Gamma-ray bursts are caused by explosive death of massive stars, new study reveals

In two papers appearing in an upcoming issue of the Astrophysical Journal, an international team of astrophysicists led by Shri Kulkarni of the California Institute of Technology reveals that new data show that supernovae are the source of gamma-ray bursts.

The new information was obtained from a gamma-ray burst that was detected in November and studied by the Hubble Space Telescope, the Australia Telescope Compact Array, the Anglo-Australian Telescope, and optical telescopes in Chile.

For the last few years astronomers have been chasing clues linking the mysterious gamma-ray bursts to their favored suspect: massive stars. Previous observations hinted at debris from an exploding star, but the observations were inconclusive.

Careful observations of gamma-ray burst GRB 011121 have uncovered remnants of the exploded star, whose signature was buried in the bright, fading embers. Now, for the first time, two compelling tell-tale signatures of the massive star were observed.

As explained by Kulkarni, who is the McArthur Professor of Astronomy and Planetary Sciences at Caltech and the head of the international team that made this discovery, "With these observations we have tied this gamma-ray burst to an exploding star. I am absolutely delighted that nature provided us with such a clean answer."

At the core of the observations, the data show that a supernova accompanied the burst. Supernovae are a natural consequence of exploding stars and difficult to make by other means. Joshua Bloom, Caltech graduate student and lead author on the supernova paper, said. "It is not often that a graduate student gets the chance to make a major discovery. I am very fortunate to be involved in this one."

The astronomers were also able to deduce that the explosion took place in a cocoon of gas fed by a "wind" of matter emanating from the progenitor star. Paul Price, graduate student at the Australian National University and a lead author on the second paper, was "intensely excited. Once it became clear that we had not only seen the supernova but also the cocoon I was very happy; I couldn't sleep for days."

The gamma-ray burst in question was detected on November 21, 2001, by the Italian-Dutch Satellite BeppoSAX in the southern-sky constellation of Chamaeleon. The position was quickly refined by a network of satellites. Astronomers from Poland and Chile, as well as another U.S. team from Harvard, used optical telescopes in Chile to rapidly identify the ``afterglow,'' or glowing embers, of the gamma-ray burst and determined that the galaxy in which the burst was located was quite near - a paltry five billion light-years from Earth.

The sensitive optical and infrared observations were in part possible because of the relatively small distance to the burst. Given the proximity, the Caltech team decided to dedicate a large portion of their allocated Hubble Space Telescope time toward observing any possible supernova component. Kulkarni says of the decision, "We simply went for broke because of the potential payoff."

Kulkarni believes that this is just the beginning of a new era in our understanding of the death of massive stars. The stars die by collapsing, and the collapse both fuels the explosion and leaves a stellar residue of neutron stars and black holes. Indeed, theorists have long speculated that gamma-ray bursts are the birth cry of spinning black holes. New facilities such as the Chandra X-Ray Observatory, and future facilities such as gravitational-wave observatories and neutrino telescopes, will allow astronomers to investigate the dramatic collapse process.

Kulkarni cautions, however, that all is still not known about gamma-ray bursts. It may be that other exotic phenomena, such as two colliding neutron stars, or a neutron star colliding with a black hole, produce some of the events that we see. "Despite extensive efforts, until now we have not seen clear signatures for a cocoon in dozens of other gamma-ray bursts, and there have been only hints of a supernova in a few other bursts," Bloom says.

Price adds, "It means there will be lots more to do in the future. I have a secure thesis now!"

In addition to Kulkarni, Bloom, and Price, members of the team reporting the results are Caltech professors S. George Djorgovski and Fiona Harrison and postdoctoral fellows and scholars Daniel Reichart, Derek Fox, Titus Galama, and Re'em Sari. Edo Berger and Sara Yost are Caltech graduate students also on the team, as are Dale Frail from the National Radio Astronomy Observatory, and many other international collaborators. Separately, P. M. Garnavich of the University of Notre Dame and his collaborators have reached similar conclusions with data taken from the Magellan telescope in Chile.

 

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Robert Tindol
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Researchers find evidence for mechanismthat creates near-Earth binary asteroids

About one in six of all near-Earth asteroids are binaries – in other words, two bodies that travel in close companionship as they orbit the sun. A new study reveals that binaries most likely form when a single asteroid swings close to Earth, is ripped apart by the planet's tidal attraction, and eventually reforms into separate bodies.

In a refereed article to be released Thursday, April 11 on the Science Express Web site of the journal Science, California Institute of Technology astronomer Jean-Luc Margot and his co-authors report detailed information on the near-Earth asteroid currently assigned the rather unpoetic name 2000 DP107, and also on four other binary asteroids. 2000 DP107 comprises two bodies that are about three kilometers apart, the larger of the two being about 800 meters in diameter and the other about 300 meters. Using particularly detailed radar data, the study is a description of the system and explains how both these particular bodies and near-Earth binaries in general can be formed.

Near-Earth asteroids were formed between Mars and Jupiter, like all other asteroids, but are kicked into elliptical orbits by the gravitational influence of Jupiter and occasionally pass near Earth. An Earth-crossing orbit is one in which the asteroid actually crosses the path that Earth follows around the sun, which means the two bodies could eventually collide.

