Press Release

Unusual Streak of Ionized Gas Hints at Galaxy's Past

March 5, 2007

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 ionized 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 ionized 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 kilometers 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 ionized 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 centers of galaxies are a typical source of long, narrow structures of ionized 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.

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

When we observe D100 today, stars are forming only at the its center. 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. Says Yagi, "the galaxy D100 and its filament of ionized 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."

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

Observation Details:
Object: D100 and vicinity in the Coma Cluster of galaxies
Telescope and Focus: Subaru Telescope (Effective aperture 8.2 meters), Prime Focus
Camera: Subaru Prime Focus Camera (Suprime-Cam)
Filter: Narrow-band H-alpha (671 nm), broadband B (450 nm), broadband R (650nm)
Observation Dates (UT): 2006/04/28, 2006/05/03, (FOCAS spectroscopy 2006/06/23)
Exposure Times: 37.5 min (B), 56 min (R), 246 min (H alpha)
Field of View: Approx. 5.1x5.1 arcminutes
Orientation: North is right; East is up
Coordinates: RA 13h 00m Dec. +27d 53m (J2000.0)
Constellation: Coma Berenices

Figure 1: D100 and its filament of ionized hydrogen gas. D100 is the lower right galaxy, among the three galaxies in the center of the image. The filament is the red streak extending to the upper right of D100. (This is a color composite image with the red, green, and blue colors assigned to the H-alpha, R, and B-band data, respectively.)

Midium resolution
High　resolution

Figure 2: An image of D100 and its filament processed to emphasize the filament. The three black dots in the center correspond to the locations of the three galaxies in Figure 1. The filament is the white streak extending from one galaxy to the upper right. (This image shows R-band broadband data subtracted from the H-alpha narrowband data. Emission of H-alpha shows up as white while absorption of H-alpha shows up as black. The fact that the galaxies are absorbing H-alpha light is an indication that they have only recently ceased active star formation.)

Note 1: Since the galaxy is 300 million light-years away, we observe the galaxy as it was 300 million years ago, the time it took light from the galaxy to reach Earth. The galaxy was actively forming stars 250 million years before it emitted the light that is now reaching us.



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