Author: Fraser Cain

Future astronomers will see this nebula in the sky. Image credit: David A. Aguilar. Click to enlarge.
Astronomers announced today that they have found the next Orion Nebula. Known as W3, this glowing gas cloud in the constellation Cassiopeia has just begun to shine with newborn stars. Shrouds of dust currently hide its light, but this is only a temporary state. In 100,000 years – a blink of the eye in astronomical terms – it may blaze forth, delighting stargazers around the world and becoming the Grand Nebula in Cassiopeia..

“The Grand Nebula in Cassiopeia will appear in our sky just as the Great Nebula in Orion fades away,” said Smithsonian astronomer Tom Megeath (Harvard-Smithsonian Center for Astrophysics), who made the announcement in a press conference at the 207th meeting of the American Astronomical Society. “Even better, its home constellation is visible year-round from much of the northern hemisphere.”

The Orion Nebula is one of the most famous and easily viewed deep-sky sights. It holds special significance for researchers as the nearest region of massive star formation.

The star formation process begins in a dark cloud of cold gas, where small lumps of material begin to contract. Gravity draws the gas into hot condensations that ignite and become stars. The most massive stars produce hot winds and intense light that blast away the surrounding cloud. But during the process of destruction, stellar radiation lights up the cloud, creating a bright nebula for stargazers to admire.

“Orion may seem very peaceful on a cold winter night, but in reality it holds very massive, luminous stars that are destroying the dusty gas cloud from which they formed,” said Megeath. “Eventually, the cloud of material will disperse and the Orion Nebula will fade from our sky.”

Orion’s Trapezium
Of special interest to Megeath is a system of four bright, massive stars at the center of Orion known as the Trapezium. These stars bathe the entire nebula with powerful ultraviolet radiation, lighting up nearby gas. Even a modest telescope reveals the Trapezium surrounded by billowing ripples of matter gleaming eerily across the vastness of space. Yet the Trapezium is only the tip of the iceberg, surrounded by more than 1000 faint, low-mass stars similar to the Sun.

“The question we want to answer is: why are these massive stars sitting in the center of the cluster?” said Megeath.

There are two competing theories to explain the Trapezium’s location. One holds that the Trapezium stars formed apart from each other but descended to the center of the cluster, ejecting a spray of low-mass stars in the process. The other leading theory is that the Trapezium stars formed together in the center of the cluster and have not moved from their birthplace.

“Obviously, we can’t go back in time and look at the Trapezium when it was still forming, so we try to find younger examples in the sky,” explained Megeath.

Such proto-Trapeziums would still be buried in their birth cocoons, hidden to visible-light telescopes but detectable by radio and infrared telescopes. Searches at those longer wavelengths have identified many regions where massive stars are forming, but could not determine whether the protostars were alone or in collections of four or more stars that could be considered Trapeziums.

Cassiopeia’s Trapezium
Megeath and his colleagues examined one such protostellar clump in W3 using the NICMOS instrument on NASA’s Hubble Space Telescope and the National Science Foundation’s Very Large Array. They discovered that the object, which was thought to be a binary star, actually contained four or five young, massive protostars, making it a likely proto-Trapezium.

These protostars are so young that they appear to be still growing by accreting gas from the surrounding cloud. All of the stars crowd into a small area only about 500 billion miles across (just under one-tenth of a light-year), making this cluster more than 100,000 times denser than stars in the Sun’s neighborhood. This suggests that the massive stars in Orion’s Trapezium formed together in the center of the cluster.

The same physical processes that have carved the Orion Nebula now are molding the W3 nebula. The massive stars in this compact group are starting to eat away at the surrounding gas with ultraviolet radiation and fast stellar outflows. Eventually, they will destroy their dense cocoon and emerge to form a new Trapezium in the center of W3. However, the final form of the nebula and the time that it will reach maximum brilliance are uncertain.

“Who knows, in 100,000 years the emerging Grand Nebula in Cassiopeia may replace the fading Orion Nebula as a favorite object for amateur astronomers,” said Megeath. “In the meantime, I think it will be a favorite target for professional astronomers trying to solve the riddle of massive star formation.”

Megeath’s colleagues on this work were Thomas Wilson (European Southern Observatory) and Michael Corbin (Arizona State University).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

Distribution of stars in galactic companion. Image credit: PSU. Click to enlarge.
A team of scientists from the Sloan Digital Sky Survey (SDSS), including a Penn State astrophysicist, has discovered a companion to the Milky Way galaxy that is so big it previously had been undetectable. The result is the topic of a press conference during the meeting of the American Astronomical Society now taking place in Washington, D.C.

