Posted on by Fraser Cain

Many Galaxies Found in the Early Universe

13 distant galaxies found in a sample of sky. Image credit: ESO. Click to enlarge.
It is one of the major goals of observational cosmology to trace the way galaxies formed and evolved and to compare it to predictions from theoretical models. It is therefore essential to know as precisely as possible how many galaxies were present in the Universe at different epochs.

This is easier to say than to do. Indeed, if counting galaxies from deep astronomical images is relatively straightforward, measuring their distance – hence, the epoch in the history of the universe where we see it [1] – is much more difficult. This requires taking a spectrum of the galaxy and measuring its redshift [2].

However, for the faintest galaxies – that are most likely the farthest and hence the oldest – this requires a lot of observing time on the largest of the telescopes. Until now, astronomers had thus to first carefully select the candidate high-redshift galaxies, in order to minimise the time spent on measuring the distance. But it seems that astronomers were too careful in doing so, and hence had a wrong picture of the population of galaxies.

It would be better to “simply” observe in a given patch of the sky all galaxies brighter than a given limit. But looking at one object at a time would make such a study impossible.

To take up the challenge, a team of French and Italian astronomers [3] used the largest possible telescope with a highly specialised, very sensitive instrument that is able to observe a very large number of (faint) objects in the remote universe simultaneously.

The astronomers made use of the VIsible Multi-Object Spectrograph (VIMOS) on Melipal, one of the 8.2-m telescopes of ESO’s Very Large Telescope Array. VIMOS can observe the spectra of about 1,000 galaxies in one exposure, from which redshifts, hence distances, can be measured. The possibility to observe two galaxies at once would be equivalent to using two VLT Unit Telescopes simultaneously. VIMOS thus effectively multiplies the efficiency of the VLT hundreds of times.

This makes it possible to complete in a few hours observations that would have taken months only a few years ago. With capabilities up to ten times more productive than competing instruments, VIMOS offers the possibility for the first time to conduct an unbiased census of the distant Universe.

Using the high efficiency of the VIMOS instrument, the team of astronomers embarked in the VIMOS VLT Deep Survey (VVDS) whose aim is to measure in some selected patch of the sky the redshift of all galaxies brighter than magnitude 24 in the red, that is, galaxies that are up to 16 million fainter than what the unaided eye can see.

In a total sample of about 8,000 galaxies selected only on the basis of their observed brightness in red light, almost 1,000 bright and vigorously star forming galaxies were discovered at an epoch 1,500 to 4,500 million years after the Big Bang (redshift between 1.4 and 5).

“To our surprise”, says Olivier Le F?vre, from the Laboratoire d’Astrophysique de Marseille (France) and co-leader of the VVDS project, “this is two to six times higher than had been found by previous works. These galaxies had been missed because previous surveys had selected objects in a much more restrictive manner than we did. And they did so to accommodate the much lower efficiency of the previous generation of instruments.”

While observations and models have consistently indicated that the Universe had not yet formed many stars in the first billion years of cosmic time, the discovery made by the scientists calls for a significant change in this picture.

Combining the spectra of all the galaxies in a given redshift range (i.e. belonging to the same epoch), the astronomers could estimate the amount of star formed in these galaxies. They find that the galaxies in the young Universe transform into stars between 10 and 100 times the mass of our Sun in a year.

“This discovery implies that galaxies formed many more stars early in the life of the Universe than had previously been thought”, explains Gianpaolo Vettolani, the other co-leader of the VVDS project, working at INAF-IRA in Bologna (Italy). “These observations will demand a profound reassessment of our theories of the formation and evolution of galaxies in a changing Universe.”

It now remains for astronomers to explain how one can create such a large population of galaxies, producing more stars than previously assumed, at a time when the Universe was about 10-20% of its current age.

Original Source: ESO News Release

Posted on by Fraser Cain

Chandra View of Tycho’s Remnant

Tycho’s Supernova as viewed by the Chandra X-Ray Observatory. Image credit: NASA. Click to enlarge.
In 1572, the Danish astronomer Tycho Brahe observed and studied the explosion of a star that became known as Tycho’s supernova. More than four centuries later, Chandra’s image of the supernova remnant shows an expanding bubble of multimillion degree debris (green and red) inside a more rapidly moving shell of extremely high energy electrons (filamentary blue).

The supersonic expansion (about six million miles per hour) of the stellar debris has created two X-ray emitting shock waves – one moving outward into the interstellar gas, and another moving back into the debris. These shock waves produce sudden, large changes in pressure and temperature, like an extreme version of sonic booms produced by the supersonic motion of airplanes.

According to the standard theory, the outward-moving shock wave should be about 2 light years ahead of the stellar debris. What Chandra found instead is that the stellar debris has kept pace with the outer shock and is only about half a light year behind.

