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B Black holes in the centres of galaxies

  1. Sep 21, 2017 #21


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    That the gravity of supermassive black holes has a large effect on the dynamics of stars in the galaxy. It doesn't.

    You seem to have the common misconception that a black hole sucks in matter. This is false. If you replaced our Sun with a black hole of the same mass, the Earth's orbit would not be changed at all.

    The only effect of the black hole's gravity comes from its mass, and that mass is minuscule compared to the stars in the galaxy once you move beyond the immediate galactic core. The spiral arms are found much further out. If the spiral arms reach very close to the center of the galaxy, that galaxy likely has a very small black hole (this is the essence of the M-sigma relation that has been discussed in the thread).

    Only in a very small region around the black hole. The spiral arms do not extend that close to the center. In fact, there's an inverse relationship between how close the spiral arms get to the center and the mass of the black hole: galaxies with larger black holes have larger "bulges", and the spiral arms only occur outside of the bulge. A galaxy's bulge is the spheroidal-shaped group of stars that make up the galaxy's center.

    Yes, I'm sure. Accretion requires a loss of energy so that the gas and dust can collapse, while the process of removing the dust adds energy. Though the process of removing most of the dust from the galaxy may result in some star formation (due to the collisions of dust clouds), in the main the heating of the dust clouds will tend to reduce the number of stars that form.

    The more massive the black hole, the less "spiral-shaped" the galaxy becomes. The really massive black holes tend to be at the centers of elliptical galaxies.
  2. Sep 21, 2017 #22
    Thanks for your elucidations, kimbyd, Vanadium 50, and Bandersnatch -- I think that the central SMBH in a spiriform galaxy owes the centrality of its location to its super-massivity -- even though it may be of extremely low mass in comparison to the whole of the galaxy -- I acknowledge the interesting point that a galaxy can be spiriform without any SMBH or other super-massive object at its center, and further, I acknowledge that greater central masses will entend galactic formations more toward spheroidicity than toward discoidosity; however, predicated upon basic understanding of a three-body system of unequally massive objects, I think that spiriform galaxies tend to establish centralities of orbits around their most massive high-density objects.
  3. Sep 21, 2017 #23


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    Three things.
    1. Centrality of orbit is a different issue entirely from whether or not the galaxy is a spiral galaxy. The stars in elliptical galaxies also tend to orbit the centers of those galaxies. They just have orbits that are in all directions, rather than mostly confined to a disk. Spiral galaxies have orbits that are confined to a disk because of the friction of gas and dust, and the spiral arms form due to the way those stars interact with one another.
    2. The supermassive black hole doesn't determine the center of mass of the galaxy. A supermassive black hole may settle near the center of mass, but it will tend to be a small fraction of the galaxy's mass, so that most of the stars won't really be orbiting the supermassive black hole at all: they'll be orbiting a different point that is determined by the center of mass of all the stars and gas and dust and dark matter that is contained within their orbits. The black hole itself may be close to that center, but won't determine the center's location, since for stars that are more than a short distance from the center all that other stuff will be more massive.
    3. Unlike our solar system, with nearly all of its mass concentrated within the Sun, the galaxy has its mass distributed throughout. This means that the orbits of individual stars don't tend to be the simple elliptical orbits that we see. They can be very irregular, as the stars get attracted by local stars and nebulae that they pass near along their complicated orbits.
  4. Sep 22, 2017 #24
    And that is why people say "almost every." They seem to be present in a large fraction of major galaxies.
  5. Sep 22, 2017 #25
    This isn't all that uncommon in observational astronomy. Its really tough to get an idea of what is going on.
  6. Sep 22, 2017 #26
    This is not how orbits work in galaxies. Every star is attracted to every other star in the galaxy. (Plus the dark matter halo, which we will ignore for now.) A SMBH can put 10,000,000 solar masses into the volume of our solar system, which is a much larger average density than the rest of galaxy. However, if pick a star at random from a galaxy of 1 trillion stars and then draw a line from my star to the central black hole and then center an arc of width 3.6 arcseconds (1/1000th the way around a full circle) on that line the arc will enclose at the very least 1/1000th of the stars in the galaxy, which for a galaxy of 1 trillion stars is 1 billion stars. The 10 million solar mass black hole then constitutes (1/100th) of the mass in the smallest possible slice which is itself only 1 of 1,000. The SMBH is such a small portion of the total mass that you could likely remove it and see minimal change in the large scale structure of the galaxy.

    Assumptions in my argument: Spiral galaxies can be treated as 2 dimensional disks. The average mass of a star is 1 solar mass. A spiral galaxy cut into "slices" like a pizza would have equal amounts of stars per slice.
  7. Sep 24, 2017 #27

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    I disagree with that. Or rather, I believe it may be true, but that the evidence is not strong enough to declare this a fact.
    What is known?

    1. Out of the few dozen SMBH's discovered in spirals, they all occur in galaxies with central bulges.
    2. There is an empirical relationship between the size of the central bulge and, if it has been detected, the size of the SMBH. This is purely empirical: there is no generl agreement that this is caused by known galaxy dynamics, although there are conjectures.

    I don't believe from 1 and 2 that the conclusion "All galaxies with central bulges have SMBH's" follows. It may be true, but it does not logically follow. I'd like to see one of two lines of evidence - either a dynamic explanation of M-sigma that shows SMBH's are inevitable, or a statement that given a known and calibrated SMBH-finding efficiency, the number of observed SMBH's is consistent with 100% of the bulgy spirals having them.

    Then there is the other fact
    3. Not all spirals have central bulges. M33, for example. The number without is maybe 20%? 25%? To me, that would mean most spirals have bulges, but not almost every spiral has a bulge.
  8. Sep 28, 2017 #28


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