The study, lead by Mario Juric of Princeton and Zeljko Ivezic of the University of Washington, found a collection of stars in the constellation Virgo that covers nearly 5,000 times the size of the full moon. Penn State Professor of Astronomy and Astrophysics Donald Schneider, a coauthor of the investigation, is the Chairman of the SDSS Quasar Science Group and the SDSS Scientific Publications Coordinator. “The star cluster is located only 30,000 light years from Earth,” noted Schneider. “This is the same distance from us as is the Galactic Center, although the cluster lies in a different direction from the Center. It is likely that the cluster is the remnant of a small galaxy that has been captured and disrupted by the gravitational field of our galaxy.”

The galaxy is a huge but very faint structure, containing hundreds of thousands of stars spread over an area area nearly 5,000 times the size of a full moon. Although the structure lies well within the confines of the Milky Way Galaxy, at an estimated distance of 30,000 light years from Earth, it does not follow any of Milky Way’s three main components: a flattened disk of stars in which the Sun resides, a bulge of stars at the center of the Galaxy, and an extended, roughly spherical, stellar halo. Instead, the discoverers believe that the most likely interpretation of the new structure is a dwarf galaxy that is merging into the Milky Way.

“Some of the stars in this Milky Way companion have been seen with telescopes for centuries,” explained Princeton University graduate student Mario Juric, who is principal author of the journal article describing what may well be our closest galactic neighbor. “But because the galaxy is so close, its stars are spread over a huge swath of the sky, and they always used to be lost in the sea of more numerous Milky Way stars. This galaxy is so big, we couldn’t see it before.”

The discovery was made possible by the unprecedented depth and photometric accuracy of the SDSS, which to date has imaged roughly 1/4 of the northern sky. “We used the SDSS data to measure distances to 48 million stars and build a 3-D map of the Milky Way,” explained Zeljko Ivezic of the University of Washington, a co-author of the study. Details of this “photometric parallax” method, which uses the colors and apparent brightnesses of stars to infer their distances, are explained in a paper titled “Milky Way Tomography,” submitted to The Astrophysical Journal.

“It’s like looking at the Milky Way with a pair of 3-d glasses,” said Princeton University co-author Robert Lupton. “This structure that used to be lost in the background suddenly snapped into view.” The new result is reminiscent of the 1994 discovery of the Sagittarius dwarf galaxy, by Rodrigo Ibata and collaborators from Cambridge University. They used photographic images of the sky to identify an excess of stars on the far side of the Milky Way, some 75,000 light years from Earth. The Sagittarius dwarf is slowly dissolving, trailing streams of stars behind it as it orbits the Milky Way and sinks into the Galactic disk.

In the ensuing decade, a new generation of sky surveys using large digital cameras has identified numerous streams and lumps of stars in the outer Milky Way. Some of these lumps are probably new Milky Way companions, while others may be shreds of the Sagittarius dwarf or of other dissolving dwarf galaxies. Earlier SDSS discoveries include an apparent ring of stars that encircles the Milky Way disk and may be the remnant of another disrupted galaxy, and the Ursa Major dwarf, the faintest known neighbor of the Milky Way.

Preliminary evidence for the new dwarf galaxy, found toward the constellation Virgo, appeared in maps of variable stars by the SDSS and by the QUEST survey (a Yale University/University of Chile collaboration). “With so much irregular structure in the outer Galaxy, it looks as though the Milky Way is still growing, by cannibalizing smaller galaxies that fall into it,” said Juric.

Another group of SDSS astronomers, led by Daniel Zucker of the Max Planck Institute of Astronomy in Heidelberg and Cambridge University’s Institute of Astronomy, has used the SDSS to find the two faintest known companions of the Andromeda Galaxy, which is the closest giant spiral galaxy similar in size to the Milky Way. “These new Andromeda companions, alongside the new Milky Way neighbors, suggest that faint satellite galaxies may be plentiful in the Local Group,” said Zucker.

While the SDSS originally was designed to study the distant universe, its wide area, high precision maps of faint stars have made it an invaluable tool for studying the Milky Way and its immediate neighborhood. The 3-D map created by Juric and his collaborators also provides strong new constraints on the shape and extent of the Milky Way’s disk and stellar halo. Another Princeton graduate student, Nick Bond, is using the subtle motions of stars detected over the 5-year span of the SDSS observations to limit the amount of dark matter in the solar neighborhood. University of Washington graduate student Jillian Meyer is mapping the distribution of interstellar dust carefully studying the colors of stars found in both the SDSS and the infrared 2MASS survey.

