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NGC 4449 and companion. Image: UCLA
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A team led by University
of California, Los Angeles (UCLA) research astronomer
Michael Rich has used a unique telescope to discover a previously unknown
companion to the nearby galaxy NGC 4449, which is some 12.5 million light years
from Earth. The newly discovered dwarf galaxy had escaped even the prying eyes
of the Hubble Space Telescope.
The research is published in Nature.
The larger, host galaxy, NGC 4449, may be
"something of a living fossil," representing what most galaxies
probably looked like shortly after the Big Bang, Rich said. The galaxy is
forming stars "so furiously" that it has giant clusters of young stars
and even appears bluish—a sign of a young galaxy—to the eye in large amateur
telescopes, he said.
NGC 4449 has a nucleus that may someday host
a black hole and an irregular structure, lacking the spiral arms characteristic
of many galaxies, he said. It is surrounded by a huge complex of hydrogen gas
that spans approximately 300,000 light years, which may be fueling its burst of
star formation.
Rich collaborated with Francis Longstaff, a
professor of finance at the UCLA Anderson School of Management and an amateur
astronomer, in acquiring and using a specialized telescope designed to take
images of wide fields of the sky. Known as the Centurion 28 (the diameter of
the mirror is 28 in), the telescope, and the observatory the astronomers used,
are located at the Polaris Observatory Association near Frazier Park, in Kern County, Calif.
With the C28 telescope, the astronomers
discovered the companion dwarf galaxy, which has "evidently experienced a
close encounter with the nucleus of NGC 4449," Rich said. Dubbed NGC
4449B, the dwarf galaxy has been stretched into a comet-like shape by this
gravitational encounter.
NGC 4449B had remained undetected because it
is more than 10 times fainter than the natural brightness of the night sky and
some 1,000 times fainter than our own Milky Way galaxy. The dwarf galaxy is in
a "transient stage," Rich said, and will soon—by astronomical
standards—be dissolved.
The Milky Way has a similar companion, known
as the Sagittarius Dwarf galaxy, which has been wrapped around our galaxy as it
orbits and which loses its stars to the Milky Way's gravitational tug.
With the help of the wide field of the C28
telescope and special image processing conducted by Christine Black, a UCLA
research assistant, and David Reitzel of the Griffith Observatory, the
astronomers were able to subtract the light of the sky and that of the outer
parts of NGC 4449 to reveal the new galaxy.
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 end would reach all the way to the sun's position. In fact, NGC
4449B is the largest dwarf galaxy known in the "local group" that also
includes the Milky Way and the Andromeda galaxy.
The deep images of the larger NGC 4449 revealed
other surprises as well: A strange arc of stars that might be an ingested
galaxy, and a "remarkable halo" of old stars that appears to consist
of two parts; the outermost part of this "halo" population was
unexpected, and makes NGC 4449 equivalent in size to the Milky Way. The origin
of these old stars is not known, but they may have been acquired when galaxies
similar to NGC 4449B fell into NGC 4449 and were shredded, Rich said.
Andrew Benson, a co-author and a senior
research fellow in theoretical cosmology at the California Institute of
Technology, said, "Our own galaxy, the Milky Way, has a host of smaller
galaxies which orbit around it. On much larger scales, we see groups and
clusters of galaxies which orbit under the pull of their mutual gravitational
attraction. Gravity has no preferred length scale, so we'd expect that dark
matter (which interacts only through gravity) should behave in more or less the
same way on all scales. For a galaxy like NGC4449, that means it should have
its own system of small dark-matter satellites orbiting around it—assuming that
dark matter works the way we think it does."
The C28 telescope used for the discovery places
the CCD camera at what is known as the "prime" focus, in front of the
light-collecting mirror. The telescope is produced by the Astro Works Corp. of Arizona, led by James
Riffle. Longstaff and Rich expect to have the telescope ready for remote
control later this year, perhaps this summer. Longstaff led the observatory and
telescope project.
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