Margot, a postdoctoral researcher in the Division of Geology and Planetary Science at Caltech, led the observations in October 2000 that uncovered 2000 DP107's binary nature, just months after the asteroid was first discovered by MIT researchers. The current study, of which Margot is lead author, employs data obtained from the 70-meter Goldstone NASA tracking telescope and the Arecibo Observatory's radio telescope in Puerto Rico, which is funded by the National Science Foundation with additional support from NASA and operated by Cornell University, to yield a much more detailed picture of the two orbiting bodies and their dynamics.

Other details from the radar data show that the two bodies are probably in a tidal lock, which means that a person standing on the larger body would always see the same face of the smaller body, but a person on the smaller body would see the larger body spinning. This is exactly like the tidal lock of the Earth-moon system.

Further, the research suggests that the tidal force applied to an asteroid by a larger planet can be the cause of its breaking apart. The process, known as "spin and fission," means that a body approaching Earth is made to change its spin rate. Specifically, the tidal force tends to make an asteroid passing nearby spin at the orbital rate, which can increase rather substantially in a close approach to a planet. This increase in spin rate, coupled with the tidal pull itself, can cause a loosely-bound, gravel-like accumulation such as the near-Earth asteroids, to sling off material. Later, the weak gravitational attraction of the particles allows the material to reform in a second body.

But the most important issue raised by the paper is that near-Earth binaries are so common, says Jet Propulsion Laboratory researcher Steve Ostro, one of the authors. "The discovery of the existence and substantial abundance of binary asteroids in Earth-crossing orbits is a major one," says Ostro, an expert on the radar characterization of asteroids. "Presumably, binary asteroids have hit Earth in the past, and will do so in the future."

"Of course, the most important thing to know about any (potentially hazardous asteroid) is whether it is two objects or one, and this is why we want to observe these binaries with radar whenever possible."

"The use of radar allows precise measurements of asteroid densities, a very important indicator of their composition and internal structure," says Margot.

"Getting (near-Earth asteroid) densities from radar is dirt-cheap compared with getting a density with a spacecraft," Ostro explains.

In addition to Margot and Ostro, the other authors are Michael Nolan of the Arecibo Observatory; Lance Benner, Raymond Jurgens, Jon Giorgini, and Martin Slade, all of JPL; and Donald Campbell of Cornell University.

The article will be available Thursday on the Science Express Web site at http://www.scienceexpress.org.

Contact: Robert Tindol (626) 395-3631

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Caltech scientists demonstratecompact silica laser

A team of applied physicists at the California Institute of Technology have demonstrated an ultrasmall Raman laser that is 1,000 times more efficient than previous devices. The device could have significant applications for telecommunications and other areas where compact, highly efficient, and tunable lasers are desirable.

Reporting in the February 7 issue of the journal Nature, Caltech applied physics professor Kerry Vahala and graduate students Sean Spillane and Tobias Kippenberg describe their progress in making the tiny device, which incorporates a small spherical glass bead and a stretched fiber-optic wire. The laser is especially efficient because of the way it stores light inside the microsphere, or resonator, as well as the manner in which the stretched optical wire permits efficient coupling of light into the sphere.

According to Vahala, the light wraps around the sphere in a ring orbit and subsequently intensifies over hundreds of thousands of orbits, resulting in extreme concentration of optical power within the sphere. In this way, very weak signals applied to the sphere from the fiber-optic wire can build to enormous intensities within the sphere itself.

At these higher power levels, the physics within the sphere enters a nonlinear regime wherein conventional rules for light propagation break down. In the Caltech work, the molecules of the glass bead itself are distorted, resulting in a process called Raman emission and lasing. Because Raman lasers require enormous intensities to function, they are usually power-hungry devices. The Caltech team uses the physics of the sphere to reduce both power and size. Normal Raman lasers turn on "with a shout"—these new devices require "only a whisper."

Central to this breakthrough was the ability to couple directly to the ring orbits, or whispering gallery modes, of the sphere while preserving the exquisite perfection of the sphere in terms of its ability to store and concentrate light. The Caltech team uses stretched optical fiber in the form of a taper to achieve coupling efficiencies, in which loss is negligible, both to and from the sphere.

Because Raman lasers and amplifiers can operate over a very broad range of wavelengths, they are important devices that extend other lasers into new or previously inaccessible wavelength bands. For example, Raman amplifiers are now used widely in commercial long-distance fiber communications systems because of this wavelength flexibility.

Also, through a process called cascading, it is possible to cover even greater wavelength bands by using one Raman laser as the pump for another. In this way, a whole series of wavelengths can be generated in a kind of domino effect. More generally, it can be used to extend the wavelength range of other laser sources into difficult-to-access wavelength bands for sensing or other purposes.

The article is titled "Ultralow-threshold Raman laser using a spherical dielectric microcavity," and is available at www.nature.com.

In the photo, the sphere has been doped, which enables observation of the ring orbit as green luminescence. The photo is by M. Cai of Vahala group.

Further discussion of this and related work can be found at the Vahala Caltech group website: www.its.caltech.edu/~vahalagr.