Building on these many successes, the SEGUE project (Sloan Extension for Galactic Understanding and Exploration) will use the SDSS telescope, its 120-megapixel digital camera, and its 640-fiber optical spectrograph to carry out detailed studies of the structure and chemical evolution of the Milky Way. SEGUE is one of three components of SDSS-II, the three-year extension of the Sloan Survey that will run through mid-2008.

Fermilab scientist Brian Yanny, one of the SEGUE team leaders, is excited at the prospect of examining its just-completed, first season of observations. “The SDSS has already told us surprising things about the Milky Way, but the most exciting discoveries should lie just ahead.”

Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.

The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions, which include the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, Cambridge University, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPA), the Max-Planck-Institute for Astrophysics (MPIA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

Original Source: Eberly College News Release

Artist illustration of Vega. Image credit: NOAO. Click to enlarge.
Strong darkening observed around the equator of Vega suggests that the fifth brightest star in Earth’s sky has a huge temperature difference of 4,000 degrees Fahrenheit from its cool equatorial region to its hot poles.

Models of the star based on these observations suggest that Vega is rotating at 92 percent of the angular velocity that would cause it to physically break apart, an international team of astronomers announced today in Washington, DC, at the 207th meeting of the American Astronomical Society.

This result confirms the idea that very rapidly rotating stars are cooler at their equators and hotter at their poles, and it indicates that the dusty debris disk known to exist around Vega is significantly less illuminated by the star?s light than previously recognized.

“These findings are significant because they resolve some confusing measurements of the star, and they should help us gain a much better understanding of Vega’s circumstellar debris disk,” says Jason P. Aufdenberg, the Michelson Postdoctoral Fellow at the National Optical Astronomy Observatory in Tucson, Arizona.

This debris disk arises mainly from the collision of rocky asteroid-like bodies. “The spectrum of Vega as viewed from its equatorial plane, the same plane as the debris disk, should be about half as luminous as the spectrum viewed from the pole, based on these new results,” Aufdenberg explains.

The team obtained high-precision interferometric measurements of the bright standard star Vega using the Center for High Angular Resolution Astronomy (CHARA) Array, a collection of six 1-meter telescopes located on Mount Wilson, California, and operated by Georgia State University.

With a maximum baseline of 330 meters (1,083 feet), the CHARA Array is capable of resolving details as small as 200 micro-arcseconds, equivalent to the angular size of a nickel seen from a distance of 10,000 miles. The CHARA Array fed the starlight of Vega to the Fiber Linked Unit for Optical Recombination (FLUOR) instrument, developed by the Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique of the Observatoire de Paris.

One major consequence of Vega’s rapid rotation is a significant drop in the effective atmospheric temperature by approximately 2,300 Kelvin (4,000 degrees Fahrenheit) from the pole to the equator. This effect, known as “gravity darkening,” was first predicted by theoretical astronomer E. Hugo von Zeipel in 1924.

The CHARA/FLUOR measurements of the brightness distribution of Vega’s surface also show it to be strongly “limb darkened.” Limb darkening refers to the diminishing brightness in the image of a star from the center of the image to the edge or “limb” of the image.

The new measurements are consistent with the “pole-on” model for Vega first proposed by Richard O. Gray of Appalachian State University, which proposes that Vega?s pole of rotation points toward Earth. The pole-on view of Vega means that the relatively cool equator corresponds to the limb of the star, such that the gravity-darkening effect further enhances the limb-darkening effect.

The CHARA/FLUOR data support the pole-on, gravity darkened model for Vega by showing that Vega’s limb darkening is 2.5 times stronger at a wavelength of 2.2 microns than expected for a star with a single effective atmosphere temperature. Archival observations from the International Ultraviolet Explorer indicate that this model for Vega is not complete. At far ultraviolet wavelengths, below 140 nanometers, the model is generally too bright.

Located at a distance of 25 light-years from Earth in the constellation Lyra, Vega rotates about its axis once every 12.5 hours. For comparison, the Sun’s average rotation period is approximately 27 Earth days. Vega is about 2.5 times more massive than the Sun, and 54 times brighter.

At Vega’s rapid rate of rotation, the star’s atmosphere is distorted, bulging 23 percent wider at its equator compared to its poles. This type of rotational distortion can be seen in images of the planet Saturn, where the planet’s equatorial diameter is roughly 10 percent wider than the polar diameter. A direct measurement of Vega’s rotational distortion is hidden by its pole-on appearance. However, the accurate angular diameter and darkening measured by CHARA/FLUOR are consistent with this distortion.

