Seeing Double: Spitzer Captures Our Galaxy's Twin

Release: June 28, 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."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.

Morphology of Our Galaxy's 'Twin'

NASA's Spitzer Space Telescope has captured these infrared images of a nearby spiral galaxy that resembles our own Milky Way. The targeted galaxy, known as NGC 7331 and sometimes referred to as our galaxy's twin, is found in the constellation Pegasus at a distance of 50 million light-years. This inclined galaxy was discovered in 1784 by William Herschel, who also discovered infrared light.

The evolution of this galaxy is a story that depends significantly on the amount and distribution of gas and dust, the locations and rates of star formation, and on how the energy from star formation is recycled by the local environment. The new Spitzer images are allowing astronomers to "read" this story by dissecting the galaxy into its separate components.

The main image, measuring 12.6 by 8.2 arcminutes, was obtained by Spitzer's infrared array camera. It is a four-color composite of invisible light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (yellow) and 8.0 microns (red). These wavelengths are roughly 10 times longer than those seen by the human eye.

The infrared light seen in this image originates from two very different sources. At shorter wavelengths (3.6 to 4.5 microns), the light comes mainly from stars, particularly ones that are older and cooler than our Sun. This starlight fades at longer wavelengths (5.8 to 8.0 microns), where instead we see the glow from clouds of interstellar dust. This dust consists mainly of a variety of carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons. Wherever these compounds are found, there will also be dust granules and gas, which provide a reservoir of raw materials for future star formation.

These shorter- and longer-wavelength views are shown separately as insets. Perhaps the most intriguing feature of the longer-wavelength image is a ring of dust girdling the galaxy center. This ring, with a radius of nearly 20,000 light-years, is invisible at shorter wavelengths, yet has been detected at sub-millimeter and radio wavelengths. It is made up in large part of polycyclic aromatic hydrocarbons. Spitzer measurements suggest that the ring contains enough gas to produce four billion stars like the Sun. Starlight was systematically subtracted from the longer-wavelength picture to enhance dust features.

Three other galaxies are seen below NGC 7331, all about 10 times farther away. From left to right are NGC 7336, NGC 7335 and NGC 7337. The blue dots scattered throughout the images are foreground stars in the Milky Way; the red ones are galaxies that are even more distant.

The Spitzer observations of NGC 7311 are part of a large 500-hour science project, known as the Spitzer Infrared Nearby Galaxy Survey, which will comprehensively study 75 nearby galaxies with infrared imaging and spectroscopy.