Category: Astronomy

Inward migration of a group of protoplanets, where they’re represented by white circles. Image credit: QMUL Click to enlarge
Astronomers think they’ve got a handle on many aspects of planetary formation. But two British researchers have discovered a problem with the formation of gas giant planets. Under their model, the cores of these massive planets should be drawn inward by their parent star in only 100,000 years – not nearly enough time to form into a stable orbit. It could be that the first generations of planets never get past the “clump” stage before they’re destroyed. It’s only the later generations that actually survive long enough to become planets.

Two British astronomers, Paul Cresswell and Richard Nelson present new numerical simulations in the framework of the challenging studies of planetary system formation. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star. Their results will soon be published in Astronomy & Astrophysics.

In an article to be published in Astronomy & Astrophysics, two British astronomers present new numerical simulations of how planetary systems form. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star.

The current picture of how planetary systems form is as follows: i) dust grains coagulate to form planetesimals of up to 1 km in diameter; ii) the runaway growth of planetesimals leads to the formation of ~100 ? 1000 km-sized planetary embryos; iii) these embryos grow in an “oligarchic” manner, where a few large bodies dominate the formation process, and accrete the surrounding and much smaller planetesimals. These “oligarchs” form terrestrial planets near the central star and planetary cores of ten terrestrial masses in the giant planet region beyond 3 astronomical units (AU).

However, these theories fail to describe the formation of gas giant planets in a satisfactory way. Gravitational interaction between the gaseous protoplanetary disc and the massive planetary cores causes them to move rapidly inward over about 100,000 years in what we call the “migration” of the planet in the disc. The prediction of this rapid inward migration of giant protoplanets is a major problem, since this timescale is much shorter than the time needed for gas to accrete onto the forming giant planet. Theories predict that the giant protoplanets will merge into the central star before planets have time to form. This makes it very difficult to understand how they can form at all.

For the first time, Paul Cresswell and Richard Nelson examined what happens to a cluster of forming planets embedded in a gaseous protoplanetary disc. Previous numerical models have included only one or two planets in a disc. But our own solar system, and over 10% of the known extrasolar planetary systems, are multiple-planet systems. The number of such systems is expected to increase as observational techniques of extrasolar systems improve. Cresswell and Nelson’s work is the first time numerical simulations have included such a large number of protoplanets, thus taking into account the gravitational interaction between the protoplanets and the disc, and among the protoplanets themselves.

The primary motivation for their work is to examine the orbits of protoplanets and whether some planets could survive in the disc for extended periods of time. Their simulations show that, in very few cases (about 2%), a lone protoplanet is ejected far from the central star, thus lengthening its lifetime. But in most cases (98%), many of the protoplanets are trapped into a series of orbital resonances and migrate inward in lockstep, sometimes even merging with the central star.

Cresswell and Nelson thus claim that gravitational interactions within a swarm of protoplanets embedded in a disc cannot stop the inward migration of the protoplanets. The “problem” of migration remains and requires more investigation, although the astronomers propose several possible solutions. One may be that several generations of planets form and that only the ones that form as the disc dissipates survive the formation process. This may make it harder to form gas giants, as the disc is depleted of the material from which gas giant planets form. (Gas giant formation may still be possible though, if enough gas lies outside the planets’ orbits, since new material may sweep inward to be accreted by the forming planet). Another solution might be related to the physical properties of the protoplanetary disc. In their simulations, the astronomers assumed that the protoplanetary disc is smooth and non-turbulent, but of course this might not be the case. Large parts of the disc could be more turbulent (as a consequence of instabilities caused by magnetic fields), which may prevent inward migration over long time periods.

This work joins other studies of planetary system formation that are currently being done by a European network of scientists. Our view of how planets form has drastically changed in the last few years as the number of newly discovered planetary systems has increased. Understanding the formation of giant planets is currently one of the major challenges for astronomers.

Original Source: Astronomy & Astrophysics

Spitzer spots galactic clusters bonding back when the universe was about 4.6 billion years old. Image credit: NASA Click to enlarge
NASA’s Spitzer Space Telescope recently turned up clusters of galaxies located 7-9 billion light years from Earth. These galaxies are located at the very limits of Spitzer’s observing ability with its 85-centimeter (33-inch) telescope. Galaxy clusters are some of the largest structures in the Universe, consisting of thousands of galaxies and trillions of stars. This discovery gives astronomers more evidence about what kinds of structures existed in the early Universe.

Astronomers using NASA’s Spitzer Space Telescope have conducted a cosmic safari to seek out a rare galactic species. Their specimens — clusters of galaxies in the very distant universe — are few and far between, and have hardly ever been detected beyond a distance of 7 billion light-years from Earth.

To find the clusters, the team carefully sifted through Spitzer infrared pictures and ground-based catalogues; estimated rough distances based on the cluster galaxies’ colors; and verified suspicions using a spectrograph instrument at the W.M. Keck Observatory in Hawaii.

Ultimately, the expedition resulted in quite a galactic catch — the most distant galaxy cluster ever seen, located 9 billion light-years away. This means the cluster lived in an era when the universe was a mere 4.5 billion years old. The universe is believed to be 13.7 billion years old.