CONTACT: Robert Tindol (626) 395-3631

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First gamma burst detected by new NASA satellite is pinpointed at Palomar Observatory

Astrophysicists have combined the Palomar Mountain 200-inch Hale Telescope with the abilities of a new NASA satellite to detect and characterize a gamma-ray burst lying at a distance of only 5 billion light-years from Earth. This is the closest gamma-ray burst ever studied by optical telescopes.

The origin of cosmic gamma-ray bursts, spectacular flashes of high-energy radiation followed by slowly decaying optical and radio emission that can be seen from great distances, is still a puzzle to astronomers. Many scientists believe that the bursts result from explosions that signal the birth of black holes; however, all agree that more data are needed before we can really know black holes' origins and nature.

NASA's new High-Energy Transient Explorer (HETE) detected a gamma-ray burst on September 21. Data indicated that the event was located in the Lacerta constellation, and refined information from the Interplanetary Network (IPN), a series of satellites with gamma-ray detectors scattered about the solar system, reduced the region astronomers needed to search to find the fading embers of the explosion. Scientists at the California Institute of Technology's Palomar Observatory, using the historic Hale 200-inch reflector, were able to locate the visual afterglow the following day. This was the first burst from the HETE satellite to be pinpointed with an accuracy sufficient to study the remains.

On October 17 the Caltech team used the Hale Telescope to obtain a redshift for the burst. This allowed a distance to be inferred, implying that the burst happened some 5 billion years ago. This makes the burst one of the closest ever found, and thus easier to study in detail. Also on October 17 the team members, led by Dale Frail from the National Radio Astronomy Observatory, detected a twinkling radio counterpart of the burst using the Very Large Array in New Mexico.

According to Shri Kulkarni, who is the MacArthur Professor of Astronomy and Planetary Science at Caltech, the team was able to find the rare optical afterglow because of the quick detection and localization abilities of the HETE satellite and the rapid follow-up with the Palomar Mountain Hale Telescope.

HETE, the first satellite dedicated to the study of gamma-ray bursts, is on an extended mission until 2004. Launched on October 9, 2000, HETE was built by MIT as a mission of opportunity under the NASA Explorer Program. The HETE program is a collaboration between MIT; NASA; Los Alamos National Laboratory, New Mexico; France's Centre National d'Etudes Spatiales, Centre d'Etude Spatiale des Rayonnements, and Ecole Nationale Superieure del'Aeronautique et de l'Espace; and Japan's Institute of Physical and Chemical Research. The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago, as well as from Brazil, India, and Italy.

"I'm very excited. I could not sleep for two nights after making the discovery," said Paul Price, the Caltech graduate student who first identified the optical afterglow from Palomar.

"With this first confirmed observation of a gamma-ray burst and its afterglow, we've really turned the corner," said George Ricker of the Massachusetts Institute of Technology, principal investigator for HETE. "As HETE locates more of these bursts and reports them quickly, we will begin to understand what causes them.

"The unique power of HETE is that it not only detects a large sample of these bursts, but it also relays the accurate location of each burst in real time to ground-based optical and radio observatories," Ricker said.

Because the enigmatic bursts disappear so quickly, scientists can best study the events by way of their afterglow. HETE detects these bursts as gamma rays or high-energy X rays, and then instantly relays the coordinates to a network of ground-based and orbiting telescopes for follow-up searches for such afterglows.

Additional observations of this event, made with the Italian BeppoSAX satellite and the Ulysses space probe, were coordinated by HETE team member Kevin Hurley at the University of California. The combination of the localization by the Interplanetary Network with the original HETE localization provided the refined information needed by ground-based observers to point their optical telescopes.

The opportunity to see the afterglow in optical light provides crucial information about what is triggering these mysterious bursts, which scientists speculate to be the explosion of massive stars, the merging of neutron stars and black holes, or possibly both. Follow-up observations of GRB 010921 using the Hubble Space Telescope and the telescopes on the ground should move us a few steps closer to the answer of this cosmic puzzle.

The team that identified the counterpart to GRB010921 includes—in addition to Caltech Professors Shri Kulkarni, Fiona Harrison, and S. George Djorgovski—postdoctoral fellows and scholars Re'em Sari, Titus Galama, Daniel Reichart, Derek Fox, and Ashish Mahabal, graduate students Joshua Bloom, Paul Price, Edo Berger, and Sara Yost, Dale Frail from the National Radio Astronomy Observatory, and many other collaborators.

More information on HETE can be found at: http://space.mit.edu/HETE Palomar Observatory: http://www.astro.caltech.edu/palomar/ Caltech Media Relations: http://pr.caltech.edu/media/.

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Robert Tindol
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Physicist Awarded Prize for Development of Superstring Theory

PASADENA, Calif.—A theoretical physicist's research lab consists of little more than a chalkboard and chalk, a world of knotty mathematical equations that seek to explain why our physical world is the way it is.

For his pioneering work in the development of one such possible explanation, known as superstring theory, John Schwarz, the Harold Brown Professor of Theoretical Physics at the California Institute of Technology, has been awarded the 2002 Dannie Heineman Prize for Mathematical Physics. The prize is awarded annually by the American Physical Society and the American Institute of Physics for, "valuable published contributions made in the field of mathematical physics." Schwarz is sharing the award with Dr. Michael B. Green of the University of Cambridge.

