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'Image' of Dark Matter

  1. Oct 3, 2004 #1

    Nereid

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    This Bell Labs site has a number of links (bottom of the page) very relevant to observational studies of the distribution of dark matter. In particular, the last three links:

    Deep Lens Survey, a programme to create maps of the distribution of dark matter, across seven 4 square degree fields, using weak lensing (shear); this technique relies on large numbers of galaxy images and statistical analyses. Among the results are dark matter filaments. The data are publicly available, so anyone who wants to test an alternative cosmological model doesn't have to fight for scarce big telescope time.

    The Dark Matter Project, a set of webpages which describes the strong and weak lensing techniques. I found the strong lensing results, from one cluster, particularly interesting ... "The majority of the dark matter is distributed broadly and smoothly in the cluster, covering a region on the sky more than 1.6 million light-years across. The mass of the individual cluster galaxies appears as pinnacles on this mountain of dark matter mass. Overall, the dark matter in the cluster outweighs all the stars in the cluster's galaxies by 250 times as shown in the mass contour plot above. We know that most of the mass is not attached to the individual galaxies in clusters."

    First High-Resolution Images of Dark Matter, a PR.
     
    Last edited: Oct 3, 2004
  2. jcsd
  3. Oct 3, 2004 #2
    I noticed the statement to the effect that the most distant of these galaxies destored by foregound clusters were bluish in color. I thought the more distant the galaxy the more redshifted it was. Is there something in the process of the lensing effect that shifts these background galaxies towards the blue?
     
  4. Oct 3, 2004 #3

    Garth

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    Nereid - thank you for those links.
    Both the Bell Labs site and the Dark Matter Project say Dark Matter "constitutes more than 90 percent of the mass in the universe". Yet I thought the LCDM model was 23% DM, 73% DE and 4% baryonic? Are they lumping DM & DE together? Later the Bell Labs site says,"Instead, their observations support an alternative universe, which contains a certain amount of vacuum energy that causes it to expand more rapidly over time" ,but they do not say how much. I have looked at some of Wittman's papers on the physics-arXiv but they do not say much more.
    I notice these pages are 4 years old now so perhaps things have moved on..
     
  5. Oct 3, 2004 #4

    selfAdjoint

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    Note the first percentage is of MASS, which to a physicist means rest mass. The second percentage is of ENERGY. Here the mass terms in the former percentage will be multiplied by c^2 and the relativistic dilation factor, and added into the non-mass forms of energy, overwhelmingly "dark energy".
     
  6. Oct 3, 2004 #5

    Garth

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    Thank you.

    The percentages normally given are those of the critical density, which are the densities of both matter and the mass equivalent of any energy, such as pressure, false vacuum, or something even more exotic.
    Quote from the Bell Labs link
    "This invisible matter makes its presence felt through its gravity and constitutes more than 90 percent of the mass in the universe." The latest LCDM value would be DM is 85% of the non-DE component of the universe.

    Later on "Instead, their observations support an alternative universe, which contains a certain amount of vacuum energy that causes it to expand more rapidly over time." spot on here then! But how much is "a certain amount"?

    My question of clarification was leading to seeking the density Omega parameter for DM and DE that this weak gravitational lensing distortion method produced independent of the WMAP flatness requirement. As these papers predated that discovery it is not surprising they did not know of it!
    Are there any later results? I couldn't find any.


    Garth
     
  7. Oct 4, 2004 #6

    Nereid

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    Good question Mike2!

    For the 'reconstructed' background galaxy, 'responsible for multiple long arcs in the z = 0.4 cluster 0024+1654', the blue colour comes from:
    1) the galaxy having a 'redshift between 1.2 and 1.8', so the original UV (above the Lyman limit) is observed by the HST as U and B
    2) its 'beaded, ring-like morphology' suggests it's not your average spiral, rather a galaxy in formation, so there is an abundance of bright, young, 'blue' stars.

    More details here.

    Generally, the redshifted colour of a distant galaxy depends on how much it is redshifted, and what the 'native' colour is (was!). For example, your average elliptical galaxy is already yellow in its rest-frame, so redshifted it'll look no bluer. A starburst galaxy (e.g. M82) is brightest in the UV, so redshifted it will look even bluer!

    You may be thinking of very distant galaxies ... below the Lyman limit (91nm - corresponds to the ionisation of a ground state H atom) intergalactic space is essentially opaque, and distant objects will be invisible at these (redshifted) wavelengths. If the redshift is great enough (how great? can you calculate it?), the Lyman limit falls in the optical (or even the NIR), so the object 'disappears' on blue(r) images - the so-called 'drop-out' objects. In synthetic RGB images (which is just about every astronomical image on the web taken by a 'big' telescope), all such objects will 'look red'.
     
