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!