Earth’s neutron-star neighbor

In August 2006, NASA’s Swift satellite spotted a bright object originally observed by the ROSAT satellite in the 1990s. The source, shown above in this artist’s illustration, has now been identified as an isolated neutron star very close to Earth.
Casey Reed (Penn State)
August 21, 2007
U sing NASA’s Swift satellite, McGill University and Penn State University astronomers have identified an object that is possibly the closest neutron star to Earth.

The object, located in the constellation Ursa Minor, is nicknamed ‘Calvera,’ after the villain in the movie “The Magnificent Seven.” If confirmed, it would be only the eighth known isolated neutron star (a neutron star not associated with a supernova remnant, a binary companion, or radio pulsations). “The seven previously known isolated neutron stars are known collectively as ‘The Magnificent Seven’ within the community, and so the name Calvera is a bit of an inside joke on our part,” says co-discoverer Derek Fox of Penn State.

Robert Rutledge of McGill University in Montreal, Quebec, originally called attention to the source. He compared a catalog of 18,000 X-ray sources from the German-American ROSAT satellite, which operated from 1990 to 1999, with catalogs of objects that appear in visible light, infrared light, and radio waves. He realized that the ROSAT source 1RXS J141256.0+792204 did not appear to have a counterpart at any other wavelength.

The group aimed Swift at the object in August 2006. Swift’s X-ray Telescope showed that the source was still there, and emitting about the same amount of X-ray energy as it had during the ROSAT era. The Swift observations enabled the group to pinpoint the object’s position more accurately, and showed that it was not associated with any known object.

“The Swift observation of this source is what got the show going,” says Penn State undergraduate Andrew Shevchuk. “As soon as I saw the data, I knew Calvera was a great neutron-star candidate.”

The team next targeted Calvera with the 8.1-meter Gemini North Telescope in Hawaii. These observations, along with a short observation by NASA’s Chandra X-ray Observatory, showed that the object is not associated with any optical counterpart down to a very faint magnitude. Chandra’s sharper X-ray vision sees the object as point-like, consistent with the neutron-star interpretation.

According to Rutledge, there are no widely accepted alternate theories for objects bright in X-rays and faint in visible light, like Calvera. Exactly which type of neutron star it is, however, remains a mystery. As Rutledge says, “Either Calvera is an unusual example of a known type of neutron star, or it is some new type of neutron star, the first of its kind.”

Calvera’s location, high above the plane of our Milky Way Galaxy, is part of its mystery. In all likelihood, the neutron star is the remnant of a star that lived in our galaxy’s starry disk before exploding as a supernova. In order to reach its current position, it had to wander some distance out of the disk. But exactly how far? “The best guess is that it is still close to its birthplace, and therefore close to Earth,” says Rutledge. If this interpretation is correct, the object is 250 to 1,000 light-years away. This would make Calvera one of the closest known neutron stars — possibly the closest.

“Because it is so bright, and probably close to Earth, it is a promising target for many types of observations,” says Fox. Indeed, to clear up the mysteries surrounding Calvera, the team will be taking a longer observation with Chandra to see if the source pulsates in X-rays, and to measure its spectrum. They also joined a group using a radio telescope to search for radio pulsations, which were not seen.

Calvera could represent the tip of the iceberg for isolated neutron stars. “There could easily be dozens,” says Fox. “The key point is that until our Swift survey, no one was able to refine the X-ray positions of large numbers of ROSAT sources to the point where it became clear which ROSAT sources were ‘missing’ their optical counterparts.”

Will Titan lose its veil?

Titan haze layers
This image of Titan reveals layers high in the moon’s atmosphere where methane-driven photochemical reactions create distinct layers of haze. If methane cannot escape from the icy layers beneath the surface, Titan’s veil will disappear.
NASA/JPL/Space Science Institute
August 20, 2007
T he question of whether or not Titan can retain its thick, organic atmosphere for the rest of its lifetime could hinge on how efficiently methane molecules were packed inside water “crates” during a period of the moon’s formation.

