Galaxy Zoo is a neato project to classify different galaxies, but one part of it mystifies me; when you classify a galaxy as a spiral, it asks whether you think it’s rotating clockwise or counterclockwise.
What difference does this make? Assuming all spirals are scattered in every different orientation, then any spiral galaxy can be said to be rotating clockwise or counterclockwise, depending on which way it happens to be facing you. (I really thought about this before posting to make sure I was visualizing it correctly!)
My WAG is that they want to see if they get a consistent answer from multiple people. If everyone thinks a particular galaxy is clockwise, that’s an indication of a very clear spiral pattern. If there is a lot of discrepancy, that’s an indication of an irregular or indistinct shape, and maybe it shouldn’t even be classified as a spiral.
There’s some recent preliminary evidence (astro-ph preprint) that spiral galaxies have a preferred rotation direction, and this classification will probably be used in extensions of that study. I don’t know how good this evidence is; the same author had another paper which made news recently, purporting to find a bias in orientations of elliptical galaxies, but that has been withdrawn with discovery of a sample bias. But it’s a cool idea, and only with the SDSS data it has become feasible to do really large-scale galaxy statistical studies like this.
If that assumption is true, then it follows that distribution is random. I think, actually, that’s the assumption they might test. If it turns out that there is a statistically significant bias towards either direction, that would indicate something to cause said bias.
As a WAG, I’d hypothesize that if the universe has some net angular momentum, then you’d see a bias in spiral galaxy rotation. I’m sure someone will come along with a more informed hypothesis soon enough, though.
Okay, here’s my question. Are there any, (many?) spiral galaxies that we see ‘edge on’, so that the spiral isn’t clearly visible from our perspective, and if so, can we tell that they’re spirals in other ways?
There are plenty of them. For example, the stunningly beautiful Sombrero Galaxy. It’s inclined just enough that you can see the spiral, but there are other galaxies that are so perpendicular that you can’t see anything but the edge of the disc and the bulge in the center. For example, NGC 891 in Andromeda.
We know they are spiral galaxies because that’s the only way you can get a bulge in the center and a long line of dust and stars bisecting it. You don’t know what kind of spiral it is, how many arms it has, and the like, but you know it’s a spiral.
