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.


 
 

 
LM-MUOS

U.S. Navy, Lockheed Martin ready to launch MUOS-4 Aug. 31

The U.S. Navy and Lockheed Martin are ready to launch the fourth Mobile User Objective System secure communications satellite, MUOS-4, from Cape Canaveral Air Force Station, Fla., Aug. 31 aboard a United Launch Alliance Atlas V...
 
 

NASA seeks proposals for extreme environment solar arrays

NASA’s space technology program is seeking proposals to develop solar array systems for space power in high radiation and low solar energy environments. In the near future, NASA will need solar cells and arrays for multiple applications in robotic and human space exploration missions. Because these systems were traditionally developed for operation near Earth, there...
 
 

NASA awards contract for construction of new mission launch command center

NASA has awarded a contract to Harkins Contracting Inc. of Salisbury, Maryland, for the construction of a new Mission Launch Command Center at the agency’s Wallops Flight Facility in Wallops Island, Va. The new 14,174 square-foot facility will serve as the hub for interfacing with and controlling rockets, their payloads and associated launch pad support...
 

 
NASA photograph

NASA concludes series of engine tests for next-gen rocket

NASA photograph The RS-25 engine fires up for a 535-second test Aug. 27, 2015 at NASA’s Stennis Space Center near Bay St. Louis, Miss. This is the final in a series of seven tests for the development engine, which will pr...
 
 
LM-satellite

Lockheed Martin makes tiny satellite cooling system

Lockheed Martin scientists are packing three times the power density into a key satellite cooling system whose previous design is already the lightest in its class. This project continues the company’s effort to reduce co...
 
 
Northrop Grumman photograph by Bob Brown

Northrop Grumman delivers telescope structure for James Webb Space Telescope

Northrop Grumman photograph by Bob Brown Northrop Grumman employees preparing the telescope structure, for NASA’s James Webb Space Telescope for shipment to Goddard Space Flight Center in Greenbelt, Md. REDONDO BEACH, Cal...
 




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=""> <s> <strike> <strong>