November 1, 2012
Fermi Helps Measure Ancient Starlight In Universe
[ Watch the Video: NASA's Fermi Explores the Early Universe ]
Astronomers have made the most accurate measurement of starlight in the universe and have used this to establish the total amount of light from all of the stars that have ever shone.
A team used data from NASA's Fermi Gamma-Ray Space Telescope to find the total sum of starlight in the universe, or the extragalactic background light (EBL).
Gamma rays are the most energetic form of light, and since Fermi's launch, it has observed the entire sky in high-energy gamma rays every three hours to create the most detailed map of the universe ever known.
"The optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine, and this creates a fossil radiation field we can explore using gamma rays from distant sources," said lead scientist Marco Ajello, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics.
EBL functions as a kind of cosmic fog to gamma rays, so the team was able to look at the EBL by studying gamma rays from 150 blazars, or galaxies powered by black holes.
"With more than a thousand detected so far, blazars are the most common sources detected by Fermi, but gamma rays at these energies are few and far between, which is why it took four years of data to make this analysis," said team member Justin Finke, an astrophysicist at the Naval Research Laboratory in Washington.
NASA said as matter falls toward a galaxy's supermassive black hole, some of it is accelerated outward at nearly the speed of light in jets pointed in opposite directions. When these jets are aimed in the direction of Earth, the galaxy appears especially bright and is classified as a blazar.
Gamma rays produced in blazar jets travel across billions of light-years to Earth, and during their journey, they pass through an increasing fog of visible and ultraviolet light.
Every now and then, a gamma ray collides with starlight and transforms into a pair of particles. After this takes place, the gamma ray light is lost, and the process dampens the gamma ray signal the same way as fog dims a lighthouse.
Scientists have used blazar studies to determine how many gamma rays should be emitted at different energies. The more distant blazars show fewer gamma rays at higher energies due to the cosmic fog.
The researchers determined the average gamma-ray attenuation across three distance ranges between 9.6 billion years ago and today.
Scientists were able to use this measurement to estimate the fog's thickness, and reported their findings in the journal Science Express.
"The Fermi result opens up the exciting possibility of constraining the earliest period of cosmic star formation, thus setting the stage for NASA's James Webb Space Telescope," according to Volker Bromm, an astronomer at the University of Texas, Austin. "In simple terms, Fermi is providing us with a shadow image of the first stars, whereas Webb will directly detect them."