Fishing for Jellyfish Galaxies

About this project

We need your help to catch elusive jellyfish galaxies in astronomical data! In this project, we have a sample of 10,000 images from the 9th data release of DECaLS (The Dark Energy Camera Legacy Survey) of bright spiral galaxies in clusters. We need your help identifying those which have been disturbed by extreme interactions with their surrounding environment.

What are jellyfish galaxies?

While most galaxies in the universe are isolated or live in small groups, many galaxies are found in galaxy clusters, which are large collections of galaxies, hot gas and dark matter, and are some of the largest gravitationally bound structures in the universe. The pull of gravity not only holds a galaxy cluster together, but can also draw in other galaxies from nearby. When a galaxy falls into a cluster, the cluster's cloud of hot gas (known as the intergalactic medium) creates a drag force, which can effectively strip the incoming galaxy's gas, in a process known as ram-pressure stripping.

Jellyfish galaxies are the most extreme examples of ram-pressure stripping at play. They are fast-moving galaxies which blast through the centres of galaxy clusters at high speed, causing material to be dislodged and removed, sometimes producing spectacular tails of material which trail behind the galaxy.

In the above example the incoming galaxy, falling along the direction of the blue arrow, collides with the cluster gas (shown in purple), and experiences a drag force (red arrows), causing the galaxy's own gas to be stripped out into a tail trailing behind it. Eventually most of the galaxy's gas will be removed and it will become part of the cluster.

Jellyfish galaxies are fairly rare and the known sample is still in the hundreds, but with large surveys of astronomical data available, many of them could already have been observed, and are just waiting to be identified! We need your help in identifying and classifying potential jellyfish galaxies from such surveys. With enough volunteers, we hope to nearly double the sample of known jellyfish galaxies.

Each galaxy we observe acts as a snapshot of the process at a moment in cosmic time. As we build the sample of known jellyfish galaxies, we can piece together a timeline of the process and get a much better understanding of how galaxies are affected, from the moment they enter a cluster to the point at which they have become fully stripped of their gas.

The unique and complex shapes of these objects make them challenging to detect using a computer, but the human eye is a fantastic tool for distinguishing signs of disturbances and trailing material. With your help, we'll be able to assemble an extensive and varied catalog of these galaxies to aid our understanding the of complex processes which produce and shape these fascinating objects. Not only that, but you can help us to further characterise and study potentially interesting galaxies by answering a few questions on the galaxy and the nature of its disturbances.

The Data

The DECaLS data (survey website) consist of images from DECam (The Dark Energy Camera) mounted at the Cerro Tololo observatory in Chile. This instrument gives very sharp, clearly resolved images of galaxies which are perfect for classifying visual disturbances such as those found in jellyfish galaxies.

From the DECaLS catalog, we have selected spiral galaxies which are close in projection to galaxy cluster centres, removing any with low-quality images or those which are too small to see their shape clearly. The motivation of this selection is to maximise the likelihood of encountering jellyfish galaxies, since clusters provide an ideal environment for ram-pressure stripping to occur, and spiral galaxies typically have large amounts of gas to form tails. From this reduced, cleaned dataset of galaxies, we have taken the brightest 10,000 objects to produce the sample for this project.

Jellyfish Galaxies in The Zooniverse

We hope to complement the great work already done by volunteers on the Cosmological Jellyfish Zooniverse project: https://www.zooniverse.org/projects/apillepich/cosmological-jellyfish
Which similarly targeted jellyfish galaxies, but using simulations from IllustrisTNG instead of observed data. Combining observations and simulations greatly improves our knowledge of the formation and processes acting in and around these fascinating objects.