These results build upon recent measurements of Vega obtained by a team lead by Deane M. Peterson of the State University of New York, Stony Brook, using the Navy Prototype Optical Interferometer.

Co-authors of this result include Antoine M?rand, Vincent Coud? du Foresto, Emmanuel Di Folco, and Pierre Kervella of the Observatoire de Paris-Meudon, France; Olivier Absil of the University of Li?ge, Belgium; Stephen T. Ridgway of the National Optical Astronomy Observatory, Tucson, Arizona and NASA; Harold A. McAlister, Theo A. ten Brummelaar, Judit Sturmann, Laszlo Sturmann, and Nils H. Turner of the Center for High Angular Resolution Astronomy, Georgia State University, Atlanta, Georgia, and Mount Wilson Observatory, California; and David H. Berger of the University of Michigan, Ann Arbor, Michigan.

This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology. The CHARA Array is operated by the Center for High Angular Resolution Astronomy, Georgia State University, Atlanta, GA. Additional support comes from the National Science Foundation, the Keck Foundation and the Packard Foundation.

The National Optical Astronomy Observatory is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the NSF.

Original Source: NOAO News Release

Computer illustration of a binary star. Image credit: Carnegie Institution. Click to enlarge.
New theoretical work shows that gas-giant planet formation can occur around binary stars in much the same way that it occurs around single stars like the Sun. The work is presented today by Dr. Alan Boss of the Carnegie Institution’s Department of Terrestrial Magnetism (DTM) at the American Astronomical Society meeting in Washington, DC. The results suggest that gas-giant planets, like Jupiter, and habitable Earth-like planets could be more prevalent than previously thought. A paper describing these results has been accepted for publication in the Astrophysical Journal.

“We tend to focus on looking for other solar systems around stars just like our Sun,” Boss says. “But we are learning that planetary systems can be found around all sorts of stars, from pulsars to M dwarfs with only one third the mass of our Sun.”

Two out of every three stars in the Milky Way is a member of a binary or multiple star system, in which the stars orbit around each other with separations that can range from being nearly in contact (close binaries) to thousands of light-years or more (wide binaries). Most binaries have separations similar to the distance from the Sun to Neptune (~30 AU, where 1 AU = 1 astronomical unit = 150 million kilometers–the distance from the Earth to the Sun).

It has not been clear whether planetary system formation could occur in typical binary star systems, where the strong gravitational forces from one star might interfere with the planet formation processes around the other star, and vice versa. Previous theoretical work had suggested, in fact, that typical binary stars would not be able to form planetary systems. However, planet hunters have recently found a number of gas-giant planets in orbit around binary stars with a range of separations.

Boss found that if the shock heating resulting from the gravitational forces from the companion star is weak, then gas-giant planets are able to form in planet-forming disks in much the same way as they do around single stars. The planet-forming disk would remain cool enough for ice grains to stay solid and thus permit the growth of the solid cores that must reach multiple-Earth-mass size for the conventional mechanism of gas-giant planet formation (core accretion) to succeed.

Boss’ models show even more directly that the alternative mechanism for gas-giant planet formation (disk instability) can proceed just as well in binary star systems as around single stars, and in fact may even be encouraged by the gravitational forces of the other star. In Boss’ new models, the planet-forming disk in orbit around one of the stars is quickly driven to form dense spiral arms, within which self-gravitating clumps of gas and dust form and begin the process of contracting down to planetary sizes. The process is amazingly rapid, requiring less than 1,000 years for dense clumps to form in an otherwise featureless disk. There would be plenty of room for Earth-like planets to form closer to the central star after the gas-giant planets have formed, in much the same way our own planetary system is thought to have formed.

Boss points out, “This result may have profound implications in that it increases the likelihood of the formation of planetary systems resembling our own, because binary stars are the rule in our galaxy, not the exception.” If binary stars can shelter planetary systems composed of outer gas-giant planets and inner Earth-like planets, then the likelihood of other habitable worlds suddenly becomes roughly three times more probable–up to three times as many stars could be possible hosts for planetary systems similar to our own. NASA’s plans to search for and characterize Earth-like planets in the next decade would then be that much more likely to succeed.

