BULK SOURCE OF UNIVERSE'S GAMMA RAYS IDENTIFIED

Scientists at Columbia University and Barnard College have found that the majority of the gamma rays outside of our galaxy are likely emitted by galaxy clusters and other massive structures. This may resolve a 30-year-old mystery as to the origin of the Universe's gamma-ray background.

The finding may also provide new insight about how structure formed in the Universe, as well as the nature of magnetic fields in the intergalactic medium, of which little is currently known.

Caleb Scharf of Columbia University and Reshmi Mukherjee of Barnard College publish this result in an upcoming issue of the Astrophysica Journal. The finding is based on the analysis of a nine-year record of gamma rays arriving at Earth from deep space, collected by NASA's Compton Gamma Ray Observatory throughout the 1990s.

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"This result not only resolves the question of where all these gamma rays are coming from, but provides a new probe of the gravity-driven picture of structure formation in the Universe," said Scharf.

Gamma rays are the highest-energy form of light. In the Milky Way galaxy, gamma rays are largely produced when cosmic rays, atomic particles moving at near light speed, collide with interstellar gas. Black holes and neutron stars are also sources of gamma rays.

Galaxy clusters are the largest gravitationally bound structures in the Universe. The clusters in this analysis contain up to several thousand Milky Way-size galaxies, and they are still accumulating material (gas and whole galaxies) from the surrounding space.

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The finding announced today supports a theory of gamma-ray production posed by Prof. Avi Loeb of Harvard University and Prof. Eli Waxman of the Weizmann Institute in Rehovot, Israel. Not unlike a black hole, the sheer mass of a cluster serves as a gravitational drain, drawing in matter at speeds of up to a thousand miles per second. Electrons in this flow are accelerated, with an additional boost from magnetic fields, to near light speed and collide with microwave light, the afterglow from the big bang known as the cosmic microwave background.

These microwave light particles, or photons, are bumped up to the gamma-ray photon energy level. The gamma rays form a halo around the galaxy clusters. Other scientists, however, have suggested that the bulk of the gamma-ray background is produced not by this mechanism but by quasar-type galaxies, called blazars, each powered by a supermassive black hole. This background was discovered by NASA's second Small Astronomy Satellite (SAS-2) in the early 1970s.

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Scharf and Mukherjee's new research compared a catalog of 2,469 galaxy clusters with the Compton database. Using sophisticated statistical techniques, they showed that the sky surrounding the most massive clusters was systematically brighter in gamma rays than other regions.

"The more massive the cluster (and greater the gravitational potential), the brighter the gamma-ray halo," said Mukherjee. "The enhancement observed was very similar to that predicted by the Loeb-Waxman theory."

The result announced today also supports the theory of the cosmic web. Scientists say that matter in the Universe forms a cosmic web, in which galaxies are formed along filaments of ordinary matter and dark matter like pearls on a string. Clusters form at the intersection of these filaments. The electrons that fuel the gamma-ray production rush into clusters along these rivers or filaments of matter connecting galaxies and clusters. Thus, gamma rays serve as probes to the early structure-forming epoch of the Universe.

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Gamma ray halos around clusters also provides a means to measure intergalactic magnetic fields. Two of the three variables to measure magnetic fields are known: the mass of galaxy clusters and the distribution of the microwave background. The third variable is electron efficiency, which can now be measured by virtue of gamma-ray production.

The Gamma-ray Large Area Space Telescope (GLAST), scheduled for launch in 2006, should resolve gamma-ray haloes around galaxies with unprecedented clarity. GLAST could measure intergalactic magnetic fields and watch the formation of structure in the universe through its gamma-ray eyes, the scientists said.

nasa spitzer first to crack open light of faraday world.

