November 13, 2012
Quasars Used To Probe Dark Energy Over 10 Billion Years In The Past
Lee Rannals for redOrbit.com - Your Universe Online
“No technique for dark energy research has been able to probe this ancient era before, a time when matter was still dense enough for gravity to slow the expansion of the universe, and the influence of dark energy hadn´t yet been felt,” BOSS principal investigator David Schlegel, an astrophysicist in the Physics Division of the U.S. Department of Energy´s Lawrence Berkeley National Laboratory (Berkeley Lab), said in a prepared statement.
“In our own time, expansion is accelerating because the universe is dominated by dark energy. How dark energy effected the transition from deceleration to acceleration is one of the most challenging questions in cosmology.”
The first priority for BOSS was to survey normal bright galaxies with redshifts up to 0.8, which are about seven billion years old. BOSS announced over 300,000 galaxies were surveyed during this phase. For the second part of the survey, the team studied quasars.
“Quasars are the brightest objects in the sky, and therefore the only credible way to measure spectra out to redshift 2.0 and beyond,” Schlegel said. “At these redshifts there are a hundred times more galaxies than quasars, but they´re too faint to use for BAO.”
As light of a quasar passes through clouds of intergalactic gas on its way towards Earth, its spectrum develops hydrogen absorption lines known as Lyman-alpha forest. Each absorption line in the "forest" reveals where the quasar's light has passed through.
Similar to a single flashlight seen through the fog, the different prominences and redshifts of the individual absorption lines varies with distance along with line of sight.
The distribution of intervening gas clouds can be mapped in three dimensions when there is enough quasars that are close enough together and covering a wide expanse of the sky.
“When I presented this idea to a conference of cosmologists in 2003, they thought it was crazy,” said Martin White, who is a professor of physics and astronomy at the University of California at Berkeley. “Nine years later, BOSS has shown that it´s an amazingly powerful technique. It has succeeded beyond our wildest dreams.”
BOSS devoted 10 percent of the two million individual targets during its five-year run to possible quasars. The targets had to be chosen from photometric data, but the team was only sure the quasars were real after the spectra was taken.
“We had one thing in our favor,” said Berkeley Lab´s Nicholas Ross, who led the target selection team. “It didn´t matter what type of quasar it was, as long as it was high redshift. To identify them we could use ultraviolet data, near-infrared data — any method, any trick — because we were only going to use them as backlights to probe the intervening clouds of gas. Ultimately we found that all our selection algorithms worked well.”
The initial Lyman-alpha result is based on a third of the volume of space that BOSS will ultimately map, and it includes 60,369 quasars confirmed by visual inspection. However, the team discarded many of them to help simplify the search for baryon acoustic oscillations (BAO).
“To get BAO out of the data, we have to remove artifacts that distort the signal we´re looking for,” said Berkeley Lab´s Bill Carithers.
The team rejected broad-absorption line quasars, which has a spectra that have been smeared out by extremely fast-moving gas from the quasar´s active center. They also rejected another class of quasars whose spectra have “damped Lyman-alpha troughs." This is when “when the light from the quasar runs into a very, very large clump of gas, so large it wipes out the forest," Carithers say.
The techniques used in the report were developed by Andreu Font-Ribera and his collaborators among BOSS´s French Participation Group. They helped produce a picture of density distributions that "gives us a first look at BAO in this previously hidden region," according to Berkeley Lab´s Stephen Bailey.
“There is no other credible way we could have measured BAO at redshifts of two or more. Five years ago it was chancy, but it was the only proposal on the table," Schelgel said. "We could have failed in any number of ways, but nature was good to us.”
BOSS will continue to move on and collect more than a million-and-a-half galaxies, and over 160,000 quasars before the third phase of SDSS is complete.