Hubble Telescope Spies a Galactic Merger

Published October 13, 2009


In a new Hubble Space Telescope image, what appears to be one galaxy is actually the product of a collision between two Milky Way-like spiral galaxies.

The result is a galactic merger called NGC 2623, or Arp 243, and it is about 250 million light-years away in the constellation of Cancer (the Crab).

Studies have revealed that as galaxies approach one another, massive amounts of gas are pulled from each galaxy towards the center of the other, until ultimately the two merge into one massive galaxy.

NGC 2623 is in the late stages of the merging process with the centers of the original galaxy pair now merged into one nucleus. Stretching out from the center are two tails of young stars, showing that a merger has taken place.

During such a collision, the dramatic exchange of mass and gases initiates star formation, seen in both the tails.

The prominent lower tail is richly populated with bright star clusters — 100 of them have been found in Hubble observations. The clusters are brighter than any seen in galaxies closer to our own. These star clusters may have formed as part of a loop of stretched material associated with the northern tail, or they may have developed from debris falling back onto the nucleus.

In addition to this active star-forming region, both galactic arms harbor very young stars in the early stages of their evolutionary journey.

Some galactic mergers (including NGC 2623) can result in an active galactic nucleus, where one of the supermassive black holes found at the centers of the two original galaxies is stirred into action. Matter is pulled toward the black hole, forming an accretion disc. The energy released by the frenzied motion heats up the disc, causing it to emit across a wide swath of the electromagnetic spectrum – typically including everything from radio waves to visible light to X-rays.

NGC 2623 is so bright in the infrared, for example, that it belongs to the group of very luminous infrared galaxies (LIRG).

The images of the merger were taken in 2007 by the Advanced Camera for Surveys (ACS) aboard Hubble.