Universe Today

Hubble Space Telescope data, analyzed by a Yale astronomer using gravitational lensing techniques, has generated a spatial map demonstrating the clumped substructure of dark matter inside clusters of galaxies.

Clusters of galaxies (about a million, million times the mass of our sun), are typically made up of hundreds of galaxies bound together by gravity. About 90 percent of their mass is darkmatter. The rest is ordinary atoms in the form of hot gas and stars.

Although little is known about it, cold dark matter is thought to have structure at all magnitudes. Theoretical models of the clumping properties were derived from detailed, high resolution simulations of the growth of structure in the Universe. Although previous evidence supported the ?concordance model? of a Universe mostly composed of cold, dark matter, the predicted substructure had never been detected.

In this study, Yale assistant professor of astronomy and physics Priyamvada Natarajan and her colleagues demonstrate that, at least in the mass range of typical galaxies in clusters, there is an excellent agreement between the observations and theoretical predictions of the concordance model.

Using gravitational lensing made it possible for the observers to visualize light from distant galaxies as it bent around mass in its way. This allowed the researchers to measure light deflections that indicated structural clumps in the dark matter.

?We used an innovative technique to pick up the effect of precisely the clumps which might otherwise be obscured by the presence of more massive structures,? said Natarajan. ?When we compared our results with theoretical expectations of the concordance model, we found extremely good agreement, suggesting that the model passes the substructure test for the mass range we are sensitive to with this technique.?

?We think the properties of these clumps hold a key to the nature of dark matter ? which is presently unknown,? said Natarajan. ?The question remains whether these predictions and observations agree for smaller mass clumps that are as yet undetected.?

Co-author on the study, funded by Yale University, is Volker Springel, MPA, Garching, Germany. Other collaborators include. Jean-Paul Kneib, LAM ? OAMP, Marseille, France, Ian Smail, University of Durham, U.K., and Richard Ellis of Caltech.

Original Source: Yale News Release

Image credit: NASA
The NASA-led Swift mission has opened its doors to a flurry of gamma-ray burst action.

Scientists were still calibrating the main instrument, the Burst Alert Telescope (BAT), when the first burst appeared on December 17. Three bursts on December 19, and one on December 20, followed.

Swift’s primary goal is to unravel the mystery of gamma ray bursts. The bursts are random and fleeting explosions, second only to the Big Bang in total energy output. Gamma rays are a type of light millions of times more energetic than light human eyes can detect. Gamma ray bursts last only from a few milliseconds to about one minute. Each burst likely signals the birth of a black hole.

“The optimists among us were hoping to detect two bursts a week, not three in one day just after turning the telescope on,” said Dr. Scott Barthelmy, the BAT lead scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md. “Maybe we got lucky, or maybe we’ve underestimated the true rate of these bursts. Only time will tell,” he added.

Once the BAT, that covers about one-seventh of the sky at any time, detects a gamma ray burst, it quickly relays a location to the ground. Within about one minute, the satellite automatically turns toward the burst. The move brings the burst within view of Swift’s two other telescopes: the X-ray Telescope (XRT) and the Ultraviolet/Optical Telescope (UVOT).

Once all three instruments are turned on and calibrated, Swift will get down to the business of analyzing gamma ray bursts. “The universe kept up its side of the bargain, and we kept up ours,” said Dr. Neil Gehrels, Swift’s Principal Investigator at Goddard. “This is going to be an exciting mission,” he said.

The Swift team tested the BAT by observing Cygnus X-1, a well-known bright source that produces gamma rays in our galaxy. It is thought to be a black hole in orbit around a star. The team called this BAT’s “first light.”

The BAT is the most sensitive gamma ray detector ever flown. The BAT employs a novel technology to image and locate gamma ray bursts. Unlike visible light, gamma rays pass right through telescope mirrors and cannot be reflected onto a detector. The BAT uses a technique called “coded aperture mask” to create a gamma ray shadow on its detectors. The mask contains 52,000 randomly placed lead tiles that block some gamma rays from reaching the detectors. With each burst, some detectors light up while others remain dark, shaded by the lead tiles. The angle of the shadow points back to the gamma ray burst.

“The BAT coded aperture mask is about the size of a pool table, the largest and most intricate ever fabricated,” said Ed Fenimore of Los Alamos National Laboratory, N.M. Los Alamos created the BAT software. “BAT can accurately pinpoint a burst within seconds and detect bursts five times fainter than previous instruments,” he added.

