Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Monday, 8 April 2013

Gassy Galaxy GIFs Galore

Galaxies, I have mentioned, are very pretty. This is an objective fact. Disputing it would be like openingly declaring that you enjoy kicking puppies - maybe it's true, but it's best kept to yourself.

For a long time astronomy (and in particular its pretty press-release pictures) has been dominated by optical images - that is, light we can see with our eyes. Which is perfectly understandable, but over time, more and more wavelengths - "colours" if you will - have become available as technology advances. So now we can see, like bees, in ultra-violet, and infra-red like some reptiles. We can also see gamma rays, x-rays, microwaves, and of course radio waves. Which is where I come in...

THESE THINGS ARE NOT THE SAME
Radio waves are not something you have to plug in headphones to listen to*. Radio telescopes do exactly the same thing as optical telescopes, but at longer (redder) wavelengths. They're just as capable of making pictures every bit as pretty as from optical telescopes. For instance, our own galaxy looks like this through a radio telescope :

* No, not even if you're Jodie Foster. Maybe if you're Ruby Payne Scott.



To be more accurate, that's what part of it looks like in neutral hydrogen. The colours are false, but the structures are as real as anything you can see with your eye. Unfortunately, to see this would require eyes 300 m wide and made of metal, so for a superpower I'd still choose X-ray vision, thanks.

But it's still better than having 300m wide metal eyes.
In astronomy, all wavelengths are useful. Hydrogen is important because there's more of it than anything else. It's also what stars are formed from. Generally speaking, lots of hydrogen means lots of stars are forming. Young star clusters contain lots of bright, blue, short-lived stars, so hydrogen is usually associated with blue, spiral galaxies. Like this one, NGC 628 :

Hydrogen in red, stars in everything else.
Great ! A nice blue galaxy full of hydrogen, just like we'd expect. Except that beyond that, things get seriously messy.

For starters, the structures seen in the stars and gas are similar, but far from identical. This is probably because in places the gas has been completely turned into stars, like in the central region. In others, it hasn't - the spiral hydrogen arms extend a lot further out than the stars. No-one's quite sure why this should be.

Things are even more confusing, because some galaxies (about a third) have lots of young stars way outside their main stellar discs, but only young stars. That's freakin' weird. Sure, star formation could have started there recently. But if that's true, it must have happened to other galaxies in the past. So why aren't there any with much more extended discs of old stars ? Eh ?

But I digress, because the real fun with hydrogen is that the data is in 3D :


This isn't what the galaxy really looks like in 3D though, because the third axis is velocity, not position. In fact the true 3D structure isn't as dramatic - the hydrogen is about 150 times as wide as it is thick, which is roughly the same as a DVD. The velocity structure is a lot more interesting to look at - but more importantly, it shows that the outermost gas is moving just as fast as the gas close to the center. Which is much, much faster than anyone was expecting. So fast, in fact, that by rights everything should just fly apart.




The speeding hydrogen could be just as bad for the laws of physics as speeding cars are for small children. Either galaxies are all just about to explode*, gravity is wrong, or there's something else holding them together. I mentioned before that the stars in galaxies appear to be moving too fast, but in fact, it's really the hydrogen that's important here. Hydrogen extends much further out than the stars, where we would expect it to be be moving a lot slower than the stars. That means this definitely isn't a problem with the observations.

* Stay tuned. Seriously.

Hydrogen in purple. Everyone likes purple.

This second GIF shows a bit more clearly how much further the hydrogen usually extends compared to the stars - about twice the diameter of the stellar disc. This is another spiral galaxy, the romantically-named NGC 2903. Hydrogen doesn't always form stars, but when it does it's usually found in spiral galaxies. Exactly how the nice spiral structure develops  is controversial (and complicated), but it's clear that there's definitely a link between gas and spiral arms.

Which means that galaxies like this one, NGC 3077, come as a bit of a shock :

Hydrogen in blue, because blue is nice.
The stars, which are much redder than in a spiral galaxy, look like a smooth round spheroid, which normally means no gas and no current star formation. Except that this one has lots of gas, but it's all outside the main group of stars and bears no resemblance whatsoever to the optical galaxy, and clearly isn't a spiral. And star formation is happening not just within the galaxy, but within its gas cloud too.

SAY WHAT ?
Our canine friend is right to be surprised. It's a strange system. It's possible that the gas did originally come from NGC 3077, but got pulled out by another passing galaxy. But what was the galaxy like before the gas was removed - was it another run-of-the-mill spiral ? If so, it must have been transformed pretty quickly, because the gas is so nearby. If not, then it must have been an elliptical galaxy but one with lots of hydrogen (enough, in fact, to form a whole new galaxy eventually). And that's weird.


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