The Adler Planetarium
University of Minnesota
University of Oxford
The Adler Planetarium
University of Minnesota
University of Oxford
From the organization:NASA Zooniverse ProjectsGalaxy Zoo
Project avatar for Overlap Zoo

Overlap Zoo

Welcome to Overlap Zoo! Please read the tutorial carefully before you begin classifying!

Research

What are overlapping galaxies?

Overlapping galaxies are pairs of galaxies that happen to overlap along our line of sight in the night sky. Although the two galaxies appear close together, these objects can actually be separated by millions of light-years. In the past few decades, this unique arrangement has proven to be a powerful tool for studying dust (tiny particles of silicates and carbon) in galaxies!

In these systems, the background galaxy essentially acts like a large cosmic flashlight. Its light shines through the overlapping region, where dust in the foreground galaxy absorbs and scatters some of that light. Astronomers can compare this light that is blocked in the foreground galaxy with the light that passes through, giving us a direct way to measure important properties of the dust within.

In the image below, you can see how the background galaxy illuminates the dust outer regions of the foreground spiral galaxy! This is one of the best examples of an overlapping pair we have to date. Hopefully, through this project, we can find even more of these absolutely beautiful systems and use them to better understand dust in galaxies.

VV191, an example of an overlapping galaxy system. Image Credits: NASA/JWST/Hubble

What are the different types of overlapping galaxies?

Overlapping galaxies come in many shapes and sizes! Since there are many types of galaxies in the Universe, we can also expect many different types of overlapping pairs. In the literature, overlapping galaxies are actually divided into more complex categories that capture additional details about their morphology and positioning. However, for the purpose of keeping things simple, we can generalise them into three broad categories:

  • Spiral-Spiral pairs (Featured Pairs)
  • Spiral-Elliptical pairs (Featured and Smooth Pairs)
  • Elliptical-Elliptical pairs (Smooth Pairs)

For our project, we are particularly interested in Spiral-Elliptical systems with a foreground spiral galaxy and background elliptical galaxy. These systems have been found to be the best for dust studies, especially those that are geometrically ideal. Below are some examples of the different types of overlapping galaxies we have found thus far.

As you can see, they are all very different from each other!

Image credits: Keel et al. (2013) / Sloan Digital Sky Survey (SDSS).

Why is studying dust so important? How do overlapping pairs help here?

Interstellar dust is extremely important in astronomical research, as its effects can influence nearly every aspect of our observations of the Universe. Whenever we take an image of a galaxy, some of the starlight we receive from stars in the galaxy is absorbed or scattered by dust throughout the galaxy’s disk. We refer to this loss of observed light as dust attenuation, a term used by astronomers to describe the way dust attenuates, or reduces, the light we observe from galaxies.

To measure properties of galaxies, astronomers need to account for this attenuation. For example, the apparent brightness of supernovae (stars that are exploding) is used to measure the distances of objects in space. If there is dust in front of these supernovae, dimming their light, these objects appear farther away than they actually are. Dust can also affect measurements of other galaxy properties, such as star formation rates and how we model galaxies.

By studying overlapping galaxies, we can use the background galaxy as a natural backlight. This gives us a more direct way to measure how dust in the foreground galaxy attenuates starlight, rather than relying only on the light coming from the galaxy's own stars. With this information, we hope to develop more general ways of correcting for the effects of dust and place better constraints on measurements of galaxy properties.

Where do we find overlapping galaxies and/or get our data?

As you might expect, the apparent overlap of two galaxies is rare, making these systems difficult to find. Automating their identification is also challenging; to our knowledge, no reliable large-scale method exists for detecting overlapping galaxies. As a result, we still depend on chance observations in astronomical surveys and volunteer classifications to discover these systems.

We are therefore extremely grateful to the Galaxy Zoo project for providing identifiers for overlapping galaxies. All overlapping galaxy pairs you see in this project were previously identified or classified by various Galaxy Zoo projects, including Galaxy Zoo 1, Galaxy Zoo 2, and Galaxy Zoo: DECaLS. Without these classifications from Galaxy Zoo volunteers, it would have been much harder to find enough candidate pairs for this project.

How does your classification help the broader community and us?

As we mentioned, an overlap between two galaxies along our line of sight is extremely rare. Among the millions of galaxies observed by astronomical surveys, only a small fraction appear to overlap. So far, we have identified around 3,000 overlapping galaxies, and only a small number of these are the ideal pairs we are looking for.

By participating in our project and classifying the candidate pairs you see in Overlap Zoo, you are helping us to identify potential ideal systems that could be used in future dust studies! With a large enough sample, we hope to begin answering questions such as: How does dust vary across different regions of a galaxy? How does the distribution of dust change from galaxy to galaxy? And how can we eventually use these measurements to develop better corrections for the effects of dust on our observations?

Every classification you make in this project helps us get closer to finding ideal overlapping pairs that may hold the answers to these questions!

Want to learn more?

For those interested in learning more about overlapping galaxies, below are some papers we recommend to learn more about the science behind the project:

Acknowledgements

This project is supported by the images provided by the DESI Legacy Survey as well as the Python module legacystamps publicly provided by Dr Frits Sweijen.

This project is soon to be supported by NASA's Citizen Science Seed Funding Program (CSSFP). For more projects, see: science.nasa.gov/citizenscience. For other ways to get involved, see: https://www.nasa.gov/get-involved/.