String theory evolved in the 1970s in an attempt to provide one all-encompassing framework that would explain the nature of nature—everything from the macro level of the cosmos to the micro level of subatomic particles (particles hundreds of times smaller than the nucleus of an atom). Further, it incorporated all the forces of nature (such as gravity) that affect the basic structure of the world.

With string theory, physicists take a different view of the fundamental units of matter. When combined, these smallest building blocks create all the physical things we see. Instead of viewing them as infinitesimally tiny points in space, though, string theorists view them as tiny, one-dimensional, stringlike bits of matter that vibrate. They are not ordinary strings, but they behave in ways that can be described mathematically, by equations that also account for two other physical laws of nature, relativity and quantum mechanics. (In a similar vein, mathematicians can write equations to describe the patterns of vibration that produce different notes from the string of a guitar; thus the term "string theory.") Each pattern of vibration of the string corresponds to a different particle of matter.

But by the late 1970s interest in string theory had faded after a number of predictions it made conflicted with the results of experiments. Most physicists therefore abandoned the theory.

But not Schwarz, Green, and a handful of others. Schwarz, for one, stuck to his guns, saying at the time that "the mathematical structure of string theory was so beautiful and had so many miraculous properties that it had to be pointing toward something deep."

In 1984, Schwarz and Green published a landmark paper that was based on more than 12 years of research. In it, they found a way to resolve these conflicts, by suggesting, among other things, that more dimensions may exist in our world then the three—height, width, and depth—we are familiar with.

Instead, they suggested a mathematical theory that included 10 dimensions. It's a world we can't experience, but that mathematically makes sense. How can we think of our world as having extra dimensions? As one physicist explained it, imagine that you can move only in two dimensions, length and width, in a big room. But the third dimension, height, isn't large like the other two but instead is curled up at each point of matter in a tiny circle, so that you don't experience it. Presumably, the additional dimensions suggested by Schwarz and Green are so small, existing at the subatomic level, that we can't experience them. However, the properties of these extra dimensions are expected to have other consequences that can be observed.

Schwarz termed this new theory superstring theory, because it incorporates a special kind of symmetry called supersymmetry. Symmetry, which is very common and important in physics, concerns the fact that equations (and sometimes nature) look the same when observed in different ways. For example, a sphere looks the same after it is rotated. Supersymmetry, which is one of the spin-offs of string theory, is a less intuitive and more quantum mechanical kind of symmetry. Their research reignited string theory, which today remains one of the hottest areas in theoretical physics. It is also the leading candidate for the elusive "theory of everything" that physicists seek. It is for this work that Schwarz and Green have been awarded the Heineman Prize.

Schwarz has worked on superstring theory for most of his professional career. In 1986 he became a Fellow of the American Physical Society. In 1987 he received a prestigious MacArthur Fellowship, and in 1997 he was elected to the National Academy of Sciences. The Dannie Heineman prize was established in 1959 to encourage further research in the field of mathematical physics. As a recipient, Schwarz joins a number of esteemed physicists, including Caltech's Murray Gell-Mann (1959) and the likes of Freeman Dyson (1965), Roger Penrose (1971), and Stephen Hawking (1977). The prize was established by the Heineman Foundation for Research, Educational, Charitable, and Scientific Purposes, Inc., and is administered jointly by the American Physical Society (APS) and the American Institute of Physics.

The prize will be awarded at the April 2002 APS meeting to be held in Albuquerque, New Mexico.

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International team uses powerful cosmic lensto find galactic building block in early universe

Exploiting a phenomenon known as gravitational lensing, an international team of astrophysicists has detected a very small, faint stellar system in the process of its formation during the first half billion years or so of the universe's existence.

The discovery is being reported in the October 20 issue of the Astrophysical Journal. According to lead author Richard Ellis, a professor of astronomy at the California Institute of Technology, the faint object is an excellent candidate for the long sought after "building blocks" thought to be abundant at early times and which later assembled to make present-day galaxies.

The discovery was made possible by examining small areas of sky viewed through a massive intervening cluster of galaxies, Abell 2218, 2 billion light-years away. The cluster acts as a powerful gravitational lens, magnifying distant objects and allowing the scientists to probe how distant galaxies assembled at very early times.

Gravitational lensing, a dramatic feature of Einstein's theory of general relativity, means that a massive object in the foreground bends the light rays radiating from one in the background because mass curves space. As a result, an object behind a massive foreground galaxy cluster like Abell 2218 can look much brighter because the foreground object has bent additional photons toward Earth, in much the same way that glass lenses in binoculars will bend more photons toward the eyes.

In the case of the system detected by Ellis and coworkers, the effect makes the image at least 30 times brighter than would be the case if the Abell 2218 cluster were not in the foreground. Without this boost, neither the Keck 10-meter Telescopes nor the Hubble Space Telescope would have detected the object.

Ellis explains, "Without the benefit of the powerful cosmic lens, the intriguing source would not even have been detected in the Hubble Deep Fields, historic deep exposures taken in 1995 and 1998."

