Hubble's Universe Unfiltered

  • February 3, 2009

    Episode 5: Through a Lens, Brightly

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    Einstein's theory of general relativity says that the presence of matter warps the space around it. The more matter there is, the stronger the warp. For really strong distortions of space-time, like black holes, the warping acts like a lens and markedly changes the path of light that passes through it. Such gravitational lenses have been found, and astronomers can use them to study very distant and very faint galaxies. Join us and see how the combination of a huge galaxy cluster and the observations from two space telescopes enable study of a galaxy whose light would otherwise be unobservable.

    • The title of this video podcast, "Through a Lens, Brightly" plays off the familiar phrase "through a lens, darkly," which is a slightly shifted version of the phrase "through a glass, darkly." Wikipedia reports that the phrase originally comes from the New Testament and is taken to mean that we have an imperfect view of our world and ourselves. The "glass" in the original version of the phrase is said to be a mirror (i.e., a looking glass), and not a glass lens. Hence, the shifted version that I have used, in order to evoke the idea of a gravitational lens, is a misinterpretation of the original. However, since Hubble uses many mirrors and lenses in observing the universe, one could come up with some weasel excuse as to why it is appropriate. One could, but not me. It just sounded cool.

    • The scientific visualization of a black hole passing through Baltimore's Inner Harbor was done with software graciously provided to me by Brian McLeod of Harvard University. The software accurately calculates the deviation of light as it bends around a point-mass black hole. Folks often ask the size of the black hole, but I don't truly know. The software allows me to specify the size of the ring in pixels, without reference to the mass of a black hole needed to produce that large of a ring. For Brian's lensed castle image, he states the mass required as being about the mass of Saturn. For the Baltimore image, the effect is bigger and the mass should be a bit larger. Perhaps Jupiter's mass would be a decent estimate.

    • Almost all of the gravitationally lensed arcs seen in the galaxy cluster Abell 1689 are short segments of circles centered on the mass of the cluster. We call them "tangential arcs," and they are the dominant type of lensed arcs seen. However, Abell 1689 is massive enough that "radial arcs," which extend out along lines from the center, can be seen as well. One radial arc can be seen at about the eight o'clock position, below and to the left of the central galaxy. In most galaxy clusters, these radial arcs are too faint or too small or too close to the central galaxy to be seen.

    • Gravitational lensing can produce multiple lensed images of the same object in the sky. Light from the object can reach us by taking multiple routes through the distorted space-time of the gravitational lens. Since these routes are of different lengths, light takes different amounts of time to travel along them. Hence, each lensed image shows the object at a slightly different time. In some lenses, we can see a change happen in one lensed image that doesn't show up until days, months, or even years later in another lensed image of that same object.

    Image notes

    Space Telescope Science Institute Muller Building
    Credit: STScI

    Baltimore Inner Harbor
    Credit: S. Westphal (STScI)

    Baltimore Inner Harbor with Black Hole
    Credit: F. Summers (STScI)

    Galaxy Cluster Abell 1689
    Credit: NASA, ESA, L. Bradley (JHU), R. Bouwens (UCSC), H. Ford (JHU), and G. Illingworth (UCSC)

    Galaxy Cluster Gravitational Lensing illustration
    Credit: A. Feild (STScI)

    Gravitational Lens Arcs in Galaxy Cluster Abell 1689
    Credit: NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA

    Abell 1689-zD1 in Visible Light from Hubble
    Credit: NASA, ESA, L. Bradley (JHU), R. Bouwens (UCSC), H. Ford (JHU), and G. Illingworth (UCSC)

    Abell 1689-zD1 in Infrared Light from Hubble
    Credit: NASA, ESA, L. Bradley (JHU), R. Bouwens (UCSC), H. Ford (JHU), and G. Illingworth (UCSC)

    Abell 1689-zD1 in Infrared Light from Spitzer
    Credit: NASA, ESA, L. Bradley (JHU), R. Bouwens (UCSC), H. Ford (JHU), and G. Illingworth (UCSC)