NASA'S Fermi Measures Cosmic 'Fog' Produced By Ancient Starlight

Nov. 1, 2012

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Lynn Chandler
Goddard Space Flight Center, Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov

RELEASE: 12-385

NASA'S FERMI MEASURES COSMIC 'FOG' PRODUCED BY ANCIENT STARLIGHT

WASHINGTON -- Astronomers using data from NASA's Fermi Gamma-ray Space
Telescope have made the most accurate measurement of starlight in the
universe and used it to establish the total amount of light from all
the stars that have ever shone, accomplishing a primary mission goal.


"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 and
Cosmology at Stanford University in California and the Space Sciences
Laboratory at the University of California at Berkeley.

Gamma rays are the most energetic form of light. Since Fermi's launch
in 2008, its Large Area Telescope (LAT) observes the entire sky in
high-energy gamma rays every three hours, creating the most detailed
map of the universe ever known at these energies.

The total sum of starlight in the cosmos is known to astronomers as
the extragalactic background light (EBL). To gamma rays, the EBL
functions as a kind of cosmic fog. Ajello and his team investigated
the EBL by studying gamma rays from 150 blazars, or galaxies powered
by black holes, that were strongly detected at energies greater than
3 billion electron volts (GeV), or more than a billion times the
energy of visible light.

"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.

As matter falls toward a galaxy's supermassive black hole, some of it
is accelerated outward at almost the speed of light in jets pointed
in opposite directions. When one of the jets happens to be 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. During their journey, the gamma rays pass
through an increasing fog of visible and ultraviolet light emitted by
stars that formed throughout the history of the universe.

Occasionally, a gamma ray collides with starlight and transforms into
a pair of particles -- an electron and its antimatter counterpart, a
positron. Once this occurs, the gamma ray light is lost. In effect,
the process dampens the gamma ray signal in much the same way as fog
dims a distant lighthouse.

From studies of nearby blazars, scientists have determined how many
gamma rays should be emitted at different energies. More distant
blazars show fewer gamma rays at higher energies -- especially above
25 GeV -- thanks to absorption by the cosmic fog.

The farthest blazars are missing most of their higher-energy gamma
rays.

The researchers then determined the average gamma-ray attenuation
across three distance ranges between 9.6 billion years ago and today.


From this measurement, the scientists were able to estimate the fog's
thickness. To account for the observations, the average stellar
density in the cosmos is about 1.4 stars per 100 billion cubic
light-years, which means the average distance between stars in the
universe is about 4,150 light-years.

A paper describing the findings was published Thursday on 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," said Volker Bromm, an
astronomer at the University of Texas, Austin, who commented on the
findings. "In simple terms, Fermi is providing us with a shadow image
of the first stars, whereas Webb will directly detect them."

Measuring the extragalactic background light was one of the primary
mission goals for Fermi.

"We're very excited about the prospect of extending this measurement
even farther," said Julie McEnery, the mission's project scientist at
NASA's Goddard Space Flight Center in Greenbelt, Md.

Goddard manages the Fermi astrophysics and particle physics research
partnership. Fermi was developed in collaboration with the U.S.
Department of Energy with contributions from academic institutions
and partners in France, Germany, Italy, Japan, Sweden and the United
States.

For images and video related to this story, please visit:

http://go.nasa.gov/RsVN4F

For more information about NASA's Fermi Gamma-ray Space Telescope,
visit:

http://www.nasa.gov/fermi


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