One of the key remaining questions about the theoretical models is the correct amount of shock heating inside the planet-forming disk, as well as the more general question of how rapidly the disk is able to cool. Boss and other researchers are actively working to better understand these heating and cooling processes. Given the growing observational evidence for gas-giant planets in binary star systems, the new results suggest that shock heating in binary disks cannot be too large, or it would prevent gas-giant planet formation.

Original Source: Carnegie News Release

Polaris with its faint companions. Image credit: Greg Bacon (STScI) Click to enlarge
We tend to think of the North Star, Polaris, as a steady, solitary point of light that guided sailors in ages past. But there is more to the North Star than meets the eye – two faint stellar companions. The North Star is actually a triple star system. And while one companion can be seen easily through small telescopes, the other hugs Polaris so tightly that it has never been seen directly – until now.

By stretching the capabilities of NASA’s Hubble Space Telescope to the limit, astronomers have photographed the close companion of Polaris for the first time. They presented their findings today in a press conference at the 207th meeting of the American Astronomical Society in Washington, DC.

“The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,” said Smithsonian astronomer Nancy Evans (Harvard-Smithsonian Center for Astrophysics).

The companion proved to be less than two-tenths of an arcsecond from Polaris – an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away. At the system’s distance of 430 light-years, that translates into a physical separation of about 2 billion miles.

“The brightness difference between the two stars made it even more difficult to resolve them,” stated Howard Bond of the Space Telescope Science Institute (STScI). Polaris is a supergiant more than two thousand times brighter than the Sun, while its companion is a main-sequence star. “With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight.”

By watching the motion of the companion star, Evans and her colleagues expect to learn not only the stars’ orbits but also their masses. Measuring the mass of a star is one of the most difficult tasks facing stellar astronomers.

Astronomers want to determine the mass of Polaris accurately because it is the nearest Cepheid variable star. Cepheids are used to measure the distance to galaxies and the expansion rate of the universe, so it is essential to understand their physics and evolution. Knowing their mass is the most important ingredient in this understanding.

“Studying binary stars is the best available way to measure the masses of stars,” said science team member Gail Schaefer of STScI.

“We only have the binary stars that nature provided us,” added Bond. “With the best instruments like Hubble, we can push farther into space and study more of them up close.”

The researchers plan to continue observing the Polaris system for several years. In that time, the movement of the small companion in its 30-year orbit around the primary should be detectable.

“Our ultimate goal is the get an accurate mass for Polaris,” said Evans. “To do that, the next milestone is to measure the motion of the companion in its orbit.”

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

The Milky Way galaxy. Image credit: Serge Brunier. Click to enlarge
The most prominent of the Milky Way’s satellite galaxies – a pair of galaxies called the Magellanic Clouds – appears to be interacting with the Milky Way’s ghostly dark matter to create a mysterious warp in the galactic disk that has puzzled astronomers for half a century.

The warp, seen most clearly in the thin disk of hydrogen gas permeating the galaxy, extends across the entire 200,000-light year diameter of the Milky Way, with the sun and earth sitting somewhere near the crease. Leo Blitz, professor of astronomy at the University of California, Berkeley, and his colleagues, Evan Levine and Carl Heiles, have charted this warp and analyzed it in detail for the first time, based on a new galactic map of hydrogen gas (HI) emissions.

They found that the atomic gas layer is vibrating like a drum, and that the vibration consists almost entirely of three notes, or modes.

Astronomers previously dismissed the Magellanic Clouds – comprised of the Large and Small Magellanic Clouds – as a probable cause of the galactic warp because the galaxies’ combined masses are only 2 percent that of the disk. This mass was thought too small to influence a massive disk equivalent to about 200 billion suns during the clouds’ 1.5 billion-year orbit of the galaxy.

Nevertheless, theorist Martin D. Weinberg, a professor of astronomy at the University of Massachusetts, Amherst, teamed up with Blitz to create a computer model that takes into account the Milky Way’s dark matter, which, though invisible, is 20 times more massive than all visible matter in the galaxy combined. The motion of the clouds through the dark matter creates a wake that enhances their gravitational influence on the disk. When this dark matter is included, the Magellanic Clouds, in their orbit around the Milky Way, very closely reproduce the type of warp observed in the galaxy.

“The model not only produces this warp in the Milky Way, but during the rotation cycle of the Magellanic Clouds around the galaxy, it looks like the Milky Way is flapping in the breeze,” said Blitz, director of UC Berkeley’s Radio Astronomy Laboratory.

“People have been trying to look at what creates this warp for a very long time,” Weinberg said. “Our simulation is still not a perfect fit, but it has a lot of the character of the actual data.”

