Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely
  Star-Forming Galaxies
  We investigate a class of rapidly growing emission line galaxies, known as
"Green Peas", first noted by volunteers in the Galaxy Zoo project because of
their peculiar bright green colour and small size, unresolved in SDSS imaging.
Their appearance is due to very strong optical emission lines, namely [O III]
5007 A, with an unusually large equivalent width of up to ~1000 A. We discuss a
well-defined sample of 251 colour-selected objects, most of which are strongly
star forming, although there are some AGN interlopers including 8 newly
discovered narrow Line Seyfert 1 galaxies. The star-forming Peas are low mass
galaxies (M~10^8.5 - 10^10 M_sun) with high star formation rates (~10
M_sun/yr), low metallicities (log[O/H] + 12 ~ 8.7) and low reddening (E(B-V) <
0.25) and they reside in low density environments. They have some of the
highest specific star formation rates (up to ~10^{-8} yr^{-1}) seen in the
local Universe, yielding doubling times for their stellar mass of hundreds of
Myrs. The few star-forming Peas with HST imaging appear to have several clumps
of bright star-forming regions and low surface density features that may
indicate recent or ongoing mergers. 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. Studying starbursting galaxies as a function of redshift is essential
to understanding the build up of stellar mass in the Universe.
