Fuel for the black hole

Figure 1: Artist's view of a dust torus surrounding the accretion disk and the central black hole in active galactic nuclei. Credit: NASA E/PO - Sonoma State University, Aurore Simonnet (http://epo.sonoma.edu/). (Click image for higher resolution).

First investigations of a galactic nucleus with the AMBER instrument of the Very Large Telescope Interferometer in Chile 

An international research team led by Gerd Weigelt from the Max-Planck-Institut für Radioastronomie in Bonn reports on high-resolution studies of an active galactic nucleus in the near-infrared. The observations were carried out with the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The use of near-infrared interferometry allowed the team to resolve a ring-shaped dust distribution (generally called "dust torus") in the inner region of the nucleus of the active galaxy NGC 3783. This dust torus probably represents the reservoir of gaseous and dusty material that "feeds" the hot gas disk ("accretion disk") and the supermassive black hole in the center of this galaxy. The resolved dust torus has an angular radius of only 0.7 milli-arcseconds on the sky, an angle that is 5 million times smaller than one degree. This angular radius corresponds to a radius of approximately 0.5 light years for a distance of 150 million light years. Studies of the physical properties of these dust tori are very important to improve our understanding of their structure and interaction with the accretion disk. To obtain these measurements, the light from up to three telescopes of the Very Large Telescope Interferometer was interferometrically combined. This method is able to achieve an angular resolution equivalent to the resolution of a telescope with a diameter of 130 Meters.

Extreme physical processes occur in the innermost regions of galactic nuclei. Supermassive black holes were discovered in many galaxies. The masses of these black holes are often a millionfold larger than the mass of our sun. These central black holes are surrounded by hot and bright gaseous disks, called "accretion disks". The emitted radiation from these accretion disks is probably generated by infalling material. To maintain the high luminosity of the accretion disk, fresh material has to be permanently supplied. The dust tori (see Fig. 1) surrounding the accretion disks are most likely the reservoir of the material that flows through the accretion disk and finally "feeds" the growing black hole.
Observations of these dust tori are very challenging since their sizes are very small. A giant telescope with a mirror diameter of more than 100 Meters would be able to provide the required angular resolution, but unfortunately telescopes of this size will not be available in the near future. This raises the question: Is there an alternative approach that provides the high resolution required?

The solution is to simultaneously combine ("interfere") the light from two or more telescopes since these multi-telescope images, which are called interferograms, contain high-resolution information. In the reported NGC 3783 observations, the AMBER interferometry instrument was used to combine the infrared light from two or three telescopes of ESO's Very Large Telescope Interferometer (VLTI, see Fig. 2). This interferometric method is able to achieve an extreme angular resolution that is proportional to the distance between the telescopes. Since the largest distance between the four telescopes of the VLTI is 130 Meters, an angular resolution is obtained that is as high as the theoretical resolution of a telescope with a mirror diameter of 130 Meters - a resolution that is 15 times higher than the resolution of one of the VLTI telescopes, which have a mirror diameter of 8 Meters.

"The ESO VLTI provides us with a unique opportunity to improve our understanding of active galactic nuclei,", says Gerd Weigelt from the Max-Planck-Institut für Radioastronomie in Bonn. "It allows us to study fascinating physical processes with unprecedented resolution over a wide range of infrared wavelengths. This is needed to derive physical properties of these sources."

And Makoto Kishimoto emphasizes: "We hope to obtain more detailed information in the next few years by additional observations at shorter wavelengths, with longer baselines, and with higher spectral resolution. Most importantly, in a few years, two further interferometric VLTI instruments will be available, which can provide complementary information."

To resolve the nucleus of the active galaxy NGC 3783, the research team recorded thousands of two- and three-telescope interferograms with the VLTI. The telescope distances were in the range of 45 to 114 Meters. The evaluation of these interferograms allowed the team to derive the radius of the compact dust torus in NGC 3783. A very small angular torus radius of 0.74 milli-arcsecond was measured, which corresponds to a radius of 0.52 light years. These near-infrared radius measurements, together with previously obtained mid-infrared measurements, allowed the team to derive important physical parameters of the torus of NGC 3783.

"The high resolution of the VLTI is also important for studying many other types of astrophysical key objects", underlines Karl-Heinz Hofmann. "It is clear that infrared interferometry will revolutionize infrared astronomy in a similar way as radio interferometry has revolutionized radio astronomy."

The research team comprises scientists from the Universities of Florence, Grenoble, Nice, Santa Barbara, and from the MPI für Radioastronomie.

Fig. 2: The Very Large Telescope Interferometer of the European Southern Observatory. Photo: Gerd Weigelt/MPIfR. (Click images for higher resolution).

Source: http://www.mpifr-bonn.mpg.de/public/pr/pr-ngc3783-may2012-en.html

Black holes turn up the heat for the Universe

A supermassive black hole is surrounded by a dust ring (torus). The collapse of gas onto the black hole launches an energetic jet of matter and radiation, which is transported over cosmological distances. A jet that is pointing into our direction is called a “blazar”Credit: ESA/NASA, the AVO project and Paolo Padovani.

Astrophysicists discover a new heating source in cosmological structure formation.

So far, astrophysicists thought that super-massive black holes can only influence their immediate surroundings. A collaboration of scientists at the Heidelberg Institute for Theoretical Studies (HITS) and in Canada and the US now discovered that diffuse gas in the universe can absorb luminous gamma-ray emission from black holes, heating it up strongly. This surprising result has important implications for the formation of structures in the universe. The results have been published today in “The Astrophysical Journal“ and „Monthly Notices of the Royal Astronomical Society”.

Every galaxy hosts a supermassive black hole at its center. Such black holes can emit high-energy gamma rays and are then called blazars. Whereas other radiation such as visible light and radio waves traverses the universe without problems, this is not the case for high-energy gamma rays. This particular radiation interacts with the optical light that is emitted by galaxies, transforming it into the elementary particles electrons and positrons. Initially, these elementary particles move almost at the speed of light. But as they are slowed down by the ambient diffuse gas, their energy is converted into heat, just like in other braking processes. As a result, the surrounding gas is heated efficiently. In fact, the temperature of the gas at mean density becomes ten times higher, and in underdense regions more than one hundred times higher than previously thought.

A Journey into the Cosmic Youth

“Blazars rewrite the thermal history of the universe”, emphasizes Dr. Christoph Pfrommer (HITS), one of the authors. But how can this idea be tested? In the optical spectra of quasars there is a plethora of lines, called the “line forest”. The forest originates from the absorption of ultra-violet light by neutral hydrogen in the young Universe. If the gas becomes hotter, weak lines in the forest are broadened. This effect represents an excellent opportunity to measure temperatures in the early Universe, while it was still growing up.

The astrophysicists at HITS checked this newly postulated heating process for the first time with detailed supercomputer simulations of the cosmological growth of structures. Surprisingly, the lines were broadened just enough so that their properties perfectly matched those of the observed lines. “This allows us to elegantly solve a long-standing problem with the quasar data”, says Dr. Ewald Puchwein, who conducted the large simulations on the supercomputer at HITS.

How Black Holes Influence the Formation of Galaxies

What are the further consequences of this new heating process? The forest of lines in the quasar spectra originates from density fluctuations in the Universe. In the course of cosmic evolution, the densest fluctuations collapse to form galaxies and galaxy clusters, as observed in the local Universe. Diffuse gas that is too hot cannot collapse. Hence, the formation of dwarf galaxies is slowed or even entirely suppressed. This could be the key to the solution of another long-standing problem in the theory of galaxy formation: why do we observe fewer dwarf galaxies in the vicinity of the Milky Way and in the underdense regions than predicted by cosmological simulations?