“Detecting a galaxy cluster 9 billion light-years away is very exciting,” said the study’s lead investigator, Dr. Peter Eisenhardt of NASA’s Jet Propulsion Laboratory. “It’s really amazing that Spitzer’s 85-centimeter telescope can see 9 billion years back in time.”

Using the same methods, the astronomers also found three other clusters living between 7 and 9 billion light-years away.

“Spitzer is an excellent instrument for detecting very distant galaxy clusters because they stand out so brightly in the infrared,” said co-investigator Dr. Mark Brodwin, also of JPL. “You can think of these distant galaxy cluster surveys as a game of ‘Where’s Waldo?’ With an optical telescope you can spot ‘Waldo,’ or the distant galaxy clusters, by carefully searching for them amongst a sea of faint galaxies.”

“But in the Spitzer data, it’s as though Waldo is wearing a bright neon hat and can be easily picked out of the crowd,” Brodwin added.

Galaxy clusters are the largest gravitationally bound structures in the universe. A typical cluster can contain thousands of galaxies and trillions of stars. Because of their huge size and mass, they are relatively rare. For example, if Earth were to represent the entire universe, then countries would be the equivalent of galaxies, and continents would be the galaxy clusters.

Galaxy clusters grow like snowballs, picking up new galaxies from gravitational interactions over billions of years. For this reason, team members say these behemoths should be even rarer in the very distant universe.

“The ultimate goal of this research is to find out when the galaxies in this and other distant clusters formed,” said co-investigator Dr. Adam Stanford, of the University of California at Davis. Stanford is the lead author of a paper on the most distant galaxy cluster’s discovery, which was published in the December 2005 issue of Astrophysical Journal Letters.

This is the second time Eisenhardt and Stanford have broken the record for capturing the most distant galaxy cluster. Both say they accidentally broke the record in 1997 when they detected a cluster located 8.7 billion light-years away. The discovery was made by a deep survey of a 0.03-degree patch of sky, or an area significantly smaller than a pea held out at arms length, for 30 nights at the Kitt Peak National Observatory in Arizona.

“We were lucky in 1997 because we weren’t looking for galaxy clusters and found the most distant one ever detected in a very small patch of sky,” said Stanford. “Because galaxy clusters are so massive and rare, you typically need to deeply survey a large area of sky to find them.”

“With Spitzer’s great infrared sensitivity we surveyed more deeply in 90 seconds than we could in hours of exposure in the 1997 observations, and we used this advantage to survey a region 300 times larger,” adds Eisenhardt.

The 9 billion-year-old cluster is just one of 25 the team captured on their Spitzer safari. They are currently preparing for more observations this spring at the W.M. Keck Observatory to confirm the distance of additional galaxy clusters from their sample. According to Eisenhardt, some of the clusters awaiting confirmation may be even more distant than the current record holder.

Original Source: Spitzer Space Telescope

The Guitar nebula. Image credit: Palomar Observatory. Click to enlarge
Since it makes up a large part of the Universe, you’d think we’d know what dark matter is by now. Sorry, it’s still a mystery, but new theories are coming out all the time. An international team of researchers are now theorizing that dark matter could be a class of particles known as “sterile neutrinos”. These particles, formed right at the Big Bang, could account for the Universe’s missing mass, and would have the handy side effect of speeding up the early formation of stars.

Dark matter may have played a major role in creating stars at the very beginnings of the universe. If that is the case, however, the dark matter must consist of particles called “sterile neutrinos”. Peter Biermann of the Max Planck Institute for Radio Astronomy in Bonn, and Alexander Kusenko, of the University of California, Los Angeles, have shown that when sterile neutrinos decay, it speeds up the creation of molecular hydrogen. This process could have helped light up the first stars only some 20 to 100 million years after the big bang. This first generation of stars then ionised the gas surrounding them, some 150 to 400 million years after the big bang. All of this provides a simple explanation to some rather puzzling observations concerning dark matter, neutron stars, and antimatter.

Scientists discovered that neutrinos have mass through neutrino oscillation experiments. This led to the postulation that “sterile” neutrinos exist – also known as right-handed neutrinos. They do not participate in weak interactions directly, but do interact through their mixing with ordinary neutrinos. The total number of sterile neutrinos in the universe is unclear. If a sterile neutrino only has a mass of a few kiloelectronvolts (1 keV is a millionth of the mass of a hydrogen atom), that would explain the huge, missing mass in the universe, sometimes called “dark matter”. Astrophysical observations support the view that dark matter is likely to consist of these sterile neutrinos.

Biermann and Kusenko’s theory sheds light on a number of still unexplained astronomical puzzles. First of all, during the big bang, the mass of neutrinos created in the Big Bang would equal what is needed to account for dark matter. Second, these particles could be the solution to the long-standing problem of why pulsars move so fast.

Pulsars are neutron stars rotating at a very high velocity. They are created in supernova explosions and normally are ejected in one direction. The explosion gives them a “push”, like a rocket engine. Pulsars can have velocities of hundreds of kilometres per second – or sometimes even thousands. The origin of these velocities remains unknown, but the emission of sterile neutrinos would explain the pulsar kicks.

