Space Warps Talk

lensing and dark matter

  • emorybraswell by emorybraswell

    What is the technique for determining dark matter from a lensed object? If too detailed may I have a reference?

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  • CountChocula by CountChocula

    I have a similar question, so I'll just append it here: Out of curiosity, how does one tell if a black hole is causing lensing? There wouldn't be a visible central object. Thanks

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  • begonjas by begonjas

    I just had the same situation. there is a clear half circle around absolutely nothing.

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  • Thomas_J by Thomas_J moderator

    At these distances, we're looking at galaxies, most of which contain a 'supermassive' black hole at the centre. I would say it's unlikely that a 'stellar mass' black hole would be noticeable at this level as they don't produce the strong gravitational lenses that we're looking for. To flip the idea around, we are looking for lensed black holes (in the form of quasars).

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  • Thomas_J by Thomas_J moderator

    With regards to determining DM from lenses, we'll have more information for you soon 😃

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  • Tom_Collett by Tom_Collett scientist

    We can't tell anything about the dark matter in the lensed source, but we can learn a lot about the dark matter in the galaxy that is doing the lensing.

    The gravitational lensing effect is caused by the total mass (dark or normal). The more massive the lens, the more the light will be deflected, so you get a bigger separation between the lensed images (or a larger diameter if it's an Einstein ring).

    So gravitational lensing tells us the total mass, but we can tell how much 'normal' matter is there because of the amount of light being emitted by the galaxy. (You have to assume things about the typical masses of stars in galaxies, but that's relatively well understood). Subtract the two numbers and you have a measure of the dark mass.

    We're also trying to learn about how the dark matter is distributed in galaxies, based on exactly where the lensed images form, but the measurement of the total mass is the easiest (and best constrained) measurement to make.

    Hope that's interesting/helpful!

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  • Tanksenior by Tanksenior

    Very interesting!

    The more massive the lens, the more the light will be deflected, so you get a bigger separation between the lensed images (or a larger diameter if it's an Einstein ring).

    I didn't initially realise that was the case, good to know.

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  • Matty-Gee by Matty-Gee

    Why is it that most lenses are blue?

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  • Budgieye by Budgieye moderator

    Most lensing galaxies are at about z=.5, and the lensed galaxy is about twice as far away at about z=1.0, where star-forming galaxies are blue. Lensed galaxies further away are purple z=1.2, then their light becomes redshifted to green,orange, yellow, red, infrared, etc. See:
    Colours of Galaxies in SDSS : Redshift chart

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  • drphilmarshall by drphilmarshall scientist, admin in response to Budgieye's comment.

    We should make some look-up charts for Space Warps, don't you think? Itd be great to be able to estimate the size, mass, distance etc of a potential lens galaxy just by looking at it!

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