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NASA Celebrates Chandra X-Ray Observatory's 10th Anniversary
ScienceDaily (July 24, 2009) — Ten years ago, on July 23, 1999, NASA's Chandra X-ray Observatory was launched aboard the space shuttle Columbia and deployed into orbit. Chandra has doubled its original five-year mission, ushering in an unprecedented decade of discovery for the high-energy universe.
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With its unrivaled ability to create high-resolution X- ray images, Chandra has enabled astronomers to investigate phenomena as diverse as comets, black holes, dark matter and dark energy.
"Chandra's discoveries are truly astonishing and have made dramatic changes to our understanding of the universe and its constituents," said Martin Weisskopf, Chandra project scientist at NASA's Marshall Space Flight Center in Huntsville, Ala.
The science that has been generated by Chandra -- both on its own and in conjunction with other telescopes in space and on the ground -- has had a widespread, transformative impact on 21st century astrophysics. Chandra has provided the strongest evidence yet that dark matter must exist. It has independently confirmed the existence of dark energy and made spectacular images of titanic explosions produced by matter swirling toward supermassive black holes.
To commemorate the 10th anniversary of Chandra, three new versions of classic Chandra images will be released during the next three months. These images, the first of which is available Thursday, provide new data and a more complete view of objects that Chandra observed in earlier stages of its mission. The image being released today is of E0102-72, the spectacular remains of an exploded star.
"The Great Observatories program -- of which Chandra is a major part -- shows how astronomers need as many tools as possible to tackle the big questions out there," said Ed Weiler, associate administrator of NASA's Science Mission Directorate at NASA Headquarters in Washington. NASA's other "Great Observatories" are the Hubble Space Telescope, Compton Gamma-Ray Observatory and Spitzer Space Telescope.
"I am extremely proud of the tremendous team of people who worked so hard to make Chandra a success," said Harvey Tananbaum, director of the Chandra X-ray Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass. "It has taken partners at NASA, industry and academia to make Chandra the crown jewel of high-energy astrophysics."
Tananbaum and Nobel Prize winner Riccardo Giacconi originally proposed Chandra to NASA in 1976. Unlike the Hubble Space Telescope, Chandra is in a highly elliptical orbit that takes it almost one third of the way to the moon, and was not designed to be serviced after it was deployed.
Marshall manages the Chandra program for NASA's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center.
Adapted from materials provided by Chandra X-ray Center.
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Chandra X-ray Center (2009, July 24). NASA Celebrates Chandra X-Ray Observatory's 10th Anniversary. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/07/090723113519.htm
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To commemorate the 10th anniversary of Chandra, this new image of the supernova remnant known as E0102 is being released. First observed with Chandra shortly after its launch in 1999, this new version gives astronomers a much more detailed look at the debris of this exploded star. (Credit: X-ray: NASA/CXC/MIT/D.Dewey et al. & NASA/CXC/SAO/J.DePasquale; Optical: NASA/STScI)
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Simulations Illuminate Universe's First Twin Stars
ScienceDaily (July 10, 2009) — The earliest stars in the universe formed not only as individuals, but sometimes also as twins, according to a paper published July 19 in Science. By creating robust simulations of the early universe, astrophysicists Matthew Turk and Tom Abel of the Kavli Institute for Particle Astrophysics and Cosmology, located at the Department of Energy’s SLAC National Accelerator Laboratory, and Brian O'Shea of Michigan State University have gained the most detailed understanding to date of the formation of the first stars.
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"We used to think that these stars formed by themselves, but now we see from our computer simulations that sometimes they have siblings," said Turk. "These stars provide the seeds of next generation star formation, so by understanding them we can better understand how other stars and galaxies formed."
To make this discovery, the researchers created an extremely detailed computer simulation of early star formation. Into this virtual universe they sprinkled primordial gas and dark matter as it existed soon after the Big Bang, data they obtained from observations of the cosmic microwave background. This mostly uniform radiation—a faint glow of radio waves spread across the entire sky—contains subtle variations that reflect the beginning of all structure in the universe.
