Sailing Among the Blues

The first name thought up for Galaxy Zoo was "Galaxy Safari". Astronomy is like a journey sometimes - through space and time, stopping at astonishing tourist sites or, further on, the wilderness. It's definitely true that Google Sky tours have suddenly become the height of fashion on the forum - now Half65, Ben Hoyle and Fermats Brother are all making them, and I expect more will follow!

I often also get the feeling that we're a little like biologists - or rather, that we've changed the whole of galaxy science from specialist zoology (sorry about that) to worldwide ecology. Traditionally, an astronomer might spend a lifetime studying up to 30 galaxies, and we've got used to the usual types - ellipticals are red, spirals are blue, galaxies are pretty, in science and view (and sorry about that too). Our discovery of blue ellipticals and red spirals was like discovering a parrot in a penguin colony, or a polar bear spotted like a leopard. We now know that, as polar creatures are white, the environment dictates galaxy colour. Being in a cluster turns galaxies red. And being alone turns galaxies blue.

That isn't due to natural selection, of course - it's due to the available gas in the local environment. And there's one type of galaxy that always seems to be blue, and that's the irregular galaxy. Irregular galaxies might be past mergers, or they might be little splodges or wisps of star formation, far smaller than your average spiral or elliptical, and with no defined shape we can so far describe scientifically.

(Credit: SDSS.)

Our brilliant Waveney wrote an Object of the Day on our irregular galaxies project yesterday. I won't try to cover all our aims or findings in this post, but one result is definite: they are nearly all blue. We've found a few that aren't, but none that are red, like ellipticals in clusters. Generally speaking, they are much, much bluer than your average Galaxy Zoo galaxy.

Irregular galaxies seem very common, although we can only see the nearest few because they're so small. As sepos stated in another Object of the Day, 90% of galaxies are the low surface brightness variety - again, not the sort we're studying. Could we be focussing on the magnificent tigers and elephants and whales of the universe, and missing out on the beetles and plankton? Do large bright galaxies depend on the existence of these dim little puffs as large organisms depend on the existence of bacteria? Or are these lowly little galaxies simply a by-product of far-flung gas, and have no effect on their larger neighbours?

Going back a moment - why are they mostly blue? It's because they're forming stars. As blue light is more energetic than red, that means the hottest stars give off blue light. That doesn't mean stars start off blue and go red later. It means that whenever star formation takes place, a few massive stars invariably form. These hydrogen-guzzling monsters outshine all the red stars in the galaxy, giving the whole galaxy a blue appearance - but they don't live very long. After a few million years, only the more sedate yellow and red ones are left.

Star formation is the subject of yet another game on the zoo (do you think I need to create a "games" label?) - not to mention Chris's current research interest at Oxford, or so the websites say. Intriguingly, his research apparently "focusses on the use of sulphur compounds". Which seems contradictory to me, since such compounds would not survive the heat and fury of stars. Perhaps they're a feature in gas clouds, or a product of something or other. This is the most informative paper Google could provide. If he ever has time, I'll ask him to talk me through it.

(Starforming region Corona Australis. Credit: NASA.)

So are all irregular galaxies young? Or are they simply like lonely blue spirals and ellipticals - continuing to drift through space and always encountering more gas? Is the Universe still too young for any area to be fully empty of gas, and will we see red irregulars in another few billion years? What about irregulars in giant superclusters, assuming they wouldn't all merge with the giant ellipticals already living there?

Some questions we already seem to have answered, at least to the best of our ability, on Galaxy Zoo so far. For example, we've established that galaxy rotation is generally random, though spirals near each other have a greater likelihood of rotating in the same direction - another indication that environment is very important in galaxy formation. (For more on galaxy rotation, I would recommend all ZookeeperKate's posts on the Galaxy Zoo Blog.) But I don't think we need worry that we've already found out everything. There is a lot more left to come!