Using the 10-meter Keck Telescopes at Mauna Kea, the collaboration found a faint signal corresponding to a pair of feeble images later recognized in a deep Hubble Space Telescope picture.

Spectroscopic studies made possible with the superior light-gathering power of the Keck confirmed that the images arise via the magnification of a single source diagnosed to be extremely distant and in the process of formation.

"The system contains about a million or so stars at a distance of 13.4 billion light-years, assuming that the universe is 14 billion years old," claims Ellis. "While more distant galaxies and quasars have been detected with the Keck Telescopes, by virtue of the magnification afforded by the foreground cosmic lens, we are witnessing a source much smaller than a normal galaxy forming its first generation of stars." " Our work is a little like studying early American history," says team member Mike Santos, a Caltech graduate student in astronomy. "But instead of focusing on prominent individuals like George Washington, we want to know how everyday men and women lived.

"To really understand what was going on in the early universe, we need to learn about the typical, commonplace building blocks, which hold important clues to the later assembly of normal galaxies. Our study represents a beginning to that understanding."

The precise location of the pair of images in relation to the lensing cluster allowed the researchers to confirm the magnification. This work was the contribution of team member Jean-Paul Kneib of the Observatoire Midi-Pyrénées near Toulouse, France, an expert in the rapidly developing field of gravitational lensing.

The team concludes that the star system is remarkably young (by cosmic standards) and thus may represent the birth of a subcomponent of a galaxy or "building block." Such systems are expected to have been abundant in the early universe and to have later assembled to form mature large galaxies like our own Milky Way.

Santos explains, "The narrow distribution of intensity observed with the Keck demonstrates we are seeing hydrogen gas heated by newly formed stars. But, crucially, there is not yet convincing evidence for a well-established mixture of stars of different ages. This suggests we are seeing the source at a time close to its formation."

In their article, the researchers infer that the stars had been forming at a rate of one solar mass per year for not much longer than a million years. Such a structure could represent the birth of a globular cluster, stellar systems recognized today to be the oldest components of the Milky Way galaxy. The work represents part of an ongoing survey to determine the abundance of such distant star-forming sources as well as to fix the period in cosmic history when the bulk of these important objects formed.

Contact:Robert Tindol (626) 395-3631

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Astronomers detect evidence of time when universe emerged from "Dark Ages"

Astronomers at the California Institute of Technology announced today the discovery of the long-sought "Cosmic Renaissance," the epoch when young galaxies and quasars in the early universe first broke out of the "Dark Ages" that followed the Big Bang.

"It is very exciting," said Caltech astronomy professor S. George Djorgovski, who led the team that made the discovery. "This was one of the key stages in the history of the universe."

According to a generally accepted picture of modern cosmology, the universe started with the Big Bang some 14 billion years ago, and was quickly filled with glowing plasma composed mainly of hydrogen and helium.

As the universe expanded and cooled over the next 300,000 years, the atomic nuclei and electrons combined to make atoms of neutral gas. The glow of this "recombination era" is now observed as the cosmic microwave background radiation, whose studies have led to the recent pathbreaking insights into the geometrical nature of the universe.

The universe then entered the Dark Ages, which lasted about half a billion years, until they were ended by the formation of the first galaxies and quasars. The light from these new objects turned the opaque gas filling the universe into a transparent state again, by splitting the atoms of hydrogen into free electrons and protons. This Cosmic Renaissance is also referred to by cosmologists as the "reionization era," and it signals the birth of the first galaxies in the early universe.

"It is as if the universe was filled by a dark, opaque fog up to that time," explains Sandra Castro, a postdoctoral scholar at Caltech and a member of the team. "Then the fires—the first galaxies—lit up and burned through the fog. They made both the light and the clarity."

The researchers saw the tell-tale signature of the cosmic reionization in the spectra of a very distant quasar, SDSS 1044-0125, discovered last year by the Sloan Digital Sky Survey (SDSS). Quasars are very luminous objects in the distant universe, believed to be powered by massive black holes.

The spectra of the quasar were obtained at the W. M. Keck Observatory's Keck II 10-meter telescope atop Mauna Kea, Hawaii. The spectra show extended dark regions, caused by opaque gas along the line of sight between Earth and the quasar. This effect was predicted in 1965 by James Gunn and Bruce Peterson, both then at Caltech. Gunn, now at Princeton University, is the leader of the Sloan Digital Sky Survey; Peterson is now at Mt. Stromlo and Siding Spring observatories, in Australia.

The process of converting the dark, opaque universe into a transparent, lit-up universe was not instantaneous: it may have lasted tens or even hundreds of millions of years, as the first bright galaxies and quasars were gradually appearing on the scene, the spheres of their illumination growing until they overlapped completely.

"Our data show the trailing end of the reionization era," says Daniel Stern, a staff scientist at the Jet Propulsion Laboratory and a member of the team. "There were opaque regions in the universe back then, interspersed with bubbles of light and transparent gas."

"This is exactly what modern theoretical models predict," Stern added. "But the very start of this process seems to be just outside the range of our data."

Indeed, the Sloan Digital Sky Survey team has recently discovered a couple of even more distant quasars, and has reported in the news media that they, too, see the signature of the reionization era in the spectra obtained at the Keck telescope.

