February 9, 2012
New Nearby Dwarf Galaxy Discovered
An astronomy team has captured images of a nearby dwarf galaxy, lying about 12.5 million light years from Earth.
University of California - Los Angeles (UCLA) research astronomer Michael Rich, leader of the team, said NGC 4449 may be "something of a living fossil", because it represents what most galaxies looked like shortly after the Big Bang.
They found that the dwarf galaxy has a dense stream of stars in its outer regions, which could be the remains of an even smaller companion galaxy in the process of merging with its host.
"This is how galaxies grow. You can see the smaller galaxy coming in and getting shredded, eventually leaving its stars scattered through the halo of the host galaxy," co-author Aaron Romanowsky, a research astronomer at the University of California - Santa Cruz (UCSC), said in a press release.
The astronomers say NGC 4449 is very similar to one of the Milky Way's satellite galaxies, the Large Magellanic Cloud.
Rich said that NGC 4449 has a nucleus that may someday host a black hole and an irregular structure.
The galaxy is surrounded by a huge complex of hydrogen gas that spans about 300,000 light years across, which could be fueling its burst of star formation.
The stream of stars located in the dwarf galaxy was first detected by another group of astronomers as a mysterious, faint smudge in digitized photographic plates from the Digitized Sky Survey project. The stream is also visible in archival images from the Sloan Digital Sky Survey.
The authors of the new study called it a "stealth merger," where an in-falling satellite galaxy is nearly undetectable by using conventional methods, but still has a substantial influence on its host galaxy.
They observed the galaxy using a specialized telescope known as Centurion 28, designed to take images of wide fields of the sky.
The astronomers discovered the companion dwarf galaxy using the Centurion 28. Rich said this galaxy has "evidently experienced a close encounter with the nucleus of NGC 4449."
This companion dwarf galaxy, NGC 4449B, has been stretched into a comet-like shape because of its gravitational encounter.
NGC 4449B is stretched into a gigantic "S" so large that if one end were placed at the center of the Milky Way, the other would reach all the way to the Sun's position. It is the largest dwarf galaxy known in the "local group", which also includes the Milky Way and Andromeda galaxy.
The team organized a follow up report with more detailed observations using the half-meter Black Bird Observatory telescope. These images confirmed the presence of a faint substructure in the halo of the galaxy.
Then, astronomers used the Subaru Telescope in Hawaii to obtain high-resolution images in which the individual stars in the stellar stream can be seen.
"I don't think I'd ever seen a picture of a galaxy merger where you can see the individual stars," Romanowsky said in a statement. "It's really an impressive image."
The researchers published their findings in the Astrophysical Journal Letters.
Image Caption: The galaxy NGC 4449 and its companion dwarf galaxy, NGC 4449B, in a false color image created by Francis Longstaff.
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