Collision of past and present

By Phil Plait | November 29, 2010 9:40 am

The European Southern Observatory just released this lovely picture of NGC 520, two galaxies in the middle of the long, long process of colliding:

eso_ngc520

[Click to galactinate.]

NGC 520 is pretty far away, about 100 million light years. Still, even at a glance you can tell something is fishy* about it. Colliding galaxies like NGC 520 are relatively common; hundreds of examples are known. These galactic train wrecks can take billions of years to unfold, and in this case the two galaxies have probably been at it for 300 million years or so. They’re well on their way to merging to become one much bigger galaxy, probably the size of the Milky Way: 100,000 light years across. We think our own galaxy grew over time in this way.

m31_merge_stillAnd if NGC 520 looks familiar to you, that may be because you’ve been reading this blog for more than a week. It was only a few days ago that I posted a stunning video showing a scientifically and mathematically-produced animation of how some scientists think two large galaxies collided and merged, forming the Andromeda Galaxy as we know it today.

Shown here is a still from that animation (flipped horizontally) which looks remarkably like NGC 520, don’t you think? The gas and dust in the real galaxies obscures the details somewhat, but you can see a lot of the same features, including the long streamer of stars (called a tidal tail, incicated by the arrow) which gets drawn out due to the gravitational interaction of the two behemoths.

The physics of galaxy collision is complex, of course, but it can be simplified quite a bit. When astronomers started modeling collisions, computers weren’t nearly as powerful as they were now, and couldn’t make as many calculations as quickly. So the astronomers would simplify hugely, assuming gravity is the only force at work, and model galaxies as a few thousand stars orbiting each other — in reality, gas flows and collides, stars form and die, and galaxies are collections of billions of stars. And yet, even with those crude models, the results looked remarkably similar to what we actually see happening in the sky!

Obviously, details are important, but sometimes you can get most of the way to your goal with just a few simplifications. Perhaps there’s a life lesson there.

Anyway, when we look at NGC 520 we may be getting a glimpse of what Andromeda looked like a billion or two years ago, and maybe what our own galaxy looked like in its distant past. That’s one of the reasons we study such objects, so that we may understand our past.

But also? Because they’re just so freaking cool.

Image credits: ESO; GEPI, Observatoire de Paris / NAOC



* … which has nothing to do with it being — and this is true — located in the constellation of Pisces.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (16)

  1. Mapnut

    Quote: “These galactic train wrecks can take billions of years to unfold, and in this case the two galaxies have probably been at it for 300 million years or so.”

    That’s why I say we should refer to these events as mergers, not collisions (or train wrecks). Gives the public a more realistic, less sensationalist impression. For instance, what’ a layman going to think when he or she hears that the Milky Way will someday collide with the Andromeda?

  2. Another Eric

    Mapnut has a point. Collision evokes the impression of objects hitting each other, such as stars slamming into each other, destroying themselves and the galaxies. Merger describes what really happens.

  3. RickJ

    Actually there’s a lot more “debris” from the collision than is shown in this image. Just before I read this post I posted my reprocessed version of this collision taken at my home observatory to the Baut forum showing far more of the stars scattered by this collision.
    http://www.bautforum.com/showthread.php/109921-Arp-157-A-major-smashup-reprocessed

    I agree with BA it is a major train wreck as the galaxies will cease to exist as we knew them before. The evolution of the galaxies will be forever changed as well, they certainly won’t be on the same evolutionary track they were before.

    Rick

  4. chris j.

    i wouldn’t get too excited about the apparent similarity. after all, the team who modeled andromeda as a merger may have used ngc 520 to calibrate their model.

  5. andy

    Elements of the past and the future combining to make something not quite as good as either…

  6. Mike Y.

    The earliest galaxy collision simulations didn’t even use computers. Holmberg in 1941 (http://adsabs.harvard.edu/abs/1941ApJ….94..385H) used light bulbs and photocells to imitate gravitational effects and modeled the collision between two “extragalactic nebulae” represented by 74 points. Ingenious, really. And his simulation showed that in some scenarios mergers are likely.

  7. andy

    Ah, Holmberg (1941). Definitely one of my favourite papers for the sheer ingenuity of doing the computation that way.

  8. Neil

    “and in this case the two galaxies have probably been at it for 300 million years or so. They’re well on their way to merging”

    BOW CHICKA BOW-BOW!!!

    300 million years of foreplay! Reading about events on a galactic scale sure does make the actions of humans seem trivial by comparison!

  9. Daniel J. Andrews

    Obviously, details are important, but sometimes you can get most of the way to your goal with just a few simplifications.

    But models can’t simulate reality, it is just too complex, models are faulty, they just fudged the models to get what they wanted, there is no such thing as galactic merging….
    ;)

  10. Jamey

    Honestly, I can see collisions superficially identical (at least to my eye) any time I want. It’s the Galaxy module in XScreensaver, and it shows this kind of thing – and all kinds of other results.

    I’m only about 30% impressed by this, really.

  11. Messier Tidy Upper

    Nice image and write-up thanks, BA. :-)

    @1. Mapnut :

    I say we should refer to these events as mergers, not collisions (or train wrecks). Gives the public a more realistic, less sensationalist impression. For instance, what’ a layman going to think when he or she hears that the Milky Way will someday collide with the Andromeda?

    Agreed. The media loves to sensationalise even “serious” science documentaries – as this favourite example :

    http://blogs.discovermagazine.com/badastronomy/2009/10/26/felicia-day-collides-galaxies/

    also involving galactic collisions mergers and M31 shows.

    That said, I don’t think the ‘collision’ word is likely to disappear anytime soon & I don’t really mind it being used in this context given that it *is* an accurate description of at least the early part of the merger process. Some of the silly sensationalising nonsense that comes with that term though .. arrgh. :roll:

  12. Georg

    “Ah, Holmberg (1941). Definitely one of my favourite papers for the sheer ingenuity of doing the computation that way.”

    Hello andy,
    right! Analog calculators are ingenious, whereas digital
    computing is brute force of large number of identical
    and boring units.
    Georg

  13. Georg

    Hello,
    Mapnut is right, an appropriate word might be “interpenetrate”.
    Georg

  14. sophia8

    As ever with these types of pictures, I’m wondering what this would look like from a planet in one of these galaxies. Makes me wish I had a Tardis to jump into.

  15. Jon Hanford

    It’s really neat to pick out some of those young blue star clusters appearing in the tidal tail in the lower portion of the image. Also, a distant anonymous galaxy cluster appears to lie behind this tidal tail too. And finally, a ring galaxy (mebbe two!) appears to the right of the plume. An image taken with the Gemini North scope shows these features a bit better: http://www.gemini.edu/images/pio/20080825_ngc520_0001.jpg

    And what do you know, Hubble Gotchu!: http://imgsrc.hubblesite.org/hu/db/images/hs-2008-16-bo-full_jpg.jpg (albeit upside down :) )

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