The Guitar Nebula contains a very fast pulsar. If dark matter is made of particles which reionized the universe – as Biermann and Kusenko suggest – the pulsar’s motion could have created this cosmic guitar.

Third, sterile neutrinos can help explain the absence of antimatter in the universe. In the early universe, sterile neutrinos could have “stolen” what is called the “lepton number” from plasma. At a later time, the lack of lepton number was converted to a non-zero baryon number. The resulting asymmetry between baryons (like protons) and antibaryons (like antiprotons) could be the reason why the universe has no antimatter.

“The formation of central galactic black holes, as well as structure on subgalactic scales, favours sterile neutrinos to account for dark matter. The consensus of several indirect pieces of evidence leads one to believe that the long sought-after dark-matter particle may, indeed, be a sterile neutrino”, says Peter Biermann

Original Source: Max Planck Society

Artist’s illustration of the northern starry river. Image credit: Caltech Click to enlarge
Astronomers have found a narrow stream of stars extending across the sky for about 45 degrees – 90 times the width of the full Moon. The stream emanates from a cluster of 50,000 stars called NGC 5466, and stretches from Ursa Major (or the Big Dipper) to the constellation Bootes. The strength of gravity from the Milky Way is different on opposite sides of the star cluster, which causes it to stretch. Outlying stars are no longer held in the cluster and fall behind, creating the stream.

Astronomers have discovered a narrow stream of stars extending at least 45 degrees across the northern sky. The stream is about 76,000 light-years distant from Earth and forms a giant arc over the disk of the Milky Way galaxy.

In the March issue of the Astrophysical Journal Letters, Carl Grillmair, an associate research scientist at the California Institute of Technology’s Spitzer Science Center, and Roberta Johnson, a graduate student at California State University Long Beach, report on the discovery.

“We were blown away by just how long this thing is,” says Grillmair. “As one end of the stream clears the horizon this evening, the other will already be halfway up the sky.”

The stream begins just south of the bowl of the Big Dipper and continues in an almost straight line to a point about 12 degrees east of the bright star Arcturus in the constellation Bootes. The stream emanates from a cluster of about 50,000 stars known as NGC 5466.

The newly discovered stream extends both ahead and behind NGC 5466 in its orbit around the galaxy. This is due to a process called tidal stripping, which results when the force of the Milky Way’s gravity is markedly different from one side of the cluster to the other. This tends to stretch the cluster, which is normally almost spherical, along a line pointing towards the galactic center.

At some point, particularly when its orbit takes it close to the galactic center, the cluster can no longer hang onto its most outlying stars, and these stars drift off into orbits of their own. The lost stars that find themselves between the cluster and the galactic center begin to move slowly ahead of the cluster in its orbit, while the stars that drift outwards, away from the galactic center, fall slowly behind.

Ocean tides are caused by exactly the same phenomenon, though in this case it’s the difference in the moon’s gravity from one side of Earth to the other that stretches the oceans. If the gravity at the surface of Earth were very much weaker, then the oceans would be pulled from the planet, just like the stars in NGC 5466’s stream.

Despite its size, the stream has never previously been seen because it is so completely overwhelmed by the vast sea of foreground stars that make up the disk of the Milky Way. Grillmair and Johnson found the stream by examining the colors and brightnesses of more than nine million stars in the Sloan Digital Sky Survey public database.

“It turns out that, because they were all born at the same time and are situated at roughly the same distance, the stars in globular clusters have a fairly unique signature when you look at how their colors and brightnesses are distributed,” says Grillmair.

Using a technique called matched filtering, Grillmair and Johnson assigned to each star a probability that it might once have belonged to NGC 5466. By looking at the distribution of these probabilities across the sky, “the stream just sort of reached out and smacked us.

“The new stream may be even longer than we know, as we are limited at the southern end by the extent of the currently available data,” he adds. “Larger surveys in the future should be able to extend the known length of the stream substantially, possibly even right around the whole sky.”

The stars that make up the stream are much too faint to be seen by the unaided human eye. Owing to the vast distances involved, they are about three million times fainter than even the faintest stars that we can see on a clear night.

Grillmair says that such discoveries are important for our understanding of what makes up the Milky Way galaxy. Like earthbound rivers, such tidal streams can tell us which way is “down,” how steep is the slope, and where the mountains and valleys are located.

By measuring the positions and velocities of the stars in these streams, astronomers hope to determine how much Dark Matter the Milky Way contains, and whether the dark matter is distributed smoothly, or in enormous orbiting chunks.

Original Source: Caltech News Release

A new detailed picture of the infant universe, where red indicates warm spots and blue for the cooler areas. Image credit: NASA/WMAP Click to enlarge
Scientists have gathered new evidence that supports the inflationary theory of expansion thanks new data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). The spacecraft has been making continuous observations of the cosmic background radiation; the afterglow of the Big Bang. These latest observations produced a map of the sky so detailed that scientists were able to trace how microscopic fluctuations in the primordial Universe were magnified in a trillionth of a second of rapid expansion to create the stars and galaxies we see today.