"It is a wonderful confirmation of our result," says Djorgovski. "The SDSS deserves much credit for finding these quasars, which can now be used as probes of the distant universe—and for their independent discovery of the reionization era."

"It is a great example of a synergy of large digital sky surveys, which can discover interesting targets, and their follow-up studies with large telescopes such as the Keck," adds Ashish Mahabal, a postdoctoral scholar at Caltech and a member of the team. "This is the new way of doing observational astronomy: the quasars were found by SDSS, but the discovery of the reionization era was done with the Keck."

The Caltech team's results have been submitted for publication in the Astrophysical Journal Letters, and will appear this Tuesday on the public electronic archive, http://xxx.lanl.gov/list/astro-ph/new.

The W. M. Keck Observatory is a joint venture of Caltech, the University of California, and NASA, and is made possible by a generous gift from the W. M. Keck Foundation.

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Robert Tindol
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Brightest Quasars Inhabit Galaxies withStar-Forming Gas Clouds, Scientists Discover

A team of scientists at the California Institute of Technology and the State University of New York at Stony Brook has found strong evidence that high-luminosity quasar activity in galaxy nuclei is linked to the presence of abundant interstellar gas and high rates of star formation.

In a presentation at the summer meeting of the American Astronomical Society, Caltech astronomy professor Nick Scoville and his colleagues reported today that the most luminous nearby optical quasar galaxies have massive reservoirs of interstellar gas much like the so-called ultraluminous infrared galaxies (or ULIRGs). The quasar nucleus is powered by accretion on to a massive black hole with mass typically about 100 million times that of the sun while the infrared galaxies are powered by extremely rapid star formation. The ULIRG "starbursts" are believed to result from the high concentration of interstellar gas and dust in the galactic centers.

"Until now, it has been unclear how the starburst and quasar activities are related," Scoville says, "since many optically bright quasars show only low levels of infrared emission which is generally assumed to measure star formation activity.

"The discovery that quasars inhabit gas-rich galaxies goes a long way toward explaining a longstanding problem," Scoville says. "The number of quasars has been observed to increase very strongly from the present back to Redshift 2, at which time the number of quasars was at a maximum.

"The higher number of quasars seen when the universe was younger can now be explained, since a larger fraction of the galaxies at that time had abundant interstellar gas reservoirs. At later times, much of this gas has been used up in forming stars.

"In addition, the rate of merging galaxies was probably much higher, since the universe was smaller and galaxies were closer together."

The new study shows that even optically bright quasar-type galaxies (QSOs) have massive reservoirs of interstellar gas, even without strong infrared emission from the dust clouds associated with star formation activity. Thus, the fueling of the central black hole in the quasars is strongly associated with the presence of an abundant interstellar gas supply.

The Scoville team used the millimeter-wave radio telescope array at Caltech's Owens Valley Radio Observatory near Bishop, California, for an extremely sensitive search for the emission of carbon monoxide (CO) molecules in a complete sample of the 12 nearest and brightest optical quasars previously catalogued at the Palomar 200-inch telescope in the 1970s. In particular, the researchers avoided selecting samples with bright infrared emissions, since that would bias the sample toward those with abundant interstellar dust clouds.

In this optically selected sample, eight out of the 12 quasars exhibited detectable CO emission-implying masses of interstellar molecular clouds in the range of two to 10 billion solar masses. (For reference, the Milky Way galaxy contains approximately two billion solar masses of molecular clouds.) Such large gas masses are found only in gas-rich spiral or colliding galaxies. The present study clearly shows that most quasars are also in gas-rich spiral or interacting galaxies, not gas-poor elliptical galaxies as previously thought.

The new study supports the hypothesis that there exists an evolutionary link between the two most luminous classes of galaxies: merging ultraluminous IR galaxies and ultraviolet/optically bright QSOs. Both the ULIRGs and QSOs show evidence of a recent galactic collision.

The infrared luminous galaxies are most often powered by prodigious starbursts in their galactic centers, forming young stars at 100 to 1,000 times the current rate in the entire Milky Way. The quasars are powered by the accretion of matter into a massive black hole at their nuclei at a rate of one to 10 solar masses per year.

The detection of abundant interstellar gas in the optically selected QSOs suggests a link between these two very different forms of galactic nuclear activity. The same abundant interstellar gases needed to form stars at a high rate might also feed the central black holes.

In normal spiral galaxies like the Milky Way, most of the interstellar molecular gas is in the galactic disk at distances typically 20,000 light-years from the center-well out of reach of a central black hole.

However, during galactic collisions, the interstellar gas can sink and accumulate within the central few hundred light-years, and massive concentrations of interstellar gas and dust are, in fact, seen in the nuclear regions of the ULIRGs. Once in the nucleus, this interstellar matter can both fuel the starburst and feed the central black hole at prodigious rates.

The discovery of molecular gas in the optically selected QSOs that do not have strong infrared emissions suggests that the QSO host galaxies might be similar systems observed at a later time after the starburst activity has subsided, yet with the black hole still being fed by interstellar gas.

