The Sciences

Spiraling tentacles of galactic doom!

Bad AstronomyBy Phil PlaitOct 30, 2007 4:00 PM


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So I'm sitting here trying to finish my book, writing about the inevitable collision of the Andromeda Galaxy with the Milky Way, when literally I take a few seconds to check my email. And what do I see? Beauty. Sheer, incredible, perfectly coincidentally-timed beauty. Behold:

Holy Haleakala! Click on it for access to a much, much more beautiful version. This small one does it no justice at all. This newly released Hubble image shows the intertwined pair called Arp 87, two big galaxies undergoing a collision. Halton Arp is an astronomer who cataloged peculiar galaxies in the 1970s, and many of them have been found to be colliding, or at least interacting in some way. This pair consists of NGC 3808A, the big spiral on the right, and NGC 3808B, the odd cigar-shaped dude on the left (actually, it's an edge-on spiral). They are about 300 million light years away in the constellation of Leo, for those keeping track at home. They are separated by about 100,000 light years. The other edge-on spiral is a background galaxy, apparently. Evidently, sometime in the past they swept past each other. Gravity from 3808B drew out a long tendril of stars, gas, and dust from 3808A. That stuff is now orbiting 3808B, forming a ring perpendicular to the plane of the galaxy's disk. In the hi-res image, you can trace the material around the galaxy at least twice! It's amazing; the dust from the tendril torn off 3808A can be seen blocking light from 3808B when it's in front, and the tendril itself disappears when it's behind 3808B. I would guess that the collision must have happened about 200 - 300 million years ago, to give the tendril time to wrap around the galaxy. Given their separation and typical speeds (200 kps), they may have passed each other as little as 160 million years ago. So those numbers are consistent. I'd love to know for sure, but oddly, I couldn't find much in the scientific literature about this pair. What's obvious from the image (to the experienced eye) is that there are scads of stars being formed in 3808A. All those bright blue blobs are areas where newly born massive stars are lighting up the gas around them. This is expected: when galaxies interact, gas clouds can collide (they're big, and smash into each other) or the conditions inside the galaxies allows clouds to collapse and form stars. These are called "starburst galaxies". Poetic, but fitting. The other galaxy, which is edge-on, looks reddish to me. That means there is a lot of dust in it, which is also an indicator of lots of star formation. Eventually, though, the activity will die down. Given how far apart they are, it looks like they may actually drift apart, and not fall back in to each other (though this is impossible to know for sure by eye). In a few billion years, we won't be so lucky. The Andromeda Galaxy is headed our way like a hundred billion solar mass freight train. It'll smash into us (well, we'll smash into each other), and the two galaxies will draw out long filaments from each other just like in Arp 87 here. But unlike these two galaxies, we won't have enough velocity to escape each others' pull. We'll draw apart some, but then recollide, maybe twice more, and eventually merge into one giant elliptical galaxy. Weirdly, stars almost never directly physically collide in galaxy mergers; stars are small and very far apart. It's the clouds that collide, forming new stars. Take a good, close look at these two galaxies that make up Arp 87. In two billion years or so, that'll be us.

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