Thursday, January 31, 2008

Astronomers have used ESO's Very Large Telescope to measure the distribution and motions of thousands of galaxies in the distant Universe. This opens fascinating perspectives to better understand what drives the acceleration of the cosmic expansion and sheds new light on the mysterious dark energy that is thought to permeate the Universe.

ESO PR Photo 04a/08Large-scale structures(Artist's Impression)

"Explaining why the expansion of the Universe is currently accelerating is certainly the most fascinating question in modern cosmology," says Luigi Guzzo, lead author of a paper in this week's issue of Nature, in which the new results are presented. "We have been able to show that large surveys that measure the positions and velocities of distant galaxies provide us with a new powerful way to solve this mystery."

Ten years ago, astronomers made the stunning discovery that the Universe is expanding at a faster pace today than it did in the past.

"This implies that one of two very different possibilities must hold true," explains Enzo Branchini, member of the team. "Either the Universe is filled with a mysterious dark energy which produces a repulsive force that fights the gravitational brake from all the matter present in the Universe, or, our current theory of gravitation is not correct and needs to be modified, for example by adding extra dimensions to space."

Current observations of the expansion rate of the Universe cannot distinguish between these two options, but the international team of 51 scientists from 24 institutions found a way that could help in tackling this problem. The technique is based on a well-known phenomenon, namely the fact that the apparent motion of distant galaxies results from two effects: the global expansion of the Universe that pushes the galaxies away from each other and the gravitational attraction of matter present in the galaxies' neighbourhood that pulls them together, creating the cosmic web of large-scale structures.

ESO PR Photo 04b/08A Cone in the Universe

"By measuring the apparent velocities of large samples of galaxies over the last thirty years, astronomers have been able to reconstruct a three-dimensional map of the distribution of galaxies over large volumes of the Universe. This map revealed large-scale structures such as clusters of galaxies and filamentary superclusters," says Olivier Le Fèvre, member of the team. "But the measured velocities also contain information about the local motions of galaxies; these introduce small but significant distortions in the reconstructed maps of the Universe. We have shown that measuring this distortion at different epochs of the Universe's history is a way to test the nature of dark energy."

Guzzo and his collaborators have been able to measure this effect by using the VIMOS spectrograph on Melipal, one of the four 8.2-m telescopes that is part of ESO's VLT. As part of the VIMOS-VLT Deep Survey (VVDS), of which Le Fèvre is the Principal Investigator, spectra of several thousands of galaxies in a 4-square-degree field (or 20 times the size of the full Moon) at epochs corresponding to about half the current age of the Universe (about 7 billion years ago) were obtained and analysed.

"This is the largest field ever covered homogeneously by means of spectroscopy to this depth," declares Le Fèvre. "We have now collected more than 13,000 spectra in this field and the total volume sampled by the survey is more than 25 million cubic light-years."

The astronomers compared their result with that of the 2dFGRS survey that probed the local Universe, i.e. measures the distortion at the present time.

Within current uncertainties, the measurement of this effect provides an independent indication of the need for an unknown extra energy ingredient in the 'cosmic soup', supporting the simplest form of dark energy, the so-called cosmological constant, introduced originally by Albert Einstein. The large uncertainties do not yet exclude the other scenarios, though.

"We have also shown that by extending our measurements over volumes about ten times larger than the VVDS, this technique should be able to tell us whether cosmic acceleration originates from a dark energy component of exotic origin or requires a modification of the laws of gravity," explains Guzzo.

"VIMOS on the VLT would certainly be a wonderful tool to perform this future survey and help us answer this fundamental question. This strongly encourages scientists to proceed with even more ambitious surveys of the distant Universe," concludes Le Fèvre.

Tuesday, January 29, 2008

An image based on data taken with ESO's Very Large Telescope reveals a triplet of galaxies intertwined in a cosmic dance.

Eso PR Photo 02/08

The three galaxies, catalogued as NGC 7173 (top), 7174 (bottom right) and 7176 (bottom left), are located 106 million light-years away towards the constellation of Piscis Austrinus (the 'Southern Fish').

NGC 7173 and 7176 are elliptical galaxies, while NGC 7174 is a spiral galaxy with quite disturbed dust lanes and a long, twisted tail. This seems to indicate that the two bottom galaxies - whose combined shape bears some resemblance to that of a sleeping baby - are currently interacting, with NGC 7176 providing fresh material to NGC 7174. Matter present in great quantity around the triplet's members also points to the fact that NGC 7176 and NGC 7173 have interacted in the past.

Astronomers have suggested that the three galaxies will finally merge into a giant 'island universe', tens to hundreds of times as massive as our own Milky Way.

Eso PR Photo 02b/08NGC 7173, 7174, and 7176

The triplet is part of a so-called 'Compact Group', as compiled by Canadian astronomer Paul Hickson in the early 1980s. The group, which is the 90th entry in the catalogue and is therefore known as HCG 90, actually contains four major members. One of them - NGC 7192 - lies above the trio, outside of this image, and is another peculiar spiral galaxy.

Compact groups are small, relatively isolated, systems of typically four to ten galaxies in close proximity to one another. Another striking example is Robert's Quartet. Compact groups are excellent laboratories for the study of galaxy interactions and their effects, in particular the formation of stars.

As the striking image reveals, there are many other galaxies in the field. Some are distant ones, while others seem to be part of the family. Studies made with other telescopes have indeed revealed that the HCG 90 group contains 16 members, most of them much smaller in size than the four members with an entry in the NGC catalogue.

NotesA list of all the work about this group of galaxies can be for example obtained through the Simbad astronomical database.

Elliptical galaxies are smooth and featureless entities, often composed of older, low mass stars, which produce very few new stars. Spiral galaxies consist of a flat, rotating disc of stars, gas and dust, and a central concentration of stars known as the bulge. The spiral arms are sites of ongoing star formation. Our own Milky Way is a spiral galaxy.

Using ESO's Very Large Telescope Interferometer, astronomers have probed the inner parts of the disc of material surrounding a young stellar object, witnessing how it gains its mass before becoming an adult.

The astronomers had a close look at the object known as MWC 147, lying about 2,600 light years away towards the constellation of Monoceros ('the Unicorn'). MWC 147 belongs to the family of Herbig Ae/Be objects. These have a few times the mass of our Sun and are still forming, increasing in mass by swallowing material present in a surrounding disc.

MWC 147 is less than half a million years old. If one associated the middle-aged, 4.6 billion year old Sun with a person in his early forties, MWC 147 would be a 1-day-old baby [1].

The morphology of the inner environment of these young stars is however a matter of debate and knowledge of it is important to better understand how stars and their cortège of planets form.

