Galactic Wrecks Far from Earth: These images from NASA's Hubble Space Telescope's ACS in 2004 and 2005 show four examples of interacting galaxies far away from Earth.
The galaxies, beginning at far left, are shown at various stages of the merger process.
The top row displays merging galaxies found in different regions of a large survey known as the AEGIS.
More detailed views are in the bottom row of images. (Credit: NASA; ESA; J. Lotz, STScI; M. Davis, University of California, Berkeley; and A. Koekemoer, STScI)
A new analysis of Hubble surveys, combined with simulations of galaxy interactions, reveals that the merger rate of galaxies over the last 8 billion to 9 billion years falls between the previous estimates.
The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies and yet, a huge discrepancy exists over how often galaxies coalesced in the past.
Measurements of galaxies in deep-field surveys made by NASA’s Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.
The study, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Md., analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time.
Lotz’s team found that galaxies gained quite a bit of mass through collisions with other galaxies.
Large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently.
In one of the first measurements of smashups between dwarf and massive galaxies in the distant universe, Lotz’s team found these mergers happened 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,” Lotz explains.
The team’s results are accepted for publication appeared in The Astrophysical Journal.
The problem with previous Hubble estimates is that astronomers used different methods to count the mergers.
“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.”
To figure out how many encounters happen over time, Lotz needed to understand how long merging galaxies would look like “wrecks” before they settle down and begin to look like normal galaxies again.
That’s why Lotz and her team turned to highly detailed computer simulations to help 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.
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