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Simple Beginner Dark Matter Question

  1. Sep 28, 2012 #1
    Simple "Beginner" Dark Matter Question

    Ok, for some reason I can't seem to get this straight, probably because its so obvious nobody spells it out. Its more a question of terminology than the real mystery of dark matter, but its proving an obstacle in my efforts to catch up on a subject i have recently become interested in.

    Very simple explanations of the existence of dark matter will often say that there isn't enough visible matter in galaxies such as ours to explain what holds it together. This leads, eventually, to the idea of a dark matter Halo etc. Now, the simple question is this: when such people talk about the visible matter in the galaxy, does this include the supermassive black hole at its centre? In other words, is this black hole considered to be part of the "observable" galaxy, or is this considered to form a part (but clearly not all) of the dark matter that holds the galaxy together.

    As simple as it sounds I have tried and failed to get this clear in my mind. I have come across discussions of whether the unknown dark matter outside the galaxy might comprise or include small black holes - but this is a different issue. My question is more straightforward - are the black holes we are aware of, in our own galaxy for instance, counted as part of the "observable" component of matter, or do they get lumped in with the dark matter component?

    Hoping you can straighten me out.

  2. jcsd
  3. Sep 28, 2012 #2


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    Re: Simple "Beginner" Dark Matter Question

    While the black hole itself is not visible in the classical sense, its mass and position is known and material in the accretion disk is visible, so I think it counts as visible matter.
    The mass of the central black hole is just a small fraction (<0.1%) of the total visible mass.
  4. Sep 28, 2012 #3
    Re: Simple "Beginner" Dark Matter Question

    iMatt, insomuch as we assume that there is a supermassive black hole at the center of every galaxy, yes, we factor this in to normal "atomic" matter.
  5. Sep 28, 2012 #4


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    Re: Simple "Beginner" Dark Matter Question

    Astrophysical black holes are part of observable.

    These are the stellar-mass black holes (and larger) formed by collapse of stars. These have been observed by their effects on surrounding---stuff orbiting them etc---well enough so that their masses can be measured and their frequency throughout the rest of the galaxy can be estimated.

    there are other categories of black holes which have never been observed and which may not exist. Primordial black holes---microscopic black holes which might have formed NOT FROM STARS but in the extreme conditions of the very early universe. These are purely theoretical and speculative and they would be included in Dark Matter. Primoridal BH from very early times are sometimes offered as a conjecture regarding what DM consists of.

    But ordinary BH that we know about and can make estimates about---they come from stars collapsing and so are called "astrophysical" black holes and they are part of ordinary matter.

    the mass of the central BH in our galaxy is modest size. Stars have been observed orbiting it over a period of 15-20 years and by plotting their orbits we can tell the mass of the central BH. It is only a few million Solar Masses. I forget how many. Something like 3 million M_sun.

    I agree with what Mfb said about its mass as a percentage.
    Last edited: Sep 28, 2012
  6. Sep 29, 2012 #5
    Re: Simple "Beginner" Dark Matter Question

    Thank you all. Very clear and very helpful. A simple seeming question but one that was making it difficult for me to put other stuff I had been reading, and trying to understand, into context. There is so much detailed discussion and speculation going on in this area that I had been finding it hard just to establish some basic foundations - kind of a "can't see the wood for the trees" sort of situation. So, again, many thanks.

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