They did this by measuring the acceleration of clouds orbiting on the outer edges of a galaxy, which enabled them to calculate the mass required to cause that acceleration. (science.howstuffworks.com)
Specifically, Yale astronomer Jeffrey Kenney looked at the way the cosmic wind is eroding the gas and dust at the leading edge of the galaxy. (sciencedaily.com)
Unlike previous SDSS surveys, they are not only mapping the centers of galaxies where supermassive black holes live, but the outer edges of the galaxies as well, which allowed them to discover the red geyser galaxy. (sciencedaily.com)