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Astronomers trace galactic crash, bang, wallop!
Posted: 08 November 2011

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The rate at which galaxies merge to mammoth sizes has been figured out by a team of astronomers led by Jennifer Lotz of the Space Telescope Institute in Baltimore, Maryland, revealing that any given Milky Way-sized galaxy has merged with another of comparable size over the past eight billion years as well as two to three dwarf galaxies over the same time period.

A galaxy merger time sequence mosaic which shows the sequence of events as two spiral galaxies similar to the Milky Way merge. Image: Patrik Jonsson/Greg Novak/Joel Primack/Nina McCurdy.

The merger rate is of a huge importance to understanding galaxy evolution, helping astronomers to understand how these giant structures have fattened up from devouring their galactic encounters. For some time, the rate at which galaxies coalesced in the past has had a question mark over it, but Lotz and her team hope that their study could help solve a combination of mysteries. “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 centres of galaxies,” says Lotz, whose team’s results have been accepted for publication in The Astrophysical Journal.

Earlier measurements made by NASA’s Hubble Space Telescope as part of deep-field surveys have generated a broad range of results revealing that anywhere from 5 percent to 25 percent of galaxies were merging. However, the problem with these estimates is that astronomers used a range of methods to count the mergers. “These different techniques probe mergers at different ‘snapshots’ in time along the merger process,” Lotz tells Astronomy Now. “It’s 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.”

This is where Lotz and her team stepped in. Figuring out how many encounters happen over time and working out how long colliding galaxies would look ‘a little worse for wear’ after their smash-up before settling down to a normal galactic state, the team turned to incredibly detailed computer simulations to make sense of Hubble’s photographs. “Normal galaxies have fairly smooth light profiles that follow their internal gravitational fields,” says Lotz. “When two galaxies merge, their gravitational fields are changing very rapidly. Their stars get thrown out of this smooth pattern and you can see tidal tails and loops, or two nuclei in one galaxy just before the final coalescence. Gradually, the stars, gas and dark matter settle back down into a regular pattern and the merged galaxy starts to look smooth again.”

The simulation, which accounted for a wide range of merger possibilities, followed galaxies for two billion to three billion years where the group investigated a range of collisions from interactions between a pair of galaxies with comparable sizes, to that of a large galaxy and a small galactic dwarf. The team were left with a combination of 57 different merger scenarios and 10 different viewing angles and found that small galaxies are more likely to collide with giant galaxies than their larger cousins. “This is mostly because there are many more small galaxies than large galaxies,” explains Lotz. “So a large galaxy like the Milky Way is more likely to encounter another small galaxy than a similarly sized large galaxy.”

“When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results,” says Lotz who also compared her simulation to images of mergers identified by the DEEP2 survey with the W M Keck Observatory in Hawaii.

For a video highlighting four merging galaxy candidates in the study region, click: here.