Gems of the Galaxy Zoos

Lore & Background

The Zoogems project originated from the Galaxy Zoo (GZ) crowdsourced astronomy project, which began in Summer 2007 and invited people to classify galaxies morphologically. Radio Galaxy Zoo started in December 2013, seeking to locate supermassive black holes and associate them with their host galaxies. Through public analysis of more than 900,000 objects, volunteers collected a 'menagerie of weird and wonderful galaxies.' The original proposal estimated 1100 targets were available, but only 300 observation slots existed, so the public voted in February 2018 to select the final list.

Observations used the Wide-Field Camera mode of the Advanced Camera for Surveys for its larger field of view. Total exposure time was 674 seconds, made of two 337-second exposures. The filter used depended on the target: F475W for mostly spiral structures, F814W for bulges, and F625W matched to SDSS r. Software calculated where the target's image would fall on the available ACS CCDs, using a coordinate offset within a 'circle of interest.' A different strategy for Green Pea systems used four filters allotted by distance.

Among the 300 Zoogems, 74 candidates were Pea galaxies. The first Zoogems study, published in May 2021, examined nine of these and found that while central star-forming clusters were up to 500 million years old, older stars possibly from the host galaxy were more than 1 billion years old. This mix of old and new stars implies Pea galaxies are not real analogues of the galaxies responsible for the Epoch of Reionisation.

Reader's Guide

The Zoogems project addressed a range of studies, which project lead Dr. William Keel noted happens rarely with galaxies. By using HST data, objects that would not normally merit an individual project were combined into an interesting study. The first study on Green Pea galaxies, published in May 2021, concluded that the substantial presence of old stars would not have been possible at the earliest stages of the first galaxies, and that the mix of old and new stars could influence galactic winds and element retention. A second study, published in December 2022, analyzed double-lobed radio-loud AGNs and found 18 spiral galaxies likely hosting such structures, concluding that galaxy morphology can no longer be a unique signpost of a galaxy's ability to generate large-scale radio jets. The project's legacy includes demonstrating the value of using small gaps in the HST schedule—as Tom Brown from the Space Telescope Science Institute said, 'It just seemed like a waste to be throwing that time on the floor'—and highlighting unusual objects such as overlapping galaxy pairs, which Samantha Brunker described as special: 'If you’re going to paint a whole picture, you can’t leave out the weird things.'

Did You Know?

• The Zoogems project used gaps in the HST schedule lasting approximately 12 to 25 minutes.
• Public voting for the final 300 targets took place in February 2018.
• The first Zoogems study, published in May 2021, focused on nine Green Pea galaxies.
• The project imaged 193 of the 300 candidates by the end of September 2023.

The Gap-Filler Innovation

The Zoogems project represents a creative solution to a practical scheduling problem at the Hubble Space Telescope. Between its primary observations, short windows of roughly 12 to 25 minutes open up where objects within the current field of view can still be captured. Rather than letting those minutes go unused, the team developed a system in which software automatically checks a pre-compiled list of candidates and selects whichever target sits closest to the telescope's current pointing. The imaging itself relies on the Wide-Field Camera mode of the Advanced Camera for Surveys, chosen for its broader field-of-view. Each observation consists of a pair of 337-second exposures totaling 674 seconds. Filter selection is target-dependent: a bluer F475W band for spiral structures, F814W for bulges, or F625W matched closely to the SDSS r band. For Green Pea systems, a distinct four-filter strategy calibrated by distance values is employed to trace continuum structure. Tom Brown of the Space Telescope Science Institute captured the philosophy behind the effort, noting that discarding even a handful of minutes felt like a waste, and that those scattered seconds add up meaningfully over time.

Crowdsourced Discovery and the Volunteer Legacy

The entire Zoogems endeavor traces its roots to the tireless work of volunteer citizen scientists. Galaxy Zoo, an ongoing crowdsourced classification project, invited the public to sort through the morphological types of vast galaxy populations. Radio Galaxy Zoo, launched in December 2013, focused specifically on locating supermassive black holes and their associated jet structures. Together, these two initiatives saw volunteers analyze more than 900,000 objects, assembling what the team described as a "menagerie of weird and wonderful galaxies," many never before encountered. The original proposal identified roughly 1,100 candidate targets, but only 300 observation slots existed. To resolve the shortfall, the public was invited to vote in February 2018 on which objects deserved priority, a decision driven by the proposal's 28 February deadline. Project lead Dr. William Keel emphasized that after volunteers sifted through images of a million galaxies, the oddities and rarities they surfaced collectively formed a study that no single object would have warranted on its own.

Green Pea Galaxies and the Reionization Question

Seventy-four of the 300 Zoogems candidates are Green Pea galaxies, a population of particular interest because they are the only known class of galaxies where hydrogen-ionizing radiation escapes in substantial quantities. This property has led astronomers to treat them as modern analogs of the distant galaxies thought to have driven the Epoch of Reionisation. However, the first Zoogems paper, published in May 2021 by Leonardo Clarke and colleagues, complicated that picture. Examining nine Pea galaxies, the team found that while central star-forming clusters were up to 500 million years old, a significant population of older stars, likely belonging to the host galaxy, exceeded one billion years in age. Such ancient stellar content would have been impossible during the universe's earliest epochs. The coexistence of old and young stars within these compact systems could alter gravitational conditions, influencing galactic winds and how elements are retained. The authors concluded that Green Peas, far from being faithful replicas of reionization-era galaxies, represent a more complex evolutionary state.

Double-Lobed Radio-Loud AGNs and the Morphology Challenge

A long-standing assumption in astrophysics held that radio-loud Active Galactic Nuclei, those powering enormous double-lobed jet structures, reside almost exclusively in elliptical or early-type galaxies. A December 2022 paper in the Astrophysical Journal, drawing on Zoogems imagery, challenged that orthodoxy. The team examined a sample of radio galaxies with extended double-lobed structures, using the high-resolution Hubble data to determine whether the optical counterparts were genuinely disk-like spirals or merely chance alignments. Applying probability statistics to rule out random superpositions or undetected faint hosts, the researchers divided their initial 32 candidates into high-confidence and low-confidence groups. Eighteen galaxies earned the high-confidence label as genuine spiral hosts of radio-loud AGNs, with a further 14 placed in the lower-confidence tier. The visibility of clear disk structures in the Zoogems images led the authors to a striking conclusion: galaxy morphology can no longer serve as a reliable indicator of a galaxy's capacity to launch large-scale radio jets.