“If Titan runs out of methane and loses its ‘veil’, it will become a completely different type of astrophysical body,” said Dr. Vasili Dimitrov, whose work will be presented at the European Planetary Science Congress. “Methane drives the chemical reactions in Titan’s atmosphere but, because it’s so highly reactive and therefore short-lived, it must be replenished. We need to find out just how much methane is stored in the primordial reserve in Titan’s interior at a level where it can escape to the surface. To do this, we need to know how efficiently the methane molecules were packed away when the reserve formed.”

The trapped methane can exist only in molecular structures called clathrates, which occur when ‘host’ water molecules form a cage-like structure around a smaller ‘guest’ molecule (in this case methane). The water crystallizes in a cubic system, rather than the hexagonal structure of normal ice, so that the cages are arranged in body-centred cubic packing. However, not all of the cages are occupied. The maximum efficiency in filling the cages is achieved only if conditions are optimal, because the structure forms slowly at temperatures close to absolute zero.

“The conditions of Titan’s accretion and evolution are poorly understood, so we cannot yet say how many of the cages were filled and how much methane is contained in the reserve,” Dimitrov said. “In addition, we need to do some more experiments in the laboratory to find out more about the transfer of materials between layers.”

Beneath Titan’s surface, there is a permafrost crust that sits on a liquid or semi-liquid mixture of ammonia, methane and water. Beneath that, an icy layer surrounds a rocky core. It is unclear whether methane trapped in the icy layer next to the core has any means of escape. “At the moment we can work out an upper and lower limit for the packing efficiency, but this doesn’t tell us which side of the critical value Titan’s methane stockpile falls. With more experiments, together with the data supplied by the Cassini-Huygens mission, we should be able to answer the question of whether this fascinating world will keep its veil of mystery,” Dimitrov said.

Being able to estimate the packing efficiency of methane in clathrates could also have important applications back on Earth. According to some estimates, the overall methane stock in the Earth’s natural clathrates may be four times higher than the oil stock. Thus, methane extracted from clathrates could one day become a major fuel source for mankind.

Dark-matter dilemma

This multi-wavelength image of Abell 520 shows the aftermath of a complicated collision of galaxy clusters, some of the most massive objects in the universe. In this image, the hot gas detected by Chandra is red. Optical data from the Canada-France-Hawaii and Subaru telescopes show the starlight from the individual galaxies (yellow and orange). The location of most of the matter in the cluster (blue) was also found using these telescopes, by tracing the subtle light-bending effects on distant galaxies. This material is dominated by dark matter.
NASA/CXC/CFHT/Univ. Victoria/A. Mahdavi et al
August 17, 2007

A stronomers have discovered a never-before-seen cosmic “train wreck” between giant galaxy clusters. NASA’s Chandra X-ray Observatory and optical telescopes revealed a dark matter core mostly devoid of galaxies that may pose problems for current theories of dark matter behavior.

“These results challenge our understanding of the way clusters merge,” said Dr. Andisheh Mahdavi of the University of Victoria in British Columbia. “Or, they possibly make us even reexamine the nature of dark matter itself.”

There are three main components to galaxy clusters: individual galaxies composed of billions of stars, hot gas in between the galaxies, and dark matter, a mysterious substance that dominates the cluster mass and can be detected only through its gravitational effects.

Optical telescopes can observe the starlight from the individual galaxies, and can infer the location of dark matter by its subtle light-bending effects on distant galaxies. X-ray telescopes like Chandra detect the multimillion-degree gas.

A popular theory of dark matter predicts that dark matter and galaxies should stay together, even during a violent collision, as observed in the case of the so-called Bullet Cluster. However, when the Chandra data of the galaxy cluster system known as Abell 520 was mapped along with the optical data from the Canada-France-Hawaii Telescope and Subaru Telescope atop Mauna Kea in Hawaii, a puzzling picture emerged. A dark matter core was found, which also contained hot gas but no bright galaxies.

“It blew us away that it looks like the galaxies are removed from the densest core of dark matter,” said Dr. Hendrik Hoekstra, also of University of Victoria. “This would be the first time we’ve seen such a thing and could be a huge test of our knowledge of how dark matter behaves.”

In addition to the dark matter core, a corresponding “light region” containing a group of galaxies with little or no dark matter was also detected. The dark matter appears to have separated from the galaxies.

