Dark energy camera aims for the cosmic dawn

There were no stars or galaxies, and the universe was filled with neutral hydrogen gas. In the next half billion years or so the first galaxies and stars appeared. Their energetic radiation ionised their surroundings, illuminating and transforming the universe.

This dramatic transformation, known as re-ionisation, occurred sometime in the interval between 300 million years and one billion years after the Big Bang. Astronomers are trying to pinpoint this milestone more precisely and the galaxies found in this study help in this determination.

"Before re-ionisation, these galaxies were very hard to see, because their light is scattered by gas between galaxies, like a car's headlights in fog," says Malhotra. "As enough galaxies turn on and 'burn off the fog' they become easier to see. By doing so, they help provide a diagnostic to see how much of the 'fog' remains at any time in the early universe."

To detect these galaxies, Malhotra and Rhoads have been using the Dark Energy Camera (DECam), one of the new powerful instruments in the astronomy field. DECam is installed at the National Optical Astronomy Observatory (NOAO)'s 4-meter Blanco Telescope, located at the Cerro Tololo Inter-American Observatory (CTIO), in northern Chile, at an altitude of 7,200 feet.

"Several years ago, we carried out a similar study using a 64-megapixel camera that covers the same amount of sky as the full moon," says Rhoads. "DECam, by comparison, is a 570-megapixel camera and covers 15 times the area of the full moon in a single image."

DECam was recently made even more powerful when it was equipped with a special narrowband filter, designed at ASU's School of Earth and Space Exploration (SESE), primarily by Rhoads and Zheng (who was a SESE postdoctoral fellow and is currently at the Shanghai Astronomical Observatory in China), with assistance from Alistair Walker of NOAO.

"We spent several months refining the design of the filter profile, optimising the design to get maximum sensitivity in our search" says Zheng, the lead author of this study.

The galaxy search using the ASU-designed filter and DECam is part of the ongoing "Lyman Alpha Galaxies in the Epoch of Reionisation" project (LAGER). It is the largest uniformly selected sample that goes far enough back in the history of the universe to reach cosmic dawn.

"The combination of large survey size and sensitivity of this survey enables us to study galaxies that are common but faint, as well as those that are bright but rare, at this early stage in the universe," says Malhotra.

Junxian Wang, a co-author on this study and the lead of the Chinese LAGER team, adds that "our findings in this survey imply that a large fraction of the first galaxies that ionised and illuminated the universe formed early, less than 800 million years after the Big Bang."

The next steps for the team will be to build on these results. They plan to continue to search for distant star forming galaxies over a larger volume of the universe and to further investigate the nature of some of the first galaxies in the universe.