The CGCG254-021 galaxy and its big X-ray tail discovered

December 25, 2015

Using data from Chandra X-ray Observatory, astronomers have discovered an enormous tail of hot gas following a galaxy known as CGCG254-021. The gas was stripped from the galaxy as it travels through the Zwicky 8338 galaxy cluster. It is likely that the X-ray tail itself will start to produce the stars. This discovery is significant in understanding evolution of galaxies inside galaxy clusters.

CGCG254-021 and its X-ray tail. Credits: NASA/CXC/University of Bonn/G. Schellenberger et al; INT

In the image above, you can see the CGCG254-021 galaxy being followed by a tail of hot gas. The gas in the tail reaches temperatures of about ten million degrees and emits X-rays that Chandra Observatory picked up. The X-ray tail is 250,000 light-years long, more than double the diameter of the Milky Way galaxy.

The study was led by Gerrit Schellenberger of the University of Bonn, Germany. The paper with the results was published in November issue of Astronomy and Astrophysics magazine.

The galaxy CGCG254-021 is part of Zwicky 8338 galaxy cluster which includes thousands of other galaxies, pools of hot gas between them and presumably dark matter. 

"Since galaxy clusters are so enormous, they play a critical role in understanding how our Universe evolves," said Schellenberger. "To understand galaxy clusters we need to understand how their galaxies change with time, and these X-ray tails provide an important element."

So, what caused the gas to be stripped from the galaxy?

These galaxies are not static of course, they move. As CGCG254-021 travels through huge pools of hot gas, this movement exerts pressure on the gas inside the galaxy. With time, this gas leaves the CGCG254-021 and begins following it as a tail.

Before this happened, CGCG254-021 possibly had the highest star birth-rate in the Zwicky 8338 cluster. Since losing most of the gas from which stars form, it is unable to produce new stars. However, its X-ray tail may begin with star production in the future.

X-ray tail has a head, shows bow shock and is separated from the CGCG254-021 galaxy.

As mentioned, the gas in the X-ray tail has the temperature ten million degrees, which is still three times cooler than the pool of gas through which both CGCG254-021 and the tail are moving. Some areas of the tail are hotter than others. The part closest to the galaxy, the head, is cooler than the rest of the tail. The head is also the richest in elements heavier than helium.

"This tail is a vivid example of how dynamic galaxy clusters are, as we may be seeing the transformation of a galaxy as it moves through the cluster," added Schellenberger. "Also, the material in the tail includes not only hydrogen but heavier elements, and could spawn a new generation of stars trailing behind the galaxy."

There’s also an indication of bow shock just in front of the head of the X-ray tail, similar to the bow shock an airplane makes when it breaks the sound barrier. Furthermore, between the galaxy and the tail, there is a gap that indicates complete separation of the two.

"The large separation between the galaxy and the tail might be telling us that the gas has been completely stripped off the galaxy," said co-author Thomas Reiprich, also from Bonn, Germany. "In effect, the tail has been cut off from the galaxy."

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