For the remaining four quasars where CO was not detected, improved future instrumentation may well yield detections of molecular gas, Scoville says. Even in the detected galaxies the CO emission was extraordinarily faint due to their great distances-typically over a billion light-years. The remaining four galaxies could well have molecular gas masses only a factor of two below those that were detected.

Future instrumentation such as the CARMA and ALMA millimeter arrays will have vastly greater sensitivity, permitting similar studies out to much greater distances.

Other members of the team are David Frayer and Eva Schinnerer, both research scientists at Caltech, Caltech graduate students Micol Christopher and Naveen Reddy and Aaron Evans at SUNY (Stony Brook).

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New Analysis of BOOMERANG Data Uncovers Harmonics of Early Universe

Cosmologists from the California Institute of Technology and their international collaborators have discovered the presence of acoustic "notes" in the sound waves that rippled through the early universe.

The existence of these harmonic peaks, discovered in an analysis of images from the BOOMERANG experiment, further strengthens results last year showing that the universe is flat. Also, the new results bolster the theory of "inflation," which states that the universe grew from a tiny subatomic region during a period of violent expansion a split second after the Big Bang.

Finally, the results show promise that another Caltech-based detector, the Cosmic Background Imager (CBI), located in the mountains of Chile, will soon detect even finer detail in the cosmic microwave background. Analysis of this fine detail is thought to be the means of precisely determining how slight fluctuations billions of years ago eventually resulted in the galaxies and stars we see today.

"We were waiting for the other shoe to drop, and this is it," says Andrew Lange, U.S. team leader and a professor of physics at Caltech. Lange was one of a group of cosmologists revealing new results on the cosmic microwave background at the American Physical Society's spring meeting April 29. Other presenters included teams from the DASI and MAXIMA projects.

The new results are from a detailed analysis of high-resolution images obtained by BOOMERANG, which is an acronym for Balloon Observations of Millimetric Extragalactic Radiation and Geophysics. BOOMERANG is an extremely sensitive microwave telescope suspended from a balloon that circumnavigated the Antarctic in late 1998. The balloon carried the telescope at an altitude of almost 37 kilometers (120,000 feet) for 10 and one-half days.

"The key to BOOMERANG's ability to obtain these new images is the marriage of a powerful new detector technology developed at Caltech and the Jet Propulsion Lab with the superb microwave telescope and cryogenic systems developed in Italy at ENEA, IROE/CNR, and La Sapienza," Lange says.

The images were published just one year ago, and the Lange team at the time reported that the results showed the most precise measurements to date of the geometry of space-time. The initial analysis revealed that the single detectable peak represented about a 1-degree expanse, which is precisely the size of large detail predicted by theorists if space-time is indeed flat. Larger peaks would have indicated that the universe is "closed" like a ball, doomed to eventually collapse in on itself, while smaller peaks would have indicated that the universe is "open," or shaped like a saddle, and would expand forever.

Cosmologists believe that the universe was created approximately 12 to 15 billion years ago in an enormous explosion called the Big Bang. The intense heat that filled the embryonic universe is still detectable today as a faint glow of microwave radiation that is visible in all directions. This radiation is known as the cosmic microwave background (CMB). Whatever structures were present in the very early universe would leave their mark imprinted as a very faint pattern of variations in brightness in the CMB.

The CMB was first discovered by a ground-based radio telescope in 1965. Within a few years, Russian and American theorists had independently predicted that the size and amplitude of structures that formed in the early universe would form what mathematicians call a "harmonic series" of structure imprinted on the CMB. Just as the difference in harmonic content allows us to distinguish between a piano and a trumpet playing the same note, so the details of the harmonic content imprinted in the CMB allow us to understand the detailed nature of the universe.

Detection of the predicted features was well beyond the technology available at the time. It was not until 1991 that NASA's COBE (Cosmic Background Explorer) satellite discovered the first evidence for structures of any sort in the CMB.

The BOOMERANG images are the first to bring the CMB into sharp focus. The images reveal hundreds of complex regions that are visible as tiny variations—typically only 100 millionths of a degree (0.0001 C)—in the temperature of the CMB. The new results, released today, show the first evidence for a harmonic series of angular scales on which structure is most pronounced.

The images obtained cover about 3 percent of the sky, generating so much data that new methods had to be invented before it could be thoroughly analyzed. The new analysis provides the most precise measurement to date of several of the parameters which cosmologists use to describe the universe.

The BOOMERANG team plans another campaign to the Antarctic in the near future, this time to map even fainter images encoded in the polarization of the CMB. Though extremely difficult, the scientific payoff of such measurements "promises to be enormous," maintains the U.S team leader of the new effort, John Ruhl, of the University of California at Santa Barbara. "By imaging the polarization, we may be able to look right back to the inflationary epoch itself—right back to the very beginning of time."

Data from the MAXIMA project is also being presented at the American Physical Society meeting, along with data from the CBI, which is also a National Science Foundation-supported mission. The CBI investigators, led by Caltech astronomy professor Tony Readhead, reported early results in the March 1 issue of the Astrophysical Journal. These results were in agreement with the finding of the other projects.