The astronomers Stefan Kraus, Thomas Preibisch, and Keiichi Ohnaka have used the four 8.2-m Unit Telescopes of ESO's Very Large Telescope to this purpose, combining the light from two or three telescopes with the MIDI and AMBER instruments.

"With our VLTI/MIDI and VLTI/AMBER observations of MWC147, we combine, for the first time, near- and mid-infrared interferometric observations of a Herbig Ae/Be star, providing a measurement of the disc size over a wide wavelength range[2]," said Stefan Kraus, lead-author of the paper reporting the results. "Different wavelength regimes trace different temperatures, allowing us to probe the disc's geometry on the smaller scale, but also to constrain how the temperature changes with the distance from the star."

The near-infrared observations probe hot material with temperatures of up to a few thousand degrees in the innermost disc regions, while the mid-infrared observations trace cooler dust further out in the disc.

The observations show that the temperature changes with radius are much steeper than predicted by the currently favoured models, indicating that most of the near-infrared emission emerges from hot material located very close to the star, that is, within one or two times the Earth-Sun distance (1-2 AU). This also implies that dust cannot exist so close to the star, since the strong energy radiated by the star heats and ultimately destroys the dust grains.

"We have performed detailed numerical simulations to understand these observations and reached the conclusion that we observe not only the outer dust disc, but also measure strong emission from a hot inner gaseous disc. This suggests that the disc is not a passive one, simply reprocessing the light from the star," explained Kraus. "Instead, the disc is active, and we see the material, which is just transported from the outer disc parts towards the forming star."

The best-fit model is that of a disc extending out to 100 AU, with the star increasing in mass at a rate of seven millionths of a solar mass per year.

"Our study demonstrates the power of ESO's VLTI to probe the inner structure of discs around young stars and to reveal how stars reach their final mass," said Stefan Kraus.

More Information

The authors report their results in a paper in the Astrophysical Journal ("Detection of an inner gaseous component in a Herbig Be star accretion disk: Near- and mid-infrared spectro-interferometry and radiative transfer modeling of MWC 147", by Stefan Kraus, Thomas Preibisch, Keichii Ohnaka").

Notes

[1] Being 6.6 times more massive than the Sun, however, MWC 147 will only live for about 35 million years, or to draw again the comparison with a person, about 100 days, instead of the 80 year equivalent of our Sun.

[2] MIDI is the mid-infrared instrument of the VLT interferometer. It operates between 8 and 13 microns. AMBER observes in the near-infrared, from 1 to 2.4 microns.

For years, researchers have observed swirling dust clouds around systems called recurring novas, which periodically explode. New images of a distant nova have now overturned astronomers' long-standing assumption that the dust originates in the blasts.

Scientists recently observed the RS Ophiuchi system, where a small white dwarf star and large red giant orbit each other. Over time, the giant sheds its outer layer of gas, which the dwarf sweeps up. The little star's mass grows gradually, eventually reaching a tipping point, when the top layer ignites in a thermonuclear explosion and expels the surface into space. The process then starts over — astronomers have already seen this system "go nova" in 1898, 1933, 1958, 1967 and 1985.

When RS Ophiuchi blew again in February 2006, researchers took advantage of a new instrument, called the Keck Nuller, at the W. M. Keck Observatory in Mauna Kea, Hawaii, to watch the event in action. The Nuller used two giant telescopes to block out the overwhelming light from the explosion so scientists could study its fainter surroundings.

They were surprised to see no dust in the bright zone around the star and only to see dust farther away, where the blast wave had not yet reached. The researchers surmised that the detonation had vaporized nearby dust particles, and that the outer dust must have been created before the bang.

"This flies in the face of what we expected," said Richard Barry, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md., who headed up the observations. "Astronomers had previously thought that nova explosions actually create dust."

The team suspects the dust is really produced when the white dwarf plows through the red giant's trail of debris, creating patches of gas where atoms are cool and dense enough to clump together into dust particles.

The findings will be detailed in the May 1 issue of the Astrophysical Journal.

This Chandra X-ray Observatory image shows Westerlund 2, a young star cluster with an estimated age of about one or two million years. Until recently little was known about this cluster because it is heavily obscured by dust and gas. However, using infrared and X-ray observations to overcome this obscuration, Westerlund 2 has become regarded as one of the most interesting star clusters in the Milky Way galaxy. It contains some of the hottest, brightest and most massive stars known.

This Chandra image of Westerlund 2 shows low energy X-rays in red, intermediate energy X-rays in green and high energy X-rays in blue. The image shows a very high density of massive stars that are bright in X-rays, plus diffuse X-ray emission.

An incredibly massive double star system called WR20a is visible as the bright yellow point just below and to the right of the cluster's center. This system contains stars with masses of 82 and 83 times that of the Sun. The dense streams of matter steadily ejected by these two massive stars, called stellar winds, collide with each other and produce copious amounts of X-ray emission. This collision is seen at different angles as the stars orbit around each other every 3.7 days. Several other bright X-ray sources may also show evidence for collisions between winds in massive binary systems.

This colored Chandra X-ray Observatory image (inset) shows Westerlund 2 in context with the Spitzer infrared observation (black & white). Westerlund 2 is a young star cluster with an estimated age of about one or two million years. Until recently little was known about this cluster because it is heavily obscured by dust and gas. However, using infrared and X-ray observations to overcome this obscuration, Westerlund 2 has become regarded as one of the most interesting star clusters in the Milky Way galaxy. It contains some of the hottest, brightest and most massive stars known.

Friday, January 25, 2008

A new study of 'GEMS' from Hubble and Spitzer reveals cosmic fireworks fizzled out when the universe reached middle age.

We all start to party less around middle age, and new studies by a team led by University of Texas at Austin astronomer Shardha Jogee now finds that the universe, as a whole, is no exception.

According to the current models of galaxy formation, dubbed "hierarchical lambda cold dark matter" models, galaxies built up to their current masses, shapes, and sizes through the successive mergers of less massive protogalaxies made of gas, stars, and dark matter. In the first quarter of the universe's lifespan, the cosmic landscape was dominated by violent galaxy mergers, which could radically transform the shape of a galaxy and convert its gas into stars at an extreme rate. More than half of bright galaxies were indulging in such violent "partying."

New research is showing that all changed when the universe hit middle age. "Our study finds that over the last 7 billion years, after the universe hit its mid-forties, so to speak, it transitioned from a violent merger-driven mode into a quieter mode," Jogee says.

She and her team find that over each billion-year interval, only 10 percent of galaxies are typically involved in strong interactions and mergers.

Jogee's team has analyzed more than 5,000 galaxies imaged by Hubble Space Telescope as part of GEMS, one of the largest-area surveys conducted with Hubble in two filters.

