars can simply orbit a barycenter which is the very center of the galaxy?

    The papers on the imaged BH explicitly say why other targets were impossible or anyway discarded.

    @user30007 : Stars (bound to a galaxy, separated widely from other galaxies) **only** orbit the barycenter of their galaxy.

    @EricTowers I see.

    I don't see how this answers the question. A supermassive black hole is not the same thing as a black hole. Are you implying "because it's really tiny and would be impractical to image"? If so, *that* would answer the question, if it were actually written.

    Thank you; that's much more accurate now.

    @user30007 same is true with the sun and the earth, the sun is also slightly wobbling and both are orbiting around a point that is not the center of the sun (also affected by other planets). All following Newton’s law of universal gravitation.

    @Mehdi But if the Sun wasn't there the planets wouldn't revolve around the location where the Sun or its barycenter (in case of Jupiter) is.

    A binary system orbits around an empty point in space. Imagine the galaxy being an n-tuple system.

    @user30007 They could; such an orbit just would be incredibly unstable. Also, if the sun were to disappear immediately, the new barycenter of the solar system wouldn't necessarily be anywhere near the sun's former location. It would depend on the distribution of the renaming bodies in the solar system.

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