Friday Galaxy : the Milky Way to outside observers (NGC 6744)

Mar 16 2007 Published by under Pretty Pictures

This is an image of NGC 6744, taken with the South African Large Telescope.

ngc6744_salt.jpg

NGC 6744 is a classic spiral galaxy, and appears similar to how the Milky Way would look if we were able to get outside of the Milky Way and look back at it. Notice that there is a bar at the center of the galaxy (oriented vertically in this picture). That is a feature that you see in a lot of spiral galaxies, and which indeed is present in our own spiral galaxy.

This image isn't exactly how the Milky Way would appear to our eyes. First of all, the surface brightness of the galaxy is low enough that our eyes wouldn't see any color, even if we were very well dark-adapted. However, suppose that our eyes could integrate; even then, this is a somewhat "false color" image. Images taken in near-ultraviolet, blue, and near-infrared light have been mapped to the blue, green, and red channels of the displayed image. As such, color contrasts have been somewhat enhanced.

6 responses so far

  • Matt says:

    Rob, is there an understood reason why there only is two bands of material circling down right to the center in this shot? As in an explanation to why spinning galaxies develop these bars near their core rather then looking like draining water in your bathtub?

  • Rob Knop says:

    Yes, although not a perfect one.
    There are two primary theories for spiral structure, which explain two different sorts of structure that is observed. Some "flocculent" spiral galaxies don't have clear arms that can be traced from the inside to the outside. This structure is currently best explained by star formation that leads to supernovae which triggers star formation in nearby clouds. Put that together in a differentially rotating disk, and you get little armlets. This theory is known as SSPSF (Stochastic Self-Propagating Star Formation.)
    When you can trace a spiral arm all the way from the inside to the outside, in that case it's clearly a system wide phenomena. In that case, spiral arms are described as spiral density waves. Just as a circularly expanding pattern of ripples is the normal wave pattern you get on the surface of water when you disturb it (e.g. by throwing a rock in it), if you disturb a differentially rotating self-gravitating disk, you get density enhancements (think of it like a traffic jam-- where the density enhancement is, al lof the stars and gas are denser than in between) that form a spiral pattern. These things are also seen in the rings of Saturn and in other disks.
    The arms are *not* material spiraling "down the drain." Yes, arms can give torques and such, and can assist in transferring angular momentum, but by and large all of the material in a spiral galaxy is orbiting in a near-circular ellipse around the center of the galaxy, and stays that way. The spiral pattern isn't stuff streaming in, but just represents waves of density (just like "the wave" in a baseball stadium isn't people running around, but is waves of standing) moving around the disk.
    The bar itself is an instability that naturally develops and can be temporarily stable in a rotating disk system. They are seen in a lot of spiral galaxies. Bars are much more efficient than spiral arms at transferring gas down to the center of the galaxy. The stars in the bar by and large don't fall in, but are in very elongated orbits that move back and forth along the bar (although the pattern itself rotates). The non-axisymmetric gravitational potential that results can be quite efficient at getting rid of angular momentum in gas and funnelling gas down to the center of the galaxy.
    -Rob

  • bigTom says:

    Very interesting reply Rob. I thought galactic collisions/mergers were supposed to play a role in forming some of this spiral structure.
    How much of these patterns of self-generated, and how much due to tidal interactions with other systems?

  • Rob Knop says:

    Very interesting reply Rob. I thought galactic collisions/mergers were supposed to play a role in forming some of this spiral structure.
    Major collisions and mergers do a lot more than that -- they actually can make elliptical galaxies, but on the shorter term they can make big tidal tails and all sorts of morphologies.
    Minor interactions *can* be the thing that triggers spiral density waves. For instance, look at an image of M51; that's a *beautiful* grand-design spiral with well-deliniated large spiral arms, and it's also interacting with a smaller galaxies.
    Since spiral density waves are the natural excitation mode of a self-gravitating disk, any sort of trigger can cause them to happen. Some galaxies with really prominent bars also have very well-delineated spiral arms; that's probably not an accident! Minor interactions are another way to do it.
    -Rob

  • Leni says:

    Rob wrote:

    The spiral pattern isn't stuff streaming in, but just represents waves of density (just like "the wave" in a baseball stadium isn't people running around, but is waves of standing) moving around the disk.

    Is this true very close to the core as well? I thought that with like, AGN or galaxies with supermassive black holes that accretion is what was fueling the high energy outputs.
    Probably it has a range though, so that I could see accretion at the core not really effecting the outer arms...But then again, it seems like there would be some sort of effect on the outer part of the galaxy. That it wouldn't be dynamically isolated.
    Or is that something not usually found in spiral galaxies?

  • Lab Lemming says:

    So are the arms of our Galaxy more tenuous than those of Andromeda?
    Also, which was does the Milky Way spin? Is it towards Canopus or towards Deneb? And how can we tell?