“The observation of this group of galaxies that is almost devoid of dark matter flies in the face of our current understanding of the cosmos,” said Dr. Arif Babul of the University of Victoria. “Our standard model is that a bound group of galaxies like this should have a lot of dark matter. What does it mean that this one doesn’t?”

In the Bullet Cluster, known as 1E 0657-56, the hot gas is slowed down during the collision but the galaxies and dark matter appear to continue on unimpeded. In Abell 520, it appears that the galaxies were unimpeded by the collision, as expected, while a significant amount of dark matter has remained in the middle of the cluster along with the hot gas.

Mahdavi and his colleagues have two possible explanations for their findings, both of which are uncomfortable for prevailing theories. The first option is that the galaxies were separated from the dark matter through a complex set of gravitational “slingshots.” This explanation is problematic because computer simulations have not been able to produce slingshots that are nearly powerful enough to cause such a separation.

The second option is that dark matter is affected not only by gravity, but also by an as-yet-unknown interaction between dark matter particles. This exciting alternative would require new physics and could be difficult to reconcile with observations of other galaxies and galaxy clusters, such as the aforementioned Bullet Cluster.

In order to confirm and fully untangle the evidence for the Abell 520 dark matter core, the researchers have secured time for new data from Chandra plus the Hubble Space Telescope. With the additional observations, the team hopes to resolve the mystery surrounding this system.

Life less likely on Enceladus

Enceladus
Enceladus, imaged by Cassini in 2006, shows off its tortured south polar terrain, crosscut by the roughly parallel furrows and ridges called “tiger stripes.” The fissures discharge plumes of ice crystals and gases. Originally, it was suggested that pockets of liquid water were the source of these plumes. Now, a new model offers another explanation.
NASA/JPL/Space Science Institute
August 16, 2007

A new model of Saturn’s icy moon Enceladus may quell hopes of finding life there. Developed by researchers at the University of Illinois, the model states the most salient observations on Enceladus can be explained without requiring the presence of liquid water.

Orbiting Saturn since June 30, 2004, the Cassini spacecraft has revealed a south polar region of Enceladus with an elaborate arrangement of fractures and ridges, intense heat radiation and geyser-like plumes consisting of ice crystals and gases such as methane, nitrogen and carbon dioxide. The plumes erupt from vents located in large fractures called “tiger stripes” that cut across the south pole.

The Cassini-monitored plumes had a rate of discharge similar to the Old Faithful geyser in Yellowstone National Park. Dubbed “Cold Faithful,” the first model proposed to explain the plumes suggested the plumes tap into shallow pockets of liquid water in a water-ice shell.

Last year, University of Illinois geology professor and planetary scientist Susan Kieffer and colleagues proposed an alternate model, which they called “Frigid Faithful.” In this model, the plumes originate in the dissociation of certain stiff compounds of ice, called clathrates, which may cover Enceladus to a depth of tens of kilometers.

“Frigid Faithful gives a straightforward account of the measured composition, including the gases left unaccounted by Cold Faithful,” said Kieffer, who holds a Charles R. Walgreen Jr. Chair at Illinois and is also a professor in the University’s Center for Advanced Study, one of the highest forms of campus recognition.

“Perhaps more important, the plumes of Frigid Faithful could remain active far below the freezing point of water, under the frigid conditions that might be surmised inside a tiny, icy moon,” Kieffer said.

Recent data from the Cassini spacecraft revealed geyser-like plumes, intense heat flux, and tectonic features including large fractures that cut across the pole, a ring of ridges (red), and radial rifts (blue) on Enceladus, imaged above. The origin of these tectonic features and the relation among the tectonic features, the heat transport, and the plumes is discussed by using a unified thermo-mechanical model of Enceladus.
NASA/JPL/Space Science Institute
Now, Kieffer, mechanical science and engineering professor Gustavo Gioia, geology research associate Pinaki Chakraborty, and geology professor and department head Stephen Marshak have expanded the model to account for both the tectonic features and the heat transport in the southern hemisphere.

By examining the deformation of a clathrate-rich shell containing a mildly warm heat source buried under the south pole, the researchers show it is possible for Enceladus to be frigid without having a shifting interior (such as plate tectonics on Earth) to develop fractures and ridges, and convey heat at the observed rate.

