We often get questions on the forum querying why we are recording the anti-clockwise/clockwise rotation direction of the spiral galaxies given that it is not an intrinsic property of the galaxy - it just depends on which side you view the galaxy. A number of answers from zoo'ers have appeared around the site, but we thought we'd put them together in one place...Clockwise or Anticlockwise: why are we investigating spiral rotation?
Who cares which way it's going round? The way we see a galaxy's rotation is only due to our point of view. It would look different from the other side. If you could look through the back of a clock, for example, the hands would be going round anti-clockwise. We admit that this is the slightly more bonkers part of the project! But there a couple of different science motivations.Firstly
, we want to check that the rotation of the galaxies are as random as we think they should be. That is, we expect (over the large parts of the sky we are observing with the Zoo) there to be an equal number of anti-clockwise and clockwise rotating galaxies. And in this case it doesn't matter that the rotation depends on the observer's position - imagine throwing down some counters which are black on one side and white on the other onto a glass table. You want to check if the counters are biased to land more one way than the other - and clearly you can do this by observing them from either side of the glass table.
If we did see a preference for galaxies to rotate one way in any part of (or all over) the sky then this would be at odds with our simple null hypothesis that the galaxies generally rotate independently of each other (over large distances). What could be happening? This is like asking 'why do most whirlpools on Earth go round anticlockwise?' (It doesn't only happen in the Northern hemisphere). Is there some kind of force making this happen? Well it'd be similar for the galaxies - perhaps there is some overall force effecting all of the galaxies (such as the presence of a magnetic field across the Universe), or perhaps the process of galaxy formation leaves a correlation over distances larger than we previously thought. In anaolgy with the counters - the first case could relate to the counters being unevenly weighted and gravity then having an effect (ie. a force is present), the second case is similar to the idea that the counters weren't actually thrown down at random (ie. all started white face up for example).
An earlier study of a few thousand galaxies from SDSS found that galaxies had a slight preference to rotate anticlockwise in one part of the sky, and clockwise in the opposite part. One of the questions Galaxy Zoo hopes to answer is whether this is true. We have many more galaxies classified, by a lot more people, and at the moment we are finding that there is a preference in our sample for galaxies to rotate anticlockwise - while it's quite close to half an half, the difference is statistically significant. We are currently stratching our heads to figure if it is a real result or if it is because of some human error or bias... I will be posting more about this work on the blog
, we are interested in knowing if the rotations of galaxies are correlated over much smaller distances - like between neighbouring galaxies. We want to measure how similar galaxy rotations tend to be as a function of distance between the galaxies (we call this the '2-point correlation function'), and this information has the potential to tell us a bit about our theories of structure formation. For example, if two neighbouring galaxies formed from the same larger swirling mass then we would expect them to usually rotate the same way. Alternatively, if they formed separately but have tugged and 'torqued' each other then we would expect them to usually have opposite rotations (to conserve angular momentum). So you see, here we are not actually concerned if they are 'clockwise' or 'anti-clockwise', but rather if they are the same or not - and this does not depend on the observer's position.In fact
, if you don't like the rotation direction idea, then you can think of the 'clock'/'anti clock' labels as just 'black'/'white' like the sides of the counters. And what we want to know is (a) are the colours distributed randomly over the sky? (b) is there a tendency for close galaxies to have the same colour?
But here's a crunch point - if we see a preference for one colour over the whole sky (eg. all the 'counters' are turned with their black side to us) then this poses a problem - it means that the statistics of our Universe are different when seen from different places, because in this example we would not have seen all the 'black' sides if we had been sitting somewhere else. In cosmology we really don't like the idea that the Universe looks different from different places or in different directions, because we want to be able to make inferences about the whole Universe from our observations. And if we cannot treat every part of the Universe identically then our equations become much more complicated!! This is one of the fab things about Galaxy Zoo - we can go some way to check that things are as random as we hope they are!
We hope this helps! Feel free to comment... Cheers, Kate & Alice
To see the quick answer: the FAQ
- second question down.
To see a discussion on the perspective: "Daily Telegraph"
- zookeeperChris and gulliver's posts towards the end.
To see another of many threads on the subject: "Clockwise or anticlockwise - why does it matter?"