Scientists peering back to the oldest light in the universe have new evidence to support the concept of inflation. The concept poses the universe expanded many trillion times its size in less than a trillionth of a second at the outset of the big bang.

This finding, made with NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), is based on three years of continuous observations of the cosmic microwave background (CMB), the afterglow light produced when the universe was less than a million years old.

WMAP polarization data allow scientists to discriminate between competing models of inflation for the first time. This is a milestone in cosmology. “We can now distinguish between different versions of what happened within the first trillionth of a second of the universe,” said WMAP Principal Investigator Charles Bennett of the Johns Hopkins University in Baltimore. “The longer WMAP observes, the more it reveals about how our universe grew from microscopic quantum fluctuations to the vast expanses of stars and galaxies we see today.”

Previous WMAP results focused on the temperature variations of this light, which provided an accurate age of the universe and insights into its geometry and composition. The new WMAP observations give not only a more detailed temperature map, but also the first full-sky map of the polarization of the CMB. This major breakthrough will enable scientists to obtain much deeper insight into what happened within the first trillionth of a second of the universe. The WMAP results have been submitted to the Astrophysical Journal and are posted at

http://wmap.gsfc.nasa.gov/results

Big bang physics describes how matter and energy developed over the last 13.7 billion years. WMAP’s observation of the blanket of cool microwave radiation that permeates the universe shows patterns that mark the seeds of what grew into stars and galaxies. The patterns are tiny temperature differences within this extraordinarily uniform light. WMAP discerns temperature fluctuations at levels finer than a millionth of a degree.

WMAP can resolve features in the cosmic microwave background based on polarization, or the way light is changed by the environment through which it passes. For example, sunlight reflecting off of a shiny object is polarized. Comparing the brightness of broad features to compact features in the microwave background, or afterglow light, helps tell the story of the infant universe. One long-held prediction was the brightness would be the same for features of all sizes. In contrast, the simplest versions of inflation predict the relative brightness decreases as the features get small, a trend seen in the new data.

“This is brand new territory,” said WMAP team member Lyman Page of Princeton University in Princeton, N.J. “The polarization data will become stronger as WMAP continues to observe the microwave background. WMAP’s new results heighten the urgency of seeking out inflation’s gravitational wave sign. If gravitational waves are seen in future measurements, that would be solid evidence for inflation.”

With a richer temperature map and the new polarization map, WMAP data favor the simplest versions of inflation. Generically, inflation posits that, at the outset of the big bang, quantum fluctuations – short-lived bursts of energy at the subatomic level – were converted by the rapid inflationary expansion into fluctuations of matter that ultimately enabled stars and galaxies to form. The simplest versions of inflation predict that the largest-sized fluctuations will also be the strongest. The new results from WMAP favor this signature.

Inflation theory predicts that these same fluctuations also produced primordial gravitational waves whose distortion of space-time leaves a signature in the CMB polarization. This will be an important goal of future CMB measurements which, if found, would provide a stunning confirmation of inflation.

“Inflation was an amazing concept when it was first proposed 25 years ago, and now we can support it with real data,” said WMAP team member Gary Hinshaw of NASA’s Goddard Space Flight Center in Greenbelt, Md.

WMAP, a partnership between Goddard and Princeton, was launched on June 30, 2001. The WMAP team includes researchers in U.S. and Canadian universities and institutes. For images and information on the Web about WMAP, visit:
http://www.nasa.gov/vision/universe/wmap_pol.html

Original Source: NASA News Release

An infrared image of the Cigar galaxy by Spitzer. Image credit: NASA/JPL Click to enlarge
NASA’s Spitzer Space Telescope has revealed a burning hot galaxy blowing out clouds of dust and smoke. The galaxy is M82, and it’s well known for vast regions of young, hot stars in stellar nurseries. In visible light, the galaxy looks fairly normal, but in Spitzer’s infrared view, it’s nestled inside an enormous cloud of dust. These clouds are the largest ever seen around a galaxy, stretching 20,000 light years from the galactic plane in both directions.

Where there’s smoke, there’s fire – even in outer space. A new infrared image from NASA’s Spitzer Space Telescope shows a burning hot galaxy whose fiery stars appear to be blowing out giant billows of smoky dust.

The galaxy, called Messier 82, or the “Cigar galaxy,” was previously known to host a hotbed of young, massive stars. The new Spitzer image reveals, for the first time, the “smoke” surrounding those stellar fires.

“We’ve never seen anything like this,” said Dr. Charles Engelbracht of the University of Arizona, Tucson. “This unusual galaxy has ejected an enormous amount of dust to cover itself with a cloud brighter than any we’ve seen around other galaxies.”

The false-colored view, online at http://www.spitzer.caltech.edu/Media , shows Messier 82, an irregular-shaped galaxy positioned on its side, as a diffuse bar of blue light. Fanning out from its top and bottom like the wings of a butterfly are huge red clouds of dust believed to contain a compound similar to car exhaust.

The smelly material, called polycyclic aromatic hydrocarbon, can be found on Earth in tailpipes, barbecue pits and other places where combustion reactions have occurred. In galaxies, the stuff is created by stars, whose winds and radiation blow the material out into space.