The most likely explanation for this behavior is that a large fraction of the energy of the outward-moving shock wave is going into the acceleration of atomic nuclei to speeds approaching the speed of light. The Chandra observations provide the strongest evidence yet that nuclei are indeed accelerated and that the energy contained in the high-speed nuclei in Tycho’s remnant is about 100 times that observed in high-speed electrons.

This finding is important for understanding the origin of cosmic rays, the high-energy nuclei which pervade the Galaxy and constantly bombard the Earth. Since their discovery in the early years of the 20th century, many sources of cosmic rays have been proposed, including flares on the sun and similar events on other stars, pulsars, black hole accretion disks, and the prime suspect – supernova shock waves. Chandra’s observations of Tycho’s supernova remnant strengthen the case for this explanation.

Original Source: Chandra News Release

Posted on by Fraser Cain

Finding the First Stars

Computer illustration of what the Universe’s first stars looked like. Image credit: CfA. Click to enlarge.
What did the very first stars look like? How did they live and die? Astronomers have ideas, but no proof. The first stars are so distant and formed so long ago that they are invisible to our best telescopes.

Until they explode. Hypernovas (more powerful cousins of supernovas) and their associated gamma-ray bursts offer astronomers the possibility of detecting light from the first generations of stars.

NASA’s Swift satellite already has seen a gamma-ray burst (GRB) with a redshift of 6.29, meaning that the progenitor star exploded about 13 billion years ago, when the universe was less than a billion years old. Theorists Volker Bromm (University of Texas at Austin) and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) predict that one-tenth of the blasts Swift will spot during its operational lifetime will come from stars at a redshift of 5 or greater, that lived and died during the first billion years of the universe.

“Most of those GRBs will come from second generation or later stars,” said Loeb. “But if we get lucky, Swift may even detect a burst from one of the very first stars that formed — a star made of only hydrogen and helium.”

Calculations suggest that such stars, which are called Population III for historical reasons, would have been behemoths weighing 50-500 times as much as the Sun. A Population III star would have gulped its nuclear fuel faster than an SUV, dying quickly and explosively.

“Our best guess right now is that the recent GRB was not from a Pop III star. However, its redshift is high enough to make it very interesting,” said Bromm.

One key question examined by Bromm and Loeb is whether a Pop III star could have generated a GRB — a blast powerful enough to be seen from a distance of more than 13 billion light-years.

The answer they derived is a qualified yes. Pop III stars were massive enough to explode violently, leaving behind a black hole in most cases. However, a Pop III star likely would have to be part of a tight binary system to generate a GRB.

A close binary companion could strip the outer layers of a dying Pop III star, leaving less material to block the star’s explosive death throes. Jets of material generated from the newborn black hole therefore could punch their way out more easily, creating a burst of gamma-ray energy detectable across the universe.

About half of all nearby stars are members of binary or multiple star systems. The frequency of binaries, particularly close binaries, among Pop III stars remains unknown.

“Astronomers will address this question of the Pop III binary frequency using a dual approach, both observational and theoretical,” said Bromm. “By searching for high-redshift GRBs, we can constrain that number empirically. We also will try to improve simulations and make them detailed enough to model those details of star formation.”

If binary star systems are common among Pop III stars, then high-redshift GRBs could offer astronomers an ideal opportunity to study the first generation of stars.

“If Pop III binaries are common, Swift will be the first observatory to probe Population III star formation at high redshifts,” said Loeb.

This research has been submitted for publication to The Astrophysical Journal and is available online at http://arxiv.org/abs/astro-ph/0509303.

Original Source: CfA News Release

Posted on by Fraser Cain

Mars Express Mission Extended

Artist illustration of Mars Express. Image credit: ESA. Click to enlarge.
ESA?s Mars Express mission has been extended by one Martian year, or about 23 months, from the beginning of December 2005.

The decision, taken on 19 September by ESA?s Science Programme Committee, allows the spacecraft orbiting the Red Planet to continue building on the legacy of its own scientific success.

Co-ordinated from the beginning with the Mars science and exploration activities of other agencies, Mars Express has revealed an increasingly complex picture of Mars.

Since the start of science operations in early 2004, new aspects of Mars are emerging day by day, thanks to Mars Express data. These include its present-day climate system, and its geological ?activity? and diversity. Mars Express has also started mapping water in its various states.

In building up a global data set for composition and characteristics of the surface and atmosphere, Mars Express has revealed that volcanic and glacial processes are much more recent than expected.

It has confirmed the presence of glacial processes in the equatorial regions, and mapped water and carbon dioxide ice, either mixed or distinct, in the polar regions. Through mineralogical analysis, it found out that large bodies of water, such as lakes or seas, might not have existed for a long period of time on the Martian surface.

Mars Express has also detected methane in the Martian atmosphere. This, together with the possible detection of formaldehyde, suggests either current volcanic activity on Mars, or, more excitingly, that there are current active ?biological? processes.

This hypothesis may be reinforced by the fact that Mars Express saw that the distribution of water vapour and methane, both ingredients for life, substantially overlap in some regions of the planet.