“As the heat source warmed at depth, it expanded and stretched the clathrate-rich shell above, giving rise to tensile stresses in the south polar cap,” said Gioia. “As a result, the shell cracked, forming the four 130 kilometer-long fractures known as tiger stripes.”

The researchers estimate the heat source could have been only 40° warmer than the surrounding shell. “In this model, the tiger stripes are analogous to the cracks that form in the glazing of a porcelain vessel when the vessel is filled with hot tea,” Gioia said.

The researchers also show that, northwards of the south polar cap (in which the stresses were tensile), the stresses turned first from tensile to compressive — forming the ring of ridges that circles the tiger stripes — and then back to tensile — forming the set of “starfish” fractures that radiates northward — from the ring of ridges. Thus the model explains the formation of the entire arrangement of fractures and ridges observed by Cassini on the southern hemisphere of Enceladus.

The Illinois researchers estimate the tiger stripes cut through the shell of Enceladus to a depth of about 35 km. After the tiger stripes formed, the clathrates exposed on the cracked surfaces of the tiger stripes were decompressed. Upon decompression, the exposed clathrates absorbed heat from the source at depth and dissociated explosively, exposing more clathrates to decompression, in a process that continues today.

The gaseous products of clathrate dissociation rush up the tiger stripes, transporting heat to the surface where they may occasionally leak in the form of plumes. The transport of heat by fast-moving gases is called “heat advection.” The cracked shell of Frigid Faithful acts as a gigantic “advection machine,” which efficiently conveys heat from the source to the surface.

In contrast to heat conduction, where the transport of heat (in a bar of steel, for example) can only occur from points at higher temperature towards points at lower temperature, heat advection takes place at a nearly uniform temperature.

The implication is that Frigid Faithful’s shell remains close to the surface temperature to a depth of about 35 km, Gioia said. According to the Cassini measurements, the surface temperature might be as many as 150° below the freezing point of water.

“This is indeed a frigid Enceladus,” Gioia said. “It appears that high heat fluxes, geyser-like activity and complex tectonic features can occur even if moons do not have hot, liquid or shifting interiors.”

While the Enceladus envisioned by the Illinois researchers is unlikely to possess liquid water and therefore unlikely to harbor life, it is compatible with the available evidence and is the only model that has been shown to explain the origin of the arrangement of fractures and ridges documented by Cassini.

Astronomy magazine podcast: International Dark-Sky Association

Light pollution above Quebec City
Jay Ouellet imaged this conjunction from Quebec City, Canada, February 23, 2005.
Jay Ouellet
August 16, 2007
Light pollution deteriorates public health, affects the ecosystem, wastes energy, and, of course, blocks the night sky. Leading the fight against the encroachment of light pollution is the International Dark-Sky Association (IDA).

In this week’s show, Bob Gent, president of the IDA’s board of directors, provides tips on what you can do to save the night sky.

After you listen, e-mail us here and let us know what you think.

To subscribe to our podcast, click here.

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Comet-like tail trails Mira

A new ultraviolet mosaic from NASA’s Galaxy Evolution Explorer shows speeding star Mira leaving an enormous tail of “seeds” for new solar systems. Mira’s whale of a tail can be found in the tail of the constellation Cetus the Whale.
NASA/JPL-Caltech
August 15, 2007
M aterial blowing off Mira is forming a wake 13 light-years long, or thousands of times the length of our solar system. The space-based Galaxy Evolution Explorer (GALEX) was scanning the star during its ongoing sky survey in ultraviolet light when astronomers noticed what looked like a comet with a gargantuan tail.

Mira, Latin for “wonderful,” has been a favorite of astronomers for about 400 years. It is a fast-moving red giant, which sheds massive amounts of surface material, but nothing like this has ever been seen before around a star.

“This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved,” said Mark Seibert of Carnegie Observatories in Pasadena, California. “We hope to be able to read Mira’s tail like a ticker tape to learn about the star’s life.”

Astronomers say Mira’s tail offers a unique opportunity to study how stars like our sun die and ultimately seed new solar systems. As Mira hurls along, its tail drops off carbon, oxygen and other important elements needed for new stars, planets, and possibly even life to form. This tail material, visible now for the first time, has been shed over the past 30,000 years.