“Usually you see smoke before a fire, but we knew about the fire in this galaxy before Spitzer’s infrared eyes saw the smoke,” said Dr. David Leisawitz, Spitzer program scientist at NASA Headquarters in Washington.

These hazy clouds are some of the biggest ever seen around a galaxy. They stretch out 20,000 light-years away from the galactic plane in both directions, far beyond where stars are found.

Previous observations of Messier 82 had revealed two cone-shaped clouds of very hot gas projecting outward below and above the center of galaxy. Spitzer’s sensitive infrared vision allowed astronomers to see the galaxy’s dust.

“Spitzer showed us a dust halo all around this galaxy,” said Engelbracht. “We still don’t understand why the dust is all over the place and not cone-shaped.”

Cone-shaped clouds of dust around this galaxy would have indicated that its central, massive stars had sprayed the dust into space. Instead, Engelbracht and his team believe stars throughout the galaxy are sending off the “smoke signals.”

Messier 82 is located about 12 million light-years away in the Ursa Major constellation. It is undergoing a renaissance of star birth in its middle age, with the most intense bursts of star formation taking place at its core. The galaxy’s interaction with its neighbor, a larger galaxy called Messier 81, is the cause of all the stellar ruckus. Our own Milky Way galaxy is a less hectic place, with dust confined to the galactic plane.

The findings will appear in an upcoming issue of the Astrophysical Journal. Other authors who contributed significantly to this work are Praveen Kundurthy and Dr. Karl Gordon, both of the University of Arizona. The image was taken as a part of the Spitzer Infrared Nearby Galaxy Survey, which is led by Dr. Robert Kennicutt, also of the University of Arizona.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech.

For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer . For more information about NASA and agency programs on the Web, visit http://www.nasa.gov/home/ .

Original Source: NASA News Release

A group of the newly discovered galaxies by the Lyman-break technique. Image credit: Astronomy & Astrophysics. Click to enlarge
An international team of astronomers have performed one of the most detailed surveys of the most distant galaxies. These galaxies are so far away, we see them as they looked when the Universe was less than half its current age. One of the big surprises of this survey; however, is how much these young galaxies match the structures we see in the current Universe. This means that galaxies probably evolved through collisions and mergers much earlier than previously believed.

A team of astronomers from France, the USA, Japan, and Korea, led by Denis Burgarella has recently discovered new galaxies in the Early Universe. They have been detected for the first time both in the near-UV and in the far-infrared wavelengths. Their findings will be reported in a coming issue of Astronomy & Astrophysics. This discovery is a new step in understanding how galaxies evolve.

The astronomer Denis Burgarella (Observatoire Astronomique Marseille Provence, Laboratoire d’Astrophysique de Marseille, France) and his colleagues from France, the USA, Japan, and Korea, have recently announced their discovery of new galaxies in the Early Universe both for the first time in the near-UV and in the far-infrared wavelengths. This discovery leads to the first thorough investigation of early galaxies. The discovery will be reported in a coming issue of Astronomy & Astrophysics.

The knowledge of early galaxies has made major progress in the past ten years. From the end of 1995, astronomers have been using a new technique, known as the “Lyman-break technique”. This technique allows very distant galaxies to be detected. They are seen as they were when the Universe was much younger, thus providing clues to how galaxies formed and evolved. The Lyman-break technique has moved the frontier of distant galaxy surveys further up to redshift z=6-7 (that is about 5% of the present age of the Universe). In astronomy, the redshift denotes the shift of a light wave from a galaxy moving away from the Earth. The light wave is shifted toward longer wavelengths, that is, toward the red end of the spectrum. The higher the redshift of a galaxy is, the farther it is from us.

The Lyman-break technique is based on the characteristic “disappearance” of distant galaxies observed in the far-UV wavelengths. As light from a distant galaxy is almost fully absorbed by hydrogen at 0.912 nm (due to the absorption lines of hydrogen, discovered by the physicist Theodore Lyman), the galaxy “disappears” in the far-ultraviolet filter. Figure 2 illustrates the ?disappearance? of the galaxy in the far-UV filter. The Lyman discontinuity should theoretically occur at 0.912 nm. Photons at shorter wavelengths are absorbed by hydrogen around stars or within the observed galaxies. For high-redshift galaxies, the Lyman discontinuity is redshifted so that it occurs at a longer wavelength and can be observed from the Earth. From ground-based observations, astronomers can currently detect galaxies with a redshift range of z~3 to z~6. However, once detected, it is still very difficult to obtain additional information on these galaxies because they are very faint.

For the first time, Denis Burgarella and his team have been able to detect less distant galaxies via the Lyman-break technique. The team collected data from various origins: UV data from the NASA GALEX satellite, infrared data from the SPITZER satellite, and data in the visible range at ESO telescopes. From these data, they selected about 300 galaxies showing a far-UV disappearance. These galaxies have a redshift ranging from 0.9 to 1.3, that is, they are observed at a moment when the Universe had less than half of its current age. This is the first time a large sample of Lyman Break Galaxies is discovered at z~1. As these galaxies are less distant than the samples observed up to now, they are also brighter and easier to study at all wavelengths thereby allowing a deep analysis from UV to infrared to be performed.

