Giving Up the Galactic Ghost

Stephan’s Quintet with red highlighting star-forming regions. Stephan’s Quintet was imaged by the Gemini Observatory using the Multi-Object Spectrograph on Gemini North. The interacting members of the cluster are almost 300 million light years away. The galaxy NGC 7320 (top-center) is thought by most astronomers to be in the foreground (about 8-times closer) and is distinguished in this image by multiple red blobs indicating hydrogen clouds where stars are forming. Orientation: North-bottom, East-right.
Credit: Gemini/NSF/USRA

A stunning image released by the Gemini Observatory captures the graceful interactions of a galactic ballet, on a stage some 300 million light years away, that might better be described as a contortionist’s dance.

The galaxies, members of a famous troupe called Stephan’s Quintet, are literally tearing each other apart. Their shapes are warped by gravitational interactions occurring over millions of years. Sweeping arches of gas and dust trace the interactions and possible ghost-like passage of the galaxies through one another. The ongoing dance deformed their structures while spawning a prolific fireworks display of star formation fueled by clouds of hydrogen gas that were shocked into clumps to form stellar nurseries.

This unprecedented image of the cluster provides a unique combination of sensitivity, high resolution and field of view. "It doesn’t take long to reach an incredible depth when you have an 8-meter mirror collecting light under excellent conditions," said Travis Rector of the University of Alaska, Anchorage who helped obtain the data with the Gemini North Telescope on Mauna Kea. "We were able to capture these galaxies at many different wavelengths or colors. This allowed us to bring out some remarkable details in the final color image that have never been seen before in one view."

Looking towards Redshift 12 galaxies, the farthest known reaches of the visible universe.
Credit: NASA

One striking element of the image is a collection of vibrant red clumps that mark star-forming regions within a galaxy called NGC 7320. Although its relation to the other galaxies in the cluster has been the subject of some controversy, most astronomers now think that the galaxy leads a relatively tranquil existence in the foreground, safely isolated from the violent quarrels of the more distant cluster.

Spectroscopic data show that NGC 7320 has an apparent velocity away from us of about 800 kilometers per second. In contrast, the rest of the group is being carried away from us by the expansion of the universe at over 6,000 kilometers per second. Using current models for the expanding universe, this would put the bulk of the cluster almost 8 times farther away from us than NGC 7320.

The vivid red patches scattered across the spiral arms of NGC 7320 in the new Gemini image provide a dramatic illustration of how these differing apparent velocities can impact our view. NGC 7320 and the other cluster galaxies have regions of intense star formation indicated by glowing clouds of hydrogen gas called HII regions. These areas appear distinctly red because a selective filter was used which only passes a special color of red light, called hydrogen alpha, that is produced in the HII regions. In the higher-velocity members of the cluster, prominent HII clumps dominate around the two closely interacting central galaxies but they do not appear red in the image. In these galaxies, the HII glow was Doppler-shifted beyond the range of the selective filter, and was therefore not detected.

The Hubble Telescope.
Credit: NASA

The interacting members of Stephan’s Quintet appear destined to continue their dance for millions more years. Eventually, this dance will probably cause some of the galaxies in the cluster to completely lose their current identity, combining into even fewer objects than we see today.

Stephan’s Quintet was discovered in 1877 by the French astronomer Edouard Stephan using the Foucault 80-centimeter reflector at the Marseilles Observatory. The cluster is listed in the Hickson Compact Group Catalog as number 92. It has been studied extensively at all wavelengths including imaging by the Hubble Space Telescope.


- Hubble Space Telescope launches aboard Space Shuttle Discovery, as Earth Orbiting Observatory

- Hubble Space Telescope finds evidence of black hole in the center of M87
- Hubble Key Project begins studying Cepheid variable stars to better define Hubble Constant, and the size of the universe

The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
Credit: NASA

- Sidney van den Bergh and Gustav Tammann debate Hubble Constant and the scale of the universe

- Jim Peebles and Michael Turner debate nature of universe and whether cosmology is solved

- John Cowan confirms age estimates of globular clusters and universe by dating metal-poor stars
- Wendy Freedman and Allan Sandage debate Hubble Constant and the scale of universe

- Hubble Space Telescope detects an atmosphere around an extrasolar planet

- Chandra X-ray Observatory finds evidence for new matter in "quark stars", matter so dense it exceeds terrestrial nuclear material with 1.2 million degree temperatures

- Final mission in NASA Great Observatory series, the infrared observatory, or Spitzer Space Telescope, finds evidence for organic molecules in intergalactic regions
- Microwave measurements precisely date the Big Bang at 13.7 billion years ago, with a remarkable 1% error prediction

- French COROT mission will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission

- Kepler, Extrasolar Terrestrial Planet Detection Mission, designed to look for transiting or earth-size planets that eclipse their parent stars [survey 100,000 stars]. Scientists expect to find thousands of planets, and perhaps 50 Earth-like candidates.
- Likely de-orbit for Hubble Space Telescope [date announced is highly fluid but assumes no planned shuttle visits from NASA]

- Planned launch for NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope.
- Planned launch for Space Interferometery Mission (SIM)

- Planned launch for TPF and Darwin missions

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