Today marks 100 years since the publication of Einstein's general theory of relativity, and tomorrow the 150th anniversary of "Alice in Wonderland." So NASA thought it would be a perfect occasion to release a visible/X-ray image of the Cheshire Cat group of galaxies.
The Cheshire Cat is the result of a small merger between two groups of galaxies. The eyes represent the biggest galaxy in each group and they will eventually merge into a single massive body. The smile and the arcs around the cat face are from four different galaxies beyond the Cheshire Cat whose light is being magnified by the galaxy in the foreground.
The image is a remarkable testament to one of the predictions of general relativity. Einstein claimed that any massive object would warp space-time around itself, which could turn the object into a cosmic magnifying glass. This phenomenon was observed for the first time in 1919 by Eddington and it was the first test for general relativity.
The purple glow in the photo represents the extent of hot intergalactic gas within the group. Hydrogen gas is being heated to millions of degrees by the powerful supermassive black holes within the "left eye" and it indicates that the galaxies are racing towards each other at 1,350 kilometers per second (more than 3 million miles per hour).
Astronomers have discussed the future of the Cheshire Cat group in a recent paper published by the Astrophysical Journal. They expect it to turn into a fossil group, a possibly transient stage, where one giant elliptical galaxy dominates over a few smaller and fainter ones. The two groups that collided to form the Cheshire Cat are most likely former fossils and astronomers are interested in understanding the evolution of such systems.
After the Milky Way and Andromeda merge in about 5 billion years, the resultant elliptical galaxy might eventually be the center of a fossil group dominating a constellation of smaller satellite galaxies. You and I will be gone by then, but maybe "Alice in Wonderland" and general relativity will still be read and appreciated.