Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the basis of Rotation Curve?

  1. Jun 25, 2014 #1
    I have been reading about rotation curves, and the I understand the basics, but I am trying to understand the basis / meaning of the curves?

    Are the findings based on redshifts of individual stars / "bins of light" at set distances from the centre of the galaxy? And given that the predominance of the stars / bins are at the outer "edge" of the galaxy, are there corrections to the data for this?

    I appreciate that these are likely "big picture" questions, and am happy to get pointers to articles / papers that can help my understanding.

    Thanking you in advance,

    Noel.
     
  2. jcsd
  3. Jun 25, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Are they?

    You can take galaxies that can be resolved in telescopes, where it is easy to measure the red/blueshift for each part of the galaxy separately.
     
  4. Jun 25, 2014 #3
    Thanks mfb, and understood, but I understand that the rotation curve is best considered for an edge-on galaxy, & if I resolve any star in an edge-on galaxy, how do I know how far from the centre it is (using a dart board example, when looked at edge-on, is the dart outside the scoring areas, in the outer double score ring, or the inner triple score ring, or one of the single score sections)?

    Even if I do know where the star is (& I appreciate that this is a simplistic view), I assume that I need to make a correction to the reading - for example to recognise that the light had to climb a distance of the gravity well of the galaxy.
     
    Last edited: Jun 25, 2014
  5. Jun 25, 2014 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    If you see it edge-on, you always see the summed effects of many stars, and need unfolding (or a fit model) to get the rotation curves.

    That should be a negligible effect, as it scales with ##v^2/c^2##, while redshift is ##v/c##.
     
  6. Jun 25, 2014 #5
    Thanks mfb. I hadn't come across unfolding previously, so that will give me some reading avenues.
     
  7. Jun 26, 2014 #6
    Sorry mfb, I've been thinking about this for a while, but it is still not clear to me.


    What is the 'it' that you are referring to?

    Thanks,

    Noel.
     
  8. Jun 26, 2014 #7

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Gravitational redshift of light ("to recognise that the light had to climb a distance of the gravity well of the galaxy."). v is the escape velocity at the point the light starts.
     
  9. Jun 27, 2014 #8

    Thanks mfb.
     
  10. Jul 6, 2014 #9
    mfb, Can I follow up on this again please? I have two questions.
    1. In the above quote, the ##v## in the first part represent escape velocity (as you have said), is the ##v## in the second part the same escape velocity?
    2. The real purpose of my OP was to try to gain an understanding of the mechanics behind the calculation of a galactic rotation curve, and in particular the nature / reason for any corrections that need to be made to the readings (in order to get a correct result). Are there any corrections that are generally made to redshift readings in order to calculate the rotation curve of a galaxy?

    As always, thanks in advance,

    Noel.
     
  11. Jul 8, 2014 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The second v is the motion of the object approximated by the escape velocity. They are not the same (otherwise the objects would escape...), but I ignored small prefactors. For a circular orbit around a central mass, those velocities differ by a factor of ##\sqrt{2}##.

    I'm sure they are, but I think they are all small. Check papers about those measurements, it should be explained there.
     
  12. Jul 8, 2014 #11
    Thanks mfb.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook