Free Astronomy Magazine March-April 2016

SPACE CHRONICLES H ST image of massive gal- axy cluster IDCS J1426.5 +3508. [NASA, ESA, and M. Brodwin (Univ. of Missouri)] win of the University of Mis- souri at Kansas City, who led the study. "As one of the earli- est massive structures to form in the universe, this cluster sets a high bar for theories that attempt to explain how clusters and galaxies evolve." Galaxy clusters are the largest objects in the universe bound together by gravity. Because of their sheer size, scientists think it should take several billion years for them to form. The distance of IDCS J1426 means astronomers are ob- serving it when the universe was only 3.8 billion years old, implying that the cluster is seen at a very young age. The data from Chandra reveal a bright knot of X-rays near the middle of the cluster, but not exactly at its center. This overdense core has been dis- lodged from the cluster cen- ter, possibly by a merger with another developing cluster 500 million years prior. Such a merger would cause the X-ray-emitting, hot gas to slosh around like wine in a glass that is tipped from side to side. "Mergers with other groups and clusters of galaxies should have been more common so early in the history of the universe," said co-author Mi- chael McDonald of the Massachu- setts Institute of Technology in Cambridge, Massachusetts. "That appears to have played an im- portant part in this young cluster's rapid formation." Aside from this cool core, the hot gas in the rest of the cluster is very smooth and symmetric. This is an- other indication that IDCS J1426 formed very rapidly. In addition, as- tronomers found possible evidence that the abundance of elements heavier than hydrogen and helium in the hot gas is unusually low. This suggests that this galaxy cluster might still be in the process of en- riching its hot gas with these ele- ments as supernovae create heavier elements and blast them out of indi- vidual galaxies. "The presence of this massive galaxy cluster in the early universe doesn't upset our current understanding of cosmology," said co-author An- thony Gonzalez of the University of Florida in Gainesville, Florida. "It does, however, give us more infor- mation to work with as we refine our models." Evidence for other mas- sive galaxy clusters at early times has been found, but none of these matches IDCS J1426, with its combi- nation of mass and youth. The mass determination used three indepen- dent methods: a measurement of the mass needed to confine the hot X-ray-emitting gas to the cluster, the imprint of the cluster's gaseous mass on the cosmic microwave back- ground radiation, and the observed distortions in the shapes of galaxies behind the cluster, which are caused by the bending of light from the gal- axies by the gravity of the cluster. These results were presented at the 227th American Astronomical So- ciety meeting being held in Kissim- mee, Florida. n