digg.com>PASADENA, Calif. --
While some galaxies are rotund and others are slender disks like our spiral
Milky Way, new observations from NASA's Spitzer Space Telescope show that the
Sombrero galaxy is both. The galaxy, which is a round elliptical galaxy with a
thin disk embedded inside, is one of the first known to exhibit characteristics
of the two different types. The findings will lead to a better understanding of
galaxy evolution, a topic still poorly understood.
"The Sombrero is more complex
than previously thought," said Dimitri Gadotti of the European Southern
Observatory in Chile and lead author of a new paper on the findings appearing
in the Monthly Notices of the Royal Astronomical Society. "The only way to
understand all we know about this galaxy is to think of it as two galaxies, one
inside the other."
The Sombrero galaxy, also known as
NGC 4594, is located 28 million light-years away in the constellation Virgo.
From our viewpoint on Earth, we can see the thin edge of its flat disk and a
central bulge of stars, making it resemble a wide-brimmed hat. Astronomers do
not know whether the Sombrero's disk is shaped like a ring or a spiral, but
agree it belongs to the disk class.
"Spitzer is helping to unravel
secrets behind an object that has been imaged thousands of times," said
Sean Carey of NASA's Spitzer Science Center at the California Institute of
Technology in Pasadena. "It is intriguing Spitzer can read the fossil
record of events that occurred billions of years ago within this beautiful and
archetypal galaxy."
Spitzer captures a different view of
the galaxy than visible-light telescopes. In visible views, the galaxy appears
to be immersed in a glowing halo, which scientists had thought was relatively
light and small. With Spitzer's infrared vision, a different view emerges.
Spitzer sees old stars through the dust and reveals the halo has the right size
and mass to be a giant elliptical galaxy.
While it is tempting to think the
giant elliptical swallowed a spiral disk, astronomers say this is highly
unlikely because that process would have destroyed the disk structure. Instead,
one scenario they propose is that a giant elliptical galaxy was inundated with
gas more than nine billion years ago. Early in the history of our universe,
networks of gas clouds were common, and they sometimes fed growing galaxies,
causing them to bulk up. The gas would have been pulled into the galaxy by
gravity, falling into orbit around the center and spinning out into a flat
disk. Stars would have formed from the gas in the disk.
"This poses all sorts of
questions," said Rubén Sánchez-Janssen from the European Southern
Observatory, co-author of the study. "How did such a large disk take shape
and survive inside such a massive elliptical? How unusual is such a formation
process?"
Researchers say the answers could
help them piece together how other galaxies evolve. Another galaxy, called
Centaurus A, appears also to be an elliptical galaxy with a disk inside it. But
its disk does not contain many stars. Astronomers speculate that Centaurus A
could be at an earlier stage of evolution than the Sombrero and might
eventually look similar.
The findings also answer a mystery
about the number of globular clusters in the Sombrero galaxy. Globular clusters
are spherical nuggets of old stars. Ellipticals typically have a few thousand,
while spirals contain a few hundred. The Sombrero has almost 2,000, a number
that makes sense now but had puzzled astronomers when they thought it was only
a disk galaxy.
NASA's Jet Propulsion Laboratory,
Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's
Science Mission Directorate, Washington. Science operations are conducted at
the Spitzer Science Center at the California Institute of Technology in
Pasadena. Data are archived at the Infrared Science Archive housed at the
Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for
NASA
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