Turk, Abel and O'Shea ran five data-intensive simulations, each of which covered a 400 quadrillion cubic mile volume of the universe and took about three weeks to run on 64 processors. The simulations focused on the first Population III stars: massive, hot stars thought to have formed a mere several hundred million years after the Big Bang.
As the researchers watched their simulated universe evolve, waves of gas and dark matter swirled through the hot, dense universe. As the universe cooled, gravity began to draw the matter together into clumps. In areas rich with matter, stars began to form. And, in one out of the researchers' five simulations, a single cloud of dust and dark matter formed into "twin" stars: one with a mass equivalent to about 10 suns, and one with a mass equivalent to about 6.3 suns. Both of them were still growing at the end of the calculation and will likely grow to many times that mass.
"We ran five of these calculations starting from the beginning of the universe, and to our surprise one of them was special," said Abel. "This opens a whole new realm of research possibilities. These stars could evolve into two black holes, which could have created gravitational waves we could detect with an instrument like the Laser Interferometer Gravitational Wave Observatory and, if they fall into bigger black holes, for the Laser Interferometer Space Antenna. Or one of the stars could evolve into a black hole that could create gamma-ray bursts that we could detect with the Swift mission and the Fermi Gamma-ray Space Telescope."
Turk added: "This will help us fine-tune our models for how structure in the universe formed and evolved. Understanding the very early stars helps us understand what we see today. It even helps explain how and when some of the atoms now on earth and in our bodies were first formed."
Adapted from materials provided by DOE/SLAC National Accelerator Laboratory.
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DOE/SLAC National Accelerator Laboratory (2009, July 10). Simulations Illuminate Universe's First Twin Stars. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/07/090709170805.htm
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This computer-simulated image shows the formation of two high density regions (yellow) in the early universe, approximately 200 million years after the Big Bang. The cores are separated by about 800 times the distance between the Earth and the Sun, and are expected to evolve into a binary--or "twin"--star system.
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Simulations Illuminate Universe's First Twin Stars
ScienceDaily (July 10, 2009) — The earliest stars in the universe formed not only as individuals, but sometimes also as twins, according to a paper published July 19 in Science. By creating robust simulations of the early universe, astrophysicists Matthew Turk and Tom Abel of the Kavli Institute for Particle Astrophysics and Cosmology, located at the Department of Energy’s SLAC National Accelerator Laboratory, and Brian O'Shea of Michigan State University have gained the most detailed understanding to date of the formation of the first stars.
See also:
Space & Time
* Astrophysics
* Stars
* Big Bang
* Cosmology
* Black Holes
* Astronomy
Reference
* Supergiant
* Nucleosynthesis
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* Astrophysics
"We used to think that these stars formed by themselves, but now we see from our computer simulations that sometimes they have siblings," said Turk. "These stars provide the seeds of next generation star formation, so by understanding them we can better understand how other stars and galaxies formed."
To make this discovery, the researchers created an extremely detailed computer simulation of early star formation. Into this virtual universe they sprinkled primordial gas and dark matter as it existed soon after the Big Bang, data they obtained from observations of the cosmic microwave background. This mostly uniform radiation—a faint glow of radio waves spread across the entire sky—contains subtle variations that reflect the beginning of all structure in the universe.
Turk, Abel and O'Shea ran five data-intensive simulations, each of which covered a 400 quadrillion cubic mile volume of the universe and took about three weeks to run on 64 processors. The simulations focused on the first Population III stars: massive, hot stars thought to have formed a mere several hundred million years after the Big Bang.
As the researchers watched their simulated universe evolve, waves of gas and dark matter swirled through the hot, dense universe. As the universe cooled, gravity began to draw the matter together into clumps. In areas rich with matter, stars began to form. And, in one out of the researchers' five simulations, a single cloud of dust and dark matter formed into "twin" stars: one with a mass equivalent to about 10 suns, and one with a mass equivalent to about 6.3 suns. Both of them were still growing at the end of the calculation and will likely grow to many times that mass.
