Giant ARC

A newly discovered crescent of galaxies spanning 3.3 billion light-years is among the largest known structures in the universe and challenges some of astronomers' most basic assumptions about the cosmos.

The epic arrangement, called the Giant Arc, consists of galaxies, galactic clusters, and lots of gas and dust. It is located 9.2 billion light-years away and stretches across roughly a 15th of the observable universe.

Its discovery was "serendipitous," Alexia Lopez, a doctoral candidate in cosmology at the University of Central Lancashire (UCLan) in the U.K., told Live Science. Lopez was assembling maps of objects in the night sky using the light from about 120,000 quasars — distant bright cores of galaxies where supermassive black holes are consuming material and spewing out energy.

As this light passes through matter between us and the quasars, it is absorbed by different elements, leaving telltale traces that can give researchers important information. In particular, Lopez used marks left by magnesium to determine the distance to the intervening gas and dust, as well as the material’s position in the night sky.

In this way, the quasars act "like spotlights in a dark room, illuminating this intervening matter," Lopez said.

In the midst of the cosmic maps, a structure began to emerge. "It was sort of a hint of a big arc," Lopez said. "I remember going to Roger [Clowes] and saying 'Oh, look at this.'"

Clowes, her doctoral adviser at UCLan, suggested further analysis to ensure it wasn't some chance alignment or a trick of the data. After doing two different statistical tests, the researchers determined that there was less than a 0.0003% probability the Giant Arc wasn't real. They presented their results on June 7 at the 238th virtual meeting of the American Astronomical Society.  TO READ MORE, CLICK HERE...