Prof. Volker Springel, scientific group leader at HITS, explains: “The process of blazar heating is especially exciting since this single effect is able to simultaneously solve several different puzzles in cosmological structure formation.” The group plans to further improve their simulation models for a still deeper understanding of the nature of blazar heating and its implications for today’s Universe.

Simulated line forest of a quasar spectrum. The blue spectrum represents a universe without blazar heating, the red one a universe with blazar heating. It is evident that the additional heating process ionizes neutral hydrogen, implying less absorption of the UV light emitted by the quasar. Credit: HITS

Source: Heidelberg Institute for Theoretical Studies (HITS)

Source: http://annesastronomynews.com/black-holes-turn-up-the-heat-for-the-universe/

Engravings of Female Genitalia May Be World's Oldest Cave Art

Eye of the beholder? An engraving of what might be female genitalia (inset) from France's Abri Castanet clocks in at 37,000 years, at least as old as the famous Chauvet Cave. Credit: Courtesy of Randall White

Since their discovery in 1994, the spectacular paintings of lions, rhinos, and other animals in southern France's Chauvet Cave have stood out as the oldest known cave art, clocking in at about 37,000 years old.^* But there have been occasional sightings of other cave art that is equally ancient, although its dating has been more uncertain. Now a team working at another site in the south of France claims to have discovered what appear to be engravings of female genitalia that are as old as or older than Chauvet, possibly making them the world's most ancient cave art.

Homo sapiens first colonized Europe from Africa around 40,000 years ago. But until the early 1990s, there was little firm evidence that our species engaged in sophisticated artistic activity that early. Many archaeologists assumed that modern humans developed their artistic skills only gradually, culminating in spectacular galleries like the 15,000-year-old painted caves at Lascaux in France and Altamira in Spain. The discovery of Chauvet changed all that and convinced most researchers that early artists had brought their skills with them from Africa.

Yet for years Chauvet seemed to stand alone, leading some archaeologists to question whether its dating—based in large part on radiocarbon samples taken directly from its charcoal paintings—was correct. Nevertheless, evidence for other art of about the same age continued to accumulate. At Fumane Cave in Italy, for example, archaeologists found depictions of animals and what appeared to be a half-human, half-beast figure, dated to about 37,000 years ago or even older, although the error ranges for the dates were fairly wide.

Since 1994, the year of Chauvet's discovery, a team led by archaeologist Randall White of New York University in New York City has been working at the Abri Castanet, a rock shelter (a shallow cave usually at the base of a cliff) in southern France's Vezere valley. Originally excavated in the early 20th century, the Abri Castanet has long been considered one of the earliest modern human sites in Europe, with occupation layers dated back to nearly 40,000 years ago. White's excavations have uncovered considerable evidence of symbolic and artistic activity at the site, including hundreds of pierced snail shells apparently used as ornaments and three limestone blocks adorned with engravings, including one the team interprets as a vulva. But the blocks, which came from the shelter's collapsed roof, were impossible to date because they do not contain the kind of organic matter necessary for radiocarbon analysis.

In 2007, however, the team began excavating another large block that had fallen from the roof and directly onto a segment of the cave floor once occupied by prehistoric humans. As White and his colleagues broke the stone slab into sections and lifted them out, they discovered that the underside had been engraved with another vulva-like image (see photo). When they sent the bones of reindeer and other animals from the cave floor to the University of Oxford's radiocarbon dating lab for analysis, the dates clustered tightly between 36,000 and 37,000 years ago. And because there was no accumulation of sediments or other deposits between the archaeological layer and the stone slab, the team argues that the painted cave ceiling must be at least as old as the bones.

That would mean that the artworks at Abri Castanet are also at least as old as those at Chauvet, White and co-workers conclude in a paper published online today in the Proceedings of the National Academy of Sciences. Because these images of vulvas are very different from the charcoal and ochre drawings at Chauvet, the team thinks that regional differences in artistic traditions were already established in Europe by that time, even at sites like Chauvet and Abri Castanet that are only a few hundred kilometers apart.

