Minor Mergers are Major Drivers of Star Formation

A NASA/ESA Hubble Space Telescope view of the spiral galaxy NGC 7714, which has been dramatically distorted in shape by a close interaction with another nearby galaxy. Minor, but frequent, disturbances such as this cause a burst of star formation, accounting for around half of all new stars being formed in the local Universe. Credit: NASA/ESA

Around half of the star formation in the local Universe arises from minor mergers between galaxies, according to data from the Sloan Digital Sky Survey. The patch of sky called Stripe 82 is observed repeatedly to produce high-quality images of spiral galaxies. Disruptions to the shapes of these galaxies, caused by interactions with their smallest neighbours, pointed to increased star formation in a study being presented at the National Astronomy Meeting at the University of Nottingham.

Gravity is a significant driver of galaxy formation. Gravity makes galaxies collide, and these collisions can affect various properties – merging drives strong star formation in the galaxies in question, increases the masses of their constituent black holes and can significantly alter the internal structure of the galaxies.

Mergers between equal mass progenitors (‘major’ mergers) have the most transformative impact on galaxies but are rare. Much more common are mergers between massive galaxies and small satellites (‘minor’ mergers). This is because small galaxies far outnumber their more massive counterparts – gravity ensures that these massive galaxies are constantly being bombarded by satellites. Studying minor mergers requires large surveys which offer ‘deep’ i.e. long exposure imaging which is able to detect the faint tidal features that are the signatures of minor mergers.

Minor mergers are important drivers of galaxy evolution eg. size growth of galaxies over the last 10-12 billion years is likely due to repeated minor mergers. Dr Sugata Kaviraj said: “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 2 less massive.” Future instuments eg Large Synoptic Survey Telescope will provide deep imaging over around half the sky, enabling the first statistically robust studies of minor merging over at least 50% of cosmic time. http://www.alphagalileo.org/ViewItem.aspx?ItemId=165563&CultureCode=en