"We ran five of these calculations starting from the beginning of the universe, and to our surprise one of them was special," said Abel. "This opens a whole new realm of research possibilities. These stars could evolve into two black holes, which could have created gravitational waves we could detect with an instrument like the Laser Interferometer Gravitational Wave Observatory and, if they fall into bigger black holes, for the Laser Interferometer Space Antenna. Or one of the stars could evolve into a black hole that could create gamma-ray bursts that we could detect with the Swift mission and the Fermi Gamma-ray Space Telescope."
Turk added: "This will help us fine-tune our models for how structure in the universe formed and evolved. Understanding the very early stars helps us understand what we see today. It even helps explain how and when some of the atoms now on earth and in our bodies were first formed."
Adapted from materials provided by DOE/SLAC National Accelerator Laboratory.
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DOE/SLAC National Accelerator Laboratory (2009, July 10). Simulations Illuminate Universe's First Twin Stars. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/07/090709170805.htm
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This computer-simulated image shows the formation of two high density regions (yellow) in the early universe, approximately 200 million years after the Big Bang. The cores are separated by about 800 times the distance between the Earth and the Sun, and are expected to evolve into a binary--or "twin"--star system.
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Antimatter Positrons Explain Gamma Ray Mystery In Milky Way Galaxy
ScienceDaily (July 9, 2009) — A team of astrophysicists has solved a mystery that led some scientists to speculate that the distribution of certain gamma rays in our Milky Way galaxy was evidence of a form of undetectable “dark matter” believed to make up much of the mass of the universe.
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In two separate scientific papers, the most recent of which appears in the July 10 issue of the journal Physical Review Letters, the astrophysicists show that this distribution of gamma rays can be explained by the way antimatter positrons from the radioactive decay of elements, created by massive star explosions in the galaxy, propagate through the galaxy. Thus, the scientists said, the observed distribution of gamma rays is not evidence for dark matter.
“There is no great mystery,” said Richard Lingenfelter, a research scientist at UC San Diego’s Center for Astrophysics and Space Sciences who conducted the studies with Richard Rothschild, a research scientist also at UCSD, and James Higdon, a physics professor at the Claremont Colleges. “The observed distribution of gamma rays is in fact quite consistent with the standard picture.”
Over the past five years, gamma ray measurements from the European satellite INTEGRAL have perplexed astronomers, leading some to argue that a “great mystery” existed because the distribution of these gamma rays across different parts of the Milky Way galaxy was not as expected.
To explain the source of this mystery, some astronomers had hypothesized the existence of various forms of dark matter, which astronomers suspect exists—from the unusual gravitational effects on visible matter such as stars and galaxies—but have not yet found.
What is known for certain is that our galaxy—and others—are filled with tiny subatomic particles known as positrons, the antimatter counterpart of typical, everyday electrons. When an electron and positron encounter each other in space, the two particles annihilate and their energy is released as gamma rays. That is, the electron and positron disappear and two or three gamma rays appear.
”These positrons are born at nearly the speed of light, and travel thousands of light years before they slow down enough in dense clouds of gas to have a chance of joining with an electron to annihilate in a dance of death,” explains Higdon. “Their slowing down occurs from the drag of other particles during their journey through space. Their journey is also impeded by the many fluctuations in the galactic magnetic field that scatter them back and forth as they move along. All of this must be taken into account in calculating the average distance the positrons would travel from their birthplaces in supernova explosions.”
”Some positrons head towards the center of the Galaxy, some towards the outer reaches of the Milky Way known as the galactic halo, and some are caught in the spiral arms,” said Rothschild. “While calculating this in detail is still far beyond the fastest supercomputers, we were able to use what we know about how electrons travel throughout the solar system and what can be inferred about their travel elsewhere to estimate how their anti-matter counterparts permeate the galaxy.”
The scientists calculated that most of the gamma rays should be concentrated in the inner regions of the galaxy, just as was observed by the satellite data, the team reported in a paper published last month in the Astrophysical Journal.
“The observed distribution of gamma rays is consistent with the standard picture where the source of positrons is the radioactive decay of isotopes of nickel, titanium and aluminum produced in supernova explosions of stars more massive than the Sun,” said Rothschild.
In their companion paper in this week’s issue of Physical Review Letters, the scientists point out that a basic assumption of one of the more exotic explanations for the purported mystery—dark matter decays or annihilations—is flawed, because it assumes that the positrons annihilate very close to the exploding stars from which they originated.
