Overlap Zoo

Why is classifying occulting galaxy pairs important?

Occulting pairs of galaxies, in which a foreground galaxy partially overlaps a background galaxy, are becoming increasingly valuable for studying dust attenuation and other dust properties in galaxies. These occulting systems present a unique opportunity to derive attenuation measurements by modelling the light from both galaxies and inferring the missing light in the overlapping regions, leaving only the light attenuated by dust remaining. This approach, first introduced by White & Keel 1992, demonstrated practical use in many areas of photometry. However, despite the success of this technique, we are still limited to a relatively small number of occulting pairs to which we can apply it and derive attenuation and other measurements. This is where Overlap Zoo comes in!

How did the project start?

This project began as an undergraduate research project inspired by the work of Keel 2013. It started as a private internal beta test with a small research group and some additional volunteers, but it has now become a full-scale citizen science project! If you want to see the results of the internal beta test, please see Butrum 2026.

When will new pairs be added?

Adding new pairs to our subject set will take some time. We rely heavily on classifications from the Galaxy Zoo project, so additional pairs can only be added once new overlaps are identified there.

Why are classifications started from scratch? Isn't your sample from Galaxy Zoo?

In the current iteration of the project (as of March 16, 2026), we are including data from Keel 2013, which was classified using SDSS. However, our project uses a survey with greater depth and higher image quality than SDSS, so we decided to redo the classifications from scratch. In the future, with data from missions such as Roman and Euclid, we plan to structure the project so that only the background galaxy is classified.