I'll be writing a lot about the colours of galaxies in several different posts; here's Chapter 1, as it were. Kind of hoping the scientists will pop in occasionally and point out any glaring errors, and anyone who'd like to will give me feedback on the clarity and style . . .
A few years ago, Kevin was a PhD student at Oxford examining a new breed of galaxy. It's well known that there are two basic galaxy types: spiral and elliptical. It has also long been well known that spirals are usually blue, and ellipticals are usually red. The beautiful arms in a spiral galaxy are really density waves, like a traffic jam on a motorway. And like the areas of congestion, they don't necessarily move along with the stars - a car might cruise along at 70 m.p.h. for a few hours, reach the traffic jam, get very cross, but eventually get to the other side of it and continue on its way. (The spiral arms do indicate a lot about the galaxy - which way it is spinning round [except in the exceptions], how big the black hole in the centre is, etc.) This means that the environment the stars and gas live in is continually changing and disrupted - which triggers off star formation.
When stars are young, they are hot - and blue. Blue light has the shortest wavelength, and the highest energy. The equivalent, in sound, is a high-pitched note. (Sound and light both have wavelengths way higher and lower than what we can hear and see, but that's another story.) In any case, a blue colour in galaxies is a good indicator of a lot of heat and star formation.
Elliptical galaxies, on the other hand, don't have star formation. There seem to be two reasons for this. One is that the orbits of the stars are "radial" - they all go their own way - so there are no large-scale disruptions to trigger clouds of gas to condense. Another is that they seem to have run out of gas anyway. Come to think of it, one thing I don't know for a fact is whether these two points are related.
Ellipticals appear dull at first, but they're incredibly peaceful, beautiful sights. SDSS pictures usually show them as a clear gold or vanilla colour. They sort of "fade out" towards the edge, and have a bright core. Of course, this bright core often contains a supermassive black hole, such as in the case of the giant elliptical galaxy M87, which won't be the pinnacle of serenity. Elliptical galaxies are thought to be remnants of past mergers. They're generally found in giant clusters of galaxies, where mergers would be more likely to take place, and they are often larger than spirals. Mergers create massive disruption and trigger off spectacular star formation, which might use up all the gas free to form stars.
So "blue galaxy" came to mean "spiral", and "red galaxy" came to mean "elliptical". It worked fine before SDSS. Then we found it wasn't that way at all.
(Is it just me or is most science based on "People have always said that things are this way. But we have found it might actually be this way!"? After you've heard that in every lecture at university for a year or so, it gets highly tedious and you wonder if all the academics want to do is prove each other wrong, rather than make new directions for themselves. On the other hand, "Eureka!" moments are . . . somewhat immature. Curiosity and research and painstaking surveys have gone on for centuries. [There are always exceptions - apparently there are no budding fungi taxonomists left in the UK, but not all British fungi have been classified yet! Who's up for filling this niche?] Most of science now seems to be refining, redefining, extending and building on what we already know. Perhaps it's irksome because you wonder what there is left to do, other than prove someone wrong and then get proved wrong by the next upstart next year. Or perhaps I'm the only one irk-able enough to be irked. Let me know.)
Kevin was studying a strange new breed of galaxy: the blue elliptical. In other words, an elliptical galaxy which was still undergoing plenty of star formation. Because these are rare, and studying a few probably wouldn't tell us the whole story of what was going on, Kevin was aiming to find as many as possible. He spent a week sorting through 50,000 galaxies, categorising them into spiral or elliptical. His office-mate, Tom Zlosnik, wrote in the Galaxy Zoo Forum that he was getting through about one per second! It must have been a trial, though, and he doesn't recommend it. "Your PhD student," Chris said in a lecture, "will classify 50,000 before telling you exactly what you can do with the other 850,000."
And so Galaxy Zoo was born. But that's definitely another story.
Many of our classifiers wrote to us to ask us, "I've been noticing that spirals are usually blue and ellipticals are usually red. Have you noticed that? Is it just me? Why is that?" I wonder if they knew what an exciting question they were asking? I didn't receive any of these myself, so I can't tell you what the team answered. The most important thing was to encourage people to classify by shape, not colour.
The blue ellipticals featured heavily on the forum, particularly in the autumn. "Here's a blue elliptical for Kevin!" we kept grinning, often posting dwarfs, irregulars and fuzzy spirals. In fact, the way they were really extracting the data wasn't using that thread but using the computer to check the colours, and extracting the ones that came out as "blue" from the computer and "elliptical" from us. (And that is why you shouldn't classify by colour, folks! If you'd followed convention you'd have clicked "spiral" and all these would have been missed!) Kevin nevertheless kept us up to date, and made us a special thread about his paper.
And that turned out to be only some of the story. Because then people started spotting the opposite: red spirals. Which I'll write about next time. One thing was getting clear - studying a few galaxies in depth won't tell you what's going on at all! Only by examining galaxies in vast quantities, on the scale of SDSS and Galaxy Zoo, can you get a whole picture. It's like studying a few animals for years until you think you know all about them - and then going and looking at the whole ecosystem. Suddenly everything we saw seemed to be introducing new exceptions.