“We clearly demonstrated this was not the case, and that the distribution of the gamma rays observed by the gamma ray satellite was not a detection or indication of a ‘dark matter signal’,” said Lingenfelter.
The scientists were supported in their studies by grants from the National Aeronautics and Space Administration.
Adapted from materials provided by University of California/San Diego.
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University of California/San Diego (2009, July 9). Antimatter Positrons Explain Gamma Ray Mystery In Milky Way Galaxy. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/07/090708201840.htm
Over the past five years, gamma ray measurements from the European satellite INTEGRAL have perplexed astronomers, leading some to argue that a "great mystery" existed because the distribution of these gamma rays across different parts of the Milky Way galaxy was not as expected. (Credit: ESA)
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Antimatter Positrons Explain Gamma Ray Mystery In Milky Way Galaxy
ScienceDaily (July 9, 2009) — A team of astrophysicists has solved a mystery that led some scientists to speculate that the distribution of certain gamma rays in our Milky Way galaxy was evidence of a form of undetectable “dark matter” believed to make up much of the mass of the universe.
See also:
Space & Time
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* Galaxy formation and evolution
* Gamma ray burst
In two separate scientific papers, the most recent of which appears in the July 10 issue of the journal Physical Review Letters, the astrophysicists show that this distribution of gamma rays can be explained by the way antimatter positrons from the radioactive decay of elements, created by massive star explosions in the galaxy, propagate through the galaxy. Thus, the scientists said, the observed distribution of gamma rays is not evidence for dark matter.
“There is no great mystery,” said Richard Lingenfelter, a research scientist at UC San Diego’s Center for Astrophysics and Space Sciences who conducted the studies with Richard Rothschild, a research scientist also at UCSD, and James Higdon, a physics professor at the Claremont Colleges. “The observed distribution of gamma rays is in fact quite consistent with the standard picture.”
Over the past five years, gamma ray measurements from the European satellite INTEGRAL have perplexed astronomers, leading some to argue that a “great mystery” existed because the distribution of these gamma rays across different parts of the Milky Way galaxy was not as expected.
To explain the source of this mystery, some astronomers had hypothesized the existence of various forms of dark matter, which astronomers suspect exists—from the unusual gravitational effects on visible matter such as stars and galaxies—but have not yet found.
What is known for certain is that our galaxy—and others—are filled with tiny subatomic particles known as positrons, the antimatter counterpart of typical, everyday electrons. When an electron and positron encounter each other in space, the two particles annihilate and their energy is released as gamma rays. That is, the electron and positron disappear and two or three gamma rays appear.
”These positrons are born at nearly the speed of light, and travel thousands of light years before they slow down enough in dense clouds of gas to have a chance of joining with an electron to annihilate in a dance of death,” explains Higdon. “Their slowing down occurs from the drag of other particles during their journey through space. Their journey is also impeded by the many fluctuations in the galactic magnetic field that scatter them back and forth as they move along. All of this must be taken into account in calculating the average distance the positrons would travel from their birthplaces in supernova explosions.”
”Some positrons head towards the center of the Galaxy, some towards the outer reaches of the Milky Way known as the galactic halo, and some are caught in the spiral arms,” said Rothschild. “While calculating this in detail is still far beyond the fastest supercomputers, we were able to use what we know about how electrons travel throughout the solar system and what can be inferred about their travel elsewhere to estimate how their anti-matter counterparts permeate the galaxy.”
The scientists calculated that most of the gamma rays should be concentrated in the inner regions of the galaxy, just as was observed by the satellite data, the team reported in a paper published last month in the Astrophysical Journal.
“The observed distribution of gamma rays is consistent with the standard picture where the source of positrons is the radioactive decay of isotopes of nickel, titanium and aluminum produced in supernova explosions of stars more massive than the Sun,” said Rothschild.
In their companion paper in this week’s issue of Physical Review Letters, the scientists point out that a basic assumption of one of the more exotic explanations for the purported mystery—dark matter decays or annihilations—is flawed, because it assumes that the positrons annihilate very close to the exploding stars from which they originated.
