Overfed Black Holes Shut Down Galactic Star-Making

PASADENA, Calif. -- The Herschel Space Observatory has shown galaxies with the most powerful, active black holes at their cores produce fewer stars than galaxies with less active black holes. The results are the first to demonstrate black holes suppressed galactic star formation when the universe was less than half its current age. Credit: Illustration: NASA/ESA/JPL-Caltech/STScI/R. Hurt (SSC)

Published : May 9, 2012
By JPL

"We want to know how star formation and black hole activity are linked," said Mathew Page of University College London's Mullard Space Science Laboratory in the United Kingdom and lead author of a paper describing these findings in this week's journal Nature. "The two processes increase together up to a point, but the most energetic black holes appear to turn off star formation."

Supermassive black holes, weighing as much as millions of suns, are believed to reside in the hearts of all large galaxies. When gas falls upon these monsters, the material is accelerated and heated around the black hole, releasing great torrents of energy. Earlier in the history of the universe, these giant, luminous black holes, called active galactic nuclei, were often much brighter and more energetic. Star formation was also livelier back then.

Studies of nearby galaxies suggest active black holes can squash star formation. The revved-up, central black holes likely heat up and disperse the galactic reservoirs of cold gas needed to create new stars. These studies have only provided "snapshots" in time, however, leaving the overall relationship of active galactic nuclei and star formation unclear, especially over the cosmic history of galaxy formation.

"To understand how active galactic nuclei affect star formation over the history of the universe, we investigated a time when star formation was most vigorous, between eight and 12 billion years ago," said co-author James Bock, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and co-coordinator of the Herschel Multi-tiered Extragalactic Survey. "At that epoch, galaxies were forming stars 10 times more rapidly than they are today on average. Many of these galaxies are incredibly luminous, more than 1,000 times brighter than our Milky Way."

For the new study, Page and colleagues used Herschel data that probed 65 galaxies at wavelengths equivalent to the thickness of several sheets of office paper, a region of the light spectrum known as far-infrared. These wavelengths reveal the rate of star formation, because most of the energy released by developing stars heats surrounding dust, which then re-radiates starlight out in far-infrared wavelengths.

The researchers compared their infrared readings with X-rays streaming from the active central black holes in the survey's galaxies, measured by NASA's Chandra X-ray Observatory. At lower intensities, the black holes' brightness and star formation increased in sync. However, star formation dropped off in galaxies with the most energetic central black holes. Astronomers think inflows of gas fuel new stars and supermassive black holes. Feed a black hole too much, however, and it starts spewing radiation into the galaxy that prevents raw material from coalescing into new stars.

"Now that we see the relationship between active supermassive black holes and star formation, we want to know more about how this process works," said Bill Danchi, Herschel program scientist at NASA Headquarters in Washington. "Does star formation get disrupted from the beginning with the formation of the brightest galaxies of this type, or do all active black holes eventually shut off star formation, and energetic ones do this more quickly than less active ones?"

Herschel is a European Space Agency cornerstone mission, with science instruments provided by consortia of European institutes and important participation by NASA. NASA's Herschel Project Office is based at JPL.

NASA's WISE mission sees skies ablaze with blazars

Fig : This image taken by NASA's Wide-field Infrared Survey Explorer (WISE) shows a blazar — a voracious supermassive black hole inside a galaxy with a jet that happens to be pointed right toward Earth. These objects are rare and hard to find, but astronomers have discovered that they can use the WISE all-sky infrared images to uncover new ones. So far, researchers have found more than 200 new blazars, and they say WISE has the potential to find many more. Active black holes are often found at the hearts of elliptical galaxies. Not all black holes have jets, but when they do, the jets can be pointed in any direction. If a jet happens to shine at Earth, the object is called a blazar. 

Published By NASA/JPL
Published on : April 16, 2012

Astronomers are actively hunting a class of supermassive black holes throughout the universe called blazars thanks to data collected by NASA’s Wide-field Infrared Survey Explorer (WISE). The mission has revealed more than 200 blazars and has the potential to find thousands more. Blazars are among the most energetic objects in the universe. They consist of supermassive black holes actively “feeding,” or pulling matter onto them, at the cores of giant galaxies. As the matter is dragged toward the supermassive hole, some of the energy is released in the form of jets traveling at nearly the speed of light. Blazars are unique because their jets are pointed directly at us.