The 36 BOOMERANG team members come from 16 universities and organizations in Canada, Italy, the United Kingdom, and the United States. Primary support for BOOMERANG comes from the Italian Space Agency, Italian Antarctic Research Programme, and the University of Rome "La Sapienza" in Italy; from the Particle Physics and Astronomy Research Council in the United Kingdom; and from the National Science Foundation and NASA in the United States.

Contact: Robert Tindol (626) 395-3631

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Distant Massive Explosion Reveals a Hidden Stellar Factory

A gamma-ray burst detected in February has led astronomers to a galaxy where the equivalent of 500 new suns is being formed each year.

The discovery of a new "starburst galaxy," made by researchers from the National Radio Astronomy Observatory and the California Institute of Technology, provides support for the theory that gamma-ray bursts are caused by exploding young massive stars. Details of the discovery are being presented today at the Gamma 2001 conference.

"This is a tremendously exciting discovery, since gamma-rays can penetrate dusty veils, and thus gamma-ray bursts can be used to locate the hitherto difficult-to-study dust galaxies at high redshifts," says Fiona Harrison, assistant professor of physics and astronomy at Caltech. "Gamma-ray bursts may offer us a new way to study how stars are formed in the early universe."

Radiation from this gamma-ray burst was first detected in the constellation Bootes by the Italian satellite observatory BeppoSAX on Feb. 21. Within hours, astronomers worldwide received the news of the burst and began looking for a visible light counterpart. The burst was one of the brightest recorded in the four years BeppoSAX has been standing watch.

Gamma-ray bursts were first detected by satellites monitoring the Nuclear Test Ban Treaty in the 1970s, and were thought for many years to represent relatively modest outbursts on nearby neutron stars. The events have now been shown to originate in the farthest reaches of the universe. They produce, in a matter of seconds, more energy than the sun will generate in its entire 10-billion-year lifetime, and represent the most luminous explosions in the cosmos.

After the February event, astronomers at the U.S. Naval Observatory discovered the visible light counterpart, pinpointing the location of the event. An international collaboration, led by Dale Frail of the National Radio Astronomical Observatory and Harrison and Shri Kulkarni of Caltech, conducted a variety of observations using the Hubble Space Telescope, the Very Large Array radio telescope, the Chandra X-ray Observatory, Institut de RadioAstronomie Millimétrique (IRAM), and the James Clerk Maxwell telescope (JCMT).

Pivotal to the detection of the starburst galaxy was the latter telescope, which sits high atop Mauna Kea in Hawaii and is designed to make measurements at the shortest radio wavelengths capable of penetrating Earth's atmosphere, called the "submillimeter" portion of the spectrum. Only five and a half hours after the first sighting, a submillimeter source was found at the burst location.

Astronomers had expected to see a rapidly brightening signal with JCMT, a sign that the shock generated by the burst was moving through the dense gas surrounding the burst. Instead, much to everyone's surprise, the signal stayed constant, never varying during this time.

Furthermore, observations conducted at a slightly lower frequency by observers on the IRAM telescope in Southern Spain showed a much fainter source, strongly suggesting that the submillimeter observation was not simply detecting the afterglow of the explosion.

"The simplest explanation is that we have detected the light from the host galaxy of the burst," says Frail, explaining that it is rare to detect galaxies at submillimeter wavelengths. Only about one in every thousand that are visible with optical telescopes are observed by short-wavelength radio telescopes.

Astronomers in Arizona found the gamma-ray burst to lie roughly 8 billion light-years from Earth. This was also confirmed almost simulataneously by the Caltech group using one of the 10-meter Keck telescopes on Mauna Kea. The light we see from it shows the galaxy when the universe was less than half its present age. At this distance, the observed submillimeter brightness implies a prodigious rate of star formation—roughly 500 solar masses of material must be turning into stars each year, meaning that one or two new stars shine forth each day. The galaxy in which the burst occurred, then, may provide a glimpse of what the Milky Way looked like in its youth.

Previous searches for starburst galaxies in the distant universe have been hampered by the imprecise positions current submillimeter telescopes provide, and by the obscuring dust and gas that largely hides such galaxies from the view of optical telescopes. Observers led by Kulkarni used the Hubble Space Telescope to observe the fading embers of the explosion, but the underlying galaxy seems ordinary as seen in visible light, since most of this light is likely absorbed by dust and converted into submillimeter radiation. Had the galaxy been observed only in the optical wavelengths, astronomers would not have guessed that so many stars were being formed in it.

The discovery of a bright gamma-ray burst in a starburst system is exciting for two reasons: it strongly supports one model for the bursts themselves—the explosive destruction of a young, massive star—and it suggests a new way to locate such galaxies. With their enormous penetrating power, the energetic gamma rays punch right through the dusty veil, pinpointing the location of vigorous star-forming activity.

By following up on the hundreds of bursts that will be detected in the next few years, astronomers will be able to collect an unbiased sample of starburst galaxies at different distances in time and space, enabling them to explain the star-formation history of the universe.

The members of the Caltech team also include: Prof. S. George Djorgovski; postdoctoral fellows Derek Fox, Titus Galama, Daniel Reichart, Re'em Sari and Fabian Walter; and graduate students Edo Berger, Joshua Bloom, Paul Price, and Sarah Yost. Astronomers from several other institutions are also involved in the collaboration.

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