“We clearly demonstrated this was not the case, and that the distribution of the gamma rays observed by the gamma ray satellite was not a detection or indication of a ‘dark matter signal’,” said Lingenfelter.
The scientists were supported in their studies by grants from the National Aeronautics and Space Administration.
Adapted from materials provided by University of California/San Diego.
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University of California/San Diego (2009, July 9). Antimatter Positrons Explain Gamma Ray Mystery In Milky Way Galaxy. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/07/090708201840.htm
Over the past five years, gamma ray measurements from the European satellite INTEGRAL have perplexed astronomers, leading some to argue that a "great mystery" existed because the distribution of these gamma rays across different parts of the Milky Way galaxy was not as expected. (Credit: ESA)
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Intense Heat Killed The Universe's Would-be Galaxies, Researchers Say
ScienceDaily (July 1, 2009) — Millions of would-be galaxies failed to develop after being exposed to intense heat from the first stars and black holes formed in the early Universe, according to new research. Our Milky Way galaxy only survived because it was already immersed in a large clump of dark matter which trapped gases inside it, scientists led by Durham University's Institute for Computational Cosmology (ICC) found.
See also:
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* Galaxies
* Astrophysics
* Dark Matter
* Astronomy
* Cosmology
* Stars
Reference
* Galaxy formation and evolution
* Supergiant
* Local Group
* Barred spiral galaxy
The research, presented at an international conference on July 1, 2009 also forms a core part of a new ICC movie charting the evolution of the Milky Way to be shown at the Royal Society.
The researchers said that the early Milky Way, which had begun forming stars, held on to the raw gaseous material from which further stars would be made. This material would otherwise have been evaporated by the high temperatures generated by the "ignition" of the Universe about half-a-billion years after the Big Bang.
Tiny galaxies, inside small clumps of dark matter, were blasted away by the heat which reached approximate temperatures of between 20,000 and 100,000 degrees centigrade, the scientists, including experts at Japan's University of Tsukuba, said.
Dark matter is thought to make up 85 per cent of the Universe's mass and is believed to be one of the building blocks of galaxy formation.
Using computer simulations carried out by the international Virgo Consortium (which is led by Durham) the scientists examined why galaxies like the Milky Way have so few companion galaxies or satellites.
Astronomers have found a few dozen small satellites around the Milky Way, but the simulations revealed that hundreds of thousands of small clumps of dark matter should be orbiting our galaxy.
The scientists said the heat from the early stars and black holes rendered this dark matter barren and unable to support the development of satellite star systems.
The findings will be presented to The Unity of the Universe conference to be held at the Institute of Cosmology and Gravitation, at the University of Portsmouth on Wednesday, July 1. The work has been funded by the Science and Technology Facilities Council (STFC) and the Japanese Society for the Promotion of Science.
The simulations also form part of a new ICC movie – called Our Cosmic Origins – which combines ground-breaking simulations with observations of galaxies to track the evolution of the Milky Way over the 13-billion-year history of the Universe.
Joint lead investigator Professor Carlos Frenk, Director of the Institute for Computational Cosmology, at Durham University, said: "The validity of the standard model of our Universe hinges on finding a satisfactory explanation for why galaxies like the Milky Way have so few companions.
"The simulations show that hundreds of thousands of small dark matter clumps should be orbiting the Milky Way, but they didn't form galaxies.
"We can demonstrate that it was almost impossible for these potential galaxies to survive the extreme heat generated by the first stars and black holes.
"The heat evaporated gas from the small dark matter clumps, rendering them barren. Only a few dozen front-runners which had a head start on making stars before the Universe ignited managed to survive."
By providing a natural explanation for the origin of galaxies, the simulations support the view that cold dark matter is the best candidate for the mysterious material believed to make up the majority of our Universe, the scientists added.
It is now up to experimental physicists to either find this dark matter directly or to make it in a particle accelerator such as the Large Hadron Collider at CERN.
Professor Frenk, added: "Identifying the dark matter is not only one of the most pressing problems in science today, but also the key to understanding the formation of galaxies."
