Searching for “tidal debris” in DECaLS images

This post was written with the help of Sugata Kaviraj, senior lecturer at the University of Hertfordshire in the UK. Sugata has been a member of the GZ science team for several years and is leading our analysis of tidal debris in the DECaLS images.

One of the biggest changes to the questions we’re asking for the new DECaLS images asks users to help us classify galaxies that are either merging and/or exhibiting “tidal debris”. While mergers have been part of Galaxy Zoo since our very first classifications, tidal debris is looking for something more specific, which the new DECaLS data is particularly suited for.

In astronomy, “tides” are a type of force exerted on an object by the effect of gravity. Specifically, it refers to the gravitational force exerted by one body on another – since the force exerted by gravity depends on the distance to the object (specifically, it’s proportional to the inverse square of the distance), the nearer side of the object will feel a stronger force than the farther side of the object. On Earth, the differential force caused (mostly) by the gravity of our Moon acts on the liquid in the oceans, causing the rise and fall in sea levels each day.

Field tidal.svg
Example of how a solid body can be deformed under tidal forces. (“Field tidal” by Krishnavedala. Licensed under CC BY-SA 3.0 via Commons.)

When the object is a solid body (like a planet or moon), tidal forces can strain and compress the body, resulting in internal heat and sometimes driving geological activity. A galaxy isn’t a solid body, but composed of individual stars/gas clouds/dark matter particles bound within its own gravitational field. When a galaxy is subjected to tidal forces, it pulls the galaxy apart, causing irregularities in shape that can take many forms depending on the magnitude and direction of the forces involved.

One of the main reasons we want to identify tidal features specifically is to make better measurements of the merger history of galaxies. A complication is that the event of merging with a galaxy isn’t an instantaneous event – depending on the relative masses, velocities, and orientations of the merging galaxies, tidal forces strip out long tails of stars and gas from the galactic centers.

The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas
The Mice (NGC 4676): colliding galaxies with tails of stars and gas distorted by tidal forces. Source:

Ultimately, the nuclei of the two galaxies will fully merge; if that happened sufficiently far in the past for the orbits of stars to relax, then it’s difficult for observers to determine if an elliptical galaxy today was the result of a merger.

Tidal debris features, however, are longer-lived signals of a merger sometime in a galaxy’s past. If we only asked about mergers, we’d be restricting the sample to galaxies that we’re lucky enough to observe “right in the act” of merging. By identifying the tidal debris as well, we can make a more complete census of galaxies that had a merger at some point in their past. One project that this is critical for is looking at the history of galaxy populations, and trying to figure out whether star formation and/or active black holes might be powered by merger events.

Since tidal debris features can be very faint (ie, having low surface brightnesses), the deeper DECaLS images that we’re currently classifying are much better at picking out these features than SDSS. That’s the main reason we’re focusing on trying to detect them in the current set of images.

Tidal debris can come in many different forms, including extended light, faint shells, dust lanes, or satellites in the process of being assimilated to clearly distorted galaxies which have presumably have had a recent interaction. The features can definitely be faint, but we’d like to ask that when you reach this question, please take a careful second look at the area around the main galaxy and see if you can spot anything. Examples are also available in the help text, and we hope that you’ll discuss features you’re not certain about with the moderators, scientists, and other volunteers in Talk.

Example 2. Faint tidal debris extension is visible to the upper left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.

Example 1. Faint tidal debris extension is visible to the upper left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.

Example 3. Faint tidal debris is visible around the main galaxy in a shell-like shape. Image courtesy DECaLS/Galaxy Zoo.

Example 2. Faint tidal debris is visible around the main galaxy in a shell-like shape (see arrows). Image courtesy DECaLS/Galaxy Zoo.

Example 4. Faint tidal debris extension is visible to the left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.

Example 3. Faint tidal debris extension is visible to the left of the main galaxy (within red ellipse). Image courtesy DECaLS/Galaxy Zoo.


Example 4. A small satellite galaxy is merging into the larger one at the center, being ripped apart and causing tidal debris. Image courtesy DECaLS/Galaxy Zoo.


Example 5. Tidal debris (within the red outline) extending to the top and left of the main galaxy. Image courtesy DECaLS/Galaxy Zoo.


Example 6. Faint tidal debris in a “shell” structure, extending in several directions from the main galaxy (comparable to Example 3). Image courtesy DECaLS/Galaxy Zoo.

Please let us know, here or on Talk, what questions you have. Thanks for your help in tackling a new and interesting scientific problem.

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About Kyle Willett

Kyle Willett is a postdoc and astronomer at the University of Minnesota. He works as a member of the Galaxy Zoo team, and gets to study galaxy morphology and evolution, AGN, blazars, megamasers, citizen science engagement, and many other cool things.

5 responses to “Searching for “tidal debris” in DECaLS images”

  1. Jean Tate says :

    Great post! Great topic!

    I do have some questions, which I’ve posted in GZ Talk, in the DECaLS – new images and science thread (link:

  2. Robert Maher says :

    Have logged on to Galaxy Zoo as I do every day but have not seen any images where we can identify tidal debris. When will they come on line?
    Robert Maher

    • Jean Tate says :

      They are DECaLS images, the ones for which the classification tree includes “tidal debris” as an option … it’s in the “Mergers” set, along with “Merging”, “Both”, and “Neither”. I don’t know the full tree, but you get this set after the degree of roundness for ‘smooth’ objects, and after all the ‘spiral’ sets for ‘features’; just before “Odd”. Hope this helps, and happy hunting! 🙂

  3. Ayesha (in)saini says :

    I have been patiently waiting for this blog on Tidal debris! Thank you. I have to say that from the pictures and guides offered while classifying(when asked’ is there Tidal debris) my understanding was completely different(wrong in fact) than what it is now after reading the blog. I feel my eyes are better equipped now to classify and give more accurate analysis. Gracias.

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