7 December 2005
Smash-Up Derby A Universal Pastime
by Kate Melville

Galactic collisions are common phenomena, according to Yale astronomer, Pieter van Dokkum, who used hundreds of images from two of the deepest sky surveys ever conducted to conclude that more than half of the largest galaxies in the nearby universe have collided and merged with another galaxy in the past two billion years.

Cosmologists believe that large galaxies were assembled primarily by mergers, rather than evolving by themselves in isolation. But the fact that the most massive galaxies appear to be the oldest, leaving minimal time since the Big Bang for the mergers to have occurred, has been a niggling inconsistency in this theory. "Our study found these common massive galaxies do form by mergers," said van Dokkum. "It is just that the mergers happen quickly, and the features that reveal the mergers are very faint and therefore difficult to detect."

Van Dokkum's study, appearing in the Astronomical Journal, used images that covered an area of the sky more than 5,000 times larger than the Hubble Deep Field, in which he looked for the telltale tidal features evident in post-collision galaxies. As it turned out, these tidal features were quite common, with 53 percent of the galaxies showing tails, broad fans of stars trailing behind them or other obvious asymmetries. "This implies that there is one galaxy that has endured a major collision and subsequent merger event for every single other 'normal' undisturbed field galaxy," Van Dokkum explained. "Remarkably, the collisions that precede the mergers are ongoing in many cases. This allows us to study galaxies before, during, and after the collisions."

While many people may be familiar with the spectacular mergers of blue spiral galaxies that were publicized in several Hubble images, Van Dokkum's red galaxy (a color selection biased to select the most massive galaxies in the local universe) mergers appear to be much more common. This ubiquity is a confirmation of predictions by the most common models for the formation of large-scale structure in the Universe, while also helping solve the apparent-age problem. "In the past, people equated stellar age with the age of the galaxy," van Dokkum explains. "We have found that, though their stars are generally old, the galaxies that result from these mergers are relatively young."

Source: Yale University