Seeing Double: Spitzer Captures Our Galaxy's Twin

The picture, which can be viewed athttp://photojournal.jpl.nasa.gov/catalog/PIA06322 , shows our twin as neverbefore. Its swirling arms spin outward from a central bulgeof light, which is outlined by a ring of actively formingstars.

"Being inside our galaxy makes it difficult to see what'sgoing on in the center," said Dr. J.D. Smith, a member ofthe team that observed NGC 7331, and an astronomer at theUniversity of Arizona, Tucson. "By looking at a very similargalaxy, we gain a bird's eye-view of what the entire MilkyWay might look like."

Such an outside perspective will teach astronomers how ourown galaxy, as well as others like it, might have formed andevolved.

The latest observations are the first in a large-scaleeffort to observe 75 nearby galaxies with Spitzer's highlysensitive infrared eyes. Called Spitzer Infrared NearbyGalaxies Survey, the program will combine Spitzer data withthat from other ground- and space-based telescopes operatingat wavelengths ranging from ultraviolet to radio to create acomprehensive map of the selected galaxies.

The program's first target, NGC 7331, was chosen in part forits striking similarities to the Milky Way. While these so-called twin galaxies do not share the same parents, theyhave many features in common, including number of stars,mass, spiral arm pattern and star-formation rate of a fewstars per year. Whether the Milky Way has an inner star-forming ring like that of NGC 7331 is not known. NGC 7331 islocated about 50 million light-years away in theconstellation Pegasus.

The new Spitzer image demonstrates the power of thetelescope's infrared eyes to dissect galaxies into theirvarious parts. Taken by the telescope's infrared arraycamera, the false-colored picture readily distinguishes NGC7331's arms (brownish red), central bulge (blue) and star-forming ring (yellow). The composition of materials makingup these regions was also revealed by the Spitzerobservations: the central bulge consists primarily of olderstars; the ring possesses a large amount of gas and dustyorganic molecules called polycyclic aromatic hydrocarbons,which typically glow when illuminated by newborn stars; andthe arms contain these same dust grains to a lesser degree.Polycyclic aromatic hydrocarbons are also found on Earth, onburnt toast and in car exhaust among other places.

Data from Spitzer's infrared spectrograph instrument werealso used to show that the center of NGC 7331 harbors eitheran unusually high concentration of massive stars, or amoderately active black hole about the same size as the onelurking at the core of our galaxy.

These findings will appear in two papers in the Septemberissue 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 detailingobservations from the infrared array camera, and Smith islead author of a paper on the infrared spectrograph results.The Spitzer Infrared Nearby Galaxies Survey project isconducted by a team of about 25 scientists from 12institutions, 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 thefourth of NASA's Great Observatories, a program that alsoincludes the Hubble Space Telescope, Chandra X-rayObservatory and Compton Gamma Ray Observatory.

JPL manages the Spitzer Space Telescope mission for NASA'sOffice of Space Science, Washington, D.C. Science operationsare conducted at the Spitzer Science Center at theCalifornia Institute of Technology in Pasadena. JPL is adivision of Caltech. Spitzer's infrared spectrograph wasbuilt by Cornell University, Ithaca, N.Y., and BallAerospace Corporation, Boulder, Colo. The instrument'sdevelopment was led by Dr. Jim Houck of Cornell. Spitzer'sinfrared array camera was built by NASA Goddard Space FlightCenter, Greenbelt, Md. The camera's development was led byDr. Giovanni Fazio of Smithsonian Astrophysical Observatory,Cambridge, Mass.

Additional information about the Spitzer Space Telescope isavailable at http://www.spitzer.caltech.edu .