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.