Blue=Young And Red=Old?
The basic division of galaxies into star-forming spiral galaxies blazing in the blue light of massive, young and short-lived stars, on the one hand, and quiescent ellipticals bathed in the warm glow of ancient low-mass stars, on the other, goes back to early galaxy surveys of the 20th century.
But, once modern surveys like the Sloan Digital Sky Survey (SDSS) began to record hundreds of thousands of galaxies, objects started emerging that didn’t quite fit into those two broad categories.
A significant number of red, quiescent galaxies aren’t elliptical in shape at all, but retain roughly a disk shape. Somehow, these galaxies stopped forming stars without dramatically changing their structure.
At the same time, blue elliptical galaxies started to surface. Their structure is similar to that of “red and dead” ellipticals, but they shine in the bright blue light of young stars, indicating that star formation is still ongoing in them.
How do these two oddballs – the red spirals and the blue ellipticals – fit into our picture of galaxy evolution?
Galaxy Zoo allows citizen scientists to classify galaxies. Screenshot by Kevin Schawinski, CC BY-ND
Send In The Citizen Scientists
As a graduate student in Oxford, I was looking for some of these oddball galaxies. I was particularly interested in the blue ellipticals and any clues they contained about the formation of elliptical galaxies in general.
At one point, I spent a whole week going through almost 50,000 galaxies from SDSS by eye, as none of the available algorithms for classifying galaxy shape was as good as I needed it to be. I found quite a few blue ellipticals, but the value of classifying all of the roughly one million galaxies in SDSS with human eyes quickly became apparent. Of course, going through a million galaxies by myself wasn’t possible.
A short time later, a group of collaborators and I launched galaxyzoo.org and invited members of the public – citizen scientists – to participate in astrophysics research. When you logged on to Galaxy Zoo, you’d be shown an image of a galaxy and a set of buttons corresponding to possible classifications, and a tutorial to help you recognize the different classes.
By the time we stopped recording classifications from a quarter-million people, each of the one million galaxies on Galaxy Zoo had been classified over 70 times, giving me reliable, human classifications of galaxy shape, including a measure of uncertainty. Did 65 out of 70 citizen scientists agree that this galaxy is an elliptical? Good! If there’s no agreement at all, that’s information too.
Tapping into the “wisdom of the crowd” effect coupled with the unparalleled human ability for pattern recognition helped sort through a million galaxies and unearthed many of the less common blue ellipticals and red spirals for us to study.