Galaxy Zoo Discovers New Group of Galaxies: ‘Green Peas’


Citizen scientists from the Galaxy Zoo project have discovered rare galaxies they’re calling the “Green Peas.” They’re small in size, bright green in color, and proficient at churning out new stars — plus, they could reveal unique insights into how galaxies form stars in the early universe.

The newly discovered galaxies appear in the image at left, from Carolin Cardamone and the Sloan Digital Sky Survey.

“These are among the most extremely active star-forming galaxies we’ve ever found,” said Cardamone, an astronomy graduate student at Yale University and lead author of a new paper on the discovery. The results will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Galaxy Zoo users volunteer their spare time to help classify galaxies in an online image bank. Cardamone said of the one million galaxies that make up Galaxy Zoo’s image bank, the team found only 250 Green Peas.

“No one person could have done this on their own,” she said. “Even if we had managed to look through 10,000 of these images, we would have only come across a few Green Peas and wouldn’t have recognized them as a unique class of galaxies.”

The Green Peas boast “some of the highest specific star formation rates seen in the local Universe,” write Cardamone and her co-authors, “yielding doubling times for their stellar mass of hundreds of millions of years.”

The authors say evidence points to recent or ongoing mergers, adding that the Peas are similar in size, mass, luminosity and metallicity to Luminous Blue Compact Galaxies.

“They are also similar to high redshift UV-luminous galaxies, e.g., Lyman-break galaxies and Lyman-alpha emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies,” they write.

The galaxies, which are between 1.5 billion and 5 billion light years away, are 10 times smaller than our own Milky Way galaxy and 100 times less massive. But they are forming stars 10 times faster than the Milky Way.

Kevin Schawinski, a postdoctoral associate at Yale and one of Galaxy Zoo’s founders, said the Green Peas would have been normal in the early universe, “but we just don’t see such active galaxies today. Understanding the Green Peas may tell us something about how stars were formed in the early universe and how galaxies evolve.”

The Galaxy Zoo volunteers who discovered the Green Peas—and who call themselves the “Peas Corps” and the “Peas Brigade”—began discussing the strange objects in the online forum. (The original forum thread was called “Give peas a chance.”)

Cardamone asked the volunteers, many of whom had no previous astronomy background or experience, to refine the sample of objects they detected in order to determine which were bona fide Green Peas and which were not, based on their colors. By analyzing their light, Cardamone determined how much star formation is taking place within the galaxies.

“This is a genuine citizen science project, where the users were directly involved in the analysis,” Schawinski said, adding that 10 Galaxy Zoo volunteers are acknowledged in the paper as having made a particularly significant contribution. “It’s a great example of how a new way of doing science produced a result that wouldn’t have been possible otherwise.”

Source: Yale University, via the American Astronomical Society press wire. The paper is here, and here is a link to the Galaxy Zoo project.

7 Replies to “Galaxy Zoo Discovers New Group of Galaxies: ‘Green Peas’”

  1. Kevin Schawinski, a postdoctoral associate at Yale and one of Galaxy Zoo’s founders, said the Green Peas would have been normal in the early universe

    I learned something new today!

  2. Galaxy Zoo is an outstanding example of Citizen Science … real contributions to contemporary science by active amateurs. In this case, the amateurs worked with data served to them via the internet, and collaborated online via an internet discussion forum.

  3. Great to see this paper published just to check out the details of what GZers had come across. I’ve logged my share of time working with Galaxy Zoo, but not specifically with the Green Pea teams.The paper notes that these galaxies might appear as Luminous Blue Compact Galaxies (LBCGs) in our local universe. A deep HI radio survey is needed to see if sufficient fuel is available to sustain star formation, given the estimated SFR of ~10 solar masses per year. Congrats to all involved with GZ 🙂

  4. 😀

    Luminous Blue Dompact Galaxies turn green if they are red-shifted.


    I think it’s funny.

  5. Yeees, it should read Compact……

    Btw: If those “green peas” really have similar proporties as LBCGs, then this is another hint to cosmological redshift. That’s great!

  6. The paper mentions that the reason for the strong green appearance (in SDSS gri color composites) is the very prominent OIII emission in these objects (the redshifted OIII line is mapped as green for the red part of the ‘gri’ images presented in Galaxy Zoo). Spectral analysis of this line indicates a velocity dispersion on the order of 1000 km/s!, another indicator of vigorous star formation in these galaxies. These objects also resemble nearby cousins of more distant Lyman-alpha emitters (LAEs) and Lyman-Break Galaxies (LBGs), offering insights into these distant star forming galaxies. Hopefully the refurbished Hubble along with Herschel may provide answers (and probably more questions) about the physical state and history of this rare subclass of galaxies. Congrats to all at GZ.

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