"It's been exciting to apply different complementary techniques in this large survey and to sift through the merger history of the universe during this elusive era," says Sarah Miller, member of the international GEMS collaboration.
In addition to estimating the frequency of mergers, Jogee and her colleagues found that contrary to what is commonly assumed, the average star formation rate in these interacting and merging galaxies is only enhanced by a modest factor of two to three compared to that in normal non-interacting galaxies.

"While extreme bursts of star formation, so-called cosmic fireworks, may happen in some galaxy mergers or interactions, they are not the norm in the vast majority of galaxy interactions taking place over the last 7 billion years," Jogee says.

The findings of Jogee's team result from a powerful synergy of data from NASA's Hubble and Spitzer space telescopes. "Mid-infrared observations from the Spitzer Space Telescope, taken by George Rieke of The University of Arizona, were key for tracing hidden star formation, obscured by dust," Jogee says. "The exquisite resolution of the GEMS Hubble data in turn allowed us to identify strongly interacting and merging galaxies at much earlier cosmological times than conventional ground-based telescopes," says team member Daniel McIntosh of the University of Massachusetts, Amherst.

Jogee and her team, in fact, find that only 20 percent of the total cosmic star formation that took place over the last 7 billion years appears to come from strongly interacting and merging galaxies. These results extend the similar trend found for a smaller sample of about 1,500 galaxies over a narrower time interval by fellow team members Christian Wolf from Oxford University and Eric Bell of the Max Planck Institute of Astronomy.

Furthermore, the results reported by Jogee and her team on the modest fraction (about 20 percent) of merger-induced star formation, and the frequency of galaxy mergers over the last 7 billion years, are in remarkably good agreement with prevailing theoretical cold dark matter models of galaxy evolution.

According to team member Rachel Somerville of the Max Planck Institute of Astronomy, "Mergers are thought to be a crucial process in transforming galaxies, causing bursts of star formation, and perhaps even feeding gas to the supermassive black holes lurking in the galaxy's nucleus.

"Although the frequency of mergers predicted by the models agrees quite well with the observed frequency," Somerville says, "these observations can also teach us much more about the effect these violent episodes have on galaxies."

In fact, Jogee says, "Our results raise many additional questions which can only be addressed with next generation facilities. For example, the cosmic star-formation rate is declining in normal galaxies, but it remains unclear what drives this decline. Are galaxies using up their internal cold gas supply, or is the accretion rate of gas from external filaments declining?"

Next-generation radio facilities, such as ALMA (the Atacama Large Millimeter/Sub-millimeter Array) will be critical for exploring how the cold gas content of galaxies changes over the last seven billion years, she said.

"Another key thing to note is that some of our results starkly disagree with prevailing hierarchical models of galaxy evolution," Jogee says. According to these models, the frequency of pure disk galaxies or so-called "bulgeless galaxies" is expected to be extremely low, because a past major merger in the life of every galaxy invariably builds a bulge.

Contrary to such predictions, postdoctoral fellow Fabio Barazza, formerly working with Jogee at The University of Texas and now at Geneva Observatory's Ecole Polytechnique Federale de Lausanne, found that about 20 percent of present-day spiral galaxies are bulgeless or disk-dominated, based on the analysis of about 1,000 galaxies from the Sloan Digital Sky Survey.

"We also see striking super-thin bulgeless galaxies in GEMS, at earlier epochs," Jogee says. "We yet have to characterize the frequency and origin of these enigmatic bulgeless galaxies at different epochs, but there is no denying their prevalence in the local universe."

All in all, "We have made important headway in piecing part of the cosmic puzzle of galaxy evolution, but daunting challenges loom ahead for both observers and theorists, " she says.

Its age implies it was formed when the universe was very young. Hubble

Some of the first massive galaxies in the universe formed when huge gas clouds rapidly collapsed, according to Elizabeth McGrath of the University of California, Santa Cruz, Alan Stockton of the University of Hawaii, and their collaborators. This discovery, which is based on new Hubble Space Telescope images, challenges the commonly held idea that all of the earliest massive galaxies formed when smaller galaxies merged.

The standard theory of galaxy formation predicts that the most massive galaxies in the universe take a long time to grow, accumulating mass through the coalescence of many smaller galaxies in a process that continues until relatively recent times. To test this theory, McGrath, Stockton, and their collaborators searched for the oldest, most massive galaxies they could find and used clues from their shape and structure to deduce how they may have formed. High-resolution images from the Hubble Space Telescope revealed galaxies more massive than our own Milky Way that existed when the universe was very young, only one-fifth its current age. It is believed that such galaxies are the distant ancestors of the most massive galaxies in the universe today.

"We expected these galaxies to look similar to the football-shaped elliptical galaxies that we see at the centers of dense groups of galaxies today, where mergers are common. We were quite surprised to find that many of them appear instead to be flattened, rotating disks of ordered material," says McGrath.

Disk galaxies are pancake or saucer-shaped, and their stars orbit in circles around the center of the galaxy, much like the planets in our solar system orbit around the Sun, or like a Frisbee spins as it moves through the air. This type of galaxy is more likely to have formed from a single massive cloud that collapses under its own gravity into a flattened disk rather than through violent collisions of previously formed galaxies. Computer simulations of the latter scenario predict that collisions would destroy disks and send stars from each merging galaxy into more chaotic, three-dimensional orbits, producing football-shaped, or elliptical galaxies. The most massive galaxies in the universe today all appear to be elliptical in shape, and therefore can be quite naturally explained through the merger hypothesis. The existence of massive disk galaxies in the early universe, however, challenges this perspective.

In total, McGrath and her collaborators observed seven of what are likely some of the first massive galaxies to form in the universe. Of these, four had shapes dominated by disk-like profiles. By age-dating the galaxies from studying properties of the stars within them, McGrath's team discovered that these disk structures have remained in pristine condition for over 1 billion years. Even so, it seems inevitable that eventually these galaxies will merge with others and be reformed into the more elliptical-shaped massive, old galaxies that are familiar to us in the nearby universe.

Arecibo Observatory finds that neutron stars can be more massive, while black holes are more rare.

Neutron stars can be considerably more massive than previously believed, and it is more difficult to form black holes, according to new research developed by using the Arecibo Observatory in Arecibo, Puerto Rico.

In the cosmic continuum of dead, remnant stars, the Arecibo astronomers have increased the mass limit for when neutron stars turn into black holes.

"The matter at the center of a neutron star is highly incompressible. Our new measurements of the mass of neutron stars will help nuclear physicists understand the properties of super-dense matter," says Paulo Freire, an astronomer from the observatory. "It also means that to form a black hole, more mass is needed than previously thought. Thus, in our universe, black holes might be more rare and neutron stars slightly more abundant."