Billions of years ago, Mira was like our Sun. Over time, it began to swell into a variable red giant — a pulsating, puffed-up star that periodically grows bright enough to see with the naked eye. Mira will eventually eject all of its remaining gas into space, forming a colorful shell, or a planetary nebula. The nebula will fade with time, leaving only the burnt-out core of the original star (a white dwarf).

Compared to other red giants, Mira is traveling unusually fast, possibly due to gravitational boosts from other passing stars. It now plows along at 291,000 mph (130 kilometers per second). Racing along with Mira is a small, distant companion thought to be a white dwarf. The pair, also known as Mira A (the red giant) and Mira B, orbit slowly around each other as they travel together in the constellation Cetus, 350 light-years from Earth.

In addition to Mira’s tail, GALEX also discovered a bow shock, a buildup of hot gas, in front of the star, and two sinuous streams of material coming out of the star’s front and back. Astronomers think hot gas in the bow shock is heating up the gas blowing off the star, causing it to fluoresce with ultraviolet light. This glowing material then swirls around behind the star, creating a turbulent, tail-like wake. The process is similar to a speeding boat leaving a choppy wake, or a steam train producing a trail of smoke.

“GALEX is so exquisitely sensitive to ultraviolet light and it has such a wide field of view that it is uniquely poised to scan the sky for previously-undiscovered ultraviolet activity,” said Barry F. Madore, senior research astronomer at the Carnegie Observatories. The fact that Mira’s tail only glows with ultraviolet light might explain why other telescopes have missed it.

“We never would have predicted a turbulent wake behind a star that glows only with ultraviolet light,” said Seibert. “Survey missions like the Galaxy Evolution Explorer can provide many surprises.”

Distant galaxies play hide-and-seek

The AzTEC submillimeter camera detected this bright source (left) in a region of the sky studied by the Cosmic Evolution Survey. The high resolution of the Smithsonian’s Submillimeter Array pinpointed the source of submillimeter radiation (center). A visible-light image by the Hubble Space Telescope (right) shows only a faint point. Combined, these data show the source in question is a very bright, distant, and dusty galaxy that existed when the universe was less than 2 billion years old.
Left: UMass Amherst. Middle: Harvard-Smithsonian CfA. Right: COSMOS/ACS Team.
August 14, 2007
B y combining the capabilities of several telescopes, astronomers have spotted extremely bright galaxies hiding in the distant, young universe. The newfound galaxies are intrinsically bright due to their large rate of star formation — 1,000 times greater than the Milky Way. However, much of that light is hidden by surrounding dust and gas and leaks out only in the infrared.

The galaxies are located about 12 billion light-years away, and existed when the universe was less than 2 billion years old. They are the most luminous and massive galaxies seen at that great distance. Smaller, dimmer galaxies were much more common in the early universe because it takes time for galaxies to form and grow.

“It’s a real surprise to find galaxies that massive and luminous existing so early in the universe,” said astronomer Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics (CfA). “We are witnessing the moment when the most massive galaxies in the universe were forming most of their stars in their early youth.”

“It’s tough to explain how such bright, massive, dusty galaxies formed so early in the lifetime of the universe,” added Harvard graduate student Josh Younger.

The hide-and-seek galaxies initially were spotted with the AzTEC imaging camera on the James Clerk Maxwell Telescope. The camera, developed by a team led by Grant Wilson and Min Yun of the University of Massachusetts at Amherst, discovered several hundred previously unseen galaxies that were bright at millimeter and submillimeter wavelengths.

A team of astronomers made follow-up observations of the seven brightest galaxies in an area of the sky studied by the Cosmic Evolution Survey (COSMOS). The Smithsonian’s Submillimeter Array pinpointed the exact location of each galaxy, allowing the team to confirm that the source was a single galaxy and not a blend of several fainter galaxies.

Once precise locations were known, additional observations were made with the Hubble Space Telescope, the Spitzer Space Telescope, and the Very Large Array of radio telescopes. Even Hubble’s powerful vision did not detect the galaxies, confirming that they are shrouded in dust that blocks visible light. Spitzer could penetrate the dust and detect the stars directly. The Very Large Array detected only the two closest galaxies.

