Thousands of Laymen Help Make A Crucial Discovery About Galaxy Evolution

Author:  Jessica Kloss
Institution:  Princeton University

Most would agree: science has progressed past the point where laymen can understand, or participate in making new discoveries. But astrophysics has recently made a breakthrough with the help of these very same laymen. For this particular field, physics and technological brilliance achieves a very simple and familiar goal: astronomers want photos of the heavenly bodies they study. In most cases – especially when studying galaxies – the most reliable way to figure out what's going on is to look at the photo.

While not everyone can make the incredibly advanced machinery used to take these photos, by looking at the photo of a galaxy, anyone, not just astronomers, can make a judgment call about the internal structure of the galaxy. In fact, the "Galaxy Zoo" project at the University of Nottingham used exactly that. Equipped with a vast number of high-resolution photos of galaxies, and knowing that just a glance at the photo is more accurate than even the most advanced computer classification algorithms, the Galaxy Zoo team asked the general public for help. The Space Telescope A901/902 Galaxy Evolution Survey (STAGES) project, also at the University of Nottingham, used images from the Hubble Space Telescope to do a study that complements the Galaxy Zoo study. Both studies found a special type of spiral galaxy that could be the missing link in our understanding of galaxy evolution.

Broadly speaking, galaxies can be separated into two huge classifications: spiral or elliptical. There are certain assumptions astronomers make when dealing with galaxies. Spiral galaxies, like the Milky Way, are usually younger and bluer, because they're still actively forming stars. Elliptical galaxies are the older, redder galaxies, which have used up all their star-forming fuel. But the teams at the University of Nottingham noticed there seemed to be something in between – red spiral galaxies, a kind of link between blue spiral and red elliptical galaxies.

The photo shows the Hubble Classification System, named for Edwin Hubble, who came up with it in order to classify the galaxy shapes that he observed. Astronomers have since come to realize that this chart could also be thought of as the changing shapes of galaxies throughout their lifetimes: newest galaxies are to the right, and they slowly move through stages until they end up on the left. Some spiral galaxies have a bar, which is an elongated core region, and some do not. Eventually, they all end up at the same position on the chart as elliptical galaxies. According to the Hubble Classification System, our own Milky Way would be an SBc galaxy, with a central bar about 27,000 light years long!

The missing link, a new flavor of galaxy that links spiral to elliptical galaxies discovered by the STAGES and Galaxy Zoo projects, would be located on the diagram around where the two branches meet. But there's no need to add a new shape to this diagram – the surprise is not the structure of the galaxy, but rather its color.


Over 150,000 Galaxy Zoo users looked at millions of galaxies over a large chunk of sky, using photos from the Sloan Digital Sky Survey, an ongoing, full-sky survey which has already mapped a quarter of the celestial sphere. Given a photo of a galaxy, like those above, users were asked to classify the galaxy as spiral or elliptical. Using this and color information, the STAGES project was able to use Galaxy Zoo results to explore in detail the regions where these red spirals seemed to be occurring with the Hubble Space Telescope.

Dr. Steven Bamford, leader of the Galaxy Zoo study and a Science and Technology Facilities Council (STFC) postdoctoral researcher at The University of Nottingham, explains: "In order to have spiral arms, they must have been normal, blue, spiral galaxies up until fairly recently. But for some reason their star formation has been stopped, and they have turned red. Whatever caused them to stop forming stars could not have been particularly violent, or it would have destroyed the delicate spiral pattern."

The STAGES and Galaxy Zoo teams found that these "new" spirals were most commonly found in the outskirts of great clusters of galaxies. Moreover, they found with further study that the despite their reddish hue, these special galaxies also host blue star-forming regions – but they are hidden behind a shroud of dust. Dust is known to have a reddening' effect on light, blocking all except the infrared part of the spectrum. So the bluish hues of star formation are hidden behind a curtain of "red".

Putting the two projects together, the teams were able to come up with a scenario about how these galaxies were evolving. As the blue spirals were gravitationally pulled into areas with higher populations of galaxies, their star formation gradually became shrouded with dust. The teams postulate that as galaxies are pulled further in, their star formation starts tapering off more and more. As a result, they begin to have the lens-like shape that corresponds, on the diagram, to where the two branches meet. But, as Dr. Bamford explained above, that doesn't explain how these galaxies lost their delicate spiral structure and became fully-fledged elliptical galaxies. In fact, more violent interactions, such as collisions, cause the galaxies to lose their features entirely and become smooth elliptical galaxies.


Meghan Gray, STFC Advanced Fellow at The University of Nottingham and leader of the STAGES survey, says, "Our two projects have approached the problem from very different directions, and it is gratifying to see that we each provide independent pieces of the puzzle pointing to the same conclusion."

Because Galaxy Zoo's 150,000 users classified such a large number of galaxies, the Galaxy Zoo and STAGES teams had enough data to make yet another discovery: the larger the mass of a spiral galaxy, the more resistant it becomes to the slow process of becoming elliptical. As Professor Bob Nichol of Portsmouth University, a member of the Galaxy Zoo team, puts it, "Just as a heavyweight fighter can withstand a blow that would bring a normal person to his knees; a big galaxy is more resistant to being messed around by its local environment. Therefore, the red spirals that we see tend to be the larger galaxies , presumably because the smaller ones are transformed more quickly."

Galaxy Zoo constantly fosters new scientific discoveries as more high-resolution photos come out from the Sloan Digital Sky survey in approximately bi-annual data releases, waiting to be classified. This discovery is only the first of many on how galaxies, the basic units of cosmology, develop over billions of years. But this story also highlights the fact that science doesn't have to be reserved to those who devote their lives to studying it. An office worker classifying galaxies at breaks could easily have contributed more to this study than a postdoctoral researcher in cosmology.

Written by: Jessica Kloss

Edited by: Jeffrey Kost

Published by: Hoi See Tsao