Galaxies Suck, Let's Get Rid Of Them

Time and again, I've stated that without dark matter galaxies would just fly apart. That's because the outermost stars and gas are just moving too dang fast, but dark matter - in a loose sense - weighs everything down. Or, if you like, it binds the galaxy together, though it won't help one whit in a battle with the Dark Side.

Still. were all the dark matter to just disappear - somehow - residents at the edge of a galaxy would be in for a big surprise. At least that's what I assume. Well, it's time to find out if that's true or if I've been unwittingly lying through my teeth. Or to put it another way, today we get to blow a up a freakin' galaxy, motherfrakkers.

World domination ? Hah, cute.

My job, in no small part, consists of looking at galaxies and sometimes working out just how much gas and dark matter is in them. This is great, because galaxies are cool, but looking at galaxies and prodding them with a big stick are two entirely different beasts. For that, I need a theoretical physicist. And that's where the charismatic and handsome (his words, not mine) Rory Smith comes in.

Rory's job is to simulate and stimulate galaxy behaviour using computer code. My minor supporting role is to make the simulations look extra pretty. One day I decided that Rory probably owed me one by now, so off went an email. And some time later, back came a simulation. Thanks, Rory !

Before I unveil the final results, a few details. For the first 8 seconds (that's 500 million years of simulation time), the defenceless galaxy is left to its own devices. Then, for no good reason other than I've always wanted to see what would happen, all of the dark matter is instantly and magically removed.

The simulation starts with the gas and stars distributed in smooth discs, embedded in a dark matter "halo" (not shown), which is basically a big spheroidal clump. The dark matter particles swarm around randomly, while the gas and stars rotate. Stars particles don't collide with one another, but the gas does, and that leads to spiral structure forming quite naturally in the simulation (the dark matter only interacts with everything else through its gravity, because that's what it's supposed to do).

Left : the galaxy at the start of the simulation. Right : After 500 Myr. Stars are blue and orange, gas is shown as diffuse white.

The stars, however, have a disc and a bulge. Disc stars just rotate, bulge stars behave more like a swarm of bees. Galaxy bulges are common, but not every galaxy has one - they're thought to form when a small galaxy is eaten by a larger one, so they don't form in simulations of isolated galaxies unless they're specially setup.

Edge-on view of the galaxy at the start of the simulation. This simulations has 50,000 gas particles and 60,000 star particles.

Here, the stars that make up the disc are shown in blue and the bulge stars are in orange. Young star clusters are full of hot, bright, blue stars, which don't live very long. As they die off, they leave behind smaller, cooler, redder stars, which live for much longer. Galaxy bulges tend to have mostly older stars, so they look redder.

I'm almost there. It's worth noting that the simulation doesn't include star formation or death as both of these slow down the calculations. That means stars stay artificially blue and red when they should, of course, change. I made the gas visible in white; in reality this is far more complicated.

Enough talk. Here's what happens. When the timer turns red, the dark matter goes poof.

Just as the galaxy is arranging itself into a nice pretty spiral, the simulation inflicts an unfortunate attack of "a wizard did it". All of the dark matter disappears instantaneously, in complete defiance of the laws of physics. And then half of the galaxy explodes.

The wizard goes by the name of Rory, and he destroys galaxies in his spare time.

The thing is, although there's more dark matter than normal matter by about a factor of 10, it's just not all that important in the inner regions. Most of the dark matter is found much further out. And that means that the inner part of the galaxy doesn't really notice that anything's amiss. They pretty much just keep spinning....

Oh, close enough.

At the edge of the galactic disc it's a different story. Here, dark matter was the only thing holding everything together. And so, as you'd expect, everything flies apart. But it's a long, slow death, like Bruce Forsythe's broadcasting career, only more interesting. It takes about 200 million years for the galaxy to double in size.

It's the bit between the inner and outer parts of the galaxy where the most interesting stuff happens. Here, some of the gas and stars aren't quite moving fast enough to escape when the dark matter goes phwoop. They move slowly outwards, but just can't quite make it, so they fall back in again. All the while their own gravity starts clumping them together, so the surviving central 'galaxy' suddenly finds it's got a few unexpected friends. These are tidal dwarf galaxies, which do exist in the real world. In reality they form when galaxies interact with each other, and not because a wizard did it.

Which just goes to show that galaxies are pretty sturdy things. Even taking away 90% of their mass  isn't enough to really stop them. Nature abhors a vacuum, and desperately wants to fill it with galaxies.

The Rhyme of the L3 Satellite (II)

They should have sent a poet... but they didn't. Only me. Sorry.


The Sun now rose upon the right:
Out of the sea came he,
He raised Tsys, and through the mist,
Went down behind the sea.

Or to put it another way, the Sun makes data bad. What a surprise.

And the other cats still came along,
But no sweet Leo did follow,
Nor any day for food or play
Came to the astronomer's gazebo !

And that was indeed a hellish thing,
And it all of them laid low:
For all averred, the sciency nerds,
Leo had set them all aglow.
To the vets ! said they, take the cats today !
Or more will surely follow.

Nor dim nor red, like God's own head,
The glorious sun uprist:
Then all averred, those sciency nerds,
That it greatly raised Tsys.
'Twas right, said they, the vet to pay,
Though Leo we all miss.

The sky so seen, in radio beam,
A unique facility !
We were the first that ever burst
That particular frequency.