“Blazars are extremely rare because it’s not too often that a supermassive black hole’s jet happens to point towards Earth,” said Francesco Massaro of the Kavli Institute for Particle Astrophysics and Cosmology near Palo Alto, California, and principal investigator of the research. “We came up with a crazy idea to use WISE’s infrared observations, which are typically associated with lower-energy phenomena, to spot high-energy blazars, and it worked better than we hoped.”

The findings ultimately will help researchers understand the extreme physics behind super-fast jets and the evolution of supermassive black holes in the early universe. WISE surveyed the entire celestial sky in infrared light in 2010, creating a catalog of hundreds of millions of objects of all types. Its first batch of data was released to the larger astronomy community in April 2011, and the full-sky data were released last month.

Massaro and his team used the first batch of data, covering more than half of the sky, to test their idea that WISE could identify blazars. Astronomers often use infrared data to look for the weak heat signatures of cooler objects. Blazars are not cool; they are scorching hot and glow with the highest-energy type of light, called gamma rays. However, they also give off a specific infrared signature when particles in their jets are accelerated to almost the speed of light. One of the reasons the team wants to find new blazars is to help identify mysterious spots in the sky sizzling with high-energy gamma rays, many of which are suspected to be blazars. NASA’s Fermi mission has identified hundreds of these spots, but other telescopes are needed to narrow in on the source of the gamma rays. Sifting through the early WISE catalog, the astronomers looked for the infrared signatures of blazars at the locations of more than 300 gamma-ray sources that remain mysterious. The researchers were able to show that a little more than half of the sources are most likely blazars. “This is a significant step toward unveiling the mystery of the many bright gamma-ray sources that are still of unknown origin,” said Raffaele D’Abrusco, a co-author of the papers from the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “WISE’s infrared vision is actually helping us understand what’s happening in the gamma-ray sky.”

The team also used WISE images to identify more than 50 additional blazar candidates and observed more than 1,000 previously discovered blazars. According to Massaro, the new technique, when applied directly to WISE’s full-sky catalog, has the potential to uncover thousands more. “We had no idea when we were building WISE that it would turn out to yield a blazar gold mine,” said Peter Eisenhardt, WISE project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who is not associated with the new studies. “That’s the beauty of an all-sky survey. You can explore the nature of just about any phenomenon in the universe.”

Milky way may swarm with nomad planets

Figure : This image is an artistic rendition of a nomad object wandering the interstellar medium. The object is intentionally blurry to represent uncertainty about whether it has an atmosphere. A nomadic object may be an icy body akin to an object found in the outer solar system, a more rocky material akin to asteroids, or even a gas giant similar in composition to the most massive solar system planets and exoplanets.

By Stanford University
Published: February 24, 2012

Our galaxy may be awash in homeless planets, wandering through space instead of orbiting a star. In fact, there may be 100,000 times more nomad planets in the Milky Way than stars, according to a new study by researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) in Stanford, California.

If observations confirm the estimate, this new class of celestial objects will affect current theories of planet formation and could change our understanding of the origin and abundance of life.

“If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” said Louis Strigari from KIPAC. Although nomad planets don’t bask in the warmth of a star, they may generate heat through internal radioactive decay and tectonic activity.

Searches over the past two decades have identified more than 500 planets outside our solar system, almost all of which orbit stars. Last year, researchers detected about a dozen nomad planets, using a technique called gravitational microlensing, which looks for stars whose light is momentarily refocused by the gravity of passing planets.

The research produced evidence that roughly two nomads exist for every typical, main sequence star in our galaxy. The new study estimates that nomads may be up to 50,000 times more common than that.

To arrive at what Strigari called “an astronomical number,” the KIPAC team took into account the known gravitational pull of the Milky Way Galaxy, the amount of matter available to make such objects, and how that matter might divvy itself up into objects ranging from the size of Pluto to larger than Jupiter. Not an easy task, considering no one is quite sure how these bodies form. According to Strigari, some were probably ejected from solar systems, but research indicates that not all of them could have formed in that fashion.

“To paraphrase Dorothy from The Wizard of Oz, if correct, this extrapolation implies that we are not in Kansas anymore, and in fact we never were in Kansas,” said Alan Boss from the Carnegie Institution for Science in Washington, D.C. “The universe is riddled with unseen planetary-mass objects that we are just now able to detect.”

A good count, especially of the smaller objects, will have to wait for the next generation of big survey telescopes, especially the space-based Wide-Field Infrared Survey Telescope and the ground-based Large Synoptic Survey Telescope, both set to begin operation in the early 2020s.

A confirmation of the estimate could lend credence to another possibility mentioned in the paper — that as nomad planets roam their starry pastures, collisions could scatter their microbial flocks to seed life elsewhere.

