Galaxy collision of superlatives

First observation of the merger of four large galaxies

Artistic representation of a galaxy collision observed from a hypothetical planet. © NASA / JPL-Caltech / T. Pyle (SSC)
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One of the largest cosmic collisions is currently taking place five billion light years away. Here, four galaxies collide and merge into a single giant galaxy, ten times larger than our Milky Way galaxy. This collision gives astronomers valuable insights into the evolution of the largest galaxies in the universe.

Collisions and mergers of galaxies are not uncommon in the cosmos. The mutual attraction brings together close together cluster of stars even closer and ultimately, over long periods of time to unification. Although the stars are thrown into each other in this process, they still have enough empty space around them to survive such events unscathed. Also, our Milky Way will probably unite with its neighbor, the Andromeda galaxy, in five billion years.

Collision of four "sand vices"

However, while mergers between one large and several small, or even two, large galaxies have been frequently observed, the now-discovered multiple collision of several large galaxies is the first documented of their kind. "Most of the galaxy blends we know are similar to compact cars, which crash into each other, "explains Kenneth Rines of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "But what we have here is more like the collision of four sand-laden vices - sand flies in all directions." The astronomers discovered the mega-collision, as the Spitzer space telescope and the WIYN telescope a distant galaxy cluster called CL0958 + 4702 took a closer look.

The four white spots in the middle are galaxies that merge into a single giant galaxy. © NASA / JPL-Caltech / K. Rines (Harvard-Smithsonian CfA)

The telescopes first registered an unusual, fan-shaped beam of light that emanated from a collection of three Milky Way galaxies and a galaxy that was about three times larger. "The WIYN images show us that the galaxies in this collision are not helical but rather elliptical, " explains Jeffrey Kenney, a professor of astrophysics at Yale University. "The galaxies have well-defined nuclei that are conserved during the fusion, much like the yolks remain more stable than the egg when mixing eggs in a eardrum."

One of the largest galaxies in the universe

Further analysis of the images revealed that the cone of light consists of billions of older stars ejected from the galaxies by the collision. However, about half of them will fall back into the galaxies. "When this merger is complete, one of the largest galaxies in the Universe will have emerged, " Rines said. Some of the ejected stars will not fall back, but stay in isolated regions outside the galaxy. If these "common" stars have planets, the night sky on them looks very different from ours: much fewer stars and more galaxies. display

Little gas and old stars

Mega-Collision in Far-Right Cluster Cluster CL0958 + 4702 - Photograph of Spitzer Space Telescope and WIYN Telescope Courtesy of Ken Rines

The observations of the Spitzer telescope also showed that the collision belongs to the more gas-poor type. While many new stars emerge from gas-rich mergers, this is not the case for the gas-poor, so old stars dominate both before and after the collision.

"The data shows that these large mergers contain little gas, unlike most collisions that we know so far, " Rines said. The data also represent the best evidence yet that the largest galaxies in the Universe were probably formed not so long ago by such collisions.

(Yale University, 08.08.2007 - NPO)