
NGC 7331: Is this what the Milky Way looks like
to its neighbours?
Credit: NASA/JPL-Caltech/STScI
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Spitzer captures our galaxy's twin
NASA/JPL PHOTO RELEASE
Posted: July 8, 2004
What would our Milky Way galaxy look like if we could travel outside
it and snap a picture? It might look a lot like a new image by NASA's
Spitzer Space Telescope of a spiral galaxy called NGC 7331 - a virtual
twin of our Milky Way.
The picture shows our twin as never before. Its swirling arms spin outward
from a central bulge of light, which is outlined by a ring of actively
forming stars.
"Being inside our galaxy makes it difficult to see what's going
on in the center," said Dr. J.D. Smith, a member of the team that
observed NGC 7331, and an astronomer at the University of Arizona, Tucson.
"By looking at a very similar galaxy, we gain a bird's eye-view of
what the entire Milky Way might look like."
Such an outside perspective will teach astronomers how our own galaxy,
as well as others like it, might have formed and evolved.
The latest observations are the first in a large-scale effort to observe
75 nearby galaxies with Spitzer's highly sensitive infrared eyes. Called
Spitzer Infrared Nearby Galaxies Survey, the program will combine Spitzer
data with that from other ground- and space-based telescopes operating
at wavelengths ranging from ultraviolet to radio to create a comprehensive
map of the selected galaxies.
The program's first target, NGC 7331, was chosen in part for its striking
similarities to the Milky Way. While these so-called twin galaxies do
not share the same parents, they have many features in common, including
number of stars, mass, spiral arm pattern and star-formation rate of a
few stars per year. Whether the Milky Way has an inner star-forming ring
like that of NGC 7331 is not known. NGC 7331 is located about 50 million
light-years away in the constellation Pegasus.
The new Spitzer image demonstrates the power of the telescope's infrared
eyes to dissect galaxies into their various parts. Taken by the telescope's
infrared array camera, the false-colored picture readily distinguishes
NGC 7331's arms (brownish red), central bulge (blue) and star-forming
ring (yellow). The composition of materials making up these regions was
also revealed by the Spitzer observations: the central bulge consists
primarily of older stars; the ring possesses a large amount of gas and
dusty organic molecules called polycyclic aromatic hydrocarbons, which
typically glow when illuminated by newborn stars; and the arms contain
these same dust grains to a lesser degree. Polycyclic aromatic hydrocarbons
are also found on Earth, on burnt toast and in car exhaust among other
places.
Data from Spitzer's infrared spectrograph instrument were also used to
show that the center of NGC 7331 harbors either an unusually high concentration
of massive stars, or a moderately active black hole about the same size
as the one lurking at the core of our galaxy. These findings will appear
in two papers in the September issue of a special supplement to the Astrophysical
Journal. Dr. Michael W. Regan of the Space Telescope Institute, Baltimore,
Md., is lead author of a paper detailing observations from the infrared
array camera, and Smith is lead author of a paper on the infrared spectrograph
results. The Spitzer Infrared Nearby Galaxies Survey project is conducted
by a team of about 25 scientists from 12 institutions, and is led by principal
investigator Dr. Robert C. Kennicutt of the University of Arizona, Tucson.
Launched August 25, 2003, the Spitzer Space Telescope is the fourth of
NASA's Great Observatories, a program that also includes the Hubble Space
Telescope, Chandra X-ray Observatory and Compton Gamma Ray Observatory.
JPL manages the Spitzer Space Telescope mission for NASA's Office of
Space Science, Washington, D.C. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in Pasadena.
JPL is a division of Caltech. Spitzer's infrared spectrograph was built
by Cornell University, Ithaca, N.Y., and Ball Aerospace Corporation, Boulder,
Colo. The instrument's development was led by Dr. Jim Houck of Cornell.
Spitzer's infrared array camera was built by NASA Goddard Space Flight
Center, Greenbelt, Md. The camera's development was led by Dr. Giovanni
Fazio of Smithsonian Astrophysical Observatory, Cambridge, Mass.
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