“Few areas of science have excited as much popular and professional interest in recent times as the prevalence of life in the universe,” said Roger Blandford from KIPAC. “What is wonderful is that we can now start to address this question quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs.”

Latest Findings on Moon’s Impact History

Figure: Post-lunar cataclysm diagram of our solar system.
Credit: LPI/Marchi/Bottke/Kring/Morbidelli

Published : February 28, 2012
By : NASA's Ames Research Center in Moffett Field, California

During Earth’s earliest days, our planet and other bodies in the inner solar system, including the Moon, experienced repeated impacts from debris that formed the building blocks of the planets. Over time, as material was swept up and incorporated into the inner planets, the rate of impacts decreased. Then, roughly 4 billion years ago, a second wave of impacts appears to have taken place, with lunar projectiles hitting at much higher speeds. This increase could reflect the origin of the debris where main belt asteroids were dislodged and sent into the inner solar system by shifts in the orbits of the giant planets.

A team of researchers from the NASA Lunar Science Institute (NLSI) at NASA’s Ames Research Center in Moffett Field, California, has discovered that debris that caused a “lunar cataclysm” on the Moon 4 billion years ago struck it at much higher speeds than those that made the most ancient craters. The scientists found evidence supporting this scenario by examining the history of crater formation on the Moon.

Scientists analyzed digital maps of the lunar surface to learn about its history. Their analysis shows that craters formed near the 533-mile-diameter (860 kilometers) Nectaris impact basin were created by projectiles hitting twice as fast as those found on more ancient terrains. This was represented by a subtle shift in crater sizes, with the craters themselves 30 to 40 percent larger on average than those found in comparable populations with older craters. The scientists believe this can be best explained by an increase in the velocities of the projectiles that produced the younger craters.

The increase in velocities may indicate a change in the solar system when the craters were created. The analysis supports the lunar cataclysm hypothesis that the brief pulse of impacting objects 4 billion years ago was due to gravitational disturbances caused by the reorganization of the giant planets as their orbits changed. Nectaris, a crater close to the Apollo 16 landing site, appears to have recorded the spike in asteroid impacts during the lunar cataclysm.

Determining the magnitude and duration of any impact cataclysm and testing that hypothesis is a top science priority for future exploration of the Moon, according to a previously published report by the National Research Council.

When Apollo astronauts gathered rock samples from the Moon, many samples had ages dating back 3.9 to 4 billion years ago, suggesting an enhanced pulse of bombardment. If a bombardment of asteroids hit the Moon as theorized, there could be indicators left on the lunar surface that would help validate the theory. Detailed mapping by the United States Geological Survey has previously identified small regions of the lunar surface that might contain clues about the bombardment. The team re-studied those ancient surfaces and measured the sizes of the impact craters using new data obtained from the Lunar Orbiter Laser Altimeter, an instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) currently orbiting around the Moon.

“This is an exciting time for lunar research with LRO and other spacecraft providing so much new data,” said Simone Marchi from NLSI. “Collaborating with scientists of different disciplines allowed us to link these observational data to dynamical models to put new constraints on solar system history.”

The inferred increase in velocity seems to have occurred after the Moon’s 1,550-mile-diameter (2,500km) South Pole-Aitken Basin was produced, but before the formation of the largest lava-filled impact basins on the lunar nearside, visible from backyards around the world.

“It is fascinating that the surface of our own Moon records evidence of orbital changes in Jupiter and Saturn that took place so long ago,” said Yvonne Pendleton from NLSI.

New tools reveal astronomical mysteries

Artist's conception of dusty disk around young star TW Hydrae.
Credit: Bill Saxton, NRAO/AUI/NSF

By NRAO, Socorro, New Mexico

Date: February 17, 2012

Two new and powerful research tools are helping astronomers gain key insights needed to transform our understanding of important processes across the breadth of astrophysics. The Atacama Large Millimeter/submillimeter Array (ALMA), and the newly expanded Karl G. Jansky Very Large Array (VLA) offer scientists vastly improved and unprecedented capabilities for frontier research.

The cutting-edge research enabled by these powerful telescope systems extends from unlocking the mysteries of star- and planet-formation processes in the Milky Way and nearby galaxies to probing the emergence of the first stars and galaxies at the universe’s “cosmic dawn,” and along the way helping scientists figure out where Earth’s water came from.

A trio of scientists outlined recent accomplishments of ALMA and the Jansky VLA, both of which are in the “early science” phase of their development, as construction progresses toward their completion.

