Help astronomers find rare cosmic jellyfish galaxies in this new Zooniverse citizen science project!

June 01, 2021

A rare kind of galaxy is at the heart of a new citizen science project that is being unveiled today: "Cosmological Jellyfish" is part of the Zooniverse platform, where volunteers can contribute to genuine scientific research projects. In the new project, participants look at the results of a cosmological simulation and identify galaxies that look somewhat like jellyfish. The jellyfish-like appearance is an indicator that the galaxy in question has interacted with gas in a galaxy cluster – which is what the creators of the project, the group of Annalisa Pillepich at the Max Planck Institute for Astronomy, want to study further.

Galaxies like our own Milky Way galaxy, consisting of millions, billions or even hundreds of billions of stars, are large-scale building blocks of our universe. While astronomers are confident they now have a reliable overall picture of how galaxies have formed over the past 13.8 billion years, after the hot Big Bang phase of the universe, many details are still in need of further research – and whenever new observations and powerful simulations become available, there are opportunities of adding pieces to the puzzle.

One region of the puzzle that is badly in need of more pieces is the case of so-called jellyfish galaxies. Such galaxies can be found in galaxy clusters, alongside with thousands of other galaxies. Such clusters not only contain the galaxies themselves, but also thin, hot intergalactic gas. As thin as that gas is, it is enough to make galaxies that are moving at high velocities through the cluster feel a "headwind".  

The missing details of jellyfish formation

Imagine someone on a motor bike, with their hair, or maybe their shawl, streaming behind as they move through the surrounding air. Galaxies moving quickly through a cluster feel a similar headwind, or "ram pressure". The stars in such a galaxy are virtually unaffected, but in extreme cases, the gas that is contained in the galaxy can be driven out, streaming behind the galaxy. The result is a galaxy that looks similar to a jellyfish: a body (made up of the galaxy's stars) with tentacles (gas) streaming behind.

We have yet to understand how this works in detail, though: Do such jellyfish galaxies form only in the most massive clusters, or can they form even around our own Milky Way? Where and how quickly do the tails form and how long do they last? What happens to the gas in these tails? How does the stripping process affect the galaxies themselves?

Computer simulations to the rescue 

Since the processes in question occur over hundreds of millions or even billions of years, it is impossible for us to observe them happening in the Universe in real time. But we can turn to computer simulations to find out more! Cosmological simulations create a virtual universe following the same laws of physics as our own cosmos. In that model universe, virtual stars and galaxies form, interact, and evolve – and for each galaxy, one can reconstruct its history! 

A key problem here are the hugely disparate scales. The physics of how stars evolve takes place on scales of thousands of kilometers. A half-way representative volume of cosmic space is hundreds of millions of light-years across, a factor of one quintillion (one with 18 zeros) larger! No computer simulation has yet managed to simulate individual stars in such a cosmological volume. But for a few years now, there have been simulations that manage to model galaxies in sufficient detail for the simulation to capture ram-pressure in clusters, and the way it can turn galaxies into jellyfish galaxies.  

Tracking jellyfish in IllustrisTNG

The first simulations that have managed to capture jellyfish creation are part of a suite called IllustrisTNG. There are three different versions of the IllustrisTNG simulation, each with a different size of the cosmic volume, a different resolution, and containing thousands to hundreds of thousands of galaxies. The two higher-resolution versions of the simulation, known as TNG50 and TNG100, are sufficiently detailed to allow for the formation of jellyfish galaxies.

But in order to study those simulated jellyfish galaxies, the researchers need to determine which of the tens of thousands of galaxies in their virtual universe are jellyfish galaxies in the first place! This requires a process that is still very difficult for computers to do automatically – but comparatively simple for human brains, with their excellent pattern recognition skills. That is why, as a first step, the researchers set out to learn which of their simulated galaxies look like jellyfish to a human observer, with a body made of stars trailing a tail made of gas.

Crowdsourcing jellyfish-galaxy identification

In a pilot study, led by Kiyun Yun, one of the group's PhD students, the team members themselves identified by eye 800 jellyfish galaxies among 2600 pre-selected candidates. But that is only a fraction of the available data – and looking at all the data in this fashion is more than a small team of scientists can handle. 

This is where the Zooniverse comes in: the world’s largest and most popular platform for people-powered research, which specializes in exactly this kind of citizen science: projects where human volunteers and their pattern-recognition-savvy brains can contribute to cutting-edge scientific research. Parsing through 38,000 images in search of rare galaxies is a considerable task, but not that difficult if thousands of volunteers take it on.

Building on work by Yun and another group member Elad Zinger, now at the Hebrew University of Jerusalem, post-doctoral researcher Gandhali Joshi transformed the problem of jellyfish galaxy identification into the Zooniverse project that is now being revealed: Cosmological Jellyfish.

Kickstarting jellyfish galaxy research

In the project, participants study pictures, each of which shows a galaxy in the middle of the image. Each picture was created from the TNG50 and TNG100 simulations, and shows a particular galaxy viewed from a random angle, along with any other gas and galaxies contained in that region Participants then need to decide: Does that particular galaxy look like a jellyfish or not?

While the project provides a tutorial, as well as classification feedback for some of the images, nature is messy – even faithfully simulated nature. There will always be cases where it is difficult to decide whether or not a specific galaxy resembles a jellyfish. But in the end, it's OK to be uncertain: During the project, each galaxy will be classified by at least twenty different participants. In the end, researchers will be able to distinguish galaxies that clearly are, or are not, jellyfish galaxies from more ambiguous specimens (where some participants saw jellyfish, others not).

Once the jellyfish galaxies are identified, the researchers know which galaxies in their simulated universe they will need to look at more closely. The simulation provides the complete formation history for each galaxy, so at that stage, the scientists should be able to find out how these galaxies were formed, how they evolved to look like jellyfish in the first place – and what went differently for the galaxies that do not look like jellyfish! 


The Cosmological Jellyfish project is available in English, in German and in Hebrew at

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