Joint lead investigator Dr Takashi Okamoto from the University of Tsukuba said: "These are still early days in trying to make realistic galaxies in a computer, but our results are very encouraging."
Presentations:
1. Constraining feedback in galaxy formation: cosmological simulations of satellite galaxy formation, Okamoto, T; Frenk CS; Jenkins A; and Theuns T, July 2009.
2. The origin of failed subhaloes and the common mass scale of the Milky Way satellite galaxies, Okamoto, T and Frenk CS, July 2009.
Adapted from materials provided by Durham University, via EurekAlert!, a service of AAAS.
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Gas around a young galaxy. (Credit: Jim Geach (Durham University) and Rob Crain (CAS/Swinburne University of Technology))
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Intense Heat Killed The Universe's Would-be Galaxies, Researchers Say
ScienceDaily (July 1, 2009) — Millions of would-be galaxies failed to develop after being exposed to intense heat from the first stars and black holes formed in the early Universe, according to new research. Our Milky Way galaxy only survived because it was already immersed in a large clump of dark matter which trapped gases inside it, scientists led by Durham University's Institute for Computational Cosmology (ICC) found.
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The research, presented at an international conference on July 1, 2009 also forms a core part of a new ICC movie charting the evolution of the Milky Way to be shown at the Royal Society.
The researchers said that the early Milky Way, which had begun forming stars, held on to the raw gaseous material from which further stars would be made. This material would otherwise have been evaporated by the high temperatures generated by the "ignition" of the Universe about half-a-billion years after the Big Bang.
Tiny galaxies, inside small clumps of dark matter, were blasted away by the heat which reached approximate temperatures of between 20,000 and 100,000 degrees centigrade, the scientists, including experts at Japan's University of Tsukuba, said.
Dark matter is thought to make up 85 per cent of the Universe's mass and is believed to be one of the building blocks of galaxy formation.
Using computer simulations carried out by the international Virgo Consortium (which is led by Durham) the scientists examined why galaxies like the Milky Way have so few companion galaxies or satellites.
Astronomers have found a few dozen small satellites around the Milky Way, but the simulations revealed that hundreds of thousands of small clumps of dark matter should be orbiting our galaxy.
The scientists said the heat from the early stars and black holes rendered this dark matter barren and unable to support the development of satellite star systems.
The findings will be presented to The Unity of the Universe conference to be held at the Institute of Cosmology and Gravitation, at the University of Portsmouth on Wednesday, July 1. The work has been funded by the Science and Technology Facilities Council (STFC) and the Japanese Society for the Promotion of Science.
The simulations also form part of a new ICC movie – called Our Cosmic Origins – which combines ground-breaking simulations with observations of galaxies to track the evolution of the Milky Way over the 13-billion-year history of the Universe.
Joint lead investigator Professor Carlos Frenk, Director of the Institute for Computational Cosmology, at Durham University, said: "The validity of the standard model of our Universe hinges on finding a satisfactory explanation for why galaxies like the Milky Way have so few companions.
"The simulations show that hundreds of thousands of small dark matter clumps should be orbiting the Milky Way, but they didn't form galaxies.
"We can demonstrate that it was almost impossible for these potential galaxies to survive the extreme heat generated by the first stars and black holes.
"The heat evaporated gas from the small dark matter clumps, rendering them barren. Only a few dozen front-runners which had a head start on making stars before the Universe ignited managed to survive."
By providing a natural explanation for the origin of galaxies, the simulations support the view that cold dark matter is the best candidate for the mysterious material believed to make up the majority of our Universe, the scientists added.
It is now up to experimental physicists to either find this dark matter directly or to make it in a particle accelerator such as the Large Hadron Collider at CERN.
Professor Frenk, added: "Identifying the dark matter is not only one of the most pressing problems in science today, but also the key to understanding the formation of galaxies."
Joint lead investigator Dr Takashi Okamoto from the University of Tsukuba said: "These are still early days in trying to make realistic galaxies in a computer, but our results are very encouraging."
Presentations:
1. Constraining feedback in galaxy formation: cosmological simulations of satellite galaxy formation, Okamoto, T; Frenk CS; Jenkins A; and Theuns T, July 2009.