One exciting area where the two facilities are expected to unlock long-standing mysteries is the study of how new stars and planets form in our Milky Way Galaxy and in its nearby neighbors.

“These new ‘eyes’ will allow us to study, at unprecedented scales, the motion of gas and dust in the disks surrounding young stars, and put our theories of planet formation to the test,” said David Wilner from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. In addition, he added, the new telescopes will help show the first stages of planet formation — the growth of dust grains and pebbles in the disks — as well as show the gravitational interactions between the disks and new planets embedded within them.

“The power of ALMA and the expanded VLA also will allow us to study many more young stars and solar systems — probably thousands — than we could before. This will help us understand the processes that produce the huge diversity we already see in extrasolar planetary systems,” Wilner said.

One set of early ALMA observations of a disk around a young star nearly 170 light-years from Earth promises to shed light on a much closer question — the origin of Earth’s oceans. Scientists think much of our planet’s water came from comets bombarding the young Earth, but aren’t sure just how much.

The key clue has been the fact that our seawater contains a higher percentage of deuterium — a heavy isotope of hydrogen — than is found in the gas between stars in our galaxy. Scientists think this enrichment of deuterium is caused by low-temperature chemical reactions in the cold outer regions of the disk surrounding the young Sun — the region from which comets arise. The new ALMA observations, however, show that in a disk surrounding the young star TW Hydrae deuterium also is found in the warmer region closer to the star.

“With further studies like this, we are on the path to more precisely measuring the percentage of Earth’s ocean water that might have come from comets,” Wilner said.

Looking beyond the Milky Way, Christine Wilson from McMaster University in Ontario, Canada, points out that ALMA and the expanded VLA will give astronomers the ability to carefully study star formation in widely different types of galaxies, from the very faint to the extremely luminous and active ones.

“This will help us understand what regulates the rate at which stars form in galaxies,” Wilson said. One result from the VLA, however, seems to add to the mystery about this. John Cannon of Macalester College in St. Paul, Minnesota, and his colleagues studied a small star-forming galaxy and found that its mass is largely dark matter rather than the gas usually thought of as the fuel for star formation. “Their sample of small, but star-forming, galaxies has low amounts of gas, and this is puzzling,” Wilson said.

The two new telescopes also will help extend the study of galaxy evolution and star formation back to the universe’s youth, 10 or 12 billion years ago.

“The Jansky VLA and ALMA are ideally suited to reveal important new facts about very distant galaxies, which we see as they were when the universe was a fraction of its current age,” said Kartik Sheth of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia. “The new capabilities of these two facilities will show us the details of dust and gas in galaxies of this early epoch, thus helping us learn how such galaxies evolved into the types we see in the current universe.”

Already, Sheth said, both instruments have provided tantalizing glimpses of both atomic and molecular gas in galaxies as distant as 12 billion light-years.

“The huge range of ages in galaxies that we will be able to observe with these facilities represents a big step in piecing together the full history of how galaxies formed, evolved, and made stars over the vast span of cosmic time,” Sheth said.

“The early research results from ALMA and the Jansky VLA show the tremendous potential of these facilities for studies of galaxies and their history,” said Fred K. Y. Lo from the NRAO. “However, this is just one area of research in which these telescopes will make landmark contributions to our understanding of astronomical processes. ALMA and the Jansky VLA are leading tools for answering the most important questions of 21st-century astrophysics.”

Giant Eruption from Eta Carinae

These images reveal light from a massive stellar outburst in the Carina Nebula reflecting off dust clouds surrounding a behemoth double-star system. The color image at left shows the Carina Nebula, a star-forming region located 7,500 light-years from Earth. The massive double-star system Eta Carinae resides near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. The images were taken over an eight-year span by the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the CTIO.

By Carnegie Institution for Science, Washington, D.C.

Date: February 15, 2012

Eta Carinae, one of the most massive stars in our Milky Way Galaxy, unexpectedly increased in brightness in the 19th century. For 10 years in the mid-1800s, it was the second-brightest star in the sky — now it is not even in the top 100. The increase in luminosity was so great that it earned the rare title of Great Eruption. New research from a team, including Carnegie’s Jose Prieto, now at Princeton University in New Jersey, has used a “light echo” technique to demonstrate that this eruption was much different than previously thought.

Eta Carinae is a Luminous Blue Variable (LBV), meaning it has periods of dimness followed by periods of brightness. The variations in brightness of an LBV are caused by increased instability and loss of mass. The Great Eruption was an extreme and unique event in which the star, which is more than 100 times the mass of the Sun, lost several times the mass of our star. Scientists have believed that this rare type of eruption was caused by a stellar wind.

