Density Waves

When you see a nice spiral galaxy it is tempting to think of the spirals arms as fixed collections of stars, gas and dust that orbit the centre of the galaxy as a solid rather as if you had drawn them onto a frisbee. However, this isn't the case.

The Whirlpool Galaxy (M51) taken with the HST's ACS. CREDIT: NASA, ESA, and G. Bacon (STScI)
If you measure the velocity of the gas, or dust, or stars you find that the velocity remains pretty much the same for stars nearer the centre as those nearer the edge of the galaxy (this isn't true for those in the middle but I'll ignore those). So if they are moving at the same speed doesn't that mean that they all go around together? Well, no and it is all to do with circles. Everything would stay together if it was going in a straight line, but in a spiral galaxy it is going in circles around the nucleus; the stuff near the middle will complete an orbit quicker than stuff further out because it has much further to travel.

If you start off with a bar of stars (or gas, or dust) sticking out from the centre of a galaxy, pretty quickly it will twist itself up so much that you can't really see the arms. By quick I mean a few orbits around the galaxy. This is considerably less than the lifetime of a galaxy, so for us to see spiral arms there must be more to it.

It turns out that the spiral arms we see are not actually solid groups of stars as you might draw on a disc. Instead they simply represent areas where there are more stars, gas and dust (sort of obvious if you think about it). So what is the difference? The individual stars (and clouds of gas and dust) move through these over-dense areas (the spiral arms) while orbiting the centre of the galaxy.

This can be a difficult concept to get your head around, but I saw a good example of it a few weeks ago. On a long stretch of very busy motorway somebody is bound to apply their brakes for one reason or another. If there is a lot of traffic, the car behind will also have to brake a bit and the car behind that one will too and so on. As well as slowing the traffic down, this reduces the space between cars and can result in a traffic jam. Now if you were to watch a long stretch of motorway from a distance you would see that the place where the jam was had more cars in it than a similar-sized patch of motorway where the traffic was flowing freely. Although the passengers may feel that they aren't getting anywhere, the cars are moving through the over-dense region. In fact, the location of the dense region (jam) may even move backwards up the motorway resulting in phantom jams - jams that you come out of wondering why it was there. From a distance you see the dense patch slowly moving backwards while the cars are moving forwards. Now replace the traffic jam with 'spiral arm' and the cars with stars and you'll not think that density waves are so odd.

So, density waves can move around the galaxy much more slowly than the stars/gas/dust and can last for quite a few orbits. This helps solve the problem of spiral arms getting wound-up too quickly. Another feature of this density wave is that the clouds of gas and dust get a bit compressed at the edge of the arm the enter at. This helps start their collapse leading to the formation of new stars. The newly formed stars will then continue through the arm but only the longer lived ones will make it out. The result is that short lifetime stars are mainly to the spiral arms.

The density wave idea seems to agree with observations of spiral arms in many galaxies and is the preferred explanation, but there are still some questions that need answers such as how do you set up a spiral arm in the first place.

Posted in astro blog by Stuart on Sunday 15th Jan 2006 (23:42 UTC) | Permalink
[an error occurred while processing this directive]
[an error occurred while processing this directive]