Archive by Author | Sandor

The clumpiness of EAGLE galaxies

We have added new galaxies from the EAGLE simulations for you to classify on To find out more about what to do if some of them appear clumpy read this blog post.

It’s important to note that while EAGLE produces some impressive galaxy images, there are still some ways in which they don’t quite resemble real galaxies. A prominent example of this is in how many star-formation “clumps” there are in galaxies. Stars form in clumps or clusters of varying size, and some observed galaxies are clumpy in appearance, so the models are reproducing a real phenomenon. It also seems that these galaxies are more common in the early Universe, and are an important part of galaxy evolution. However, the clumpy galaxies may be too common within EAGLE.


Some EAGLE galaxies that appear clumpy. Clumps appear bright blue, because they have formed recently and contain the hottest and brightest (but shortest-lived) stars. From left to right, you can see clumpy galaxies that may appear disk-like, rounded or more chaotic in shape.

We have an understanding of why this happens: clumps can result from the limited detail with which galaxies can be modelled (even in the most powerful supercomputers), and the simplifications that need to be made to how gas interacts. This doesn’t affect other things we can learn from classifying these images. If you come across a galaxy that looks super-clumpy like the above images, the best thing to do is just ignore the clumpiness and classify the rest of the galaxy (If you would like to learn more about clumps, read about our sister project Galaxy Zoo: Clump Scout).

An EAGLE eye on galaxy formation

We have added new galaxies from the EAGLE simulations for you to classify on To find out more about why we need your help with this task please read this blog post.

Modern telescopes allow us to marvel at the diverse galaxies scattered through the vast expanse of space. Each galaxy appears unique, but many share common features with others billions of light years away. These stunning images pose some fundamental questions. How did these galaxies come to be? What will happen to them? What does their appearance tell us about their past?  We know it takes a very long time to build a galaxy; most of the nearby galaxies have been evolving for over 10 billion years. While galaxy evolution is exciting, we can hardly sit and wait for a galaxy to evolve in front of our eyes! Instead, the remarkably realistic simulated universes that are now being generated with modern supercomputers could hold the key to answering some of these questions. We are excited to announce a new image set of simulated galaxies from the EAGLE project. With your help, this will let us track how individual galaxies take their shape in a sophisticated simulated universe. 


Figure 1: The Hubble telescope peers back in time to reveal a diversity of galaxies, each one at an instant in their evolution. The Hubble ultra-deep field, reproduced courtesy of NASA, ESA, S. Beckwith (STScI) and the HUDF Team.

Computer models are increasingly powerful tools in astronomy, providing a tantalising glimpse into how galaxies evolve. We can follow the formation of galaxies in a simulated universe, once we include relevant processes such as the formation of stars, the growth of black holes and supernova explosions. The EAGLE project is a modern example of this, produced by a large international collaboration. EAGLE was run on a supercomputer using 4000 computer processors simultaneously over 4 months to generate a model universe. EAGLE is one of the most detailed model universes to date, and, along with the Illustris project, represents a historical advance in understanding various aspects of galaxy formation theory. This allows us to go through the 14 billion year history of the Universe in record time; from minuscule variations in the temperature of the first light of the Universe, to the emergence of the galaxies we see today. These detailed simulated galaxies have complex structure, particularly for galaxies as massive as our Milky Way. In EAGLE we can follow each galaxy’s complex family tree, providing a model for the direct evolution of individual galaxies. The EAGLE researchers ‘light up’ these simulated galaxies by modelling how stars shine, and how their light is obscured by dust. 


Figure 2: Simulated images of  EAGLE galaxies, showing the ancestors of three recognisable galaxy types we see in the local universe; a spiral, a lenticular, and an elliptical. The gradual evolution of their forms over time encodes information about the elusive physical processes that shape galaxies. Images are made to emulate data from the Sloan Digital Sky Survey, allowing direct comparison with real galaxies.

To enable these simulated galaxy images to tell us more about the galaxies in our real Universe, we can harness the power of Galaxy Zoo. Collecting Galaxy Zoo classifications of the EAGLE galaxies will help our understanding of how the physical properties of galaxies translate to what we see through our telescopes. What’s more, by classifying simulated galaxies at different stages of their lives, we get an idea of how each galaxy took its shape, and insights into what physical processes are working behind the scenes. Could an unassuming elliptical galaxy be the faded remnant of a once grand spiral? Or even a relic from a catastrophic collision between galaxies? By following the evolution of galaxies in EAGLE, we may find this out. 

For this experiment, we make images of all the simulated galaxies that have as many stars as the Milky Way or more at EAGLE’s ‘present day’ (14 billion years after the Big Bang) and use their galaxy family trees to take snapshots of the galaxy throughout its life, back to when the Universe was less than half its age. We make these images appear in the same way as those from the Sloan Digital Sky Survey (SDSS), which will let us compare directly to real data. Example images for three present-day galaxies can be seen in the figure above. An important aspect of the experiment is that some galaxies taken from the early EAGLE universe, which we would struggle to detect even with our most powerful telescopes, are shown as if they were local galaxies. These can take more unusual or chaotic forms. Classifying these galaxies under the same conditions as their descendents will give exciting new insight into why galaxies appear the way they do, and how they took their shape. 

This is not the first time Galaxy Zoo has classified galaxies from a simulated universe: you may remember classifying images from the Illustris project, which produced valuable insight into both the models and our real Universe at the present day (see Hugh’s previous blog). We are optimistic that the different imaging techniques and inclusion of dust effects in these new images will improve the resemblance between real and simulated galaxies, and the new approach of looking at galaxies through cosmic time will lead to new discoveries.

Most of the galaxies you see on Galaxy Zoo will continue to come from our survey of the Southern sky, but EAGLE galaxies will appear no more than 20% of the time. Your classifications of these images will help scientists tremendously in understanding the evolution of galaxies. Computer experiments are the closest thing we have to a laboratory where we can test our theories of how galaxies form, and, thanks to the Galaxy Zoo, everyone can play a part!