Swift, a medium-class explorer mission managed by Goddard, was launched from Cape Canaveral on November 20, 2004. The mission is in participation with the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom.

Swift was built at Goddard in collaboration with General Dynamics, Ariz.; Penn State University, College Station, Pa.; Sonoma State University, Rohnert Park, Calif.; Los Alamos; Mullard Space Science Laboratory, Surrey, England; the University of Leicester, England; the Brera Observatory, Milan, Italy; and ASI Science Data Center, Rome.

Original Source: NASA News Release

Planetary nebulae are expanding gas shells that are ejected by Sun-like stars at the end of their lifetimes. Sun-like stars spend most of their lifetime burning hydrogen into helium. At the end of this hydrogen fusion phase, these stars increase their diameter by about a factor of 100 and become “red giant stars”. At the end of the red giant phase, the outer layers of the star are blown away. The ejected gas continues to expand out from the remaining central star, which later evolves into a “white dwarf” when all nuclear fusion has ceased. Astronomers believe that a planetary nebula forms when a fast stellar wind that comes from the central star catches up a slower wind produced earlier when the star ejected most of its outer layers. At the boundary between the two winds, a shock occurs that produces the visible dense shell characteristic of planetary nebulae. The gas shell is excited and lighted up by the light emitted by the hot central star. The light from the central star is able to light up the planetary nebula for some 10,000 years.

The observed shapes of planetary nebulae are very puzzling: most of them (about 80%) are bipolar or elliptical rather than spherically symmetric. This complexity has lead to beautiful and amazing images obtained with modern telescopes. The pictures below compare planetary nebulae with bipolar (left) and spherical (right) shapes.

The reason why most planetary nebulae are not spherical is not well understood. Several hypotheses have been considered so far. One of them suggests that the strange shapes of planetary nebulae might be due to some centrifugal effect that results from the fast rotation of red giants. Another theory is that the symmetry of the star’s wind may be affected by a companion star. However, the most recent and convincing theories explaining the shapes of the nebulae involve magnetic fields.

The presence of magnetic fields would nicely explain the complicated shapes of planetary nebulae, as the ejected matter is trapped along magnetic field lines. This can be compared to iron filings trapped along the field lines of a bar magnet – a classic demonstration in high school physics classrooms. Since strong magnetic fields at the surface of the star also exert pressure on the gas, matter can more easily leave the star at the magnetic poles where the magnetic field is strongest.

There are several ways magnetic fields can be created in the vicinity of planetary nebulae. Magnetic fields can be produced by a stellar dynamo during the phase when the nebula is ejected. For a dynamo to exist, the core of the star must rotate faster than the envelope (as is the case in the Sun). It is also possible that the magnetic fields are fossil relics of previous stages of stellar evolution. Under most circumstances, the matter in stars is so highly electrically conductive that magnetic fields can survive for millions or billions of years. Both mechanisms, combined with the interaction of the ejected matter with the surrounding interstellar gas, would be able to shape the planetary nebulae.

Until recently, the idea that magnetic fields are an important ingredient in the shaping od planetary nebulae was a purely theoretical claim. In 2002, the first indications of the presence of such magnetic fields were found. Radio observations revealed magnetic fields in circumstellar envelopes of giant stars. These circumstellar envelopes are indeed progenitors of planetary nebulae. However, no such magnetic field has ever been observed in the nebulae themselves. To obtain direct clue of the presence of magnetic fields in planetary nebulae, astronomers decided to focus on the central stars, where the magnetic fields should have survived.

This first direct evidence has now been obtained. For the first time, Stefan Jordan and his team detected magnetic fields in several central stars of planetary nebulae. Using the FORS1 spectrograph of the 8-m class Very Large Telescope (VLT, European Southern Observatory, Chile), they measured the polarization of the light emitted by four of these stars. The polarization signatures in the spectral lines make it possible to determine the intensity of the magnetic fields in the observed stars. In the presence of a magnetic field, atoms change their energy in a characteristic way; this effect is called the Zeeman effect and was discovered in 1896 by Pieter Zeeman in Leiden (Netherlands). If these atoms absorb or emit light, the light becomes polarized. This makes it possible to determine the strength of the magnetic field by measuring the strength of the polarization. These polarization signatures are usually very weak. Such measurements require very high quality data that can only be obtained using 8-meter class telescopes such as the VLT.