2. The origin of failed subhaloes and the common mass scale of the Milky Way satellite galaxies, Okamoto, T and Frenk CS, July 2009.
Adapted from materials provided by Durham University, via EurekAlert!, a service of AAAS.
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Durham University (2009, July 1). Intense Heat Killed The Universe's Would-be Galaxies, Researchers Say. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/06/090630202127.htm
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Gas around a young galaxy. (Credit: Jim Geach (Durham University) and Rob Crain (CAS/Swinburne University of Technology))
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New Light Shed On 'Dark' Gamma-ray Bursts
ScienceDaily (June 21, 2009) — Gamma-ray bursts are the universe's biggest explosions, capable of producing so much light that ground-based telescopes easily detect it billions of light-years away. Yet, for more than a decade, astronomers have puzzled over the nature of so-called dark bursts, which produce gamma rays and X-rays but little or no visible light. They make up roughly half of the bursts detected by NASA's Swift satellite since its 2004 launch.
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At the American Astronomical Society meeting in Pasadena, Calif., an international team of astronomers have reported surprising new insight into dark bursts. The study finds that most occur in normal galaxies detectable by large, ground-based optical telescopes.
"One possible explanation for dark bursts was that they were occurring so far away their visible light was completely extinguished," said Joshua Bloom, associate professor of astronomy at the University of California (UC), Berkeley and the study's senior author. Thanks to the expansion of the universe and a thickening fog of hydrogen gas at increasing cosmic distances, astronomers see no visible light from objects more than about 12.9 billion light-years away. Another possibility: Dark bursts were exploding in galaxies with unusually thick amounts of interstellar dust, which absorbed a burst's light but not its higher-energy radiation.
Using one of the world's largest optical telescopes, the 10-meter Keck I in Hawaii, the team looked for unknown galaxies at the locations of 14 Swift-discovered dark bursts. "For eleven of these bursts, we found a faint, normal galaxy," said Daniel Perley, the UC Berkeley graduate student who led the study. If these galaxies were located at extreme distances, not even the Keck telescope could see them. Most gamma-ray bursts occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets -- driven by processes not fully understood -- punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths, including visible light.
For 11 of these 14 dark bursts, the team successfully detected a distant galaxy hosting the explosion, while the remaining three bursts without detectable hosts had faint optical counterparts. This indicates that none of these bursts had come from the most distant regions of the universe, since at distances greater than about 12.9 billion light years all the detectable light from both the afterglow and the host galaxy would be shifted into the infrared due to the expansion of the universe.
''And while 12.9 billion light years is a large distance even by most astronomers' standards, gamma-ray bursts are so powerful that if these were frequent occurrences 13 billion years ago, we ought to be detecting large numbers of those same explosions today as high redshift events," Cenko said. "We don't, which indicates that the first stars formed at a less frenzied pace than some models suggested."
The lack of any very high redshift events in the sample indicates that these distant explosions cannot comprise more than a few percent of all gamma-ray bursts, Cenko said. However, such distant bursts are known to exist. Just two months ago, a gamma-ray burst at a distance of 13.1 billion years was discovered.
"Putting this recent event together with the others in our study, for the first time we can provide both an upper and lower limit to the fraction of gamma-ray bursts at very high redshift," Perley said. Specifically, the authors conclude that the high redshift fraction is between 0.2 and 7 percent.
Because none of the 14 bursts studied in the survey is at this distance, by far the most likely cause of the bursts' optical dimness is dust inside the host galaxy absorbing light from the afterglow before it escapes, the team concluded. However, the starlight shows no obvious signatures of dust, indicating that the dust may be hiding in patches or clouds where it is difficult to detect.
Consequently, there could be much more dust than has been suspected as the result of measurements using other techniques, and "dark gamma-ray bursts could provide a complementary way of answering the question of how much star formation was going on inside galaxies in the early universe," Perley said.
A paper on the findings has been submitted to The Astronomical Journal.
Adapted from materials provided by NASA/Goddard Space Flight Center.