NASA's Spitzer First to Crack Open Light of Faraway Worlds
NASA's Spitzer Space Telescope has captured for the first time enough light from planets outside our solar system, known as exoplanets, to identify molecules in their atmospheres. The landmark achievement is a significant step toward being able to detect possible life on rocky exoplanets and comes years before astronomers had anticipated. "This is an amazing surprise," said Spitzer project scientist Dr. Michael Werner of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We had no idea when we designed Spitzer that it would make such a dramatic step in characterizing exoplanets." Spitzer, a space-based infrared telescope, obtained the detailed data, called spectra, for two different gas exoplanets. Called HD 209458b and HD 189733b, these so-called "hot Jupiters" are, like Jupiter, made of gas, but orbit much closer to their suns. Image right: NASA's Spitzer Space Telescope was recently used to capture spectra, or molecular fingerprints, of two "hot Jupiter" worlds like the one depicted here. This is the first time a spectrum has ever been obtained for an exoplanet, or a planet beyond our solar system. Image credit: NASA/JPL-Caltech + Animation and caption + Related images and briefing materials The data indicate the two planets are drier and cloudier than predicted. Theorists thought hot Jupiters would have lots of water in their atmospheres, but surprisingly none was found around HD 209458b and HD 189733b. According to astronomers, the water might be present but buried under a thick blanket of high, waterless clouds. Those clouds might be filled with dust. One of the planets, HD 209458b, showed hints of tiny sand grains, called silicates, in its atmosphere. This could mean the planet's skies are filled with high, dusty clouds unlike anything seen around planets in our own solar system. "The theorists' heads were spinning when they saw the data," said Dr. Jeremy Richardson of NASA's Goddard Space Flight Center, Greenbelt, Md. "It is virtually impossible for water, in the form of vapor, to be absent from the planet, so it must be hidden, probably by the dusty cloud layer we detected in our spectrum," he said. Richardson is lead author of a Nature paper appearing Feb. 22 that describes a spectrum for HD 209458b. In addition to Richardson's team, two other groups of astronomers used Spitzer to capture spectra of exoplanets. A team led by Dr. Carl Grillmair of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, Calif., observed HD 189733b, while a team led by Dr. Mark R. Swain of JPL focused on the same planet in the Richardson study, and came up with similar results. Grillmair's results will be published in the Astrophysical Journal Letters. Swain's findings have been submitted to the Astrophysical Journal Letters. A spectrum is created when an instrument called a spectrograph splits light from an object into its different wavelengths, just as a prism turns sunlight into a rainbow. The resulting pattern of light, the spectrum, reveals "fingerprints" of chemicals making up the object. Until now, the only planets for which spectra were available belonged in our own solar system. The planets in the Spitzer studies orbit stars that are so far away, they are too faint to be seen with the naked eye. HD 189733b is 370 trillion miles away in the constellation Vulpecula, and HD 209458b is 904 trillion miles away in the constellation Pegasus. That means both planets are at least about a million times farther away from us than Jupiter. In the future, astronomers hope to have spectra for smaller, rocky planets beyond our solar system. This would allow them to look for the footprints of life -- molecules key to the existence of life, such as oxygen and possibly even chlorophyll. "With these new observations, we are refining the tools that we will one day need to find life elsewhere if it exists," said Swain. "It's sort of like a dress rehearsal." Spitzer was able to tease out spectra from the feeble light of the two planets through what is known as the "secondary eclipse" technique. In this method -- first used by Spitzer in 2005 to directly detect the light from an exoplanet for the first time ( http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/index.shtml ) -- a so-called transiting planet is monitored as it circles behind its star, temporarily disappearing from our Earthly point of view. By measuring the dip in infrared light that occurs when the planet disappears, Spitzer can learn how much light is coming solely from the planet. The technique will work only in infrared wavelengths, where the planet is brighter than in visible wavelengths and stands out better next to the overwhelming glare of its star. In the new studies, Spitzer's spectrograph, which measures infrared light at a range of wavelengths, stared at the two transiting planets as they orbited their stars. This allowed the astronomers to subtract the spectra of the stars from the spectra of the planets plus their stars to obtain spectra of the planets alone. "When we first set out to make these observations, they were considered high risk because not many people thought they would work," said Grillmair. "But Spitzer has turned out to be superbly designed and more than up to the task." Previous observations of HD 209458b by NASA's Hubble Space Telescope revealed individual elements, such as sodium, oxygen, carbon and hydrogen, that bounce around the very top of the planet, a region higher up than that probed in the Spitzer studies and a region where molecules like water would break apart. To do this, Hubble measured changes in the light from the star, not the planet, as the planet passed in front. The observations indicated less sodium than predicted, which again supports the idea that the planet is socked in with high clouds. Astronomers hope to use Spitzer for additional studies of transiting exoplanets, which are those that cross in front of their stars from our point of view. Of the approximately 200 known exoplanets, 14 are transiting. At least three of these in addition to HD 209458b and HD 189733b are candidates for obtaining spectra. Further spectral studies of HD 209458b and HD 189733b will also yield more information about the planets' atmospheres. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph was built by Cornell University, Ithaca, N.Y. Its development was led by Dr. Jim Houck of Cornell.