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.

Infographic : Galaxy Size Comparison Chart

EDIT : Several typos were found my myself and others over the last few months. To prevent links from breaking, the old charts are still online. For the latest, correct versions, see my website.

Type in "asteroid sizes" into Google and you'll quickly find a bunch of  images comparing various asteroids, putting them all next to each at the same scale. The same goes for planets and stars. Yet the results for galaxies are useless. Not only do you not get any size comparisons, but scroll down even just a page and you get images of smartphones, for crying out loud.

Well, it's time to correct this disgusting  nay, bigoted oversight with the following infographics (which is what I gather is now the cool term for "posters").  These were prompted partly because no similar images exist that I can find, and also by recent claims that the largest spiral galaxy has been discovered.

The images I used were selected purely on an ad-hoc basis. Obviously, the Milky Way had to be there. Since really giant galaxies are many times larger than the Milky Way, and those were the ones I particularly wanted to show, that basically ruled out showing any dwarf galaxies (like the LMC and SMC for example). I tried to get a nice selection of well-known, interesting objects. I was also a little limited in that I needed high-resolution images which completely mapped the full extent of each object (often, because of a small field of view, only the central regions are mapped).

Still, I think the final selection has a decent mix, and I reckon it was a productive use of a Saturday.

Zoomable version here.

As will be evident from the poster, "my galaxy is bigger than your galaxy" claims should be treated with caution. The latest hoo-ha is about NGC 6872 (very bottom of the poster), which, though indeed enormous, has been stretched by an interaction with another object. Is it really fair to claim it's the largest if it's been stretched ?

Even defining the edge of ordinary galaxies can be tricky. Especially since their various components (gas, stars, dust, dark matter) extend to different distances from their centers. In some cases, truly enormous radio jets extend many times further out than the stars. Should they be included as part of the galaxy ?

To my mind the gigantic (but very faint) Malin 1 has a better claim to the throne than NGC 6872, as its disc hasn't been temporarily stretched by some interaction. How such a large disc formed is a bit of a mystery, but it is at least a true spiral disc, even if it's very faint and not remotely photogenic (it's barely visible with Hubble, for heaven's sake). 

But to some extent, all this is just semantics, and it really doesn't matter which galaxy is the largest, any more than it matters who landed on the Moon second or if Pluto is a planet or not. In any case, spirals are puny. The hands-down largest galaxy of any kind is IC 1101. And it is truly, in every way, monstrous.

Zoomable version here 

I don't just mean it's monstrous in that it's staggeringly vast, although that's part of it. I mean monstrous because it probably got so large by eating its neighbours. cD galaxies like this are found at the center of rich galaxy clusters, where there are plenty of smaller galaxies falling in that can be absorbed. That makes the galaxy heavier, which means their gravity can pull in more and more galaxies. It's like a cannibalistic orgy on steroids*. 

* So just like every episode of True Blood then.

Perhaps surprisingly, there aren't many pretty pictures of IC 1101. The galaxy's morbid obesity is offset by its great distance from us, although there are a few nice ones with Hubble (see below).  So for the chart, I took the image of M87 (another giant elliptical) and scaled it up. This isn't such a cheap way to do it, because both galaxies are pretty smooth structures, and in any case no image exists that's large enough to display at the massive resolution required.

IC 1101, as you will have guessed, is the big bright one. More images can be found through the Hubble archive and this website.

Another point is that while colliding galaxies might be initially spectacular, eventually they run out of gas and stop forming new stars (during the collision, the gas gets compressed, triggering star formation). Eventually, there's nothing left but a huge ball of old, red stars, the blue (short-lived) ones having died off aeons ago. With no gas and no new stars being formed, over time the random motions of the long-lived red stars make the galaxy nothing more than a titanic stellar swarm. And that's why our cannibalistic juggernaut isn't going to win any beauty contests.

ACHIEVEMENT UNLOCKED

GOOGLEWHACK !