By combining these measurements, the astronomers showed that five of the seven AzTEC galaxies are located at redshifts greater than 3, which corresponds to a distance of 12 billion light-years.

“These results suggest that the brightest submillimeter galaxies may be the most distant,” said Fazio.

The galaxies’ large infrared brightness indicates that they are forming new stars rapidly, probably due to collisions and mergers.

“The source of the infrared radiation seems to be very compact, which suggests that they are colliding galaxies that may eventually evolve into quasars,” said Younger.

In the future, the astronomers plan to image more sources of submillimeter radiation in different cosmic environments to better understand the population.

“We also plan to use the most extended configuration of the SMA to zoom in and try to resolve these objects, and really narrow down the source of their extreme infrared luminosity,” added Younger.

Star caught smoking

An R Coronae Borealis (R CrB) star is surrounded by a large “puff” of dust in this artist’s rendering, as inferred from the observations obtained with ESO’s Very Large Telescope. Such stars show erratic variability thought to arise from the presence of such dust clouds in their envelopes.
ESO
August 7, 2007
U sing ESO’s Very Large Telescope Interferometer, astronomers from France and Brazil have detected a huge cloud of dust around a star. This observation supports the theory that such stellar puffs are the cause of the repeated extreme dimming of the star.

R Coronae Borealis (R CrB) stars are supergiants exhibiting erratic variability. They are more than 50 times larger than our Sun. R CrB stars can see their apparent brightness unpredictably decline to one thousandth of their nominal value within a few weeks, with the return to normal light levels being much slower. It has been accepted for decades that such fading could be due to obscuration of the stellar surface by newly-formed dusty clouds.

This ‘Dust Puff Theory’ suggests that mass is lost from the R CrB star and then moves away until the temperature is low enough for carbon dust to form. If the newly-formed dust cloud is located along our line of sight, it eclipses the star. As the dust is blown away by the star’s strong light, the ‘curtain’ vanishes and the star reappears.

RY Sagittarii is the brightest member in the southern hemisphere of this family of weird stars. Located about 6,000 light-years away toward the constellation Sagittarius, its peculiar nature was discovered in 1895 by famous Dutch astronomer Jacobus Cornelius Kapteyn.

In 2004, near-infrared adaptive optics observations made with NACO on ESO’s Very Large Telescope allowed astronomers Patrick de Laverny and Djamel Mekarnia to clearly detect the presence of clouds around RY Sagittarii. This was the first direct confirmation of the standard scenario explaining the light variations of R CrB stars by the presence of heterogeneities in their envelope surrounding the star.

However, the precise place where such dust clouds would form was still unclear. The brightest cloud detected was several hundred stellar radii from the centre, but it had certainly formed much closer. But how much closer?

To probe the vicinity of the star, the astronomers then turned to ESO’s Very Large Telescope Interferometer. Combining two different pairs of the 8.2-meter Unit Telescopes, the astronomers explored the inner 110 astronomical units around the star. Given the remoteness of RY Sagittarii, this corresponds to looking at details on a one-euro coin about 75 kilometers away!

The astronomers found that a huge envelope, about 120 times as big as RY Sagittarii itself, surrounds the supergiant star. But more importantly, the astronomers also found evidence for a dusty cloud lying only about 30 astronomical units away from the star, or 100 times the radius of the star.

“This is the closest dusty cloud ever detected around a R CrB-type variable since our first direct detection in 2004,” says de Laverny, leader of the team. “However, it is still detected too far away from the star to distinguish between the different scenarios proposed within the Dust Puff Theory for the possible locations in which the dusty clouds form.”

If the cloud moves at the speed of 300 km/s, as one can conservatively assume, it was probably ejected more than 6 months before its discovery from deeper inside the envelope. The astronomers are now planning to monitor RY Sagittarii more carefully to shed more light on the evolution of the dusty clouds surrounding it.

“Two hundred years after the discovery of the variable nature of R CrB, many aspects of the R CrB phenomenon remain mysterious,” concludes de Laverny.

