More than half of the largest galaxies in the nearby universe have collided and merged with another galaxy in the past two billion years, according to a Yale astronomer in a study using hundreds of images from two of the deepest sky surveys ever conducted.
The panels show several of the newly found galaxy collisions in the nearby universe, using the NOAO Deep Wide-Field Survey (NDWFS) and the Multiwavelength Survey by Yale/Chile (MUSYC). The collisions (occurring in different galaxy pairs) are seen in different stages of the merger process, which taken together show the sequence that occurs. In (a) and (b) [top left and top right], the galaxies are still separated, but huge tidal forces of gravity are already at work pulling stars from the galaxies into enormous broad fans that stretch hundreds of thousands of light-years in space.
In (c) and (d) [bottom left and bottom right], the colliding galaxies have merged into single, larger galaxies. The violent past of these galaxies can be inferred from the tidal "debris" that still surrounds the newly formed galaxies. Images (a), (b), and (d) are from the NDWFS; image (c) is from MUSYC. Credit: P. van Dokkum/Yale University and NOAO/AURA/NSF
The idea of large galaxies being assembled primarily by mergers rather than evolving by themselves in isolation has grown to dominate cosmological thinking. However, a troubling inconsistency within this general theory has been that the most massive galaxies appear to be the oldest, leaving minimal time since the Big Bang for the mergers to have occurred.
"Our study found these common massive galaxies do form by mergers. It is just that the mergers happen quickly, and the features that reveal the mergers are very faint and therefore difficult to detect," said Pieter van Dokkum , assistant professor of astronomy at Yale University, and sole author of the paper appearing in the December 2005 issue of the Astronomical Journal.
Janet Rettig Emanuel | EurekAlert!
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