Levine, a graduate student, will present the results of the work in Washington, D.C., on Jan. 9 during a 10 a.m. session on galactic structure at the American Astronomical Society meeting. Blitz will summarize the work later that day during a 12:30 p.m. press briefing in the Wilson C Room of the Marriott Wardman Park Hotel.

The interaction of the Magellanic Clouds with the dark matter in the galaxy to produce an enigmatic warp in the hydrogen gas layer is reminiscent of the paradox that led to the discovery of dark matter some 35 years ago. As astronomers built better and better telescopes able to measure the velocities of stars and gas in the outer regions of our galaxy, they discovered these stars moving far faster than would be expected from the observed number and mass of stars in the entire Milky Way. Only by invoking a then-heretical notion, that 80 percent of the galaxy’s mass was too dark to see, could astronomers reconcile the velocities with known theories of physics.

Though no one knows the true identity of this dark matter – the current consensus is that it is exotic matter rather than normal stars too dim to see – astronomers are now taking it into account in their simulations of cosmic dynamics, whether to explain the lensing effect galaxies and galaxy clusters have on the light from background galaxies, or to describe the evolution of galaxy clusters in the early universe.

Some physicists, however, have come up an alternative theory of gravity called Modified Newtonian Dynamics, or MOND, that seeks to explain these observations without resorting to belief in a large amount of undetected mass in the universe, like an invisible elephant in the room. Though MOND can explain some things, Weinberg thinks the theory will have a hard time explaining the Milky Way’s warp.

“Without a dark matter halo, the only thing the gas disk can feel is direct gravity from the Magellanic Clouds themselves, which was shown in the 1970s not to work,” he said. “It looks bad for MOND, in this case.”

Because many galaxies have warped disks, similar dynamics might explain them as well. Either way, the researchers say their work suggests that warps provide a way to verify the existence of the dark matter.

The starting point for this research was new spectral data released this past summer about hydrogen’s 21-centimeter emissions in the Milky Way. The survey, the Leiden-Argentina-Bonn or LAB Survey of Galactic HI, merged a northern sky survey conducted by astronomers in the Netherlands (the Leiden/Dwingeloo Survey) with a southern sky survey from the Instituto Argentino de Radioastronom?a. The data were corrected by scientists at the Institute for Radioastronomy of the University of Bonn, Germany.

Blitz, Levine and Heiles, UC Berkeley professor of astronomy, took these data and produced a new, detailed map of the neutral atomic hydrogen in the galaxy. This hydrogen, distributed in a plane with dimensions like those of a compact disk, eventually condenses into molecular clouds that become stellar nurseries.

With map in hand, they were able to mathematically describe the warp as a combination of three different types of vibration: a flapping of the disk’s edge up and down, a sinusoidal vibration like that seen on a drumhead, and a saddle-shaped oscillation. These three “notes” are about 3 million octaves below middle C.

“We found something very surprising, that we could describe the warp by three modes of vibration, or three notes, and only three,” Blitz said, noting that this rather simple mathematical description of the warp had escaped the notice of astronomers since the warp’s discovery in 1957.

“We were actually trying to analyze a more complex ‘scalloping’ structure of the disk, and this simple, elegant vibrational structure just popped out,” Levine added.

The current warp in the gas disk is a combination of these three vibrational modes, leaving one-half of the galactic disk sticking up above the plane of stars and gas, while the other half dips below the disk before rising upward again farther outward from the center of the galaxy. The results of this analysis will be published in an upcoming issue of the Astrophysical Journal.

Weinberg thought he could explain the observed warp dynamically, and used computers to calculate the effect of the Magellanic Clouds orbiting the Milky Way, plowing through the dark matter halo that extends far out into the orbit of the clouds.

What he and Blitz found is that the clouds’ wake through the dark matter excites a vibration or resonance at the center of the dark matter halo, which in turn makes the disk embedded in the halo oscillate strongly in three distinct modes. The combined motion during a 1.5-billion-year orbit of the Magellanic Clouds is reminiscent of the edges of a tablecloth flapping in the wind, since the center of the disk is pinned down.

“We often think of the warp as being static, but this simulation shows that it is very dynamic,” Blitz said.

Blitz, Levine and Heiles are continuing their search for anomalies in the structure of the Milky Way’s disk. Weinberg hopes to use the UC Berkeley group’s data and analysis to determine the shape of the dark matter halo of the Milky Way.

The research of the UC Berkeley group is supported by the National Science Foundation. Weinberg is partly supported by NASA and the NSF.

Original Source: UC Berkeley News Release