The Green Peas: An Introduction

A few years ago, a  citizen science project called the Galaxy Zoo found some really neat galaxies.  While the group of volunteers were busy classifying the morphological (shape) properties of galaxies in the Sloan Digital Sky Survey (SDSS), they discovered these round, green blobs, that look like this:

SDSS images of four Green Pea galaxies.

The Galaxy Zoo named these objects Green Peas, and professional astronomers took a closer look.  What they discovered were some pretty extreme star-forming galaxies.   In short, the emission from ionized gas is extraordinarily strong.    Normally, the light in galaxy images is dominated by the continuum emission from stars.    But in these cases, the discrete emission lines from ionized oxygen and hydrogen are so strong that they dominate the light from stars.    This means that, on one hand, there must be quite a lot of ongoing star-formation, because young hot stars are needed to ionize the gas.  And on the other hand, there must not be much stellar mass, since the ionized gas is able to dominate the stellar continuum.  To the best of our understanding, these galaxies do not contain Active Galactic Nuclei (supermassive black hole centers).

Green Peas are extremely rare in our local Universe.  Out of the more than 800,000 galaxies covered by SDSS spectroscopy, only 80 were found.  But that does not mean that they are unimportant.   As it turns out, they have many similarities to galaxies that are much, much more distant.   They may even be local analogs to the most distant galaxies ever observed.   Since they are much closer than these high-redshift record holders, there is a lot that we can learn about them.   This is where my research comes into focus:  I am using Green Peas as local laboratories for high-redshift astrophysics.

In upcoming posts I’m going to talk a lot about spectroscopy.  These type of observations are my primary tool, because they can really help us to learn a lot more about galaxies than just looking at images.   For basic background information on light and spectroscopy, check out this article by @astropixie Amanda Bauer.