Astronomers now have a more accurate estimate of how often galaxies combined in the last eight to nine billion years. New supercomputer simulations can predict how merging galaxies will look at various stages of interaction and from various perspectives. Jennifer Lotz of the Space Science Telescope Institute and others searched through four astrological surveys, comparing images of thousands of galaxies to the simulated models. This study led them to a more accurate value of the merger rate.
A major question in astronomy is how galaxies and stars form. The combining of two galaxies is a phenomenon that has been observed in many instances throughout the universe, and is thought to affect the mass and shape of different galaxies, as well as the formation of stars in those galaxies. Galactic collisions may also drive the mass of the black holes that lie at the center of almost all observable galaxies.
Though galaxy collisions have been observed in the past, the rate at which they occur has not been determined with certainty. Some studies looked only for galaxies that were in the process of merging, while others only looked for galaxies that seemed likely to merge soon. Without enough images of galaxies to look at, and without the correct methods of identifying mergers, these studies could not obtain an accurate value for the galactic merger rate. Past estimates of how many observed galaxies were merging ranged from 5 percent to 25 percent.
"With the galaxy merger rate, we want to know the ‘number' of galaxies colliding and joining together per unit time," Lotz wrote in an email to The News-Letter. They measured the merger rate in two ways: by finding the fraction of galaxies that were merging, known as the fractional merger rate, and secondly by finding the volume-averaged merger rate, which is the average number of galaxies that are merging in a "box" or fixed volume in space.
The study focused on galaxies' behavior eight billion to nine billion years ago. The time of the merge was identified by its redshift. Since the universe is expanding, the light from distant objects bends on its way to earth, creating a red-shift. Because this bending is well understood, a "redshift is a proxy for distance," Lotz wrote.
It also indicates the time of the event being viewed. "Because the speed of light is finite, looking farther away is like looking back in time ... we are now seeing the light from galaxies at high redshifts/large distances as they were billions of years ago," Lotz wrote.
Before you can count the mergers, it is necessary to know what they look like. Lotz's lab used new supercomputer simulations to visualize the process of galaxy collision. They simulated 57 different varieties of galaxy combinations from 10 different angles. The simulations showed the behavior of galaxies for two to three billion years during collision.
Two large spiral galaxies, such as the Milky Way galaxy and the Andromeda galaxy, combine to form an elliptical galaxy. Another form of merger is when a small galaxy is ‘swallowed' by a larger galaxy in an interaction that does not change the shape of the larger galaxy.
Two small galaxies are now thought to be in the process of merging with the Milky Way galaxy. Mergers do not involve actual collision of stars because galactic bodies are so dispersed, but gases and mass redistribute during the interaction, and in active galaxies matter is consumed by the central black hole.
Lotz's team searched for merging galaxies in the Hubble's All-Wavelength Extended Groth Strip International Survey (AEGIS) and Cosmological Evolution Survey (COSMOS). They also used the Great Observatories Origins Deep Survey (GOODS) and the DEEP2 survey taken at the W.M. Keck Observatory in Hawaii. The team looked for galaxies that were in the middle of collision as well as those that were beginning to coalesce but had not yet collided. This gave them a broad view of galactic interaction, correcting a limitation of previous studies that only identified galaxies in mid-collision.
This study also used a more realistic timescale over which the mergers take place. "Different ways of finding galaxy mergers catch the merger for different periods of time. . . To calculate the merger rate, we need to both count up the number of observed galaxy mergers and then divide this by the right timescale," Lotz wrote.
The study showed that in the last nine billion years small or ‘dwarf' galaxies underwent mergers an average of three times, and larger galaxies combined on average only once during that time. These mergers greatly increased the mass of the galaxies.
The merging of galaxies may have contributed to star formation, so Lotz wants to study galaxy collisions 11 billion years ago when a peak number of stars were being formed.