One key difference, White says, is that whereas the paintings at Chauvet are hidden deep within that cave and away from living areas, the depictions at Abri Castanet were on the rock shelter ceiling right above the spaces where prehistoric humans slept and ate, making them a kind of everyday and public art.

Harold Dibble, an archaeologist at the University of Pennsylvania, says the team's dating of the vulva engraving appears sound because it cannot be any younger than the surface onto which it fell and might even be older. "The context of the find is quite clear," Dibble says. As for the long-standing tradition among archaeologists working in France of interpreting such images as vulvas, Dibble says, "Who the hell knows" what they really represent? Dibble adds that such interpretations could be colored by the worldview of Western archaeologists whose culture probably differs greatly from that of prehistoric peoples. "Maybe it's telling us more about the people making those interpretations" than the artists who created the images, Dibble says. On the other hand, he says, the repeated use of this image at other sites in the Vezere valley suggests that it was some sort of "shared iconography" that might identify specific groups of people. Indeed, archaeologists have also identified differences in the styles of personal ornaments and other artifacts that might also reflect different groups or tribes, much as people express their group identities by the way they dress today.

Paul Pettit, an archaeologist at the University of Sheffield in the United Kingdom, agrees that the new work "provides admirable independent verification of the age of the Castanet rock art that has been suspected for decades." What's more, argues Pettit, a leader of a small but vocal group of archaeologists who have questioned the dating of the Chauvet paintings, the discovery at Abri Castanet helps make their case that the Chauvet art is too sophisticated to be 37,000 years old. "The only other examples of convincingly dated rock art in this period are the painted block from Fumane, which in terms of technical achievement is similar to the Castanet examples," he says. The reason there are so many stylistic differences between the spectacular Chauvet paintings and the relatively simple engravings at Abri Castanet, he insists, is that the Chauvet images are much younger.

Source: http://news.sciencemag.org/sciencenow/2012/05/engravings-of-female-genitalia.html?ref=em

The Solar System's Missing Boundary

Since the 1980's, space scientists have generally accepted that the bubble of gas and magnetic fields generated by the sun –the heliosphere – moves through space, creating three distinct boundary layers that culminate in an outermost bow shock. This shock is similar to the sonic boom created ahead of a supersonic jet. Earth itself certainly has one of these bow shocks on the sunward side of its magnetic environment, as do most other planets and many stars. A collection of new data from NASA's Interstellar Boundary Explorer (IBEX), however, now indicate that the sun does not have a bow shock, creating more of a “wake” as it travels through space. The image above taken by multiple telescopes shows bow shocks that exist around other astrospheres. 

The boundaries of the heliosphere have largely been assumed to be a series of three. The first is a fairly spherical boundary called the termination shock -- the point where the solar wind streaming from the sun slows down below supersonic speeds. From there the wind continues more slowly until it collides with the material in the rest of the galaxy and is pushed back, deflecting around the outskirts of the heliosphere, streaming back toward the tail of the moving bubble. This second boundary is called the heliopause. The third boundary was thought to be the bow shock, formed as the heliosphere plowed its way through the local galactic cloud the same way a supersonic jet pushes aside the air as it moves.

In a paper appearing online in Science Express on May 10, 2012, scientists compile data from IBEX, NASA's twin Voyager spacecraft, and computer models to show that the heliosphere just isn't moving fast enough to create a bow shock in the tenuous and highly magnetized region in our local part of the galaxy.

"IBEX gives a global view. It shows the whole of this region," says Eric Christian who is the mission scientist for IBEX at NASA's Goddard Space Flight Center in Greenbelt, Md. and who was formerly the program scientist for Voyager. "At the same time the Voyager spacecraft are actually there, in situ, measuring its environment at two locations. The combination of IBEX and Voyager gives you great science and now the new IBEX results strongly indicate that there is no bow shock."

