Space

November 2, 2012

NASA’s Fermi measures cosmic ‘fog’ produced by ancient starlight

This plot shows the locations of 150 blazars (green dots) used in the EBL study. The background map shows the entire sky and was constructed from four years of gamma rays with energies above 10 billion electron volts detected by Fermi. The plane of our Milky Way galaxy runs along the middle of the plot. The Fermi LAT instrument is the first to detect more than 500 sources in this energy range.

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 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. 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, 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 Nov. 1 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.




All of this week's top headlines to your email every Friday.


 
 

 

Headlines July 23, 2014

News: Israel’s Iron Dome defense in line for tripled U.S. spending - Israel’s iron Dome missile defense system may end up getting triple the U.S. funding that the Defense Department sought for it in March. Ukraine asked U.S. for systems to counter Russian missiles - A month before the United States says a Russian missile likely brought...
 
 

News Briefs July 23, 2014

U.S. military deaths in Afghanistan at 2,194 As of July 22, 2014, at least 2,194 members of the U.S. military had died in Afghanistan as a result of the U.S.-led invasion of Afghanistan in late 2001, according to an Associated Press count. The AP count is three less than the Defense Department’s tally. At least...
 
 
Raytheon photograph

Raytheon completes key Air, Missile Defense Radar reviews

Raytheon photograph Partially-populated, full-sized Air and Missile Defense Radar array. Raytheon has completed two critical program reviews for the new Air and Missile Defense Radar, the U.S. Navy’s next generation integ...
 

 
Insitu photograph

Insitu demonstrates long endurance capabilities of Integrator unmanned aircraft

Insitu photograph Insitu’s Integrator unmanned aircraft recovers via SkyHook; the aircraft recently completed a 24-hour endurance flight. Insitu announced July 22 the successful 24-hour flight of its Integrator unmanned a...
 
 

NASA partners punctuate summer with spacecraft development advances

Spacecraft and rocket development is on pace this summer for NASA’s aerospace industry partners for the agency’s Commercial Crew Program as they progress through systems testing, review boards and quarterly sessions under their† Space Act Agreements with the agency. NASA engineers and specialists continue their review of the progress as the agency and partners move...
 
 

U.S. Navy selects Northrop Grumman for ship self-defense system

The U.S. Navy has awarded Northrop Grumman a $12 million task order for a full range of engineering services to continue modernizing the Ship Self-Defense System Mark 2. The contract has a potential value of $61 million over five years, if all options are exercised. SSDS MK2 is a combat system designed for anti-air defense...
 




0 Comments


Be the first to comment!


Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>