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Concept
Pea galaxy
Summary
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.
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.
Official source
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