Binge Drinking vs Binge Coding

Binge drinking is a bit like wrestling a shark, it might be fun, but it's not necessarily a good idea to video your activities and post them on YouTube. Seriously, read the links.

Binge coding - that is, doing nothing but writing computer code day in, day out - can also have deleterious effects. Currently, moments after waking (usually around 6:30, which doesn't help) and overcoming the customary momentary existential crisis - a sort of mental flailing of the arms - the first thing I find myself thinking is something like, "Hmm, I wonder how I can translate the bounding box values in a postscript file into the world coordinate system of an HI data cube ?"

And yes, you're right, that is a little worrying.

I continue to be unwaveringly preoccupied with such esotericisms (no, spellchecker, I did not mean eroticisms) until very late at night. At which point, while shaking slightly and murmuring things like, "renzograms...  matplotlib...  mesh geometry all wrong..." I reluctantly surrender consciousness. And that's on a Saturday.

Alright, I'll confess. My ulterior motive for this post is that I'm really hoping to generate a Googlewhack*. It turns out that there are currently zero results returned in Google for "renzogram matplotlib." Matplotlib is a popular graph-plotting package for Python. Renzograms are a curiously unpopular (though hardly unknown) type of plot, of which more in a subsequent post, perhaps (they can be really quite pretty, especially in 3D).

* With luck, this will be the least viewed post ever !

They also look quite a lot like the time vortex in Dr Who. This may well be the first ever renzogram generated by matplotlib posted on the internet. GO ME !

Anyway, the effects of binge drinking and binge coding can be very similar. For one thing, both cause anti-social behaviour, though of radically different natures. An angry drunk may tend to hit people. A happy drunk may tend to hit on people, or possibly just sing loud obnoxious songs. Whereas the binge coder will simply refuse natural light, shun all human contact and occasionally wonder why no-one's calling any more.

The effects are also similarly dissimilar on health. In the short term, neither does very much harm except to cause an unpleasant sense of confusion and a headache. In the longer term, full-blown alcoholism leads to all kinds of problems, like death. I can't imagine that it's physically possible to code oneself to death -

if self.alive == true:
   reality.unlink(self)

- probably doesn't work, for instance, but it's certainly not going to convey much in the way of health benefits to be shut away in a darkened room gorging on all manner of snack food with nothing to drink except tea and root beer. Well.... not if this continues for a whole year, at any rate.

However, binge drinking can be (except in extreme cases) self-limiting. Eventually, you run out of money, or throw up, or pass out, or possibly all three (but not necessarily in that order). Unfortunately, there's no such in-built safety net with binge coding, In fact, binge coding will more often than not leave you with more money that when you started.

Case in point.

Granted (to pick an example ENTIRELY AT RANDOM) generating, oh, I don't know, renzograms with matplotlib is unlikely to lead to the next Angry Birds, but it's still earning a salary rather than burning one. While I suppose sheer exhaustion might cause you to pass out eventually, it's probably not possible to code oneself into puking :

if self.coding >= excessive:
   for x in self.stomach:
      remove.x

No, the only way to beat binge coding is to see it through to the bitter end. To fix ALL of the bugs so that generating renzograms with matplotlib becomes foolproof. And then eventually you find that all is done and you can resume science and write more coherent blog posts.

SPIKE

Thanks, reddit ! You rock !

Welcome, new followers ! May your visit be educational and your hovercraft full of eels !

Wait, what ? NEW followers ? NEW followers ??? Oh crap oh crap oh crap oh crap oh crap.... this means I have to make more content.... umm.... wasn't really prepared for this... bugger. Err, in the meantime, here are some popular articles you might enjoy, or might enjoy posting on reddit, or might not.

Project Orion : How to Nuke a Spaceship Without Killing Anyone

Deep Space Force

Galaxies are Pretty

Beginner's Guide to the Universe

Excuse me while I scramble to find something else to write about.... err.... back in a bit !