When galaxies strip for each other

The entire universe in blog form
Oct. 19 2012 7:00 AM

When galaxies strip for each other

It's been a while since I've posted a gorgeous shot of a weird and beautiful galaxy, and I have one that fits the bill perfectly: NGC 660, what's called a polar ring galaxy:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies! Follow him on Twitter.

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How awesome is that? [Click to galactenate.]

This picture, taken using the Gemini 8-meter telescope and put together by my old pal Travis Rector, is really pretty, and really pretty weird. Reading about it, in fact, I learned something! Learn it with me:

Ring galaxies are odd. I've always thought they were the result of galactic collisions, literally where two galaxies collide. If one is smaller, traveling rapidly, and pierces right through the heart of the other, the weird gravitational effects wind up creating a gigantic ring of material, and you get something that looks like a fried egg. The best and coolest example I know of this is Hoag's Object, seen here in a Hubble image (put together by another friend of mine, Tiffany Davis).

In general with these kinds of objects, there are tell-tale signs of the collision. The gas in the galaxies gets slammed around a lot, which means it can collapse and form lots of stars, for example. Usually, the ring is perpendicular to the central galaxy, too - that's why they're called polar ring galaxies.

But NGC 660 is an oddity. It shows the ring - which is 40,000 light years across, a little less than half the size of our Milky Way - and it has lots of star formation: the ring is studded with red gas clouds, stellar factories, furiously churning out stars. But the ring isn't aligned right; it's tilted at a funny angle. It's shape is off-kilter, too. Plus, although you can't see it in this image, other observations show that the central galaxy is creating lots of stars in its core too. That's not easy to explain in the head-on collision scenario, and is weird all by itself, since usually the cores of galaxies stopped forming stars billions of years ago.

But here's where I found out something new to me: some polar ring galaxies are formed when two galaxies pass near each other. The gravity from one galaxy can strip the gas from the other, and if the geometry is right that gas can form a huge, long arm which wraps around the first galaxy. That can form a ring of material that cranks out stars... just like in NGC 660.

Also, a near pass like that tends to be slower than a head-on collision. That means there's more time for the gravity of the two galaxies to shake each other up. That dumps gas into the center of the galaxies, where it can form stars. So this explains why we see newborn stars in NGC 660, too.

Well, that was new to me! I've written about galaxy collisions before, and the distorted but elegant and lovely shapes they take on (see Related Posts below). And it's obvious to me now that the disturbed material can form a ring, but it's just something I'd never run across before. So I learned something.

And now you did too.

So there you go. A picture of a galaxy with a big telescope is high art, beautiful and graceful. But there's a whole lot of science behind it, too. So, as usual, the appreciation you can have for something like NGC 660 only gets deeper and better when you understand what's actually going on. You're seeing two entire galaxies colliding, hundreds of billions of stars, octillions of megatons of gas being tossed around, new stars being born... and all because they are held sway under the invisible grasp of gravity.

See? Isn't that cooler than just seeing a pretty picture?

Image credit: Gemini Observatory/AURA



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