More Often Than Not, Massive Galaxies Form by Mergers

colliding galaxies

New data seem to show that galaxies collide all the time. In fact, the oldest and largest galaxies in the universe most likely formed from such intergalactic combinations.

Astronomer Pieter van Dokkum of Yale University used some of the longest and deepest sky surveys ever conducted to try to determine whether the oldest, largest galaxies--called ellipticals because they lack the swirling arms of the spiral type, like our own Milky Way--formed from the collapse of ancient clouds of gas or the accretion of smaller galaxies bumping into each other. Of the 126 galaxies of all varieties van Dokkum looked at, 67 showed telltale signs of impact, such as trailing tails of stars, or a collision in progress. And of the 86 oldest galaxies in the survey, 61 showed some trace of a cosmic crash.

"Our study found these common massive galaxies do form by mergers," Van Dokkum explains. "It is just that the mergers happen quickly and the features that reveal the mergers are very faint and therefore difficult to detect."

"Quickly" on a galactic scale means just a few hundred million years--a small fraction of the 13.7 billion years the universe has been in existence--and, because such collisions rarely involve head-to-head star crashes, they leave few traces behind except in the shape of the resulting galaxy and a general slowing in its formation of new stars. The survey results appear in the current issue of the Astronomical Journal.

None of the six spiral galaxies in the survey showed any after-crash damage, but that doesn't mean that our own galaxy is free and clear. "The Milky Way will indeed undergo a collision in the near future as we are heading toward M31, the Andromeda Nebula," van Dokkum adds. "'Near future' in this case is about four billion years from now though."

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