Galaxy Cluster [HGO2008]SDSS J1531+3414

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This new Hubble Space Telescope image shows two galaxies from the cluster SDSS J1531+3414. The two galaxies have been found to be merging into one and a "chain" of young stellar superclusters are seen winding around the galaxies' nuclei. The galaxies are surrounded by an egg-shaped blue ring caused by the immense gravity of the cluster bending light from other galaxies beyond it. Finding two elliptical galaxies merging is rare, but it is even rarer to find a merger between ellipticals rich enough in gas to induce star formation. Galaxies in clusters are generally thought to have been deprived of their gaseous contents; a process that Hubble has recently seen in action. Yet, in this image, not only have two elliptical galaxies been caught merging but their newborn stellar population is also a rare breed. The stellar infants - thought to be a result of the merger - are part of what is known as "beads on a string" star formation. This type of formation appears as a knotted rope of gaseous filaments with bright patches of new stars and the process stems from the same fundamental physics which causes rain to fall in droplets, rather than as a continuous column. Nineteen compact clumps of young stars make up the length of this "string", woven together with narrow filaments of hydrogen gas. The star formation spans 100,000 light years, which is about the size of our galaxy, the Milky Way. The strand is dwarfed, however, by the ancient, giant merging galaxies that it inhabits. They are about 330,000 light years across, nearly three times larger than our own galaxy. This is typical for galaxies at the center of massive clusters, as they tend to be the largest galaxies in the Universe. The electric blue arcs making up the spectacular egg-like shape framing these objects are a result of the galaxy cluster's immense gravity. The gravity warps the space around it and creates bizarre patterns using light from more distant galaxies. Astronomers have ruled out the possibility that the blue strand is also just a lensed mirage from distant galaxies and now their challenge is to understand the origin of the cold gas that is fuelling the growth of the stellar superclusters. Was the gas already in the merging galaxies? Did it condense like rain from the rapidly cooling X-ray plasma surrounding the two galaxies? Or, did it cool out of a shock in the X-ray gas as the ten-million-degree gaseous halos surrounding the galaxies collided together? Future observations with both space- and ground-based observatories are needed to unravel this mystery.
Image Credit: NASA, ESA/Hubble and Grant Tremblay (European Southern Observatory)
Image enhancement: Jean-Baptiste Faure