JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011
March 2, 2006
A Shocking Surprise in Stephan's Quintet
This false-color composite image of the Stephan's Quintet galaxy cluster
clearly shows one of the largest shock waves ever seen (green arc),
produced by one galaxy falling toward another at over a million miles
per hour. It is made up of data from NASA's Spitzer Space Telescope and
a ground-based telescope in Spain.
Four of the five galaxies in this image are involved in a violent
collision, which has already stripped most of the hydrogen gas from the
interiors of the galaxies. The centers of the galaxies appear as bright
yellow-pink knots inside a blue haze of stars, and the galaxy producing
all the turmoil, NGC7318b, is the left of two small bright regions in
the middle right of the image. One galaxy, the large spiral at the
bottom left of the image, is a foreground object and is not associated
with the cluster.
The titanic shock wave, larger than our own Milky Way galaxy, was
detected by the ground-based telescope using visible-light wavelengths.
It consists of hot hydrogen gas. As NGC7318b collides with gas spread
throughout the cluster, atoms of hydrogen are heated in the shock wave,
producing the green glow.
Spitzer pointed its infrared spectrograph at the peak of this shock wave
(middle of green glow) to learn more about its inner workings. This
instrument breaks light apart into its basic components. Data from the
instrument are referred to as spectra and are displayed as curving lines
that indicate the amount of light coming at each specific wavelength.
The Spitzer spectrum showed a strong infrared signature for incredibly
turbulent gas made up of hydrogen molecules. This gas is caused when
atoms of hydrogen rapidly pair-up to form molecules in the wake of the
shock wave. Molecular hydrogen, unlike atomic hydrogen, gives off most
of its energy through vibrations that emit in the infrared.
This highly disturbed gas is the most turbulent molecular hydrogen ever
seen. Astronomers were surprised not only by the turbulence of the gas,
but by the incredible strength of the emission. The reason the molecular
hydrogen emission is so powerful is not yet completely understood.
Stephan's Quintet is located 300 million light-years away in the Pegasus
This image is composed of three data sets: near-infrared light (blue)
and visible light called H-alpha (green) from the Calar Alto Observatory
in Spain, operated by the Max Planck Institute in Germany; and 8-micron
infrared light (red) from Spitzer's infrared array camera.
The Spitzer Space Telescope is a NASA mission managed by the Jet
[NOTE: Images and supporting weblinks are available at