Green Pea galaxies: a part of diet for cosmic reionization

Six of the Green Pea galaxies found in galaxy survey data by citizen scientists. These galaxies may resemble forms in the early Universe, which produced the radiation that ionized much of the gas in the cosmos.

Anne Jaskot

A major triumph of citizen science was the identification of the Green Pea galaxies by people involved with the GalaxyZoo project. Named for their unusual green color and small size, these strange objects were unknown until 2007. That's when the regular people who were helping out scientists by looking at galaxy photos started discussing the Green Peas in the project's forum.

Since then, scientists have discovered that the Green Peas are aggressive star-forming galaxies, which means they are a source of high-energy ultraviolet radiation. Unusual as they are, Green Pea galaxies could help resolve the problem of reionization of the Universe, the gradual stripping of electrons from atoms that made the cosmos transparent some 380,000 years after the Big Bang. Ultraviolet radiation is thought to have driven the reionization, but most star-forming galaxies reabsorb too much of that light to be useful for studying the process.

A new paper by A. E. Jaskot and M. S. Oey argues that galaxies much like the Green Peas could be responsible for the reionizing radiation. They analyzed the light emissions from the galaxies, and determined that their gas is thinner than in typical star-forming galaxies, which could allow more ultraviolet light into intergalactic space. The researchers also found signs in a few Green Peas of extremely massive stars, the ones most responsible for ionizing radiation.