When the cores of massive stars run out of nuclear fuel, their enormous gravitation then causes their collapse then becomes a supernova. The core, typically with a mass 1.4 times larger than that of the sun is compressed into a neutron star. These extreme objects have a radius about 10 to 16 kilometers and a density on the order of a billion tons per cubic centimeter. Freire says that a neutron star is like one single, giant atomic nucleus with about 460,000 times the mass of the Earth.

Astronomers had thought the neutron stars needed a maximum mass between 1.6 and 2.5 Suns in order to collapse and become black holes. However, this new research shows that neutron stars remain neutron stars between the mass of 1.9 and up to possibly 2.7 Suns.

"The matter at the center of the neutron stars is the densest in the universe. It is one to two orders of magnitude denser than matter in the atomic nucleus. It is so dense we don't know what it is made out of," says Freire. "For that reason, we have at present no idea of how large or how massive neutron stars can be."

From June 2001 to March 2007, Freire used Arecibo's "L-wide" receiver (sensitive to radio frequencies from 1100 to 1700 MHz) and the Wide-Band Arecibo Pulsar Processors, a very fast spectrometer on the Arecibo telescope, to examine a binary pulsar called M5 B, in the globular cluster M5, which is located in the constellation Serpens. Like a lighthouse emits light, a pulsar is a strongly magnetized neutron star that emits large amounts of electromagnetic radiation, usually from its magnetic pole. As in the case of a lighthouse, distant observers perceive a sequence of pulsations, which are caused by the rotation of the pulsar. In the case of M5 B, these radio pulsations arrive at the Earth every 7.95 milliseconds.

These radio pulsations were scanned by the wide-band spectrometers once every 64 microseconds for 256 spectral channels, and then recorded to a computer disk, with accurate timing information. The precise arrival time of the pulses were then used by the astronomers to accurately measure the orbital motion of M5 B about its companion. This allowed the astronomers to estimate the mass (1.9 solar masses) of the pulsar.

A distant galaxy cluster has turned into a giant particle accelerator, spinning electrons over vast distances at high speeds.

Scientists discovered this phenomenon by observing highly energetic X-rays emanating from the Ophiuchus cluster of galaxies.

The European Space Agency's orbiting gamma-ray observatory Integral detected the X-rays, which are too energetic to originate from the inert gas in the cluster and must instead come from accelerated particles.

Previous observations have been able to detect only lower-energy radio waves released in other clusters-turned-particle accelerators.

"This is the first time we have detected significant high-energy X-ray radiation from a cluster," said Stephane Paltani, an astrophysicist at Geneva Observatory in Switzerland, who was involved in the finding. "Only now are we reaching the sensitivity that we need to detect this radiation."

The Ophiuchus cluster must have recently merged with a smaller galaxy cluster, Paltani said. The collision would have mixed the gases in each cluster, producing rippling shock waves. As electrons bounced back and forth in the chaotic merger, they likely picked up energy and accelerated.

This cosmic particle accelerator is 20 times more powerful than the largest man-made atom smasher, the Large Hadron Collider being constructed at CERN, the particle physics lab in Switzerland, Paltani said.

"Of course the Ophiuchus cluster is somewhat bigger," Paltani said. "While LHC is 27 kilometers [17 miles] across, the Ophiuchus galaxy cluster is over two million light-years in diameter."

The scientists don't know for sure why the sped-up electrons release X-rays, but there are two possibilities. Perhaps the electrons created synchrotron radiation, which is produced when charged particles fly though magnetic fields. Or maybe the electrons collided with the Cosmic Microwave Background radiation left over in the universe from the big bang. When the sped-up particles hit the radiation they would have given it an energy boost, pumping its frequency up to the X-ray range of the electromagnetic spectrum.

New observations will be needed to tell which scenario occurred, the scientists said.

"These findings will help us better understand the properties of these clusters," Paltani told LiveScience. "This has important consequences for the history of the cluster itself. We will be able to put constraints on when the particle acceleration takes place and understand better what happens when these clusters merge."

New observations from NASA's Spitzer Space Telescope suggest that galaxies prefer to raise stars in cosmic suburbia rather than in "big cities."

Galaxies across the universe reside in cosmic communities, big and small. Large, densely populated galactic communities are called galaxy clusters. Like big cities on Earth, galaxy clusters are scattered throughout the universe, connected by a web of dusty "highways" called filaments. While thousands of galaxies live within the limits of a cluster, smaller galactic communities are sprinkled along filaments, creating celestial suburbs. Over time, astronomers suspect that all galactic suburbanites will make their way to a cluster by way of filaments.

For the first time, Spitzer's supersensitive eyes have caught an infrared glimpse of several galaxies traveling along two filamentary roads into a galaxy cluster called Abell 1763.

"This is the first time we've ever seen a filament leading into a cluster with an infrared telescope," says Dario Fadda, of the Herschel Science Center, which is located at the California Institute of Technology in Pasadena, Calif.

"Our observations show that the fraction of starburst galaxies in the filaments is more than double the number of starburst galaxies inside the cluster region," he adds.

According to Fadda, clusters and the filaments that connect them are among the largest structures in the cosmos. To see them, astronomers need instruments that can map large areas of sky and have the sensitivity to resolve individual galaxies.

Luckily, instruments aboard Spitzer can do both. Using the telescope's multiband imaging photometer, Fadda and his colleagues saw structures spanning 23 million light-years. They used the observatory's infrared array camera to collect a census of each galaxy's star formation and used a ground-based telescope at the Kitt Peak National Observatory near Tucson, Ariz. to determine which galaxies belonged to the cluster and surrounding filaments. Ultimately, Fadda found that galaxies in the filaments form stars at a higher rate than their cluster counterparts.

"The new Spitzer findings will provide valuable insights into how galaxies grow and change as they leave cosmic suburbia for the big cities," says Fadda.

He notes that future infrared missions will be able to follow in Spitzer's footsteps and study how filaments and clusters affect the growth of galaxies in greater detail. One such mission is the European Space Agency's Herschel Space Telescope, which has significant NASA involvement.

His paper on this topic has been accepted for publication in Astrophysical Journal Letters. Co-authors on the paper include Andrea Biviano of the INAF/Osservatorio Astronomico di Trieste, Italy; Florence Durret of Institut d'Astrophysique de Paris, France; and Francine Marleau and Lisa Storrie-Lombardi of the Spitzer Science Center, Pasadena, Calif.

Thursday, January 24, 2008

The illustration below is courtesy of amateur astronomer Dr. Dale Ireland from Silverdale, WA. The illustration shows the asteroid's track on the sky for 3 days near the time of the close Earth approach as seen from the city of Philadelphia. Since the object's parallax will be a significant fraction of a degree, observers are encouraged to use our on-line Horizons ephemeris generation service for their specific locations.