The team of scientists, led by Armin Rest of the Space Telescope Science Institute in Baltimore, Maryland, used images of Eta Carinae over eight years to study light echoes of the Great Eruption. For the first time, they observed light from the eruption that bounced, or echoed, off interstellar dust tens of light-years from the star. Those extra light-years mean that the light is reaching Earth now rather than in the 1800s when people on Earth observed the light that traveled here directly.

They then used the Magellan and du Pont telescopes at Las Campanas Observatories in Chile to obtain spectra of the echoes of light. The spectra allow them to precisely separate the light into its constituents, much like a drop of rain naturally acts as a prism and separates sunlight into the colors of the rainbow. These observations give important information about the chemical composition, temperature, and velocity of the material ejected during the 19th century Great Eruption.

Most surprisingly, their observations show that the Great Eruption is different from “supernova impostors,” events in nearby galaxies that are thought to be eruptions from LBVs. For example, the Great Eruption was significantly cooler than allowed by simple stellar-wind models used to explain supernova impostors.

“This star’s Giant Eruption has been considered a prototype for all supernova imposters in external galaxies,” Prieto said. “But this research indicates that it is actually a rather unique event.”

Scientists still don’t know what phenomenon caused Eta Carinae to erupt and lose such a quantity of mass without being destroyed. Further research is necessary to determine whether other proposed models could have triggered this activity instead.

Venus is spinning slower than before

Figure : Venus Express
Research done By ESA, Noordwijk, Netherlands

Published: February 10, 2012

The European Space Agency’s (ESA) Venus Express spacecraft has discovered that our cloud-covered neighbor spins a little slower than previously measured. Peering through the dense atmosphere in the infrared, the orbiter found surface features were not quite where they should be.

Using the VIRTIS instrument at infrared wavelengths to penetrate the thick cloud cover, scientists studied surface features and discovered that some were displaced by up to 12 miles (20 kilometers) from where they should be given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s.

These detailed measurements from orbit are helping scientists determine whether Venus has a solid or liquid core, which will help our understanding of the planet’s creation and how it evolved.

If Venus has a solid core, its mass must be more concentrated towards the center. In this case, the planet’s rotation would react less to external forces.

The most important of those forces is due to the dense atmosphere — more than 90 times the pressure of Earth’s, and high-speed weather systems, which are believed to change the planet’s rotation rate through friction with the surface.

Earth experiences a similar effect, where it is largely caused by wind and tides. The length of an Earth day can change by roughly a millisecond, and it depends seasonally upon wind patterns and temperatures over the course of a year.

In the 1980s and 1990s, the Venera and Magellan orbiters made radar maps of the surface of Venus, long shrouded in mystery as well as a dense, crushing, and poisonous atmosphere. These maps gave us our first detailed global view of this unique and hostile world.

Over its four-year mission, Magellan was able to watch features rotate under the spacecraft, allowing scientists to determine the length of the day on Venus as being equal to 243.0185 Earth days.

However, surface features seen by Venus Express some 16 years later could only be lined up with those observed by Magellan if the length of the Venus day is on average 6.5 minutes longer than Magellan measured.

This also agrees with the most recent long-duration radar measurements from Earth.

“When the two maps did not align, I first thought there was a mistake in my calculations as Magellan measured the value very accurately, but we have checked every possible error we could think of,” said Nils Müller from the DLR German Aerospace Center.

Scientists, including Özgur Karatekin from the Royal Observatory of Belgium, looked at the possibility of short-term random variations in the length of a Venus day, but concluded these should average themselves out over longer timescales.

On the other hand, other recent atmospheric models have shown that the planet could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotation period. Other effects could also be at work, including exchanges of angular momentum between Venus and Earth when the two planets are relatively close to each other.

“An accurate value for Venus’ rotation rate will help in planning future missions because precise information will be needed to select potential landing sites,” said Håkan Svedhem from ESA.

While further study is needed, it’s clear that Venus Express is penetrating far deeper into the mysteries of this enigmatic planet then anyone dreamed.

Latest computer model explains lakes and storms on Titan

Figure: Titan is covered in a thick atmosphere with abundant methane. Credit: NASA/JPL/Space Science Institute

By California Institute of Technology, Pasadena

Published: January 5, 2012

Saturn’s largest moon, Titan, is an intriguing alien world that’s covered in a thick atmosphere abundant with methane. With an average surface temperature of a brisk –300° Fahrenheit (–185° Celsius) and a diameter just less than half of Earth’s, Titan boasts methane clouds and fog as well as rainstorms and plentiful lakes of liquid methane. It’s the only place in the solar system, other than Earth, that has large bodies of liquid on its surface.