The two Voyager spacecraft have confirmed the existence of the first boundary, and have seen evidence for the second as they move toward it. However, each Voyager spacecraft has seen different things on their respective trips – one moving in a more northerly direction, one moving more to the south. They've encountered different regions at different distances from the sun, suggesting the very shape of the heliosphere is squashed and asymmetrical. Scientists believe this asymmetry is caused by the force and direction of magnetic fields ramming into the heliosphere from outside, the same way a hand pushing on a balloon will force it out of shape.

This was the first clue that there's a strong magnetic field exerting pressure on the outskirts of the solar system. Independently, IBEX has seen a well-defined band, or ribbon, at the edge of the heliosphere, believed to be defined by this external magnetic field. Other studies from IBEX have helped quantify the magnitude of the magnetic field, showing that it is on the strong end of what was previously thought possible.

"We've seen one after another signature of a very strong magnetic field in the galactic environment," says Nathan Schwadron, a space scientist at the University of New Hampshire in Durham who is one of the authors on the paper. "That magnetic field influences the structure of the heliosphere and the boundaries themselves. That leads to a whole new paradigm."

Along with increased evidence for a strong external magnetic field, IBEX has also provided a new measurement for the speed of the heliosphere itself with respect to the local cloud.

"We recently analyzed two years worth of IBEX data, and they showed that the speed of the heliosphere – with respect to the local cloud of material – is only 52,000 miles per hour, instead of the previously believed 59,000," says David McComas at the Southwest Research Institute in San Antonio, Texas, who is first author on this paper and also the principal investigator for IBEX. "That might not seem like a huge difference, but it translates to a quarter less pressure exerted on the boundaries of the heliosphere. This means there's a very different interaction, a much weaker interaction, than previously thought."

The heliosphere's boundaries lie roughly 10 billion miles away from Earth, but are nonetheless crucial for understanding our place in the universe. The heliopause provides some protection for our solar system from the harsh, radiation environment surrounding it. By knowing the nature of these boundaries, scientists can start to better understand the propagation of particles that do have enough energy and speed to make it into our environment.

"Imagine the point at which Voyager crosses the threshold of the heliopause and either does or does not see what IBEX is predicting," says Schwadron. "There will be enormous opportunities for scientific advancement."

Source: http://www.dailygalaxy.com/my_weblog/2012/05/the-solar-systems-missing-boundary.html

Is There a 'Fifth Force' that Alters Gravity at Cosmic Scales?

Radical new research is attempting to characterize the properties of a fifth force that disrupts the predictions general relativity makes outside our own galaxy, on cosmic-length scales. University of Pennsylvania astrophysicist Bhuvnesh Jain, says the nature of gravity is the question of a lifetime. As scientists have been able to see farther and deeper into the universe, the laws of gravity have been revealed to be under the influence of an unexplained force.

By innovatively analyzing a well-studied class of stars in nearby galaxies, Jain and his colleagues — Vinu Vikram, Anna Cabre and Joseph Clampitt at Penn and Jeremy Sakstein at the University of Cambridge — have produced new findings that narrow down the possibilities of what this force could be. Their findings, published on the Arxiv (see below), are a vindication of Einstein’s theory of gravity.

Having survived a century of tests in the solar system, it has passed this new test in galaxies beyond our own as well.In 1998, astrophysicists made an observation that turned gravity on its ear: the universe’s rate of expansion is speeding up. If gravity acts the same everywhere, stars and galaxies propelled outward by the Big Bang should continuously slow down, like objects thrown from an explosion do here on Earth.

This observation used distant supernovae to show that the expansion of the universe was speeding up rather than slowing down. This indicated that something was missing from physicists’ understanding of how the universe responds to gravity, which is described by Einstein’s theory of general relativity.

Two branches of theories have sprung up, each trying to fill its gaps in a different way. One branch — dark energy — suggests that the vacuum of space has an energy associated with it and that energy causes the observed acceleration. The other falls under the umbrella of “scalar-tensor” gravity theories, which effectively posits a fifth force (beyond gravity, electromagnetism and the strong and weak nuclear forces) that alters gravity on cosmologically large scales.

