Why Different Galaxies Matter

Galaxies come in many familiar forms — elegant spirals, smooth ellipticals, diffuse irregulars — but some are far stranger. These peculiar galaxies are snapshots of galaxies in transition, often showing the scars of powerful forces that reshape them over cosmic time. Mergers can twist stellar disks and ignite starbursts, while the hot gas in clusters can strip galaxies of their fuel, producing spectacular jellyfish galaxies with long, one-sided tails. Other systems show warped disks, asymmetric or unwinding spiral arms, and shells or rings that hint at past interactions.

By studying these “strange” systems, we can uncover the physical processes that drive galaxy evolution — the interplay between gravity, gas, and environment that transforms galaxies from blue, star-forming disks into red, quiescent spheroids.

How You Can Help

Your task is simple but powerful: visually classify galaxies and flag anything that looks unusual.
Does a galaxy show two bright cores or stretched tidal tails?
Is it a spiral whose arms seem to unwind or twist asymmetrically?
Does it have a single bright disk but trails of material on one side, like a jellyfish?

Each classification helps us pinpoint galaxies undergoing transformation — mergers, tidal encounters, ram-pressure stripping, or other processes yet to be understood. Your input will guide astronomers toward the most intriguing systems for deeper study, including follow-up with instruments such as 4MOST and MUSE.

Where the Images Come From

The galaxies you see come from Data Release 10 (DR10) of the DESI Legacy Imaging Surveys, a public imaging data set designed to map a large fraction of the extragalactic sky in optical bands (and matched infrared data). DR10 includes extensive Dark Energy Camera (DECAM) imaging in the southern footprint. DECam is a wide-field imager mounted on the Víctor M. Blanco 4-meter telescope at Cerro Tololo in Chile.

Which DECam/Legacy Survey objects become “subjects” in this project?

We are not drawing random galaxies from the full survey. Instead, this project focuses on a targeted subset: galaxies selected to match the objects planned for spectroscopy by CHANCES (the CHileAN Cluster galaxy Evolution Survey), a survey within the 4MOST program. CHANCES will provide the spectroscopic view; your classifications provide the complementary morphological view.

How We Prepare the Images

1. To make the images uniform and easy to compare, we process them before upload:

2. Create r-band cutouts with a fixed size of 1 arcminute.

3. Use the Galaxy Morphology Extractor (galmex) python package to estimate a galaxy’s Petrosian radius.

4. Build the RGB cutout using a field-of-view set by the galaxy size (we use 8× the Petrosian radius).

5. Rescale images (keeping the same angular scale) so they appear at a consistent size in the Zooniverse interface, using ImageMagick.

Humans and Machine Learning

Machine learning is on our roadmap — but the key ingredient is a reliable training set. Your volunteer classifications will create high-quality labels of galaxy types and rare features (tails, mergers, asymmetries, jellyfish-like morphologies). These labels can then be used to train and validate models that can extrapolate to much larger imaging samples, while the human classifications remain essential for (1) catching truly rare/odd objects, and (2) verifying borderline cases and unexpected morphologies.

Why Your Classifications Matter

Every “strange” galaxy you identify helps us trace the story of change in the Universe. Together, we’ll build a catalogue of peculiar and transforming galaxies in DECam-based Legacy Survey imaging (DR10), tied directly to CHANCES/4MOST spectroscopic targets — a powerful combination for understanding how gravity, gas, and environment sculpt galaxies across cosmic time.