Previous observations of distant galaxies have led to the discovery of two classes of galaxies, one of which includes galaxies that emit light in the near-UV and visible wavelength ranges. The other type of galaxy emits light in the infrared (IR) and submillimeter ranges. The UV galaxies were not observed in the infrared range, while IR galaxies were not observed in the UV. It was thus difficult to explain how such galaxies could evolve into present-day galaxies that emit light at all wavelengths. With their work, Denis Burgarella and his colleagues have taken a step toward solving this problem. When observing their new sample of z~1 galaxies, they found that about 40% of these galaxies emit light in the infrared range as well. This is the first time a significant number of distant galaxies were observed both in the UV and IR wavelength ranges, incorporating the properties of both major types.

From their observations of this sample, the team also inferred various information about these galaxies. Combining UV and infrared measurements makes it possible to determine the formation rate for stars in these distant galaxies for the first time. Stars form there very actively, at a rate of a few hundred to one thousand stars per year (only a few stars currently form in our Galaxy each year). The team also studied their morphology, and show that most of them are spiral galaxies. Up to now, distant galaxies were believed to be mainly interacting galaxies, with irregular and complex shapes. Denis Burgarella and his colleagues have now shown that the galaxies in their sample, seen when the Universe had about 40% of its current age, have regular shapes, similar to present-day galaxies like ours. They bring a new element to our understanding of the evolution of the galaxies.

Original Source: Astronomy & Astrophysics News Release

An artist’s impression of two colliding galaxies. Image credit: ESO Click to enlarge
Dark matter is a mysterious substance that appears to account for 25% of the mass of the Universe. We can’t see it, but we can measure the effect of its gravity; this can reveal information about galactic structure and formation. European astronomers have measured the amount of dark matter in several galaxies, and found that a large portion of them are out of balance; their internal motions are very disturbed. This means that many galaxies – as much as 40% – have recently gone through mergers or near collisions.

Studying several tens of distant galaxies, an international team of astronomers found that galaxies had the same amount of dark matter relative to stars 6 billion years ago as they have now. If confirmed, this suggests a much closer interplay between dark and normal matter than previously believed. The scientists also found that as many as 4 out of 10 galaxies are out of balance. These results shed a new light on how galaxies form and evolve since the Universe was only half its current age.

“This may imply that collisions and merging are important in the formation and evolution of galaxies”, said Francois Hammer, Paris Observatory, France, and one of the leaders of the team.

The scientists were interested in finding out how galaxies that are far away – thus seen as they were when the Universe was younger – evolved into the ones nearby. In particular, they wanted to study the importance of dark matter in galaxies.

“Dark matter, which composes about 25% of the Universe, is a simple word to describe something we really don’t understand,” said Hector Flores, co-leader. “From looking at how galaxy rotates, we know that dark matter must be present, as otherwise these gigantic structures would just dissolve.”

In nearby galaxies, and in our own Milky Way for that matter, astronomers have found that there exist a relation between the amount of dark matter and ordinary stars: for every kilogram of material within a star there is roughly 30 kilograms of dark matter. But does this relation between dark and ordinary matter still hold in the Universe’s past?

This required measuring the velocity in different parts of distant galaxies, a rather tricky experiment: previous measurements were indeed unable to probe these galaxies in sufficient details, since they had to select a single slit, i.e. a single direction, across the galaxy.

Things changed with the availability of the multi-object GIRAFFE spectrograph, now installed on the 8.2-m Kueyen Unit Telescope of ESO’s Very Large Telescope (VLT) at the Paranal Observatory (Chile).

In one mode, known as “3-D spectroscopy” or “integrated field”, this instrument can obtain simultaneous spectra of smaller areas of extended objects like galaxies or nebulae. For this, 15 deployable fibre bundles, the so-called Integral Field Units (IFUs) , cf. ESO PR 01/02 , are used to make meticulous measurements of distant galaxies. Each IFU is a microscopic, state-of-the-art two-dimensional lens array with an aperture of 3 x 2 arcsec2 on the sky. It is like an insect’s eye, with twenty micro-lenses coupled with optical fibres leading the light recorded at each point in the field to the entry slit of the spectrograph.

“GIRAFFE on ESO’s VLT is the only instrument in the world that is able to analyze simultaneously the light coming from 15 galaxies covering a field of view almost as large as the full moon,” said Mathieu Puech, lead author of one the papers presenting the results. “Every galaxy observed in this mode is split into continuous smaller areas where spectra are obtained at the same time.”

The astronomers used GIRAFFE to measure the velocity fields of several tens of distant galaxies, leading to the surprising discovery that as much as 40% of distant galaxies were “out of balance” – their internal motions were very disturbed – a possible sign that they are still showing the aftermath of collisions between galaxies.

When they limited themselves to only those galaxies that have apparently reached their equilibrium, the scientists found that the relation between the dark matter and the stellar content did not appear to have evolved during the last 6 billions years.

