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.


 
 

 
nasa-hubble

NASA unveils celestial fireworks as official image for Hubble 25th Anniversary

The brilliant tapestry of young stars flaring to life resemble a glittering fireworks display in the 25th anniversary NASA Hubble Space Telescope image, released to commemorate a quarter century of exploring the solar system an...
 
 
NASA illustration

NASA awards radiation challenge winners, launches next round

NASA illustration This illustration depicts our heliosphere, showing the approximate locations of Voyager 1 and Voyager 2 spacecraft. Galactic cosmic rays originate outside the heliosphere and stream in uniformly from all direc...
 
 
NASA photograph

Celebrate with NASA as agency commemorates Hubble’s 25th anniversary

NASA’s Hubble Space Telescope is turning 25 this year. The observatory has transformed our understanding of our solar system and beyond, and helped us find our place among the stars. NASA is celebrating the Hubble Space T...
 

 

ULA unveils America’s new rocket

https://www.youtube.com/watch?v=emmeil-0u5k&feature=player_embedded United Launch Alliance unveiled its Next Generation Launch System April 13 at the 31st Space Symposium. The new rocket, Vulcan, will transform the future of space by making launch services more affordable and accessible. The NGLS brings together decades of experience on ULA’s reliable Atlas and Delta vehicles, combin...
 
 
NASA/JHU APL/Carnegie Institution of Washington

NASA spacecraft achieves unprecedented success studying Mercury

NASA/JHU APL/Carnegie Institution of Washington NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft traveled more than six and a half years before it was inserted into orbit around Merc...
 
 

NASA selects American small business, research institution projects for further development

NASA has selected 149 research and technology proposals from American small businesses and research institutions that will enable NASA’s future missions into the solar system and beyond while benefiting America’s technology-driven economy right here on Earth. The selected proposals now will enter into negotiations for contract awards as part of Phase II of the agency’s...
 




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>