“These two possibilities are both radical in their own way,” Jain said. “One is saying that general relativity is correct, but we have this strange new form of energy. The other is saying we don't have a new form of energy, but gravity is not described by general relativity everywhere.”

Jain’s research is focused on the latter possibility; he is attempting to characterize the properties of this fifth force that disrupts the predictions general relativity makes outside our own galaxy, on cosmic length scales.

Jain’s recent breakthrough came about when he and his colleagues realized they could use the troves of data on a special property of a common type of star as an exquisite test of gravity.

Astrophysicists have been pursuing tests of gravity in the cosmos for many years, but conventional tests require data on millions of galaxies. Future observations are expected to provide such enormous datasets in the coming data. But Jain and his colleagues were able to bypass the conventional approach.

“We’ve been able to perform a powerful test using just 25 nearby galaxies that is more than a hundred times more stringent than standard cosmological tests,” Jain said.The nearby galaxies are important because they contain stars called cepheids that are bright enough to be seen individually. Moreover, cepheids have been used for decades as a kind of interstellar yardstick because their brightness oscillates in a precise and predictable way.

“You can measure the brightness of a light bulb at some distance and know that, if you move it twice as far, it will be four times as faint. So you can tell just by the difference in its observed brightness how much further you moved it,” Jain said. “But you need to know how intrinsically bright the bulb is first to determine its actual distance from us.”

Cepheids have a unique trait that allows astrophysicists to get this critical information: their luminosity oscillates over the course of days and weeks. The known relationship between a cepheid’s rate of oscillation and intrinsic brightness serves as that baseline for calculating its distance from Earth, which in turn serves as a baseline for calculating the distance of other celestial objects.

The accelerating universe observation, for example, relied upon cepheid data for scale.“Now that we understand a little bit more about what makes the cepheids pulsate — a balance of gravity and pressure — we can use them to learn about gravity, not just distance,” Jain said. “If the fifth force enhances gravity even a little bit, it will make them pulsate faster.”

Because of their usefulness, there was already more than a decade of data on cepheids based on the Hubble Space Telescope and other large telescopes in Chile and Hawaii. Using that data, Jain and his colleagues compared nearly a thousand stars in 25 galaxies. This allowed them to make comparisons between galaxies that are theoretically “screened” or protected from the effects of the hypothetical fifth force and those that are not. Larger galaxies and ones that belong to galaxy clusters are screened, while smaller, isolated galaxies are not.

“If we compare galaxies that don't permit this extra force, like our own galaxy, with others that do, then we should see a difference in the way those galaxies’ cepheids behave,” Jain said. “Because this new force would increase the speed of their oscillations and because we can use the rate of their oscillations to their measure distance from us, the measurement we get from cepheids in unscreened galaxies should be smaller than distance measurements made with different techniques.”

Jain and his colleagues ultimately did not see variation between their control sample of screened galaxies and their test sample of unscreened ones. Their results line up exactly with the prediction of Einstein’s general relativity. This means that the potential range and strength of the fifth force is severely constrained.

“We find consistency with Einstein’s theory of gravity and we sharply narrow the space available to these other theories. Many of these theories are now ruled out by the data,” Jain said.With better data on nearby galaxies in the coming years, Jain expects that an entire class of gravity theories could essentially be eliminated. But there remains the exciting possibility that better data may reveal small deviations from Einstein’s gravity, one of the most famous scientific theories of all time.

The image at the top of the page shows the most distant X-ray cluster of galaxies yet found by astronomers using NASA's Chandra X-ray Observatory. Approximately 10 billion light years from Earth, the cluster 3C294 is 40 percent farther than the next most distant X-ray galaxy cluster previously known. The existence of such a distant galaxy cluster is important for understanding how the Universe evolved.

Astrophysical tests of_modified_gravity

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Source: http://www.dailygalaxy.com/my_weblog/2012/05/is-there-a-fifth-force-that-alters-gravity-at-cosmos-scales-.html