Asteroid 2007 TU24, discovered by the Catalina Sky Survey on October 11, 2007 will closely approach the Earth to within 1.4 lunar distances (334,000 miles) on 2008 Jan. 29 08:33 UT. This object, between 150 and 600 meters in diameter, will reach an approximate apparent magnitude 10.3 on Jan. 29-30 before quickly becoming fainter as it moves further from Earth. For a brief time the asteroid will be observable in dark and clear skies with amateur telescopes of 3 inch apertures or larger.

Given the estimated number of near-Earth asteroids of this size (about 7,000 discovered and undiscovered objects), an object of this size would be expected to pass this close to Earth, on average, about every 5 years or so. The average interval between actual Earth impacts for an object of this size would be about 37,000 years. For the January 29th encounter, near Earth asteroid 2007 TU24 has no chance of hitting, or affecting, Earth.

2007 TU24 will be the closest currently known approach by a potentially hazardous asteroid of this size or larger until 2027. Plans have been made for the Goldstone planetary radar to observe this object Jan 23-24 and for the Arecibo radar to observe it Jan 27-28 and then Feb 1-4. High resolution radar imaging is expected, which may permit later 3-D shape reconstruction.

Wednesday, January 23, 2008

Detailed analysis of two continent-sized storms that erupted in Jupiter's atmosphere in March 2007 shows that Jupiter's internal heat plays a significant role in generating atmospheric disturbances. Understanding this outbreak could be the key to unlock the mysteries buried in the deep Jovian atmosphere, say astronomers.

Understanding these phenomena is important for Earth's meteorology where storms are present everywhere and jet streams dominate the atmospheric circulation. Jupiter is a natural laboratory where atmospheric scientists study the nature and interplay of the intense jets and severe atmospheric phenomena.

An international team coordinated by Agustin Sánchez-Lavega from the Universidad del País Vasco in Spain presents its findings about this event in the January 24 issue of the journal Nature.

The team monitored the new eruption of cloud activity and its evolution with an unprecedented resolution using NASA's Hubble Space Telescope, the NASA Infrared Telescope Facility in Hawaii, and telescopes in the Canary Islands (Spain). A network of smaller telescopes around the world also supported these observations.

According to the analysis, the bright plumes were storm systems triggered in Jupiter's deep water clouds that moved upward in the atmosphere vigorously and injected a fresh mixture of ammonia ice and water about 20 miles (30 kilometers) above the visible clouds. The storms moved in the peak of a jet stream in Jupiter's atmosphere at 375 miles per hour (600 kilometers per hour). Models of the disturbance indicate that the jet stream extends deep in the buried atmosphere of Jupiter, more than 60 miles (approximately100 kilometers) below the cloud tops where most sunlight is absorbed.

Friday, January 18, 2008

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)Acknowledgment: D. de Mello (Catholic University of America and GSFC)

This loose collection of stars is actually a dwarf irregular galaxy, called Holmberg IX. It resides just off the outer edge of M81, a large spiral galaxy in Ursa Major. This image was taken with Hubble's Advanced Camera for Surveys in early 2006. Holmberg IX is of the so-called Magellanic type of galaxy, as its size and irregularity in structure are similar to the Small Magellanic Cloud, a neighbor to our own Milky Way. Holmberg IX was first discovered by astronomer Sidney van den Bergh in 1959, and cataloged as DDO 66. The galaxy received its “Holmberg IX” naming when it was discussed in Eric Holmberg's study of groups of galaxies ten years later. It is suspected that the dwarf galaxy was created as a result of a galactic interaction between M81 and neighboring galaxy M82.

Of the more than 20,000 stars that can be resolved in this Hubble image, only about 10% are considered to be old stars with ages of billions of years. The rest are thought to be young stars with ages of only 10 – 200 million years. Due to the Advanced Camera for Surveys' resolution in this image, astronomers have noted that the old and the young stars have distinct spatial distributions which might be related to their origin.

Simulations predict that the triplet M81, M82, and nearby NGC 3077 had a close passage 200-300 million years ago. This close encounter may have triggered the newer star formation that has occurred in Holmberg IX.

The bluish-white fuzz in the space surrounding M81 and Holmberg IX is new star formation triggered by gravitational interactions between the two galaxies. There are many low mass galaxies that form stars in nearby space. While none of these are as dominated by recently produced stars as Holmberg IX, they might be related to the same family. By understanding how Holmberg IX was formed, scientists hope to understand their role as building blocks of large galaxies.

The stellar explosions known as supernovae are among the most powerful events in the universe. Triggered by the collapsing core of a massive star or the nuclear demise of a white dwarf, supernovae occur in average spiral galaxies only about once every century.

But the remarkable spiral galaxy NGC 2770 has lately produced more than its fair share. Two still bright supernovae and the location of a third, originally spotted in 1999 but now faded from view, are indicated in this image of the edge-on spiral.

All three supernovae are now thought to be of the core-collapse variety, but the most recent of the trio, SN2008D, was first detected by the Swift satellite at more extreme energies as an X-ray flash (XRF) or possibly a low-energy version of a gamma-ray burst on January 9th.

Located a mere 90 million light-years away in the northern constellation Lynx, NGC 2770 is now the closest galaxy known to host such a powerful supernova event.

Wednesday, January 16, 2008

Figure 1: Two images of V1647 Orionis and McNeil’s Nebula. The image on the left is an optical color composite taken about four years ago with GMOS-North on UT 2004 February 14. The image on the right is also an optical color image taken about one year ago on UT 2007 February 22.

Figure 2: Expanded view of the 2.12-2.35 micron region of the near infrared spectroscopy of V1647.

A “new” star appeared in the constellation of Orion in late 2003 when the young pre-main sequence star V1647 Orionis went into outburst. The eruption and huge increase in brightness of the object resulted in the appearance of a reflection nebula called “McNeil’s Nebula,” named after the amateur astronomer, Jay McNeil, who discovered the object and alerted the world.

During the outburst the star and nebula remained bright for approximately 18 months before fading rapidly over a six month period. By early 2006 the star and its environment were very similar to their pre-burst stage. The event was monitored and observed with many ground- and space-based facilities and Gemini Observatory played a key role in monitoring the event during its eruptive and quiescent phases. A team led by Colin Aspin (IfA/University of Hawaii), Tracy Beck (STScI) and Bo Reipurth (IfA/University of Hawai‘i) spearheaded the monitoring campaign of this unique event.

The eruption of V1647 Orionis is most likely associated with a mass dumping of the inner regions of a heated circumstellar disk onto the young stellar photosphere. The spectacular flaring in brightness of the object is due to a significant increase in accretion luminosity and the clearing or destroying of surrounding dust by an energetic wind that made the star visible. These eruptions are thought to be repetitive and indicative of periods when a significant fraction of the final star’s mass is accreted.