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Dense knots of dust in otherwise normal galaxies dim the light of a dark gamma-ray burst (center). The dust absorbs most or all of a burst's visible light but not higher-energy X-rays and gamma rays. (Credit: NASA/Swift/Aurore Simonnet)
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New Light Shed On 'Dark' Gamma-ray Bursts
ScienceDaily (June 21, 2009) — Gamma-ray bursts are the universe's biggest explosions, capable of producing so much light that ground-based telescopes easily detect it billions of light-years away. Yet, for more than a decade, astronomers have puzzled over the nature of so-called dark bursts, which produce gamma rays and X-rays but little or no visible light. They make up roughly half of the bursts detected by NASA's Swift satellite since its 2004 launch.
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At the American Astronomical Society meeting in Pasadena, Calif., an international team of astronomers have reported surprising new insight into dark bursts. The study finds that most occur in normal galaxies detectable by large, ground-based optical telescopes.
"One possible explanation for dark bursts was that they were occurring so far away their visible light was completely extinguished," said Joshua Bloom, associate professor of astronomy at the University of California (UC), Berkeley and the study's senior author. Thanks to the expansion of the universe and a thickening fog of hydrogen gas at increasing cosmic distances, astronomers see no visible light from objects more than about 12.9 billion light-years away. Another possibility: Dark bursts were exploding in galaxies with unusually thick amounts of interstellar dust, which absorbed a burst's light but not its higher-energy radiation.
Using one of the world's largest optical telescopes, the 10-meter Keck I in Hawaii, the team looked for unknown galaxies at the locations of 14 Swift-discovered dark bursts. "For eleven of these bursts, we found a faint, normal galaxy," said Daniel Perley, the UC Berkeley graduate student who led the study. If these galaxies were located at extreme distances, not even the Keck telescope could see them. Most gamma-ray bursts occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets -- driven by processes not fully understood -- punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths, including visible light.
For 11 of these 14 dark bursts, the team successfully detected a distant galaxy hosting the explosion, while the remaining three bursts without detectable hosts had faint optical counterparts. This indicates that none of these bursts had come from the most distant regions of the universe, since at distances greater than about 12.9 billion light years all the detectable light from both the afterglow and the host galaxy would be shifted into the infrared due to the expansion of the universe.
''And while 12.9 billion light years is a large distance even by most astronomers' standards, gamma-ray bursts are so powerful that if these were frequent occurrences 13 billion years ago, we ought to be detecting large numbers of those same explosions today as high redshift events," Cenko said. "We don't, which indicates that the first stars formed at a less frenzied pace than some models suggested."
The lack of any very high redshift events in the sample indicates that these distant explosions cannot comprise more than a few percent of all gamma-ray bursts, Cenko said. However, such distant bursts are known to exist. Just two months ago, a gamma-ray burst at a distance of 13.1 billion years was discovered.
"Putting this recent event together with the others in our study, for the first time we can provide both an upper and lower limit to the fraction of gamma-ray bursts at very high redshift," Perley said. Specifically, the authors conclude that the high redshift fraction is between 0.2 and 7 percent.
Because none of the 14 bursts studied in the survey is at this distance, by far the most likely cause of the bursts' optical dimness is dust inside the host galaxy absorbing light from the afterglow before it escapes, the team concluded. However, the starlight shows no obvious signatures of dust, indicating that the dust may be hiding in patches or clouds where it is difficult to detect.
Consequently, there could be much more dust than has been suspected as the result of measurements using other techniques, and "dark gamma-ray bursts could provide a complementary way of answering the question of how much star formation was going on inside galaxies in the early universe," Perley said.
A paper on the findings has been submitted to The Astronomical Journal.
Adapted from materials provided by NASA/Goddard Space Flight Center.
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NASA/Goddard Space Flight Center (2009, June 21). New Light Shed On 'Dark' Gamma-ray Bursts. ScienceDaily. Retrieved August 24, 2009, from http://www.sciencedaily.com /releases/2009/06/090608131150.htm
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Dense knots of dust in otherwise normal galaxies dim the light of a dark gamma-ray burst (center). The dust absorbs most or all of a burst's visible light but not higher-energy X-rays and gamma rays. (Credit: NASA/Swift/Aurore Simonnet)
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