Thanks to its exquisite spectral resolution, GIRAFFE also allows for the first time to study the distribution of gas as a function of its density in such distant galaxies. The most spectacular results reveal a possible outflow of gas and energy driven by the intense star-formation within the galaxy and a giant region of very hot gas (HII region) in a galaxy in equilibrium that produces many stars.

“Such a technique can be expanded to obtain maps of many physical and chemical characteristics of distant galaxies, enabling us to study in detail how they assembled their mass during their entire life,” said Fran?ois Hammer. “In many respects, GIRAFFE and its multi-integral field mode gives us a first flavour of what will be achieved with future extremely large telescopes.”

Original Source: ESO News Release

The double helix nebula. Image credit: NASA/UCLA Click to enlarge
Astronomers have discovered an unusual helix-shaped nebula near the centre of the Milky Way. This peculiar nebula stretches 80 light years, and looks like the classic image of a DNA molecule. The nebula formed because it’s so close to the supermassive black hole at the heart of the Milky Way, which has a very powerful magnetic field. This field isn’t as powerful as the one surrounding the Sun, but it’s enormous, containing a tremendous amount of energy. It’s enough to reach out this incredible distance and twist up this gas cloud with its field lines.

Astronomers report an unprecedented elongated double helix nebula near the center of our Milky Way galaxy, using observations from NASA’s Spitzer Space Telescope. The part of the nebula the astronomers observed stretches 80 light years in length. The research is published March 16 in the journal Nature.

“We see two intertwining strands wrapped around each other as in a DNA molecule,” said Mark Morris, a UCLA professor of physics and astronomy, and lead author. “Nobody has ever seen anything like that before in the cosmic realm. Most nebulae are either spiral galaxies full of stars or formless amorphous conglomerations of dust and gas – space weather. What we see indicates a high degree of order.”

The double helix nebula is approximately 300 light years from the enormous black hole at the center of the Milky Way. (The Earth is more than 25,000 light years from the black hole at the galactic center.)

The Spitzer Space Telescope, an infrared telescope, is imaging the sky at unprecedented sensitivity and resolution; Spitzer’s sensitivity and spatial resolution were required to see the double helix nebula clearly.

“We know the galactic center has a strong magnetic field that is highly ordered and that the magnetic field lines are oriented perpendicular to the plane of the galaxy,” Morris said. “If you take these magnetic field lines and twist them at their base, that sends what is called a torsional wave up the magnetic field lines.

“You can regard these magnetic field lines as akin to a taut rubber band,” Morris added. “If you twist one end, the twist will travel up the rubber band.”

Offering another analogy, he said the wave is like what you see if you take a long loose rope attached at its far end, throw a loop, and watch the loop travel down the rope.

“That’s what is being sent down the magnetic field lines of our galaxy,” Morris said. “We see this twisting torsional wave propagating out. We don’t see it move because it takes 100,000 years to move from where we think it was launched to where we now see it, but it’s moving fast – about 1,000 kilometers per second – because the magnetic field is so strong at the galactic center – about 1,000 times stronger than where we are in the galaxy’s suburbs.”

A strong, large-scale magnetic field can affect the galactic orbits of molecular clouds by exerting a drag on them. It can inhibit star formation, and can guide a wind of cosmic rays away from the central region; understanding this strong magnetic field is important for understanding quasars and violent phenomena in a galactic nucleus. Morris will continue to probe the magnetic field at the galactic center in future research.

This magnetic field is strong enough to cause activity that does not occur elsewhere in the galaxy; the magnetic energy near the galactic center is capable of altering the activity of our galactic nucleus and by analogy the nuclei of many galaxies, including quasars, which are among the most luminous objects in the universe. All galaxies that have a well-concentrated galactic center may also have a strong magnetic field at their center, Morris said, but so far, ours is the only galaxy where the view is good enough to study it.

Morris has argued for many years that the magnetic field at the galactic center is extremely strong; the research published in Nature strongly supports that view.

The magnetic field at the galactic center, though 1,000 times weaker than the magnetic field on the sun, occupies such a large volume that it has vastly more energy than the magnetic field on the sun. It has the energy equivalent of 1,000 supernovae.

What launches the wave, twisting the magnetic field lines near the center of the Milky Way? Morris thinks the answer is not the monstrous black hole at the galactic center, at least not directly.

Orbiting the black hole like the rings of Saturn, several light years away, is a massive disk of gas called the circumnuclear disk; Morris hypothesizes that the magnetic field lines are anchored in this disk. The disk orbits the black hole approximately once every 10,000 years.

“Once every 10,000 years is exactly what we need to explain the twisting of the magnetic field lines that we see in the double helix nebula,” Morris said.

Co-authors on the Nature paper are Keven Uchida, a former UCLA graduate student and former member of Cornell University’s Center for Radiophysics and Space Research; and Tuan Do, a UCLA astronomy graduate student. Morris and his UCLA colleagues study the galactic center at all wavelengths.

NASA’s Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer Space Telescope mission for the agency’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology. JPL is a division of Caltech. NASA funded the research.

