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New map of the galaxy (Levine Blitz Heiles)

  1. Jun 6, 2006 #1


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    ===sample quote===
    Levine and colleagues took previously obtained data from the hydrogen "21-cm emission lines" and applied a technique called unsharp masking to increase the contrast between high and low signal regions. They found that the spiral arms reach out as far as 25 kpc, or 80,000 light-years, away from the centre of the galaxy.

    "We have been able to trace the spiral structure of the entire disk of gas beyond the orbit of the Sun around the Galactic Center to the edge of the disk," explains Blitz. "This gives the clearest and most complete picture of the spiral structure of the disk so far, and shows that the spiral structure continues well beyond the radius at which we expect stars to be. This is not expected from theory, and will require a deeper understanding of the origin of spiral structure."

    The sharpened map of the galaxy also raises new questions, such as how gravity affects the spiral structure in galaxies. The spiral pattern in the Milky Way is like most other "grand design" spirals in that it is approximately logarithmic, or shaped like a cyclone. Scientists believe that the spiral structure is driven by the gravity of the disk. "But we are seeing spiral structure even where gravity seems to be too weak to drive it. Why? How?" asks Blitz.

    The Spiral Structure of the Outer Milky Way in Hydrogen
    E.S. Levine, Leo Blitz, Carl Heiles
    10 pages with 4 color figures. Accepted for publication in Science. Embargoed for discussion in the popular press until publication in ScienceXpress. Higher resolution versions of figures 3 and 4 are available at this http URL

    "We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk demonstrating that the Galaxy is a non-axisymmetric multi-armed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kpc, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20-25 degrees."
    Last edited: Jun 6, 2006
  2. jcsd
  3. Jun 6, 2006 #2
    And this on Andromeda...

    http://cosmiclog.msnbc.msn.com/archive/2006/06/05/115.aspx [Broken]

    "The Andromeda Galaxy — the nearest spiral to our own — is all dressed up in reddish, dusty swirls in a new infrared portrait from NASA's Spitzer Space Telescope. The picture, which has plenty of scientific as well as aesthetic value, is just one of the visual delights coming out of this week's meeting of the American Astronomical Society." ...

    This in particular, further on in the piece, TOTALLY baked my noodle...

    "The readings confirm that Andromeda, 2.5 million light-years away in the constellation of the same name, puts our own Milky Way galaxy to shame in the star department: Andromeda has roughly 1 trillion stars, compared with the Milky Way's 400 billion stars."

    One ta-ta-ta TRILLION stars? As in a thousand billion stars?

    So, how many square miles of the Sahara do you suppose you'd have to be looking at to have line of sight on one TRILLION grains of sand?

    Heh. How much would YOU pay for a 30ft by 10ft back-lit transparency of that image of Andromeda for the livingroom?
    Okay, so, the fish tank would have to go,.. and the TV, and the bookshelves,.. and the window that looks out onto the backyard.

    And my girlfriend would disown me.

    ... but, I mean,.. ya know,..

    ... (sigh) DAMN.
    Last edited by a moderator: May 2, 2017
  4. Jun 7, 2006 #3


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    "neutrino" has priority on this topic

    he started a thread on 06-05

    I didnt see it, he happened to start it in "Astrophysics"

    since I didnt start this thread until 06-06, his thread is the right one to post comment on
  5. Jun 7, 2006 #4


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    I think this could be another clue to the "MOND" mystery.

    It seems as if several clues have surfaced saying that where gravity is expected to be very weak, it turns out to be NOT QUITE SO WEAK as anticipated------out at the fringes of things. and of course several possible explanations
  6. Jun 8, 2006 #5


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    Extended DM halos do a better job of predicting this result than MOND, IMO. It is, however, an important observation that will push both models to new limits.
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