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Benchmark redshifts to tell your kid brother or sister

  1. Sep 14, 2008 #1

    marcus

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    Can anyone find us an online photo of a galaxy that is z = 1.4?
    I'd like to be able to point in the direction and show a kid the galaxy (too far away to see without telescope, but still possible to imagine.)

    A galaxy at redshift 1.4 is now, at this moment, receding at 1.0 c.

    (this is according to the standard model with Hubble 71, dark energy 0.73, matter 0.27---the usual, as in Ned Wright's calculator)

    A galaxy at redshift 1.65 was then, when it emitted the light we see from it, receding at 1.0 c.

    I'd like to know of some nice-looking galaxies at those redshifts. It shouldn't be hard to find them because thousands of galaxies have been found at all different redshifts out as far as z = 7.
     
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  3. Sep 15, 2008 #2

    chemisttree

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  4. Sep 19, 2008 #3

    marcus

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    Thanks that is not a bad place to start. It focuses on a very small patch of sky in which there are scores of galaxies, some with the right redshifts.
    I found an article on arxiv that listed the same galaxies in a conventional format that I could run my eye down (instead of interactively with HTML links)
    All the galaxies in the GOODS-North survey are right around coordinates alpha= 12:36 h
    and delta=62 degrees.

    So the next thing (I don't know whether this would actually work with a kid, it seems a bit abstract) would be for me to learn how to point to that spot in the night sky.

    There are two galaxies there called BM1053 and BX1095 which are both receding at about the speed of light. One has z = 1.46 and the other has z = 1.45.

    In both cases the picture is relatively nice (for something that far away).

    With standard parameters it would be z=1.41 for something exactly Hubble distance, receding exactly at speed c. So these two are racing away at just a bit MORE than speed c.
    But they are approximately what I was asking for.

    Thanks.

    Around the same spot in the sky there is also one called BX1287 which is a nicelooking galaxy which you could just barely get to if you could travel at speed c indefinitely (no limit, thousands of billions of years if necessary...)
    BX1287 has redshift z=1.675.
    The cosmological horizon is at z = 1.7. A galaxy at that redshift is one that we can see OK (we can see much further things than that!) but you could never get to, no matter how long you traveled, because of accelerated expansion. Expansion carries it away too fast. If we flashed a light at them to send them a message, the message would never get there.

    Remember these things are too far away to see individual stars or even much structure (as far as I know we just see them as little blotches, maybe someone else has pictures with more shape to them)
     
  5. Sep 19, 2008 #4

    marcus

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    Thanks that is not a bad place to start. It focuses on a very small patch of sky in which there are scores of galaxies, some with the right redshifts.
    I found an article on arxiv that listed the same galaxies in a conventional format that I could run my eye down (instead of interactively with HTML links)
    All the galaxies in the GOODS-North survey are right around coordinates alpha= 12:36 h
    and delta=62 degrees.

    So the next thing (I don't know whether this would actually work with a kid, it seems a bit abstract) would be for me to learn how to point to that spot in the night sky*

    There are two galaxies there called BM1053 and BX1095 which are both receding at about the speed of light. One has z = 1.46 and the other has z = 1.45.

    In both cases the picture is relatively nice (for something that far away).

    With standard parameters it would be z=1.41 for something exactly Hubble distance, receding exactly at speed c. So these two are racing away at just a bit MORE than speed c.
    But they are approximately what I was asking for.

    Thanks.

    Around the same spot in the sky there is also one called BX1287 which is a nicelooking galaxy which you could just barely get to if you could travel at speed c indefinitely (no limit, thousands of billions of years if necessary...)
    BX1287 has redshift z=1.675.
    The cosmological horizon is at z = 1.7. A galaxy at that redshift is one that we can see OK (we can see much further things than that!) but you could never get to, no matter how long you traveled, because of accelerated expansion. Expansion carries it away too fast. If we flashed a light at them to send them a message, the message would never get there.

    Remember these things are too far away to see individual stars or even much structure (as far as I know we just see them as little blotches, maybe someone else has pictures with more shapte to them)

    *I guess it's above the big dipper, above the star where the handle joins the bowl, above by an amount like the depth of the bowl of the dipper. So it is a patch of sky that is easy to point to. then you'd have to have a printout in your pocket, of the pictures of BM1053 and BX1095. Because those are the two that are receding at speed c. (and of course you'd need a telescope to see them really, so pictures have to do)
     
    Last edited: Sep 19, 2008
  6. Sep 24, 2008 #5

    chemisttree

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    How did you do the conversion from alpha and delta into RA and DEC?
     
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