The authors describe three phases for the V1647 Orionis latest eruption:

1. Before November 2004 is the pre-outburst phase2. From November 2004 to February 2006 is the outburst phase3. From February 2006 is the quiescent phase

The Gemini observing campaign led by Aspin has revealed some interesting results, particularly for the quiescent period. These include:

* McNeil’s Nebula is faintly visible in these GMOS-N images (Figure 1 right) indicating that the nebular material is still weakly illuminated by the star V1647 Orionis. At the time of acquisition of the GMOS-N imaging and spectroscopic data , V1647 Orionis had an r’ magnitude of 23.3.* NIRI spectroscopy has revealed for the first time in this type of object the presence of molecular overtone absorption from CO and other key diagnostic atoms like Na and Ca (possibly betraying the photosphere of the star), see Figure 2. The 2um spectroscopy shown in the paper is from IRTF not NIRI. We did publish NIRI spectroscopy but from just after the outburst, not in quiescence.* The star has a mass of about 0.8 solar mass and its age is about half a million years or less.* V1647 Orionis in this pre-main sequence phase is about five times more luminous than the Sun.* Material is falling onto the star at a rate of about one millionth of a solar mass per year.* Mid infrared observation with MICHELLE/Gemini show evidence of silicate dust evolution over the outburst-to-quiescence period, see Figure 3.

In a previous article on V1647 Orionis, Aspin studied a previous outburst of the star which occurred in 1966. It seems that perhaps V1647 Orionis ‘wakes up’ every 37 years but soon (after 1 to 2 years) tires and takes another long nap!

For more details, read the article "V1647 Orionis: One year into quiescence", by C. Aspin, T. Beck and B. Reipurth in The Astronomical Journal, January 2008, pp. 423-440.

For more details on the 1966 outburst of V1647 Orionis, read the article "The 1966-1967 Outburst of V1647 Orionis and the Appearance of McNeil's Nebula", by C. Aspin and others in The Astronomical Journal, Volume 132, Issue 3, pp. 1298-1306.

Tuesday, January 15, 2008

Results from NASA's Chandra X-ray Observatory, combined with new theoretical calculations, provide one of the best pieces of evidence yet that many supermassive black holes are spinning extremely rapidly. The images on the left show 4 out of the 9 large galaxies included in the Chandra study, each containing a supermassive black hole in its center.

The Chandra images show pairs of huge bubbles, or cavities, in the hot gaseous atmospheres of the galaxies, created in each case by jets produced by a central supermassive black hole. Studying these cavities allows the power output of the jets to be calculated. This sets constraints on the spin of the black holes when combined with theoretical models.Click for large jpgIllustration of Black Hole Engine

The Chandra images were also used to estimate how much fuel is available for each supermassive black hole, using a simple model for the way matter falls towards such an object. The artist's impression on the right side of the main graphic shows gas within a "sphere of influence" falling straight inwards towards a black hole before joining a rapidly spinning disk of matter near the center. Most of the material in this disk is swallowed by the black hole, but some of it is swept outwards in jets (colored blue) by quickly spinning magnetic fields close to the black hole.

Previous work with these Chandra data showed that the higher the rate at which matter falls towards these supermassive black holes, the higher their power output is in jets. However, without detailed theory the implications of this result for black hole behavior were unclear. The new study uses these Chandra results combined with leading theoretical models for the production of jets, plus general relativity, to show that the supermassive black holes in these galaxies must be spinning at close to the maximum rate. If black holes are spinning at this limit, material can be dragged around them at close to the speed of light, the speed limit from Einstein's theory of relativity.

(Credit: NASA/CXC/M.Weiss)

Illustration of Black Hole Engine

The first artist's illustration shows a close-up view of a supermassive black hole in a galaxy's center. Gas becomes hotter as it approaches the black hole, turning from red to yellow to white. Most of the gas is swallowed by the black hole, but some is launched in jets away from the black hole at almost the speed of light. The next illustration shows a larger area where gas is first attracted to the black hole, a region about a million times larger than the black hole's event horizon. The final illustration shows enormous cavities -- a hundred times larger -- that have been created in the galaxy's hot gas by jets from the black hole.

Monday, January 14, 2008

Looping eruptions on the Sun, like this one on July 24, 1999,create antimatter. Earth is shown for size comparison.

Antimatter, which annihilates matter upon contact, seems to be rare in the universe. Still, for decades, scientists had clues that a vast cloud of antimatter lurked in space, but they did not know where it came from.

The mysterious source of this antimatter has now been discovered — stars getting ripped apart by neutron stars and black holes.

While antimatter propulsion systems are so far the stuff of science fiction, antimatter is very real.

What it is

All elementary particles, such as protons and electrons, have antimatter counterparts with the same mass but the opposite charge. For instance, the antimatter opposite of an electron, known as a positron, is positively charged.

When a particle meets its antiparticle, they destroy each other, releasing a burst of energy such as gamma rays. In 1978, gamma ray detectors flown on balloons detected a type of gamma ray emerging from space that is known to be emitted when electrons collide with positrons — meaning there was antimatter in space.

"It was quite a surprise back then to discover part of the universe was made of antimatter," researcher Gerry Skinner, an astrophysicist at Goddard Space Flight Center in Greenbelt, Md., told SPACE.com.

These gamma rays apparently came from a cloud of antimatter roughly 10,000 light-years across surrounding our galaxy's core. This giant cloud shines brightly with gamma rays, with about the energy of 10,000 suns.

What exactly generated the antimatter was a mystery for the following decades. Suspects have included everything from exploding stars to dark matter.

Now, an international research team looking over four years of data from the European Space Agency's International Gamma Ray Astrophysics Laboratory (INTEGRAL) satellite has pinpointed the apparent culprits. Their new findings suggest these positrons originate mainly from stars getting devoured by black holes and neutron stars.

As a black hole or neutron star destroys a star, tremendous amounts of radiation are released. Just as electrons and positrons emit the tell-tale gamma rays upon annihilation, so too can gamma rays combine to form electrons and positrons, providing the mechanism for the creation of the antimatter cloud, scientists think.

Billions and billions

The researchers calculate that a relatively ordinary star getting torn apart by a black hole or neutron star orbiting around it — a so-called "low mass X-ray binary" — could spew on the order of one hundred thousand billion billion billion billion positrons (a 1 followed by 41 zeroes) per second. These could account for a great deal of the antimatter that scientists have inferred, reducing or potentially eliminating the need for exotic explanations such as ones involving dark matter.

"Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries," said researcher Georg Weidenspointner of the Max Planck Institute for Extraterrestrial Physics in Germany.

Now that they have witnessed the death of antimatter, the scientists hope to see its birth.

"It would be interesting if black holes produced more matter than neutron stars, or vice versa, although it's too early to say one way or the other right now," Skinner explained. "It can be surprisingly hard to tell the difference between an X-ray binaries that hold black holes and neutron stars."

