* Astronomy

Researchers from the National Astronomical Observatory of Japan (NAOJ) and the University of Tokyo used Subaru's Suprime-Cam camera to discover an unusual streak of ionised hydrogen gas associated with a galaxy 300 million light-years from Earth. The filament of gas is only 6 thousand light-years wide, yet extends 200,000 light-years, about the distance between the Milky Way Galaxy and its companion, the Large Magellanic Cloud. Finding such an extremely narrow and long ionised gas cloud is a first in astronomy.
The filament is associated with a galaxy called D100. It is one of thousands that belong to the Coma Cluster of galaxies, which lies in the direction of the constellation Coma Berenices. The Coma Cluster appears to be moving away from Earth at a speed of about 7,000 kilometres per second. When the researchers discovered the unusual filament, they were using Subaru's prime focus camera (Suprime-Cam) with a special filter designed to pass visible light from ionised hydrogen gas that is moving away from Earth at the same speed as the Coma Cluster. Once they discovered the filament, follow-up observations with Subaru's Faint Object Camera and Spectrograph (FOCAS) showed that the recession speed of the filament is closest to the recession speed of D100, which is located at one end of the filament.
The researchers had no particular reason to expect that D100 would be associated with a filamentary gas cloud. Jets emanating from black holes that are actively swallowing matter at the centres of galaxies are a typical source of long, narrow structures of ionised gas on galactic scales. However, D100 doesn't have such an active galactic nucleus, nor does it emit x-rays or radio waves that typically accompany jets observed in visible light.


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D100 and its filament of ionised hydrogen gas. D100 is the lower right galaxy, among the three galaxies in the centre of the image. The filament is the red streak.

"We don't know exactly what made this remarkable filament, but its unique structure and sheer existence holds clues to D100's past" - Masafumi Yagi, NAOJ researcher and first author of the scientific paper announcing the discovery.

When we observe D100 today, stars are forming only at the its centre. However, there is evidence that 250 million years in its past, stars were actively forming throughout D100. The abrupt halt in star formation is probably related to D100's location in a cluster of galaxies where interactions with ambient gas and other galaxies can interfere with the star formation within a galaxy.

"The galaxy D100 and its filament of ionised gas tell an important story about what actually happens in a cluster of galaxies. As we look deeper for hydrogen emission, we'll probably find other similarly intriguing phenomena that may lead us to some unexpected scientific discovery" - Masafumi Yagi.

This research will be published in the April 20, 2007, edition of the Astrophysical Journal.

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