Down came the data, it was sent down,
'Twas as useful as could be;
The Astronomer could not take lunch-break:
So reduced data quietly !

I couldn't possibly comment.

All tropical heat and azure sky,
The blazing Sun, at noon,
Right up above the platform did stand,
No bigger than the Moon.

Which is a unique cosmic coincidence.

Day after day, day after night,
The data reduced only in slow-motion,
With GRIDZILLA slower than any cargo ship
Adrift upon the ocean.

GRIDZILLA is the software of choice for processing HI data. But it is indeed slower than a tranquillised tortoise with three broken legs because it isn't parallelised.

Data, data, every where,
And certain scientists think -
Data, data every where,
And lots of Scotch to drink !

I couldn't possibly comment.

The wealth of data did indicate,
That there should be much to do.
And furry cats did crawl with legs
The mother and kittens two.

About, about, in reel and rout
ALFA spun by day and night;
The WAPPs, restarting as though cursed,
Gave us all a fright !

ALFA is the receiver what detects hydrogen and stuff. WAPPs are processing instruments for which we can't get any more replacement parts.

And some in dreams assured were
Of the villain that plagued us so;
Bringing students unbidden he followed us
All across Puerto Rico !

I couldn't possibly comment.

And every tongue, through utter drought,
Was withered at the root;
We could not speak, no more than if
We had been choked with soot.

There didn't seem to be any need to change that last verse from the Coleridge original, so I didn't.

And O ! the fearful looks
He wrought in old and young !
Instead of data, a student's unwrit paper
About my neck was hung.

DISCLAIMER : Unless I'm suddenly given a student who can't spell their own name, supervising them won't be all that bad. This is an exaggeration, for the lols and whatnot.

I Checked My Spam Folder And It Was Full Of Science

While waiting for a life-or-death email (well, nearly) I decided to check my spam folder, just in case. Low and behold it wasn't there. In fact there wasn't much of anything, thanks to gmail's handy "automatically exterminate spam after 30 days" feature. Less helpful was its "throw the baby out with the bathwater" feature. About 15 emails were actually genuinely relevant, which is quite a lot out of 56 in total.

Most of these 15 were people asking, "can I buy a poster ?" which is nice, and I guess gmail thinks that people trying to buy things from you is equivalent to people trying to sell things to you. Understandable. So was the one, "can I put your image in a magazine about UFOs ?"*

* I said yes. Time will tell if I regret it, but I rather enjoy hearing crackpot theories from time to time.

Thankfully the most important graphic-related email, from Ciel et Espace (French readers take note, COUGH COUGH COUGH LORRAINE! COUGH COUGH COUGH) survived the spam filter intact. So at the end of next month there should be a nice little article featuring moi. Vive la France ! Ou est la gare ? Je voudrais une Origina et poulet parce que je suis une idiot. Je detest le cochon d'inde mais j'aime les mouton parce que je suis Gallois !

Errm.... moving on,,,

The other important email was not so fortunate. Someone had spotted a mistake with the galaxy charts. And it's a whopper, because a light year isn't 6 billion miles. That would make it about the size of Pluto's orbit, which it isn't. It's 6 trillion miles. I was off by a factor of a thousand.

Being off by a factor of a thousand is quite a lot, even for astronomy (where a factor of 5 is de rigueur). That would be like saying that, "This non-stop flight from London to New York is expected to take about a year" instead of 8 hours. It's equivalent to saying that the Battle of Hastings happened about a million years ago, a level of historical inaccuracy that not even 300 and Xena combined could hope to match.

Wait, you mean history wasn't full of scantily-clad warriors fighting monsters ?!?

The saving grace is that only a very very small part of the text was wrong. OK, it's ludicrously wrong, but the rest is all fine - all the relative galaxy sizes are correct. Otherwise I'd have to go and whip myself, or something.

Fortunately the images in the two blog posts were too small to read the text, but the ones on my website are now corrected. I kept the file names the same so any external links should still work. The one exception are the zoomable images - those had to be replaced. Which they have been in both the blog and my own website. I can't speak for other websites though.

Space Station V

I continue my quest to kick old projects dragging and screaming into the 21st Century with Space Station V, the big round whirly thing from 2001 : A Space Odyssey. Readers will, or should, already be familiar with my unhealthy obsession with this movie*, which very nearly rivals that of Star Trek. Anyway, here's the original model I made about 8 years ago, or thereabouts :

*Reassuringly, while I think it's one of the greatest movies ever made, this guy seems to think it's one of the Greatest Human Achievements of All Time.

It isn't too hideous, I suppose, but it's totally unsuitable for modern, hi-def renders, let alone close-ups. So this time I went a little bit nuts with the greebles, and it's now my most detailed model ever with a vertex count of 3.8 million (my previous personal best was the gargantuan, 3-mile long Mayflower II with a mere 1.9 million). Of course, a lot of this was from liberal use of the duplicate tool. But a lot of it wasn't.

Click here for full-size image.

And of course, I couldn't very well rest easy if I didn't animate this. So I did, and here's the result. Not too awful, but I think it will look much better in the larger scene I'm planning which will use all my spaceships (and also RAM, and possibly sanity).

Well, that's it. Stay tuned, because next week I get to destroy a galaxy, Seriously.


UPDATE : Inspired by some very nice renders of the battleship Tirpitz, here are some clay renders to show the mesh geometry in even more detail.

Larger version here.

Larger version here.

Milking It

Last one, I promise. The next post won't have any galaxies at all !