The origins of many of these features, however, remain puzzling to scientists. Now, researchers at the California Institute of Technology (Caltech) have developed a computer model of Titan’s atmosphere and methane cycle that, for the first time, explains many of these phenomena in a relatively simple and coherent way.

In particular, the new model explains three baffling observations of Titan. One oddity was discovered in 2009 when researchers found that Titan’s methane lakes tend to cluster around its poles, and noted that there are more lakes in the northern hemisphere than in the south.

Secondly, the areas at low latitudes near Titan’s equator are known to be dry, lacking lakes and regular precipitation. But when the Huygens probe landed on Titan in 2005, it saw channels carved out by flowing liquid, possibly runoff from rain. And in 2009, Caltech researchers discovered raging storms that may have brought rain to this supposedly dry region.

Finally, scientists uncovered a third mystery when they noticed that clouds observed over the past decade — during summer in Titan’s southern hemisphere — cluster around southern middle and high latitudes.

Scientists have proposed various ideas to explain these features, but their models either can’t account for all of the observations, or do so by requiring exotic processes such as cryogenic volcanoes that spew methane vapor to form clouds. The Caltech researchers say their new computer model, on the other hand, can explain all these observations and does so using relatively straightforward and fundamental principles of atmospheric circulation.

“We have a unified explanation for many of the observed features,” said Tapio Schneider from Caltech. “It doesn’t require cryovolcanoes or anything esoteric.”

Schneider said the team’s simulations were able to reproduce the distribution of clouds that’s been observed, which was not the case with previous models. The new model also produces the right distribution of lakes. Methane tends to collect in lakes around the poles because the sunlight there is weaker on average, he said. Energy from the Sun normally evaporates liquid methane on the surface, but since there’s generally less sunlight at the poles, it’s easier for liquid methane there to accumulate into lakes.

But then why are there more lakes in the northern hemisphere? Schneider points out that Saturn’s slightly elongated orbit means that Titan is farther from the Sun when it’s summer in the northern hemisphere. Kepler’s second law says that a planet orbits more slowly the farther it is from the Sun, which means that Titan spends more time at the far end of its elliptical orbit, when it’s summer in the north. As a result, the northern summer is longer than the southern summer. And since summer is the rainy season in Titan’s polar regions, the rainy season is longer in the north. Although the summer rains in the southern hemisphere are more intense — triggered by stronger sunlight because Titan is closer to the Sun during southern summer — there’s more rain over the course of a year in the north, filling more lakes.

In general, however, Titan’s weather is bland, and the regions near the equator are particularly dull, the researchers say. Years can go by without a drop of rain, leaving the lower latitudes of Titan parched. It was a surprise, then, when the Huygens probe saw evidence of rain runoff in the terrain. That surprise only increased in 2009 when Schaller, Brown, Schneider, and Henry Roe discovered storms in this same, supposedly rainless, area.

No one really understood how those storms arose, and previous models failed to generate anything more than a drizzle. But the new model was able to produce intense downpours during Titan’s vernal and autumnal equinoxes — enough liquid to carve out the type of channels that Huygens found. With the model, the researchers can now explain the storms. “It rains very rarely at low latitudes,” Schneider said. “But when it rains, it pours.”

The new model differs from previous ones in that it’s 3-D and simulates Titan’s atmosphere for 135 Titan years — equivalent to 3,000 years on Earth — so that it reaches a steady state. The model also couples the atmosphere to a methane reservoir on the surface, simulating how methane is transported throughout the moon.

The model successfully reproduces what scientists have already seen on Titan, but perhaps what’s most exciting, Schneider said, is that it also can predict what scientists will see in the next few years. For instance, based on the simulations, the researchers predict that the changing seasons will cause the lake levels in the north to rise over the next 15 years. They also predict that clouds will form around the north pole in the next two years. Making testable predictions is “a rare and beautiful opportunity in the planetary sciences,” Schneider said. “In a few years, we’ll know how right or wrong they are.”

“This is just the beginning,” he adds. “We now have a tool to do new science with, and there’s a lot we can do and will do.”

