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 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 (~3,200 million solar masses), a star formation rate of /yr (~10 solar masses a year), an [O III] equivalent width of 69.4 nm and a low metallicity. A GP is purely star-forming, rather than having an active galactic nucleus. 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 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 present 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 (J1219 and 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. More details here: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 the Hubble Space Telescope to examine interesting images from the citizen science website 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 HST. These new HST images have produced one study in 2021 which challenges theories that GPs are similar to the first galaxies in the re-ionisation epoch, in that the stellar population in GPs may be older than any early galaxies that might have existed. Quoting from the abstract: "A substantial presence of old stars would indicate that the mechanisms allowing large escape fractions in these local galaxies may be different from those at play during the reionization epoch."