We’re happy to announce the launch of a new project in Galaxy Zoo, called “What the Shell?”. This project will focus on shell galaxies – a class of galaxy that has shown up as a classification option in many Galaxy Zoo projects, but hasn’t been the star of the show until now. We’re initially launching this project with 1000 candidate shell galaxies from Euclid Quick Data Release 1 and DESI DR1, with potential to increase this number based on volunteer feedback. We welcome everyone to contribute to this unique project!

While we still need you to classify galaxy images based on whether or not they contain shells, we’re excited by an additional feature that’s shown up in Galaxy Zoo now and again: Drawing. After identifying that a galaxy image contains shells, you’ll be asked to draw where you see each structure. Since the locations and shapes of shells in a galaxy contain information about the history, dynamics, and content of that galaxy, we need you to break out your artistic talent and share it with us!

The Science

Galaxies grow over time through collisions, or mergers, with other galaxies. When a smaller galaxy hits a larger galaxy in a head-on collision, it is completely torn apart, but its stars keep traveling through the host galaxy. As these stars turn around in their orbits, like a bungee jumper reaching the end of their rope, they pile up and make large concentric rings, which we call shells. These shells are large, spherical, concentric structures that surround the central galaxy to give them their unique, ethereal appearance.

Stars inside a galaxy move according to their energy and the gravitational field of that galaxy. This gravity is created by the combination of the luminous material in a galaxy (stars, gas) plus the “dark” component of the galaxy, made up of unknown material that doesn’t emit any light. If we’re able to measure the gravity inside a galaxy somehow, then we’re able to figure out how much matter is inside that galaxy, and roughly where it’s located.

Shells provide a wonderful opportunity to study this. As the dwarf galaxy shreds apart in the center of the larger galaxy during collision, all of its stars have roughly the same amount of energy, setting them on nearly identical orbits. As the stars move outwards to become shells, the location that they turn around is set only by the strength and shape of the gravitational field. So, understanding the shape and locations of the shells inside a shell galaxy tells you the properties of the gravitational field for that galaxy. The shape and number of shells in a galaxy also contains a wealth of information about how long ago the collision happened, the properties of the colliding galaxies, and the orientation of the galactic collision.

It’s relatively straightforward to understand where the stars and gas are located in a galaxy (because you can see them), and as a result we can estimate how much gravity we expect there to be from everything we can see. This can then be subtracted out of the total gravity that we observe from our analysis of shells, and we are only left with the amount and location of the dark matter within that galaxy.Different theories for what dark matter is made of make different predictions for how it is distributed in galaxies. Some of them predict a smooth distribution, while others predict there may be clumps of dark matter that could be identified from measurements of shells! Knowing more about the distribution of dark matter in shell galaxies will help us narrow down different possibilities for our best guesses about what dark matter actually is.

In reality, shell galaxies don’t always look very clean. The host galaxy might move in the merger, or have some existing structure that makes the shells take on more messy shapes. Alternatively, the galaxy can interact with other neighboring galaxies while shells are forming or after shells have formed, which can further complicate the shapes of shells. Shell galaxies can take on a wide variety of shapes, sizes, colors, and environments, making them an extremely rich dynamical playground to study the life cycle of galaxies throughout the Universe.

Why do we need your help?

Determining where shells are located in a galaxy is hard to do automatically. There are some algorithms that can sometimes tell you where shells are located, but these usually only work for the nice-looking shells that are made from ideal conditions. Only a small fraction of observed shell galaxies fit in this category. Additionally, sometimes it can be hard to tell if a galaxy even contains shells or not from a glance! It would take researchers an extremely long amount of time to properly characterize and model each shell galaxy.

Therefore, we are asking for help from volunteers to help classify shells in galaxies. We have a substantial number of galaxy images from DESI and Euclid, where we have used Galaxy Zoo’s Zoobot model to select likely shell galaxies. As you help us determine which galaxies do and don’t contain shells, we will be able to refine our search to better select only shell galaxies. This will provide us with an extensive catalog of shell galaxies plus the locations and shapes of their shells, at a scale that has never been done before.

With this new catalog of shell galaxy data, we will be able to characterize the amount and shape of the dark matter in each galaxy using the process described above. We hope to use the data for this large population of shell galaxies to learn about how spherical dark matter halos are, put a limit on how “clumpy” the dark matter is in each galaxy, and also learn about the events that lead to shell galaxies and galactic collisions in the first place.

As always, thank you for your help and support!

Tom Donlon & the “What the Shell” Team

Click here to get started!