The largely-empty Universe can be a crowded place sometimes. Entire clusters of galaxies, whizzing by each other through the dark alleyways of space, will sometimes smash together, and, surprisingly, a superior force may be driven out from the center of the engagement. That seems to be what's happening in the case of "El Gordo", an object formed as one large galaxy cluster collides with a smaller one. El Gordo is an astounding object - it is the most massive, the hottest, brightest in X-rays of any known galaxy cluster at its distance or beyond. The image above, a composite optical image by HST and an X-ray image by Chandra, details the "dark" and "light" side of the ferocious battle between the two clusters of galaxies in El Gordo. Analysis of the HST optical image shows distinct warping of background galaxies by the gravitating mass of El Gordo. This distortion, caused by gravitational lensing, can be used to derive the spatial distribution of "dark matter" in the cluster. The distribution of dark matter is shown in blue in the image above. The X-ray data from the Chandra X-ray Observatory, shown in pink, reveals the spatial distribution of "normal" matter (the familiar type of stuff made of protons, neutrons and electrons). The Chandra and Hubble images show that, as in the case of the "Bullet Cluster", the dark matter is distributed outside the X-ray emitting "normal" matter. Astronomers believe this indicates that pressure produced by the collision in El Gordo caused the "normal" matter to slow down near the center of the collision; the "dark matter was not slowed down by the collision to the same degree, showing that dark matter interacts only via gravity.
CXC: Monster "El Gordo" Galaxy Cluster is Bigger than Thought
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