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Gravitational lensing anomaly?

  1. Dec 21, 2011 #1

    The recent Astronomy Picture of the Day:


    Has the following caption:

    ...the gravity of a luminous red galaxy (LRG) has gravitationally distorted the light from a much more distant blue galaxy.

    I was under the impression that the more distant a galaxy is then the more red-shifted it becomes. So my question is how can the more distant galaxy be blue?

    Thanks in advance.
  2. jcsd
  3. Dec 21, 2011 #2

    You appear to have a misconception with regards to redshift. Redshift does not mean becomes redder, it corresponds to a longer wavelength - moving to the "red" end of the spectrum so through the visible spectrum to eventually x rays, gamma rays.

    Please see the following link for clarification:


    Extract: "In physics (especially astrophysics), redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum. More generally, where an observer detects electromagnetic radiation outside the visible spectrum, "redder" amounts to a technical shorthand for "increase in electromagnetic wavelength" — which also implies lower frequency and photon energy in accord with, respectively, the wave and quantum theories of light."
  4. Dec 21, 2011 #3

    Wouldn't that still mean that the light appears redder to the observer?

    http://www.setterfield.org/000docs/Redshift.htm [Broken]

    My emphasis.
    Last edited by a moderator: May 5, 2017
  5. Dec 21, 2011 #4
    My guess is one of the side effects of gravitational lensing would be a change in wavelength. Thus the further galaxy (the blue one that's now an Einstein Ring) would appear blue-shifted to us simply because of the distortion caused by the gravitational lensing.
  6. Dec 21, 2011 #5


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    The "color" of a galaxy relies on more than just its Doppler shift. A galaxy of mostly hot stars will be bluer than one with cooler stars or one that is dusty. It takes quite a sever doppler shift to cause a visible reddening of a galaxy. The light at the blue end of the spectrum that is shifted to the red is replaced by ultraviolet that shifts from being invisible to being blue.
  7. Dec 22, 2011 #6
    Thanks Janus, that does seem to make sense.

    It would imply that the lensed galaxy, now appearing blue (and much further away) consisted of extremely hot stars whose ultra-violet radiation is now visible to us.

    My faith in science is restored!
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