Almost 55 percent of the star formation arises from minor mergers between galaxies, according to an astronomer who analyzed data collected as part of the Sloan Digital Sky Survey (SDSS).
A Hubble image 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. Image credit: NASA / ESA / A. Gal-Yam, Weizmann Institute of Science.
Astronomers have often assumed that mergers between equal mass galaxies (‘major’ mergers) have the most transformative impact on galaxies. However, such events are rare.
Much more common are mergers between massive galaxies and dwarf satellites — ‘minor’ mergers. This is because small galaxies far outnumber their more massive counterparts.
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.
The new study, conducted by Dr. Sugata Kaviraj from the University of Hertfordshire, UK, 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. Just over half of the cosmic star formation budget is directly driven by minor mergers,” Dr. Kaviraj said.
“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.’
Dr. Kaviraj captured high-quality images of spiral galaxies located in the patch of sky dubbed SDSS Stripe 82.
Disruptions to the shapes of these spirals, caused by interactions with their smallest neighbors, pointed to increased star formation.
“We estimate that about 40 percent of the star formation in spiral galaxies in the local Universe is directly triggered by minor mergers. The disturbed spirals also exhibit higher nuclear-accretion rates, implying that minor mergers enhance the growth rate of the central black hole,” the astronomer explained.
“Combining our results with the star formation in early-type galaxies – which is minor-merger-driven and accounts for about 14 percent of the star formation budget – suggests that around half of the star formation activity in the local Universe is triggered by the minor-merger process.”
“Without a deeper comprehension of this key process, our understanding of galaxy evolution will remain incomplete.”
The results were presented June 27 at the National Astronomy Meeting 2016 in Nottingham, UK.
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Sugata Kaviraj. Minor mergers: fundamental but poorly-understand drivers of galaxy evolution. NAM 2016
