---
title: "Pea galaxy"
chunk: 1/11
source: "https://en.wikipedia.org/wiki/Pea_galaxy"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:15:28.328105+00:00"
instance: "kb-cron"
---

A Pea galaxy, also referred to as a Pea or Green Pea, might be a type of luminous blue compact galaxy that is undergoing very high rates of star formation. Pea galaxies are so-named because of their small size and greenish appearance in the images taken by the Sloan Digital Sky Survey (SDSS).
"Pea" galaxies were first discovered in 2007 by the volunteer citizen scientists within the forum section of the online astronomy project Galaxy Zoo (GZ), part of the Zooniverse web portal.

== Description ==
The Pea galaxies, also known as Green Peas (GPs), are compact oxygen-rich emission line galaxies that were discovered at redshift between z = 0.112 and 0.360.  These low-mass galaxies have an upper size limit generally no bigger than 16,300 light-years (5,000 pc) across, and typically they reside in environments less than two-thirds the density of normal galaxy environments. An average GP has a redshift of z = 0.258, a mass of ~3,200 million M☉ (~3,200 million solar masses), a star formation rate of ~10 M☉/yr (~10 solar masses a year), an [O III] equivalent width of 69.4 nm and a low metallicity. They have a strong emission line at the [OIII] wavelength of 500.7 nm. [OIII], O++ or doubly ionized oxygen, is a forbidden mechanism of the visible spectrum and is only possible at very low density. When the entire photometric SDSS catalogue was searched, 40,222 objects were returned, which leads to the conclusion the GPs are rare objects.
GPs are the least massive and most actively star-forming galaxies in the local universe. "These galaxies would have been normal in the early Universe, but we just don't see such active galaxies today", said astrophysicist Dr. Kevin Schawinski. "Understanding the Green Peas may tell us something about how stars were formed in the early Universe and how galaxies evolve".
GPs exist at a time when the universe was three-quarters of its current age and so are clues as to how galaxy formation and evolution took place in the early universe. With the publication of Amorin's GTC paper in February 2012, it is now thought that GPs might be old galaxies having formed most of their stellar mass several billion years ago. Old stars have been spectroscopically confirmed in one of the three galaxies in the study by the presence of magnesium.

In January 2016, a study was published in the journal Nature identifying J0925+1403 as a Lyman continuum photons (LyC) 'leaker' with an escape fraction of ~8% (see section below). A follow-up study using the same Hubble Space Telescope (HST) data identifies four more LyC leakers, described as GPs.  In 2014–15, two separate sources identified two other GPs to be likely LyC leaking candidates (GP J1219 and GP J0815), suggesting that these two GPs are also low-redshift analogs of high-redshift Lyman-alpha and LyC leakers. Finding local LyC leakers is crucial to theories about the early universe and reionization. (for more details see Izotov et al. 2016)
The image to the right shows Pea galaxy GP_J1219. This was observed in 2014 by a HST team whose principal investigator was Alaina Henry, using the Cosmic Origins Spectrograph and the Near Ultraviolet channel. The scale bar in the image shows 1 arc second (1"), which corresponds to ~10,750 light years at the distance of 2.69 billion light years for GP_J1219. When using the COS Multi-Anode Micro-channel Array, in NUV imaging mode, the detector plate scale is ~40 pixels per arcsecond (0.0235 arcseconds per pixel).
GPs feature significantly within the Zoogems project, which uses HST to examine images of interest from the citizen science websites Galaxy Zoo and Radio Galaxy Zoo, collected since 2007. Among the ~300 possible candidates for the Zoogems observations are 75 GPs. The original GP classifications used SDSS images, which are not as good quality as the HST examples.

== History of discovery ==

=== Years 2007 to 2009 ===
Galaxy Zoo (GZ) is a project online since July 2007 that seeks to classify up to one million galaxies. On July 28, 2007, two days after the start of the Galaxy Zoo Internet forum, citizen scientist 'Nightblizzard' posted two green objects thought to be galaxies. A discussion, or thread, was started on this forum by Hanny Van Arkel (cf. Hanny's Voorwerp) on the 12th of August 2007 called "Give peas a chance" in which various green objects were posted. This thread started humorously, as the name is a word play of the title of the John Lennon song "Give Peace a Chance", but by December 2007, it had become clear that some of these unusual objects were a distinct group of galaxies. These "Pea galaxies" appear in the SDSS as unresolved green images. This is because the Peas have a very bright, or powerful, spectral line in their spectra for highly-ionized oxygen, which in SDSS color composites increases the luminosity, or brightness, of the "r" color band with respect to the two other color bands "g" and "i". The "r" color band shows as green in SDSS images. Enthusiasts, calling themselves the "Peas Corps" (another humorous play on the Peace Corps), collected over a hundred of these Peas, which were eventually placed together into a dedicated discussion thread started by Carolin Cardamone in July 2008. The collection, once refined, provided values that could be used in a systematic computer search of the GZ database of one million objects, which eventually resulted in a sample of 251 Pea galaxies, also known as Green Peas (GPs).