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.