Thermo-camera sees first light

The Large Bolometer Camera (LABOCA) is installed on the APEX telescope at the 5,100-meter-high Chajnantor site in Chile. LABOCA is a thermometer camera with 295 detectors and an 11.4-arcminute field of view.
ESO
August 10, 2007
The world’s largest bolometer camera for submillimeter astronomy is now in service at the 12-meter Atacama Pathrinder Experiment (APEX) telescope, located on the 5,100-meter-high Chajnantor plateau in the Chilean Andes. The Large Bolometer Camera (LABOCA) was specifically designed for the study of extremely cold astronomical objects. With its large field of view and very high sensitivity, LABOCA will expand our knowledge of how stars form and how the first galaxies emerged from the Big Bang.

“A large fraction of all the gas in the universe has extremely cold temperatures of around minus 250° Celsius, a mere 20° above absolute zero,” says Karl Menten, director at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany. “Studying these cold clouds requires looking at the light they radiate in the submillimetre range, with very sophisticated detectors.”

Astronomers use bolometers for this task, which are essentially thermometers. They detect incoming radiation by registering the resulting rise in temperature. More specifically, a bolometer detector consists of an extremely thin foil that absorbs the incoming light. Any change of the radiation’s intensity results in a slight change in temperature of the foil, which can then be registered by sensitive electronic thermometers. To be able to measure such minute temperature fluctuations requires the bolometers to be cooled down to less than 0.3° above absolute zero (–272.85° Celsius).

“Cooling to such low temperatures requires using liquid helium, which is no simple feat for an observatory located at 5,100-meter altitude,” says Carlos De Breuck, the APEX instrument scientist at ESO.

Nor is it simple to measure the weak temperature radiation of astronomical objects. Millimeter and submillimeter radiation opens a window into the enigmatic, cold universe, but the signals from space are heavily absorbed by water vapor in Earth’s atmosphere. “It is a bit as if you were trying to see stars during the day,” explains Axel Weiss of the MPIfR and leader of the team that installed LABOCA on APEX.

This is why telescopes for this kind of astronomy must be built on high, dry sites, and why the 5,100-meter-high plateau at Chajnantor in the extremely dry Atacama Desert was chosen. Even under such optimal conditions, the heat from Earth’s atmosphere is still a hundred thousand times more intense than the tiny astronomical signals from distant galaxies. Very special software is required to filter such weak signals from the overwhelming disturbances.

LABOCA and its associated software were developed by MPIfR. “Since so far there are no commercial applications for such instruments we have to develop them ourselves,” explains Ernst Kreysa, head of the group that built the new instrument.

A bolometer camera combines many tiny bolometer units into a matrix, much like how pixels are combined in a digital camera. LABOCA observes at the wavelength of 0.87 millimeters, and consists of 295 channels arranged in 9 concentric hexagons around a central channel. The angular resolution is 18.6 arcseconds, and the total field of view is 11.4 arcmin, a remarkable size for instruments of this kind.

“The first astronomical observations with LABOCA have revealed its great potential. In particular, the large number of LABOCA’s detectors is an enormous improvement over earlier instruments,” says Giorgio Siringo, from MPIfR and member of the LABOCA team. “LABOCA is the first camera that will allow us to map large areas on the sky with high sensitivity.”

APEX where LABOCA is installed is a new-technology, 12-meter telescope, based on an ALMA prototype antenna, and operating at the ALMA site. It has modified optics and an improved antenna surface accuracy, and is designed to take advantage of the excellent sky transparency working with wavelengths in the 0.2 to 1.4 mm range.

“APEX is located a mere 2 kilometers from the center of the future ALMA array. The new LABOCA camera will be very complementary to ALMA, as its very wide view will find thousands of galaxies which will be observed in great detail with ALMA,” says De Breuck.

A new type of active galaxy

In the newly discovered type of AGN, the disk and torus surrounding the black hole are so deeply obscured by gas and dust that no visible light escapes, making them very difficult to detect. This illustration shows the scene from a more distant perspective.
Aurore Simonnet (Sonoma State University)
August 9, 2007
An international team of astronomers using NASA’s Swift satellite and the Japanese/United States Suzaku X-ray observatory has discovered a new class of active galactic nuclei (AGN).