El Gordo : largest galaxy cluster in early universe

Figure: Composite image of the El Gordo galaxy cluster. An exceptional galaxy cluster, the largest seen in the distant universe, has been found using NASA’s Chandra X-ray Observatory and the Atacama Cosmology Telescope (ACT) in Chile. Credit: X-ray: NASA/CXC/Rutgers/J. Hughes et al; Optical: ESO/VLT & SOAR/Rutgers/F. Menanteau; IR: NASA/JPL/Rutgers/F. Menanteau

By Chandra X-ray Center, Cambridge, Massachusetts

Published: January 10, 2012

Officially known as ACT-CL J0102-4915, the galaxy cluster has been nicknamed “El Gordo” (“the big one” or “the fat one” in Spanish) by the researchers who discovered it. The name, in a nod to the Chilean connection, describes just one of the remarkable qualities of the cluster, which is located more than 7 billion light-years from Earth. This large distance means that it is being observed at a young age.

“This cluster is the most massive, the hottest, and gives off the most X-rays of any known cluster at this distance or beyond,” said Felipe Menanteau from Rutgers University in New Brunswick, New Jersey.

Galaxy clusters, the largest objects in the universe that are held together by gravity, form through the merger of smaller groups or sub-clusters of galaxies. Because the formation process depends on the amount of dark matter and dark energy in the universe, clusters can be used to study these mysterious phenomena.

Dark matter is material that can be inferred to exist through its gravitational effects, but it does not emit and absorb detectable amounts of light. Dark energy is a hypothetical form of energy that permeates all space and exerts a negative pressure that causes the universe to expand at an ever-increasing rate.

“Gigantic galaxy clusters like this are just what we were aiming to find,” said Jack Hughes, also from Rutgers. “We want to see if we understand how these extreme objects form using the best models of cosmology that are currently available.”

Although a cluster of El Gordo’s size and distance is extremely rare, it is likely that its formation can be understood in terms of the standard Big Bang model of cosmology. In this model, the universe is composed predominantly of dark matter and dark energy and began with the Big Bang about 13.7 billion years ago.

The team of scientists found El Gordo using ACT thanks to the Sunyaev-Zel’dovich effect. In this phenomenon, photons in the cosmic microwave background interact with electrons in the hot gas that pervades these enormous galaxy clusters. The photons acquire energy from this interaction, which distorts the signal from the microwave background in the direction of the clusters. The magnitude of this distortion depends on the density and temperature of the hot electrons and the physical size of the cluster.

X-ray data from Chandra and the European Southern Observatory’s Very Large Telescope, an 8-meter optical observatory in Chile, show that El Gordo is, in fact, the site of two galaxy clusters running into one another at several million miles per hour. This and other characteristics make El Gordo akin to the well-known object called the Bullet Cluster, which is located almost 4 billion light-years closer to Earth.

As with the Bullet Cluster, there is evidence that normal matter, mainly composed of hot, X-ray bright gas, has been wrenched apart from the dark matter in El Gordo. The hot gas in each cluster was slowed down by the collision, but the dark matter was not.

According to 'Felipe Menanteau' of Rutgers University who led the study says, "This cluster is the most massive, the hottest, and gives off the most X-rays of any known cluster at this distance or beyond."

Based on European Southern Observatory's Very Large Telescope and Chandra X-ray Observatory findings, El Gordo is composed of two separate galaxy subclusters , colliding at several million kilometers per hour. Their observation based on X-ray data and other characteristics suggests that, 'El Gordo' most probably formed like Bullet Cluster and make akin to the Bullet Cluster, located 4 billion light years to Earth. According to 'Cristobal Sifon' from Pontifical Catholic University of Chile says, "This is the first time we've found a system like the Bullet Cluster at such a large distance."

“This is the first time we’ve found a system like the Bullet Cluster at such a large distance,” said Cristobal Sifon from Pontificia Universidad de Catolica de Chile (PUC) in Santiago. “It’s like the expression says: If you want to understand where you’re going, you have to know where you’ve been.”

The Astrophysical Journal has accepted the results for publication and will announce its findings and results on 'El Gordo' at its 219th meeting in Austin of Texas.

Fermi shows that Tycho's star shines in gamma rays

Fig: Gamma rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines in the highest-energy form of light. This portrait of the shattered star includes gamma rays (magenta), X-rays (yellow, green, and blue), infrared (red) and optical data. Gamma ray, NASA/DOE/Fermi LAT Collaboration; X-ray, NASA/CXC/SAO; Infrared, NASA/JPL-Caltech; Optical, MPIA, Calar Alto, O. Krause et al. and DSS

By NASA's Goddard Space Flight Center, Greenbelt, Maryland

Published: December 16, 2011

In early November 1572, observers on Earth witnessed the appearance of a “new star” in the constellation Cassiopeia, an event now recognized as the brightest naked-eye supernova in more than 400 years. It’s often called “Tycho’s supernova” after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. Now, years of data collected by NASA’s Fermi Gamma-Ray Space Telescope reveal that the shattered star’s remains shine in high-energy gamma rays.

