Abell 1644 is located about 15 degrees north of the center of the Shapley
supercluster, at the same distance; this corresponds to a physical separation of
only about 50 Mpc (~160 Million lightyears). A1644 as well as other X-ray
emitting galaxy clusters, primarily from the supercluster, are shown below
(circles) on top of a distribution of 2MASS galaxies (Galactic coordinates).
A1644 itself is a rather exceptional complex double cluster. The figure below shows the X-ray surface brightness distribution (contours) and the intracluster gas temperature distribution (blue = cool = 30 Million degrees and yellow/red = hot = 60 Million degrees) as obtained with data from the ESA X-ray satellite observatory XMM-Newton. Note the cool trail that the upper subcluster appears to be leaving behind.
The cool cluster centers of both subclusters, as imaged by NASA's Chandra mission, are shown below. In these false color images - in order to provide for some confusion - blue signifies hotter emission than red. One notes that the complexity visible on the largest scales (the supercluster environment), continues through the cluster scale (previous figure) down to the very centers.
Now here's an animation of a hydrodynamic simulation of what we think might have happened in A1644: a smaller subcluster has passed by the main subcluster off-axis and a fraction of its gas has been stripped off during this process (click on image; darker = cooler temperature). This would make A1644 the cluster exhibiting one of the largest scale stripping processes ever observed.
Below are the XMM-Newton temperature map plus X-ray surface brightness contours and a snapshot of the temperature and surface brightness distribution of the hydrodynamical simulation combined in a single image.
Click here to check out the ESA XMM-Newton Image Gallery take on A1644.
Click here to get the HEASARC Picture of the Week view.
Check out a more technical description, published in The Astrophysical Journal (Reiprich, Thomas H., Sarazin, Craig L., Kempner, Joshua C., and Tittley, Eric, ApJ, 608, 179) in Summer 2004. Please cite paper (ApJ, 608, 179) and webpage (http://A1644.dark-energy.net) when using any of the above.