Press Release

Subaru Witnesses Galactic Cannibalism in Action

November 18, 2004

The following release was received from Tohoku University Astronomical Institute and is reprinted here in its entirety for the convenience of our readers:
(Original Article:

Subaru telescope has witnessed a large galaxy in the act of devouring a small companion galaxy in a new image obtained by Yoshiaki Taniguchi (Tohoku University), Shunji Sasaki (Tohoku University), Nicolas Scoville (California Institute of Technology) and colleagues. The evidence is a wispy band of stars extending over 500 thousand light years, the faintest and longest known example of its kind.

Current theories of galaxy formation suggest that large galaxies like the Milky Way grow by consuming smaller dwarf galaxies. Evidence of this process can be found in our own galactic neighborhood. Some stars in the Milky Way appear to have once belonged to a small nearby galaxy called the Sagitarius Dwarf. Our closest large neighbor galaxy Andromeda also shows evidence for past galactic astronomy. However, in both cases these conclusions are inferred from "post-digestive" observations.

The destruction of dwarf galaxies is difficult to observe because dwarf galaxies are inherently faint and their light becomes increasingly diffuse as stars get pulled away by a larger galaxy. The only previously known observation of the destruction of a dwarf galaxy in progress is from the Advanced Camera for Surveys on the Hubble Space Telescope.

Taniguchi, Sasaki, Scoville and colleagues serendipitously discovered the large elliptical galaxy (COSMOS J100003+020146) pulling apart the dwarf galaxy (COSMOS J095959+020206) while observing an area of sky in the constellation Sextans to study the properties of galaxies over large scales in space and time. The pair of galaxies is about one billion light years away and the distance between the two galaxies is about 330 thousand light years.

The thin band of stars extending from the dwarf galaxy both toward and away from the large elliptical galaxy reveals that the gravity of the elliptical is tidally tearing the dwarf apart. Stars that are closest to the elliptical galaxy experience a stronger pull than stars in the center of the dwarf galaxy, and stars on the opposite side experience a weaker pull. As a result, the dwarf galaxy becomes stretched and looks as if it's being pulled from two opposite directions even though there is only one galaxy doing the pulling. This effect is comparable to how two areas on the opposite sides of Earth experience high tide at the same time even though there is only one Moon tugging on Earth's oceans.

The tidally torn strip of stars in the newly observed pair of galaxies is five times more extended and three times fainter in surface brightness than the one observed with Hubble Space Telescope. Subaru telescope's ability to gather large amounts of light and focus it into a superbly sharp image was essential for this new discovery.

As astronomers find more examples of galactic cannibalism in action, our knowledge of the history of galaxies should become increasingly vivid. Although no human alive today will be able to witness the ultimate of fate of the newly discovered pair, chances are the elliptical galaxy will be able to complete the meal it's begun and fully consume its neighbor.

Figure 1: The giant elliptical galaxy in the left hand side is COSMOS J100003+020146. The small galaxy that is linked to this giant galaxy with the faint tidal tail is COSMOS J095959+020206. Another tidal tail can also be seen at the opposite (right hand) side of this small galaxy.

Figure 2: The Andromeda galaxy (M 31) is shown at the left corner for reference (Sandage & Bedke 1994, The Carnegie Atlas of Galaxies (Washington: Carnegie Inst.)

Figure 3: The first galaxy threshing system found by Dr. Duncan Forbes's group (Forbes, D. et al. 2003, Science, 301, 1217)

Figure 4: The comparison between our galaxy threshing system (right) and that found by Dr. Duncan Forbes' group (left).

Figure 5: Our new galaxy system is located in a direction toward Sextans.

Search members

  • Shunji Sasaki (Tohoku University)
  • Yoshiaki Taniguchi (Tohoku University)
  • Takashi Murayama (Tohoku University)
  • Yasuhiro Shioya (Tohoku University)
  • Tohru Nagao (Arcetri Observatory)
  • Masaru Ajiki (Tohoku University)
  • Ryoko Sumiya (Tohoku University)
  • Norio Kaifu (NAOJ)
  • Hiroshi Karoji (NAOJ)
  • Nobuo Arimoto (NAOJ)
  • Yutaka Komiyama (NAOJ)
  • Satoshi Miyazaki (NAOJ)
  • Sadanori Okamura (University of Tokyo)
  • Kouji Ohta (Kyoto University)
  • Nick Scoville (Caltech)
  • Dave Sanders (IfA)
  • Bahram Mobasher (STScI)
  • Herve Aussel (IfA)



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