AUSTIN, TEXAS--Astronomers have identified what may be the most massive single structure yet seen in the universe. The structure, a ribbon of exceptionally rich clusters of galaxies, stretches across at least 400 million light-years of the southern hemisphere sky, according to research presented today at a meeting of the American Astronomical Society.
Researchers parse the universe into a hierarchy of structures. Galaxies like our own Milky Way tend to clump into tight groups of a few to dozens of galaxies, which form looser clusters with hundreds or thousands of members. Finally, ten or more clusters can assemble into vast superclusters, which astronomers consider the largest coherent "objects" in the cosmos. These structures seem to form spidery filaments, leaving behind gaping voids.
A team led by astronomer David Batuski of the University of Maine, Orono, performed the most thorough analysis to date of a supercluster in the constellation Aquarius, using a 3.6-meter telescope at the European Southern Observatory in Chile. To map the supercluster, they gauged the distances to 737 galaxies in 46 of its clusters. About one-third of the richest clusters--those containing at least 50 bright galaxies and thousands of fainter ones--line up in a thick chain between 1 billion and 1.4 billion light-years from Earth. Astronomers have never spotted filaments of galaxies that approached this supercluster's total mass, Batuski says. A knot of six rich clusters within the chain is crammed into a space just a few tens of millions of light-years across--100 times more tightly packed than other galaxies in the region.
Because gravity's attraction is relentless but modest, it requires almost the universe's lifetime to form superclusters like this one, notes astronomer J. Patrick Henry of the University of Hawaii, Honolulu. Its formation probably was triggered by subtle irregularities in the big bang, adds Batuski. "They are quite difficult to find, because they are in the process of being born," Henry says. "We are watching the birth of the newest, largest things in the universe."