    Last edited: Oct 4, 2004
  8. Oct 4, 2004 #7
    Your link give me an error. But the bright, young, 'blue' stars does it for me. Thanks. Thought I thought spiral galaxies were caused by collisions and were thought to be older galaxies.
     
  9. Oct 4, 2004 #8

    Nereid

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    I'll look, but IIRC, the Bell Labs team lost its funding following the telecoms sector meltdown (and Lucent laid off 2/3rds of its workforce). AFAIK, the LSST (aka 'dark matter telescope') also didn't get any funding. The Deep Lens Survey has already made 3 releases; it is, of course, an observational project; how any team chooses to analyse the results is not within the remit of the Survey. Did you check the "Publications and Presentations" link on the Brown University page? It gives a series of interesting links to January 2003 AAS posters.
     
  10. Oct 4, 2004 #9
    Nereid,

    I have read that spherical galaxies do not have dark matter. Rather than searching thru your links, I wonder if you or anyone can tell me if this is true, and if so, is there an explanation?

    Richard
     
  11. Oct 4, 2004 #10

    turbo

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  12. Oct 5, 2004 #11
    The data mentioned above where Dark Matter fills entire clusters of galaxies with peaks in each galaxy, which suggests that spherical galaxies contain Dark Matter, refutes the contention of Cahill in that previous post that spherical galaxies contain no Dark Matter. I was just hoping to get a more definitive statement to that effect.

    Richard
     
  13. Oct 5, 2004 #12

    Nereid

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    Of the papers referenced, I was most impressed with the Romanowsky one ("A Dearth of Dark Matter in Elliptical Galaxies" - not linked in yanniru's original post). They studied five (well, three) elliptical galaxies, to find the equivalent of the spiral rotation curves which MOND has been so successful with, and their results are quite cautious, and very interesting - apparently not only no DM halo as LCDM models would expect (or a much smaller one), but also some hints that galaxy formation isn't quite [jk]'according to Hoyle'[/jk].

    However, to go from 3 (or 5) ellipticals to 'all ellipticals' is flat out wrong; indeed, Romanowsky et al were very careful to limit both their study and conclusions to only one kind of elliptical. Second, at least one of the ellipticals (and maybe all?) are NOT spherically symmetrical. And lastly, three middling ellipticals doth not an iconoclast's summer make (though another 20 years of detailed research will very likely continue to turn up plenty of results showing that we don't really have a good handle on galaxy formation yet).

    Which brings us to Moffat: I'm sure Garth would enjoy that paper! To me it follows in the fine tradition of taking GR and tweaking it, to see where it takes you.

    And finally Cahill: let's just say that he has rather a lot more work to do (one can only imagine what the same blockhead reviewer Garth got would say about Cahill's paper!).
     
  14. Oct 5, 2004 #13

    Garth

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    I find it hard to understand why in the non-symmetric gravitational theory (NGT) spherical galaxies in particular should not contain DM, whatever it is. NGT modifies Newton rather like MOND so no DM exists in the first place anywhere, (apart from a some dark baryons and neutrinos.) However, the weak gravitational lensing plots imaging DM, post #1 by Nereid, would seem to falsify that idea.


    Garth

    [Edit: crossed with #12 in the post]
     
    Last edited: Oct 5, 2004
  15. Oct 8, 2004 #14
    Thanks Nereid. That was the kind of answer I was looking for. My interest in Natural Theology requires the existence of both Dark Matter and Dark Energy.

    Richard
     
  16. Oct 9, 2004 #15

    Chronos

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    My understanding is that dark matter is less abundant in older, less interacting galaxies. Spherical galaxies are usually considered older, less interacting galaxies. I know the observational evidence is still rather tenuous, but, it appears to make sense.
     
  17. Oct 9, 2004 #16

    Garth

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    Richard, at the risk of being moved to the philosophy forum, may I ask what is the connection between Natural Theology and the existence of both DM & DE?
    Garth
     
  18. Oct 9, 2004 #17

    Garth

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    Because in the older galaxies the DM has had time to condense into HII regions and stars??? (i.e. DM is baryonic)

    Garth
     
  19. Oct 9, 2004 #18

    Chronos

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    This relates more to dark matter distribution than quantity. Mature structures do not necessarily have less dark matter, rather it is more uniformly dispersed.
     
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