Credit: X-ray: NASA/CXC/MSU/M.Sun et al; H-alpha/Optical: SOAR (MSU/NOAO/UNC/CNPq-Brazil)/M.Sun et al.

NASA's space-based Chandra X-ray Observatory (CXO) recently pictured a curious tailed galaxy. The tail formed as the galaxy plunged into a cluster of galaxies, expelling material and forming stars behind it. In the CXO image, the galactic debris extends for 200,000 light years, twice the breadth of the Milky Way Galaxy.

The tailed galaxy is known as ESO 137-001. The above image is a composite of CXO's X-ray image and visual light images obtained with the Southern Astrophysical Research telescope (SOAR) in Chile.

The tail formed as gas was stripped from ESO 137-001 as it fell toward the center of a giant cluster of galaxies, known as Abell 3627. The cluster is approximately 220 million light years away. This is relatively near by the cosmic distance scale. The X-ray image reveals an extremely hot gas of multimillion degrees temperatures. Optical images obtained in the hydrogen alpha (H-alpha) light show a much cooler gas.

Astronomers found evidence that stars have been forming in the tail of gas. This discovery suggests that such "orphan" stars may be much more common in the Universe than previously thought.

"This is one of the longest tails we have ever seen," said Ming Sun of Michigan State University (MSU), the principal author of the study. "It turns out that this is a giant wake of creation, not of destruction."

Evidence for star formation in the tail includes 29 regions of ionized hydrogen that are glowing in H-alpha, emitted by newly formed stars. These regions are all downstream of the galaxy, lying in or near the tail. The researchers believe that the stars formed in the tail within the last 10 million years or so. This is a very short period of time by cosmic standards. Also, two X-ray sources near these regions provide additional evidence for star birth activity.

The observations show that the gas in the tail has formed millions of stars. As star formation needs large amounts of gas and dust, typically found within galaxies, astronomers have previously thought it unlikely that large numbers of stars would form outside a galaxy.

"This is not the first time that stars have been seen to form between galaxies," said team member Megan Donahue, also of MSU. "However the number of stars forming here is unprecedented."

The gas that formed the orphan stars was stripped out of its parent galaxy due to the pressure resulting from the motion of the galaxy through the multimillion degree gas that pervades the intergalactic space of the galaxy cluster. Eventually most of the gas will be worn out from the galaxy, depleting the raw material for new stars, and effectively terminating further star formation in the galaxy.

Further Reading

Chandra Press Release
http://chandra.harvard.edu/press/07_releases/press_092007.html

Aymen Mohamed Ibrahem
Senior Astronomy Specialist