Galaxies have been growing over most of the 13.7 billion year history of the universe. Some of the growth is due to intergalactic gas gradually swept up by an existing galaxy and then driving star formation in the galaxy. But another growth mechanism is the merger of two (and sometimes more) existing galaxies into one. In this case, star formation in the merged galaxies increases as the gas within the galaxies is stirred up during the merger.
Bursts of star formation are important, since in most cases our only indication of the size of a galaxy is its brightness resulting from visible stars. In general, there’s no good way to determine the mass of a galaxy that is due to dark matter and the presence of gas outside stars.
Astrophysicists generally assume that large, symmetrical spiral galaxies (such as our Milky Way) have not merged with other galaxies of similar size, though they may have incorporated several smaller galaxies. However, a large elliptical galaxy is expected to result from the merger of a large spiral and another galaxy of similar size.
For a large elliptical galaxy, it’s natural to wonder about the relative importance of the two possible growth mechanisms in the galaxy’s history – growth by merger and growth by accretion of intergalactic gas. That in turn naturally raises the question of how often mergers occur between galaxies of varying relative sizes.
New research that is soon to be published gives much better estimates of merger rates.
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.
It has not been easy to get good estimates of these merger rates. The problem is that all we can observe, for any given possible merger, is a snapshot of the action.
Even though galaxies can be observed that are close together, it’s not clear whether they are approaching or receding from each other. Only if they are approaching is a collision possible in their future, and even then they might actually be on different paths that don’t intersect. It’s necessary to be able to see other evidence of an impending merger, such as distortions in the galaxies’ shapes, in order to count close galaxy pairs as a likely merger. Consequently, surveys that are based on studying galaxy pairs tend to underestimate galaxy mergers.
Other surveys have instead tried to count mergers based on tallying galaxies that have irregular shapes, on the assumption that this is evidence of a recent merger. This method tends to overestimate the number of mergers within a particular time interval, since not all galaxies with disturbed shapes are a product of a relatively recent merger.
The latest research has made essential use of computer simulations of galaxy mergers of a large range of types in order to match the expected shapes of galaxies about to merge or the distorted shapes of single galaxies against optical images of apparent mergers that have already happened or seem about to happen. This makes it possible to eliminate cases where mergers probably either will not occur or have not occurred. And for the remaining cases, an estimate can be made of the time duration before or after a merger.
Galaxies were selected from previous surveys out to a redshift of z≈1.5 (about 9 billion light-years). The net result is a much improved estimate of how often mergers occur within a given period of time. The answer, not very surprisingly, is that the rate is between estimates from earlier surveys based on less rigorous analysis.
Much effort went into the computer simulations, in order to best interpret, from the optical images, what was happening and how far along the merger process had progressed.
Creating the computer models was a time-consuming process. Lotz’s team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. “Viewing the simulations was akin to watching a slow-motion car crash,” Lotz says.
The simulations followed the galaxies for 2 billion to 3 billion years, beginning at the first encounter and continuing until the union was completed, about a billion years later.
|Jennifer M. Lotz, Patrik Jonsson, T. J. Cox, Darren Croton, Joel R. Primack, Rachel S. Somerville, & Kyle Stewart (2011). The Major and Minor Galaxy Merger Rates at z<1.5 Astrophysical Journal arXiv: 1108.2508v1|