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.
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