Author Topic: Why record the galaxy rotation?  (Read 16816 times)

zookeeper_Kate

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Why record the galaxy rotation?
« on: January 04, 2008, 12:01:45 pm »
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 next Thurs.

Secondly, 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 in www.galaxyzoo.org - 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?"
« Last Edit: January 04, 2008, 02:54:56 pm by zookeeper_Kate »

EricFDiaz

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Re: Why record the galaxy rotation?
« Reply #1 on: January 04, 2008, 12:17:34 pm »
Thanks, Kate. That makes perfect sense. Now I understand. :)

Mark OConnell

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Re: Why record the galaxy rotation?
« Reply #2 on: January 04, 2008, 03:27:55 pm »
Whereas the project looks at the uniformity of Clock/Anti-Clock why is the uniformity not similarly equal between spirals verses eliptical as the distance increases? We see way more elipticals then spirals the further out we look. Is that because at a certain distance blue spirals look like yellow elipticals due to the red shift and the blurryness?
« Last Edit: January 04, 2008, 05:00:40 pm by Mark OConnell »

zookeeper_Kate

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Re: Why record the galaxy rotation?
« Reply #3 on: January 04, 2008, 04:03:34 pm »
is the uniformity not similarly equal between spirals verses eliptical as the distance increases? We see way more elipticals then spirals the further out we look. Is it just that we can't really see well enough and the blue spirals at a certain distance look like yellow elipticals to use because of the red shift and the blurryness?

We do not expect the same number of ellipticals as spirals. But indeed as you point out, whatever the ratio is then we expect it to stay roughly the same with distance from us. What we actually find is that the proportion of ellipiticals increases with distance from us, and this is mainly because as things get fainter and fuzzier then they tend to look more like ellipticals! One of the Zoo papers out later this month is exactly on this point - Steven has shown that we can statistically correct for this kind of elliptical biasing that happens with distance: he finds the bias correction as a function of size on sky, and luminosity. And once you correct for this then you find the ratio is constant with distance (redshift). Hoorah!
I think Steven will be writing about this on the blog at somepoint., and see his thread about it here
« Last Edit: January 04, 2008, 04:29:28 pm by zookeeper_Kate »

Mark OConnell

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Re: Why record the galaxy rotation?
« Reply #4 on: January 04, 2008, 05:01:47 pm »
I changed my question so it read better.  :D

You answered it well.  Thanks for the link. :)

mushroom

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Re: Why record the galaxy rotation?
« Reply #5 on: January 09, 2008, 03:37:03 pm »
Cor blimey you boffins must get through a lot of Paracetamol! If a 'deepest' existence model is ever found that even has a 'beyond reasonable doubt' level of weight to it this would surely be an amazing feat. It's not surprising really that sometimes our use of words can appear to essentially point to nothing at all (i.e. bull**it!).

Well done and doing boffins.  :)

FermatsBrother

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Re: Why record the galaxy rotation?
« Reply #6 on: January 25, 2008, 12:53:04 pm »
Hi Kate & Alice - You make no mention as to why there isn't a computer program to look at this CW/ACW distribution.
A very short program has been used previously to analyse >100K of asteroids and to classify/correlate them !!
Surely it can't be all that difficult to write a piece of software do differentiate CW from ACW galaxies ?
Anything judged to be ambiguous can/will be discarded in equal (CW:ACW) proportions, in the same way as the human assessment.
Are there theoretical problems with this approach, if so, what are they  ?
If it has already been attempted, what were the results/problems ?
Cheers - Fermats Brother
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Alice

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Re: Why record the galaxy rotation?
« Reply #7 on: January 28, 2008, 03:35:42 pm »
Well, it's in the introduction of Galaxy Zoo generally - computers are useless at recognising patterns, whilst our brains are very good at it. Even if they recognised a lot of the basics, they wouldn't be able to deal with the weirder shapes such as mergers, irregulars and unclear spirals.

And besides, why turn over such enjoyable work to a computer when we could be the one to see all those pretty pictures!

That's it, really.

Alice

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Re: Why record the galaxy rotation?
« Reply #8 on: February 04, 2008, 07:54:47 pm »
Nothing like looking up Who's Online every so often, to see what the guests are all doing. Absolutely fascinating discussion here - it took place right back in the first couple of days, when I was still busy trying to figure out the way around this forum and I missed it at the time.

I wonder why the guests often seem to go to the earliest topics?

ElisabethB

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Re: Why record the galaxy rotation?
« Reply #9 on: February 04, 2008, 08:44:12 pm »
Well, maybe they like to read up on everything we've been at about !  ;D

Hanny

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Re: Why record the galaxy rotation?
« Reply #10 on: February 04, 2008, 09:48:21 pm »
I've wondered about that too :-\

GeoHani

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Re: Why record the galaxy rotation?
« Reply #11 on: August 26, 2008, 02:59:16 am »
I've asked this question before in this thread... but why can't the galaxies be flipping end over end?

weezerd

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Re: Why record the galaxy rotation?
« Reply #12 on: August 26, 2008, 07:48:47 am »
We do not expect the same number of ellipticals as spirals. But indeed as you point out, whatever the ratio is then we expect it to stay roughly the same with distance from us. What we actually find is that the proportion of ellipiticals increases with distance from us, and this is mainly because as things get fainter and fuzzier then they tend to look more like ellipticals!

This is one of those doh! comments.

It is the predominent view (I gather from reading other comments in the forum over the past year) that ellipticals are older than spirals. Also, the further we look into the Universe, the older pictures we see. Is it not then logical that the further distant galaxies we view, the older they are? ...and if they are older isn't there a bias towards them being elliptical?

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where time is not and angels play the paeans of the galaxies;
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Edd

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Re: Why record the galaxy rotation?
« Reply #13 on: August 26, 2008, 08:41:21 am »
I've asked this question before in this thread... but why can't the galaxies be flipping end over end?

Two things: one - if they're spinning around one axis it's hard to make them spin around a second, as they're basically acting like gyroscopes or spinning tops.
two - if they were at any great rate you'd be able to see this in spectroscopy across the galaxy and I'm not aware of anything pointing to this
When I look up at the night sky and think about the billions of stars out there, I think to myself: I'm amazing. - Peter Serafinowicz

FermatsBrother

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Re: Why record the galaxy rotation?
« Reply #14 on: August 26, 2008, 11:06:00 am »
I've asked this question before in this thread... but why can't the galaxies be flipping end over end?

Two things: one - if they're spinning around one axis it's hard to make them spin around a second, as they're basically acting like gyroscopes or spinning tops.
two - if they were at any great rate you'd be able to see this in spectroscopy across the galaxy and I'm not aware of anything pointing to this
Hi GeoHani & Edd - "...it's hard to make them spin around a second (axis) ...".
The objects in a galaxy are held together only by gravity. Nothing is rigid.
If you attempt to make such a group of objects rotate around a second axis (such as passing close to another galaxy at an appropriate angle) the angular momenta added to the original angular momenta of each object will result in random motion. All the spiral structure will be lost and an elliptical will result.
So, it's not only difficult, it's impossible !

It's also impossible for a rigid object, like a bicycle wheel, to spin on two axes in the way you would like.
The two angular momenta would combine to give you only one angular momentum axis !

Fermats Brother
« Last Edit: August 26, 2008, 09:32:26 pm by FermatsBrother »
A spectrum, many spectra. A Supernova, many supernovae. A datum, many data. A nebula, many nebulae. SATELLITE.
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