Original Source: UCLA News Release

Harlan J. Smith Telescope. Image credit: Marty Harris/McDonald Observatory. Click to enlarge
An international group of astronomers has used the Hubble Space Telescope to determine the origin of an unusual class of objects called extreme helium stars. These objects are formed when two white dwarf stars merge together. Since they were first discovered more than 60 years ago, less than two dozen have found. They contain almost no hydrogen, and are dominated by helium and other heavier elements. When two white dwarf stars merge together, the resulting star swells up to become a supergiant star rich in helium.

An international group of astronomers including Dr. David L. Lambert, director of The University of Texas at Austin McDonald Observatory, has used Hubble Space Telescope to determine the origin of a very unusual and rare type of star. The group’s studies indicate that the so-called “extreme helium stars” are formed by the merger of two white dwarf stars. The work has been published in the February 10 issue of The Astrophysical Journal.

The team was led by Dr. Gajendra Pandey of the Indian Institute of Astrophysics (IIA) in Bangalore, and also includes Dr. C. Simon Jeffery of Armagh Observatory in Northern Ireland, and Professor N. Kameswara Rao, also of IIA.

“It’s taken more than 60 years after the first discovery at McDonald to get some idea of how these formed,” Rao said. He has been studying these types of stars for more than 30 years. “We are now getting a consistent picture.”

The nature of the first extreme helium star, HD 124448, was discovered at McDonald Observatory in 1942 by Daniel M. Popper of The University of Chicago. Since then, fewer than two dozen of these stars have been identified. They are supergiant stars – less massive than the Sun but many times larger and hotter – and remarkable for their strange compositions. They contain almost no hydrogen, the most abundant chemical element in the universe, and the most basic component of all stars. Instead, they are dominated by helium, with significant amounts of carbon, nitrogen, and oxygen, and traces of all other stable elements.

The origin of extreme helium stars cannot be traced back to formation in a cloud of helium gas, since no such clouds exist in our Milky Way galaxy. Nuclear reactions in a star like the Sun convert hydrogen to helium to provide sunlight or starlight. Since the helium is confined the hot core of a star, the star must lose vast amounts of gas before the helium is at the star’s surface – and thus detectable by telescopes. No known mechanism inside the star can drive off the overlying layers to expose the helium.

Two decades ago, astronomers Ronald Webbink and Icko Iben of the University of Illinois introduced the theory that extreme helium stars formed from the merger of two white dwarfs.

White dwarfs are the end product of the evolution of stars like the Sun. They don’t contain much hydrogen. Some are rich in helium, and others in carbon and oxygen. A pair of white dwarfs can result from the evolution of a normal binary star (two normal stars in orbit around each other).

Webbink and Iben supposed that, in some cases, one star in the binary may evolve as a helium-rich white dwarf, and the other as a carbon-oxygen-rich white dwarf. Over billions of years of orbiting each other, the two stars lose energy and move steadily closer to each other. Eventually, the helium white dwarf is consumed by the more massive carbon-oxygen white dwarf. The resultant single star swells up to become a helium-rich supergiant star.

To test this theory, astronomers needed to uncover the exact chemical composition of extreme helium stars. This is what Pandey, Lambert, and their colleagues set out to do. They obtained crucial observations with NASA’s Hubble Space Telescope, and made supporting observations from the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory and the 2.3-meter Vainu Bappu Telescope in India.

“As an aside,” Lambert said, “it’s interesting to note that the namesakes of these two telescopes, Harlan J. Smith and Vainu Bappu, were the very best of friends in graduate school at Harvard.” Later, Smith served as director of McDonald Observatory from 1963 to 1989. Vainu Bappu founded the Indian Institute of Astrophysics. “Today, with collaborations like this project,” Lambert said, “we’re maintaining the important international and personal ties that astronomy thrives upon.”

The group made detailed studies of the ultraviolet light coming from seven extreme helium stars with Hubble Space Telescope’s STIS instrument (the Space Telescope Imaging Spectrograph) and of the optical light from the telescopes in Texas and India. This data provided them with the specific amounts of at least two dozen different chemical elements present in each star they studied.

According to Rao, it is the advance in technology of being able to observe the spectra of these stars in ultraviolet light with Hubble that made this breakthrough study possible more than 60 years after extreme helium stars were discovered.

The Hubble results match up well with predicted compositions from models of the composition of a star formed through the merger of two white dwarf stars in which the helium-core white dwarf is torn apart, and forms a thick disk around the carbon-oxygen white dwarf. Then, in a process taking only a few minutes, the disk is gravitationally pulled into the carbon-oxygen white dwarf.

What happens next depends of the mass of the new, resulting star. If it is above a certain mass, called the Chandrasekar limit, it will explode (specifically, it will explode as a Type Ia supernova). However, if the mass is below this limit, the new merged star will balloon up into a supergiant, eventually becoming an extreme helium star.

Pandey, Lambert, Jeffery, and Rao plan to continue their research on extreme helium stars, using both the Smith and Hobby-Eberly Telescopes at McDonald Observatory. They hope to identify more extreme helium stars, and discover even more chemical elements in these stars.

This research was supported by grants from the Robert A. Welch Foundation of Houston, Texas and the Space Telescope Science Institute in Baltimore, Maryland.

Original Source: University of Texas at Austin