The rate at which galaxies produce new stars is not constant based on the age of the universe, but instead depends considerably on the conditions that the galaxies are in.
While the interaction of galaxies' flows of cold gas plays a role, one of the major drivers of star formation in the universe is actually "minor mergers", according to new research. These minor mergers are responsible for about half of the star formation in the local universe. These findings are being announced at the National Astronomy Meeting at the University of Nottingham this week.
Galaxy mergers are considered to be one of the fundamental steps in a galaxy’s formation and evolution. Mergers occur when two or more galaxies collide. This interaction can radically alter the overall structure of the parent galaxies due to both the tidal forces between the galaxies and the friction between the gas and dust that gets moved around during the process.
Minor mergers occur when one of the parent galaxies is at least three times larger than the other. In this scenario, the bigger one will often "eat" the smaller one, absorbing most of its gas and stars, and this combined with gravitational interaction is what triggers more star formation.
“The results are striking,” said lead author Dr. Sugata Kaviraj of the University of Hertfordshire in a statement. “Just over half of the cosmic star formation budget is directly driven by minor mergers. In other words, if this process did not take place then galaxies in today’s universe would be at least a factor of two less massive.”
Minor mergers don’t disrupt the bigger galaxies, so spotting these collisions requires high-quality imaging. The current data comes from the Sloan Digital Sky Survey, specifically from a patch of sky called Stripe 82 that has been observed several times with many different telescopes.
Future surveys like the Large Synoptic Survey Telescope will help deliver these high-quality images over a larger area of the sky in order to fully understand the role of minor mergers over cosmic time.