Image 1 - Astronomers Confirm Collision Rate Of Galaxies
October 27, 2011

Astronomers Confirm Collision Rate Of Galaxies

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Astronomers have confirmed that the collision rate of galaxies over the last 8 to 9 billion years falls between the previous estimates.

NASA said the galaxy merger rate is one of the fundamental measures of galaxy evolution.  However, there has been a discrepancy in the scientific community about how often galaxies collided in the past.

The study analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time.

The team found mergers between dwarf and massive galaxies in the distant universe happen three times more often than encounters between two hefty galaxies.

“Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies,”  Jennifer Lotz of the Space Telescope Science Institute and study leader, said in a press release.

Astronomers used different methods to count the mergers in previous Hubble estimates.

“These different techniques probe mergers at different ℠snapshots´ in time along the merger process,” Lotz says. “It is a little bit like trying to count car crashes by taking snapshots. If you look for cars on a collision course, you will only see a few of them."

"If you count up the number of wrecked cars you see afterwards, you will see many more. Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers than those that searched for galaxies with disturbed shapes, evidence that they´re in smashups.”

Lotz and her team used detailed computer simulations to make sense of the Hubble photographs.  The team made simulations of the many possible galaxy collision scenarios and then mapped them to Hubble images of galaxy interactions.

The researchers also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other.

“Our simulations offer a realistic picture of mergers between galaxies,” Lotz said in a press release.

The researchers also analyzed encounters among small galaxies, which can be a difficult task because the objects are so dim relative to their companions.

“When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results,” Lotz said in a statement.

She said her next goal is to analyze galaxies that were interacting about 11 billion years ago. 

The study was published in The Astrophysical Journal.


Image 1: Merging Galaxies -- 2.4 Billion Light-Years from Earth. Credit: NASA, ESA, and J. Lotz (STScI)

Image 2: Merging Galaxies in the Extended Groth Strip. Credit: NASA, ESA, J. Lotz (STScI), M. Davis (University of California, Berkeley), and A. Koekemoer (STScI)


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