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 ] \lambda 5007 Ã , with an unusually large equivalent width of up to \sim 1000 Ã .
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 \sim 10 ^ { 8.5 } -10 ^ { 10 } { M _ { \odot } } ) with high star formation rates ( \sim 10 { M _ { \odot } yr ^ { -1 } } ) , low metallicities ( log [ O/H ] + 12 \sim 8.7 ) and low reddening ( { E ( B - V ) \leq 0.25 } ) and they reside in low density environments .
They have some of the highest specific star formation rates ( up to \sim 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 .