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Minor mergers are major drivers of star formation

Last Updated on Tuesday, 28 June 2016 13:16
Published on Tuesday, 28 June 2016 08:15

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 ofspiral 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.

NGC4417A 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.Gravity, the ubiquitous attractive force that pervades our Universe, 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.
Our classical paradigm has often assumed that mergers between equal mass progenitors (‘major’ mergers) have the most transformative impact on galaxies. However, such events 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 – the attractive nature of gravity then ensures that these massive galaxies are constantly being bombarded by satellites.
While major mergers are more spectacular and easier to study because they tend to be brighter, 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.
Recently, circumstantial evidence is accumulating that suggests that minor mergers are indeed important drivers of galaxy evolution e.g. the observed size growth of galaxies over the last 10-12 billion years is likely due to repeated minor mergers. This study is the first to use a deep survey to quantify what fraction of the star formation in the nearby Universe is likely to be driven by the minor-merger process. “The results are striking”, according to Dr Sugata Kaviraj of the University of Hertfordshire, the scientist behind this work. “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.”
Without a good comprehension of the minor-merger process, therefore, our understanding of galaxy evolution will remain incomplete. This paper is a precursor to work that can be done using future instrumentation like the Large Synoptic Survey Telescope which will, for the first time, provide deep imaging over around half the sky, enabling the first statistically robust studies of minor merging over at least 50% of cosmic time.
Media contacts
Dr Robert Massey
Royal Astronomical Society
Mob: +44 (0)794 124 8035
Ms Anita Heward
Royal Astronomical Society
Mob: +44 (0)7756 034 243
Dr Sam Lindsay
Royal Astronomical Society
Mob: +44 (0)7957 566 861
Science contacts
Dr Sugata Kaviraj
University of Hertfordshire
Images and captions
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.
Notes for editors
The RAS National Astronomy Meeting 2016 (NAM 2016, takes place this year at the University of Nottingham from 27 June to 1 July. NAM 2016 brings together more than 550 space scientists and astronomers to discuss the latest research in their respective fields. The conference is principally sponsored by the Royal Astronomical Society and the Science and Technology Facilities Council. Follow the conference on Twitter via @rasnam2016
The Royal Astronomical Society (RAS,, founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @royalastrosoc
The Science and Technology Facilities Council (STFC, is keeping the UK at the forefront of international science and has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. STFC's Astronomy and Space Science programme provides support for a wide range of facilities, research groups and individuals in order to investigate some of the highest priority questions in astrophysics, cosmology and solar system science. STFC's astronomy and space science programme is delivered through grant funding for research activities, and also through support of technical activities at STFC's UK Astronomy Technology Centre and RAL Space at the Rutherford Appleton Laboratory. STFC also supports UK astronomy through the international European Southern Observatory. Follow STFC on Twitter via @stfc_matters