Virgos have beautiful eyes

By Phil Plait | August 29, 2011 6:30 am

No, don’t fret: I’m not betraying everything I know to be true and suddenly supporting astrology! I’m just having a little joke at the expense of NGC 4435 and 4438, two galaxies in the Virgo Cluster known as "The Eyes", and seen in lovely detail by the Very Large Telescope:

[Click for orbus giganticus, and you really should; the details are beautiful.]

Clearly, these guys know each other. NGC 4438 (upper left) is distorted and drawn out, which is a sure bet that it’s undergone a collision with another galaxy in the recent past. Given how close NGC 4435 (lower right) is to it, that seems like the culprit (though M86, not seen in this shot, is also close by and may be to blame). They may have actually passed right through each other as recently as 100 million years ago! Direct hits between galaxies aren’t like car accidents where the vehicles stop dead; galaxies are mostly empty space, and stars are so small compared to the galaxies themselves that a direct impact between two stars is incredibly unlikely.

But the gravitational pulls from the opposing galaxies can affect each other, teasing out long tails of material just like the one streaming from NGC 4438 . The scattering of dust is also another clue. Although stars don’t collide, gas clouds are much larger, some dozens of light years across. Those do in fact slam into each other, causing them to collapse and form stars (though there’s some evidence that’s not always the case). Vigorous star formation can cause lots of dust to be created, and that’s what we’re seeing in NGC 4438. And it’s all weird and distorted too, clinching the case.

You may notice NGC 4435 is a bit featureless. That’s actually common in disk galaxies that live in clusters. As they move through the cluster at high speed, the intergalactic medium — thin gas expelled from the galaxies — can strip away the gas and dust in a galaxy, like opening a car window can blow out stale air inside.

Galaxy collisions are pretty cool, and a rich field for study. And if you’re patient, you’ll get a great view of one: our galaxy is headed for a close encounter with the Andromeda Galaxy. Given that it and the Milky Way are among the biggest and most massive spiral galaxies in the local Universe, it’ll be a spectacular show. Better reserve your seats now, though. You only have a billion or two years to wait!


Related posts:

- Gorgeous galaxies celebrate Hubble’s 21st birthday
- Evidence and theory collide with galactic proportions
- When beauty and science collide
- Bang!

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (16)

  1. Diederick

    I know that stars won’t collide when their galaxies do, but what about their central black holes? Do they actually attract each other and can they collide, and what happens then?

  2. Kevin

    If two stars did somehow collide (I’d imagine it’s happened at some point, unless there is a mechanism specifically preventing it), what would occur? A supernova, or something else?

  3. Efthimios

    Two years! Now, that’s what I call an official confirmation of the 2012 scenarios. May Quetzalcoatl, Kukulcan, or whoever else cares have mercy on our iPads.

  4. Fiddlepixels

    Interesting question from Diederick. I would think that the central black holes, though massive, are small, and are more likely to slingshot around each other than collide or be captured in mutual orbit. What do you say Phil?

  5. dartigen

    Aw, damn. I’d want to see it happen. Like you said, the odds of stars or planets (or hell, even dust particles) smashing into each other is small…but we are talking galaxies here. With a sufficient density of Things in it, it’s probable that somewhere it’s going to hit something else.

    I don’t know what would happen if stars did collide but it probably looks pretty. (At least, to someone not in the immediate vicinity – like so many other things.) Black holes I’d imagine would sort of merge together, although again I don’t know what happens. (Does anyone? I’d imagine the odds of two supermassive black holes colliding would be tiny, and the odds of it being observable even smaller, but I’m not trying very hard to find any info.)

  6. Chris A.

    @Kevin (#2):
    The result of a stellar collision is thought to be a “blue straggler” star (i.e. a larger, hotter star).

  7. VinceRN

    Two fascinating questions asked by #1&2. We need more info here!

  8. Valerie

    Wil be sharing this. Bee-you-ti-ful!

  9. David

    Dumb question: “They may have actually passed right through each other.” I assume a galaxy’s structure comes from its stars’ gravitational attraction. So when the galaxies passed right through each other, there would be a period when most of both galaxies’ stars would become gravitationally bound, right? Why wouldn’t that new binding be permanent? Is it because each galaxy’s stars’ momentum keeps the galaxies separate?

  10. Nice image, but I don’t see any gigantic orphans in the image, Phil.

  11. I am Elenin

    lovely! Never saw them looking quite so eye-like. Love the couple little satellite galaxies, too.

  12. Robin Byron

    “…our galaxy is headed for a close encounter with the Andromeda Galaxy.”

    Oh, what an unimaginable night (or even day) sky that would be. I would so love to see that but it’s just “not in the stars”. :(

  13. Rae

    actually I think it was more like 3 billion years phil

  14. Messier Tidy Upper

    .. And if you’re patient ..

    And live for near eternity! ;-)

    Beautiful image. :-)

    @1. Diederick asks :

    I know that stars won’t collide when their galaxies do, but what about their central black holes? Do they actually attract each other and can they collide, and what happens then?

    They merge into one larger supermassive black hole. Or can often do so anyhow. As (#5) dartigan has noted already.

    @5. dartigen :

    I don’t know what would happen if stars did collide but it probably looks pretty. (At least, to someone not in the immediate vicinity – like so many other things.) Black holes I’d imagine would sort of merge together, although again I don’t know what happens.

    Stellar collisions~wise it depends on the mass and type of colliding stars.

    Blue stragglers known from globularcluster studies are one result as are FK Comae Berenice variables which, I gather, result from the merger of contact binaries.

    The collision – merger – of two white dwarfs is one widely acccepted explanation for type Ia supernovae eruptions.

    A white dwarf star passing through a red giant probably strips away its outer atmosphere and produces a white dwarf pairing.

    I’ve seen some suggestions that a smaller star can actually orbit inside a red supergiants thin atmosphere. (Betelguese specifically.)

    Two neutron stars colliding – merging – probably creates a Black hole as well as being a good test for Relativity in the lead up.

  15. Jon Hanford

    Although Phil mentions that NGC 4435 is the likely culprit for the disturbed shaped of NGC 4438, a 2008 paper pretty strongly implicates M 86 (also mentioned above by Phil): http://arxiv.org/PS_cache/arxiv/pdf/0810/0810.0711v2.pdf

    A striking version of Fig 1 of that paper is available here: http://www.noao.edu/outreach/press/pr08/images/M86.jpg

    Note the “low-velocity” red streamers of hydrogen gas between M 86 and NGC 4438. The green “high-velocity” streamers seen near NGC 4388 at lower right may actually result from “stripping” by the IGM of the Virgo Cluster, but this is somewhat unclear. Also, the authors of the paper find NGC 4435 an unlikely player, as the galaxy has a higher cluster velocity than NGC 4438 and it shows no obvious signs of gravitational distortions that should be visible in this low-mass galaxy if it was interacting.

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