The detection gives astronomers another clue in understanding the origin of cosmic rays, subatomic particles — mainly protons — that move through space at nearly the speed of light. Exactly where and how these particles attain such incredible energies has been a long-standing mystery because charged particles speeding through the galaxy are easily deflected by interstellar magnetic fields. This makes it impossible to track cosmic rays back to their sources.

“Fortunately, high-energy gamma rays are produced when cosmic rays strike interstellar gas and starlight,” said Francesco Giordano from the University of Bari and the National Institute of Nuclear Physics in Italy. “These gamma rays come to Fermi straight from their sources.”

Better understanding the origins of cosmic rays is one of Fermi’s key goals. Its Large Area Telescope (LAT) scans the entire sky every three hours, gradually building up an ever-deeper view of the gamma-ray sky. Because gamma rays are the most energetic and penetrating form of light, they serve as signposts for the particle acceleration that gives rise to cosmic rays.

“This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays,” said Stefan Funk from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) in Stanford, California.

In 1949, physicist Enrico Fermi — the satellite’s namesake — suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of interstellar gas clouds. In the decades that followed, astronomers showed that supernova remnants might be the galaxy’s best candidate sites for this process.

When a star explodes, it is transformed into a supernova remnant, a rapidly expanding shell of hot gas bounded by the blast’s shock wave. Scientists expect that magnetic fields on either side of the shock front can trap particles between them in what amounts to a subatomic Pingpong game.

“A supernova remnant’s magnetic fields are very weak relative to Earth’s, but they extend across a vast region, ultimately spanning thousands of light-years,” said Melitta Naumann-Godo from Paris Diderot University and the Atomic Energy Commission in Saclay, France. “They have a major influence on the course of charged particles.”

As they shuttle back and forth across the supernova shock, the charged particles gain energy with each traverse. Eventually, they break out of their magnetic confinement, escaping the supernova remnant and freely roaming the galaxy.

The LAT’s ongoing sky survey provides additional evidence favoring this scenario. Many younger remnants, like Tycho’s, tend to produce more high-energy gamma rays than older remnants. “The gamma-ray energies reflect the energies of the accelerated particles that produce them, and we expect more cosmic rays to be accelerated to higher energies in younger objects because the shock waves and their tangled magnetic fields are stronger,” Funk said. By contrast, older remnants with weaker shock waves cannot retain the highest-energy particles, and the LAT does not detect gamma rays with corresponding energies.

The supernova of 1572 was one of the great watersheds in the history of astronomy. The star blazed forth at a time when the starry sky was regarded as a fixed and unchanging part of the universe. Tycho’s candid account of his own discovery of the strange star gives a sense of how radical an event it was.

The supernova first appeared around November 6, but poor weather kept it from Tycho until November 11, when he noticed it during a walk before dinner. “When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star. ... They immediately replied with one voice that they saw it completely and that it was extremely bright,” he said.

The supernova remained visible for 15 months and exhibited no movement in the heavens, indicating that it was located far beyond the Sun, Moon, and planets. Modern astronomers estimate that the remnant lies between 9,000 and 11,000 light-years away.

After more than 2.5 years of scanning the sky, LAT data clearly show that an unresolved region of GeV (billion electron volt) gamma-ray emission is associated with the remnant of Tycho’s supernova. (For comparison, the energy of visible light is between about 2 and 3 electron volts.)

“We knew that Tycho’s supernova remnant could be an important find for Fermi because this object has been so extensively studied in other parts of the electromagnetic spectrum,” said Keith Bechtol from SLAC. “We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons.” he said.

The science team’s model of the emission is based on LAT observations along with higher-energy TeV (trillion electron volt) gamma rays mapped by ground-based facilities and radio and X-ray data. The researchers conclude that a process called pion production best explains the emission. First, a proton traveling close to the speed of light strikes a slower-moving proton. This interaction creates an unstable particle — a pion — with only 14 percent of the proton’s mass. In just 10 millionths of a billionth of a second, the pion decays into a pair of gamma rays.

If this interpretation is correct, then somewhere within the remnant protons are being accelerated to near the speed of light, and then interacting with slower particles to produce gamma rays, the most extreme form of light. With such unbelievable goings-on in what’s left of his “unbelievable” star, it’s easy to imagine that Tycho Brahe himself might be pleased.