Weidenspointner, Skinner and their colleagues, detailed their findings in the Jan. 10 issue of the journal Nature.

Why does Jupiter have rings? Jupiter's rings were discovered in 1979 by the passing Voyager 1 spacecraft, but their origin was a mystery. Data from the Galileo spacecraft that orbited Jupiter from 1995 to 2003 later confirmed that these rings were created by meteoroid impacts on small nearby moons.

As a small meteoroid strikes tiny Adrastea, for example, it will bore into the moon, vaporize and explode dirt and dust off into a Jovian orbit. Pictured above is an eclipse of the sun by Jupiter, as viewed from Galileo. Small dust particles high in Jupiter's atmosphere, as well as the dust particles that compose the rings, can be seen by reflected sunlight.

Friday, January 11, 2008

This image of Pickering’s Triangle was taken with the National Science Foundation’s Mayall 4-meter telescope at Kitt Peak National Observatory. T.A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF

A new wide-field image of Pickering's Triangle is being released today in Austin, Texas, at the 211th meeting of the American Astronomical Society. The image was taken with the National Science Foundation's Mayall 4-meter telescope at Kitt Peak National Observatory.

Pickering's Triangle is part of the Cygnus Loop supernova remnant, which includes the famous Veil Nebula. It is located about 1,500 light-years from Earth, in the constellation Cygnus, the Swan. Astronomers estimate that the supernova explosion that produced the nebula occurred between 5,000 to 10,000 years ago; the entire shell stretches more than six full Moons in width across the sky.

This new image was obtained September 2007 by Travis Rector and Heidi Schweiker by combining two full pointings of the 64-megapixel NOAO Mosaic-1 imager, mounted on the historic Mayall telescope.

Thursday, January 10, 2008

An artist's concept of the accretion disk around the binary star system WZ Sge, with the previous concept above and the revised concept using new data from Kitt Peak National Observatory and the Spitzer Space Telescope below, which includes an asymmetric outer disk of dark matter.

Observations of the interacting binary star using telescopes at Kitt Peak National Observatory and NASA’s Spitzer Space Telescope suggest that the disks of hot gas that accumulate around a wide variety of astronomical objects—from degenerate stars in energetic binary systems to supermassive black holes at the hearts of active galaxies—are likely to be much larger than previously believed.

The target of this specific investigation, named WZ Sagittae (WZ Sge), is an interacting binary star located in the constellation Sagitta, the arrow of the archer Sagittarius. As part of a program called the Spitzer-NOAO Observing Program for Teachers and Students, Steve B. Howell and a team of astronomers and educators imaged WZ Sge using the National Science Foundation’s 2.1-meter telescope and the WIYN 0.9-meter telescope, both located at Kitt Peak, and the Infrared Array Camera (IRAC) on Spitzer.

“We were very surprised to see the contrasting results obtained with the optical telescopes on the ground and the infrared telescope in space,” says Howell, an astronomer at the National Optical Astronomy Observatory (NOAO) and leader of the team who made the discovery being reported today in Austin, TX, at the 211th meeting of the American Astronomical Society (AAS). “The much larger size of the infrared-emitting portion of the accretion disk around WZ Sge was immediately obvious in the data. Our observations strongly imply the presence of dark matter in these structures, which are ubiquitous throughout the Universe.”

Interacting binary stars such as WZ Sge contain a white dwarf star (a compact star about the size of the Earth, but with a mass near that of the Sun) and a larger, but less massive and much cooler companion star. The companion, usually a low-mass star or a brown dwarf, has material ripped off its surface by the stronger gravity of the white dwarf. This material flows toward the more massive star and, in the process, forms a disk surrounding the white dwarf, known as an accretion disk.

Stars such as WZ Sge are called cataclysmic variables due to their rapid and often large changes in brightness, all caused by variations in the accretion disk. The two stars in such systems orbit about each other at a similar distance to that between Earth and the Moon, but with tremendous angular momentum that results in orbital periods ranging from a few hours down to as short as tens of minutes (the period of WZ Sge is 81 minutes).

Whether they form in cataclysmic variable systems or they surround the massive black hole hearts of active galaxies, accretion disks have been well observed and modeled using measurements obtained across much of the electromagnetic spectrum, from X-rays to the near-infrared. The derived picture of the “standard accretion disk” model is a geometrically thin disk of gaseous material surrounding the white dwarf or black hole. Accretion disk models, bolstered by observation, are generally composed of hot gas having a temperature distribution within them, being hottest near the center and falling off in temperature toward the outer edge.

In order to confirm the general accretion disk models and extend them into the mid-infrared portion of the spectrum, Howell’s team obtained the first time series observations of an accretion disk system at 4.5 and 8 microns with the Spitzer Space Telescope. At nearly the same time, they obtained optical observations of WZ Sge at Kitt Peak. The optical observations confirmed the standard view of the accretion disk size and temperature, values known for over a decade.

The mid-infrared observations, however, were completely unexpected and revealed that a larger, thicker disk of cool dusty material surrounds much of the gaseous accretion disk. This outer dust disk likely contains as much mass as a medium-sized asteroid. The newly discovered outer disk extends about 20 times the radius of the gaseous disk.

“This discovery suggests that our current model for accretion disks of all kinds is wrong,” says team member Donald Hoard of the Spitzer Science Center. “We will need to rethink and recast these models for accretion disks, not only in interacting binary stars but also in distant, highly luminous active galaxies.”

The implications from such a discovery are far reaching, affecting not only the theoretical models (since the formation and evolution of the disks are modeled based on their size, temperature, and composition—all quantities that now need to be revised), but also nearly all previous observations of systems containing accretion disks.

In addition, the dust disk (which is thicker than the known gaseous disk) blocks infrared light emitted by the compact central object and the inner hot regions of the gaseous disk. Not knowing that some mid to far infrared light is blocked by the newly discovered outer dust ring can lead observers to significantly underestimate the total luminosity of the central object. “The amount of this underestimation is not yet accurately known from our initial discovery, but may be as large as 50 percent,” Howell says.

An artist’s concept comparing the previous view and the new view of the accretion disk around WZ Sge is available above.

The observational program making this discovery was a joint effort between research scientists Howell, Hoard, and Carolyn Brinkworth of Spitzer Science Center, and high school teacher Beth Thomas and student Kimmerlee Johnson (Great Falls Public Schools, Great Falls, MT), teacher Jeff Adkins and student John Michael Santiago (Deer Valley High School, Antioch, CA), and teacher Tim Spuck and student Matt Walentosky (Oil City High School, Oil City, PA).

The work was funded by Spitzer Science Center as part of a joint project with NOAO to expand and extend the national observatory’s Research Based Science Education (RBSE) teacher professional development program to include observations with the Spitzer Space Telescope. RBSE has been training groups of 20 teachers in the research process (including regular observations at Kitt Peak National Observatory) every year for more than a decade, using funding support from NSF.

Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, based in Tucson, AZ, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF.

Like people, galaxies come in different shapes and sizes. There are thin spirals both with and without central bulges of stars, and more rotund ellipticals that are themselves like giant bulges. Scientists have long held that all galaxies except the slender, bulgeless spirals harbor supermassive black holes at their cores. Furthermore, bulges were thought to be required for black holes to grow.

The new Spitzer observations throw this theory into question. The infrared telescope surveyed 32 flat and bulgeless galaxies and detected monstrous black holes lurking in the bellies of seven of them. The results imply that galaxy bulges are not necessary for black hole growth; instead, a mysterious invisible substance in galaxies called dark matter could play a role.

"This finding challenges the current paradigm. The fact that galaxies without bulges have black holes means that the bulges cannot be the determining factor," said Shobita Satyapal of the George Mason University, Fairfax, Va. "It's possible that the dark matter that fills the halos around galaxies plays an important role in the early development of supermassive black holes."

Satyapal presented the findings today at the 211th meeting of the American Astronomical Society in Austin, Texas. A study from Satyapal and her team will be published in the April 10 issue of the Astrophysical Journal.

Our own Milky Way is an example of a spiral galaxy with a bulge; from the side, it would look like a plane seen head-on, with its wings out to the side. Its black hole, though dormant and not actively "feeding," is several million times the mass of our sun.

Previous observations had suggested that bulges and black holes flourished together like symbiotic species. For instance, supermassive black holes are almost always about 0.2 percent the mass of their galaxies' bulges. In other words, the more massive the bulge, the more massive the black hole. Said Satyapal, "Scientists reasoned that somehow the formation and growth of galaxy bulges and their central black holes are intimately connected."

But a wrinkle appeared in this theory in 2003, when astronomers at the University of California, Berkeley, and Observatories of the Carnegie Institution of Washington, Pasadena, Calif., discovered a relatively "lightweight" supermassive black hole in a galaxy lacking a bulge. Then, earlier this year, Satyapal and her team uncovered a second supermassive black hole in a similarly svelte galaxy.

In the latest study, Satyapal and her colleagues report the discovery of six more hefty black holes in thin galaxies with minimal bulges, further weakening the "bulge-black hole" theory. Why hadn't anybody seen these black holes before? According to the scientists, bulgeless galaxies tend to be very dusty, letting little visible light escape. But infrared light can penetrate dust, so the team was able to use Spitzer's infrared spectrograph to reveal the "fingerprints" of active black holes lurking in galaxies millions of light years away.

"A feeding black hole spits out high-energy light that ionizes much of the gas in the core of the galaxy," said Satyapal. "In this case, Spitzer identified the unique fingerprint of highly ionized neon -- only a feeding black hole has the energy needed to excite neon to this state." The precise masses of the newfound black holes are unknown.

If bulges aren't necessary ingredients for baking up supermassive black holes, then perhaps dark matter is. Dark matter is the enigmatic substance that permeates galaxies and their surrounding halos, accounting for up to 90 percent of a galaxy's mass. So-called normal matter makes up stars, planets, living creatures and everything we see around us, whereas dark matter can't be seen. Only its gravitational effects can be felt. According to Satyapal, dark matter might somehow determine the mass of a black hole early on in the development of a galaxy.

"Maybe the bulge was just serving as a proxy for the dark matter mass -- the real determining factor behind the existence and mass of a black hole in a galaxy's center," said Satyapal.

Other authors of this study include: D. Vega of the George Mason University; R.P. Dudik of the George Mason University and NASA Goddard Space Flight Center, Greenbelt, Md.; N.P. Abel of the University of Cincinnati, Ohio; and Tim Heckman of the Johns Hopkins University, Baltimore, Md.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell.

What superficially resembles a giant moth floating in space is giving astronomers new insight into the formation and evolution of planetary systems.

This is not your typical flying insect. It has a wingspan of about 22 billion miles. The wing- like structure is actually a dust disk encircling the nearby, young star HD 61005, dubbed "The Moth." Its shape is produced by starlight scattering off dust.

Dust disks around roughly 100-million-year-old stars like HD 61005 are typically flat, pancake-shaped structures where planets can form. But images taken with NASA's Hubble Space Telescope of "The Moth" are showing that some disks sport surprising shapes.

"It is completely unexpected to find a dust disk with this unusual shape," said senior research scientist Dean Hines of the Space Science Institute in Corrales, New Mexico, and a member of the Hubble team that discovered the disk. "We think HD 61005 is plowing through a local patch of higher-density gas in the interstellar medium, causing material within HD 61005's disk to be swept behind the star. What effect this might have on the disk, and any planets forming within it, is unknown."

Hines called this possible collision "unusual, because we don't expect very much interstellar material to be in the solar neighborhood. That's because the area through which our Sun is moving was evacuated within the past few million years by at least one supernova, the explosion of a massive star. Yet, here's evidence of dense material that's very close, only 100 light-years away."

Astronomers have found evidence that the environment in which a star forms influences its prospects for planet formation. Hubble has actually seen that young planet-forming disks can be affected directly by their environment. The harsh stellar radiation from the Trapezium stars in the Orion Nebula has altered some disks. It is unclear, however, what effect passage through a cloud similar to the one in which HD 61005 finds itself would have on planet formation. Researchers have speculated that passage through dense regions of the interstellar medium could impact the atmospheres of evolving planets.

The Moth is part of a survey of Sun-like stars that Hines and collaborators observed with Hubble's Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) and NASA's Spitzer Space Telescope to study the formation and evolution of planetary systems. Under the lead of Michael Meyer of the University of Arizona in Tucson, the team initially used Spitzer to look for heat radiation—the tell-tale sign of dust warmed by the star—to identify interesting star systems.

Hines then teamed with Glenn Schneider of the University of Arizona to use Hubble's high- contrast imaging capability of the NICMOS coronagraph to image these disks and reveal where the dust detected by Spitzer resides. The NICMOS coronagraph blocked out the starlight so that astronomers could see details in the surrounding disk.

Added Meyer: "Combining observations from these two spacecraft gives us information about the composition of the dust grains, whether they're icy or sandy, or whether they're like the sooty smoke particles rising from a chimney. The composition and sizes of the dust can tell us a lot about the dynamics and evolution of a solar system. In our solar system, for example, astronomers have evidence of rocks smashing into each other and generating dust, as in the asteroid and Kuiper belts. We're seeing these same processes unfold in other planetary systems."

Hines and his collaborators will report their finding on Jan. 10 at the 211th meeting of the American Astronomical Society in Austin, Texas. The result also appeared in the December 20 issue of the Astrophysical Journal Letters.