By now, you’d think astronomers would have found all the different classes of AGN — extraordinarily energetic cores of galaxies powered by accreting supermassive black holes. AGN such as quasars, blazars, and Seyfert galaxies are among the most luminous objects in our Universe, often pouring out the energy of billions of stars from a region no larger than our solar system.

But by using Swift and Suzaku, the team has discovered that a relatively common class of AGN has escaped detection — until now. These objects are so heavily shrouded in gas and dust that virtually no light gets out.

“This is an important discovery because it will help us better understand why some supermassive black holes shine and others don’t,” says astronomer and team member Jack Tueller of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Evidence for this new type of AGN began surfacing over the past two years. Using Swift’s Burst Alert Telescope (BAT), a team led by Tueller has found several hundred relatively nearby AGNs that were previously missed because their visible and ultraviolet light was smothered by gas and dust. The BAT was able to detect high-energy X-rays from these heavily blanketed AGNs because, unlike visible light, high-energy X-rays can punch through thick gas and dust.

To follow up on this discovery, Yoshihiro Ueda of Kyoto University in Japan, Tueller, and a team of Japanese and American astronomers targeted two of these AGNs with Suzaku. They were hoping to determine whether these heavily obscured AGNs are basically the same type of objects as other AGN, or whether they are fundamentally different. The AGNs reside in the galaxies ESO 005-G004 and ESO 297-G018, which are about 80 million and 350 million light-years from Earth, respectively.

Suzaku covers a broader range of X-ray energies than BAT, so astronomers expected Suzaku to see X-rays across a wide swath of the X-ray spectrum. But despite Suzaku’s high sensitivity, it detected very few low- or medium-energy X-rays from these two AGN, which explains why previous X-ray AGN surveys missed them.

According to popular models, AGNs are surrounded by a donut-shaped ring of material, which partially obscures our view of the black hole. Our viewing angle with respect to the donut determines what type of object we see. But team member Richard Mushotzky, also at NASA Goddard, thinks these newly discovered AGN are completely surrounded by a shell of obscuring material. “We can see visible light from other types of AGN because there is scattered light,” says Mushotzky. “But in these two galaxies, all the light coming from the nucleus is totally blocked.”

Another possibility is that these AGN have little gas in their vicinity. In other AGN, the gas scatters light at other wavelengths, which makes the AGN visible even if they are shrouded in obscuring material.

“Our results imply that there must be a large number of yet unrecognized obscured AGNs in the local universe,” says Ueda.

In fact, these objects might comprise about 20 percent of point sources comprising the X-ray background, a glow of X-ray radiation that pervades our Universe. NASA’s Chandra X-ray Observatory has found that this background is actually produced by huge numbers of AGNs, but Chandra was unable to identify the nature of all the sources.

By missing this new class, previous AGN surveys were heavily biased, and thus gave an incomplete picture of how supermassive black holes and their host galaxies have evolved over cosmic history. “We think these black holes have played a crucial role in controlling the formation of galaxies, and they control the flow of matter into clusters,” says Tueller. “You can’t understand the universe without understanding giant black holes and what they’re doing. To complete our understanding we must have an unbiased sample.”

Astronomy magazine podcast: Touring Cygnus

Cygnus
Use this finder chart to locate objects in and near Cygnus. Remember to be patient and hold your binoculars steady.
Astronomy: Roen Kelly
August 9, 2007
Are you looking for a quick way to learn how to navigate the night sky? Let Senior Editor Michael Bakich narrate your ride through the sky. In this series, he will profile constellations one-by-one, telling you about bright — and not so bright — objects within their boundaries. He also will provide a few fun facts along the way.

In this show, Michael profiles Cygnus the Swan.

After you listen, e-mail us here and let us know what you think.

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Downloadable File(s)
Additional constellation observing tools from Astronomy magazine

  • Video: How to use a star chart — Senior Editor Rich Talcott helps you navigate the night sky using Astronomy magazine’s pullout star chart, located in the center of each monthly issue.
  • Constellation observing — Downloadable articles that highlight all 88 constellations in the sky and explain how to observe each constellation’s deep-sky targets.
  • The starry sky — Learn a few constellations and you’ll have a familiar friend every clear night.
  • Learn the constellations — Constellations can help you sort the twinkling dots scattered across the night sky. Connect the stars to see what deep-sky wonders emerge.