Four central stars of planetary nebulae were observed by the team and magnetic fields were found in all of them. These four stars were chosen because their associated planetary nebulae (named NGC 1360, HBDS1, EGB 5, and Abell 36) are all non-spherical. Therefore, if the magnetic field hypothesis to explain the shapes of planetary nebulae is correct, these stars should have strong magnetic fields. These new results show that it is indeed the case: the strengths of the detected magnetic fields range from 1000 to 3000 Gauss, that is about one thousand times the intensity of the Sun’s global magnetic field.

These new observations published by Stefan Jordan and his colleagues support the hypothesis that magnetic fields play a major role in shaping planetary nebulae. The team now plans to search for magnetic fields in the central stars of spherical planetary nebulae. Such stars should have weaker magnetic fields than the ones just detected. These future observations will allow astronomers to better quantify the correlation between magnetic fields and the strange shapes of planetary nebulae.

In the few past years, polarimetric observations with the VLT have led to the discovery of magnetic fields in a large number of stellar objects in late evolutionary stages. In addition to improving our understanding of these beautiful planetary nebulae form, the detection of these magnetic fields allows science to take a step forward towards the clarification of the relationship between magnetic fields and stellar physics.

Original Source: NASA Astrobiology Story

Like the annual New Year?s fireworks display, astronomers at Gemini Observatory are ushering in 2005 with a striking image that dazzles the eye with stellar pyrotechnics.

In the image, the face-on spiral galaxy NGC 6946 is ablaze with colorful galactic fireworks fueled by the births and deaths of multitudes of brilliant, massive stars. Astronomers suspect that massive stellar giants have been ending their lives in supernova explosions throughout NGC 6946 in rapid-fire fashion for tens of millions of years.

?In order to sustain this rate of supernova activity, massive, quickly evolving stars must form or be born at an equally rapid rate in NGC 6946,? said Gemini North Associate Director, Jean-Ren? Roy. ?Its stars are exploding like a string of firecrackers!?

Astronomers speculate that if just a million years of this galaxy?s history were compressed into a time-lapse movie lasting a few seconds, there would be nearly constant outbursts of light as new stars flare into view, while old ones expire in spectacular explosions. Over the past century, eight supernovae have exploded in the arms of this stellar metropolis, occurring in 1917, 1939, 1948, 1968, 1969, 1980, 2002, and 2004. This makes NGC 6946 the most prolific known galaxy for supernovae during the past 100 years.

By comparison, the average rate for such catastrophic stellar outbursts in the Milky Way is about one per century, and only four have been recorded over the last thousand years. The last known supernova went off in our galaxy in the constellation Ophiuchus in 1604.

Yet, it is the ubiquitous occurrence of starbirth throughout NGC 6946 and not its supernovae that lend this galaxy its blazingly colorful appearance. For reasons not completely understood, it experiences a much higher rate of star formation than all the large galaxies in our local neighborhood. The prodigious output of stellar nurseries in this galactic neighbor eventually leads to accelerated numbers of supernova explosions.

Starbirth regions exist in most galaxies, particularly in spirals, and are obvious as clouds of predominantly hydrogen gas called H II regions. These areas coalesce over millions of years to form stars. Young, hot, massive stars formed in these regions emit copious amounts of ultraviolet radiation, which strip the electrons from hydrogen atoms in which they are embedded. When these ionized hydrogen atoms re-associate with electrons they radiate in a deep red color (at a wavelength of 656.3 nanometers) as the electrons transition back to lower energy levels.

This Gemini image of NGC 6946 utilizes a selective filter specifically designed to detect the radiation emanating from the starbirth regions. Additional filters help to distinguish other details in the galaxy, including clusters of massive blue stars, dust lanes, and a yellowish core where older more evolved stars dominate.

NGC 6946 lies between 10 and 20 million light-years away on the border between the constellations of Cepheus and Cygnus, and was discovered by Sir William Herschel (1738-1822) on September 9, 1798. It continues to fascinate astronomers, who estimate that it contains about half as many stars as the Milky Way. They often use it to study and characterize the evolution of massive stars and the properties of interstellar gas. As viewed in the new Gemini optical image, we see only the ?tip of the iceberg? of this galaxy. Its optical angular diameter is about 13 arcminutes, but viewed at radio wavelength at the frequency of neutral hydrogen (1420 Mhz or 21-cm line), it extends considerably more than the angular diameter of the Moon.

Original Source: Gemini News Release