Furthermore, the mission detected aurorae for the first time on the Red Planet. It has made global mapping of the density and pressure of the atmosphere between 10 and 100 kilometres altitude, and studied atmospheric escape processes in the upper layers of the atmosphere. This is contributing to our understanding of the weather and climate evolution of the planet.

There is still much to be discovered by the extraordinary set of instruments on board Mars Express. First, the 23-month extension will enable the Mars Express radar, MARSIS, to restart Martian night-time measurements in December this year.

MARSIS will continue its subsurface studies mainly in the search for liquid and frozen water. By combining subsurface, surface and atmospheric data, Mars Express will provide an unprecedented global picture of Mars and, in particular, its water.

So far, the High Resolution Stereo Camera has imaged only 19% of the Martian surface at high resolution. In the extended phase, it will be able to continue the 3D high-resolution colour imaging. After the Viking missions, Mars Express is building today?s legacy of Mars imagery for present and future generations of scientists.

Thanks to the extension, Mars Express will also be able to study for a second year the way the atmosphere varies during different seasons, and to observe again variable phenomena such as frost, fog or ice.

Finally, Mars Express will be able to revisit those areas where major discoveries, such as new volcanic structures, sedimentary layering, methane sources, nightglow and auroras, have been made, thus allowing to confirm and understand all aspects related to these discoveries.

Original Source: ESA News Release

Posted on by Fraser Cain

Sweeping View of the Rings

Sweeping view of Saturn’s rings. Image credit: NASA/JPL/SSI. Click to enlarge.
A grandiose gesture of gravity, Saturn’s icy rings fan out across many thousands of kilometers of space. The moon Pan (26 kilometers, or 16 miles across) dutifully follows its path, like the billions and billions of particles comprising the rings. The little moon is seen at the center of this view, within the Encke gap.

The famous Cassini Division spans upper left corner of the scene. The Cassini Division is approximately 4,800-kilometers-wide (2,980 miles) and is visible in small telescopes from Earth.

The narrow, knotted F ring is thinly visible just beyond the main rings.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 20, 2005, at a distance of approximately 2.1 million kilometers (1.3 million miles) from Saturn. The image scale on Pan is 13 kilometers (8 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Mark Mortimer

Magnificent Desolation: Walking on the Moon 3D

Twelve people have had the experience of visiting our Moon’s surface. Probes and sensors tried to fill in our knowledge before landing but there’s nothing like first hand observation. Pockmarked with craters, ripped by rills and coated in dust, the gray, still surface stretches onto the horizon. Nothing marks distances, no clues to perception tell you where you are. Valleys may be a gentle slope down a few steps or a brusque drop down a one kilometre precipice. Without trees, bushes or human markers, you feel lost in an ever changing yet hauntingly, similar realm. People’s senses, tuned to the pace of life on Earth, got shocked by this vista but nevertheless were sufficiently able to perform on the Moon.

People and our Moon are the ingredients for this film. Join these two together and we get a taste of philosophy and a bucket full of technology. The beginning features children in a sparse film set responding to history questions. Who were the Apollo astronauts? What did they eat? Would you like to go to the Moon? These all obviously lead to the point of the film: that exploration is in our blood and we need to keep satisfying our craving. The innocense, lack of knowledge and desire of these children mirrors that of the unproven astronauts in the mid 60s.

With this basis, the film jumps into the good stuff. Using the full expanse of the IMAX screen combined with superb audio, the Moon arrives. Step by step, from landing the lunar module, opening the hatch and placing the foot onto the dusty plain, we have the impact of the desolation spreading about. All is still, quiet, transcendent, then a foot falls nearby and a shower of rocks and dust shoot towards us. Ducking, we smile and remember we’re just in a theatre. Interspersed with realistic but artificial events are vignettes of actual Apollo footage. The astronauts do strange shuffles to cross craters or jump with both feet high in the air, experiencing the delicious freedom of the lesser gravity. The film smoothly interweaves these with computer generations to deliver a thorough lunar presence.

The authenticity of the film’s computer generation appears both lifelike and technically accurate. Perhaps this is in part from the sponsorship of Lockheed Martin, a very knowledgeable partner. Renditions of the lunar module have creases in the right locations, antennas pop out appropriately and even dust and smear markings age the vehicle where it stands. As another example, the separation of the ascent stage from the descent stage stresses the technical imagination as no one saw this event from the vantage point shown in the film. Also, the audio wizards were busily at work as well. Actual capcom flow gets precisely overlaid onto computer generated images to increase the feel of authenticity.

Given this technical wonder, you might be worried about too dry a film. It isn’t. Comedy is present as live footage shows astronauts tripping, falling and playing golf. Animation puts us on the rear spoiler while the electric lunar buggy zooms between boulders, through craters all the while bouncing over exotic rough terrain. A little suspense highlights the challenges. Our animated astronauts have a vehicular accident from which to extricate themselves. Though only computer generated, there remains the overlying message, ‘what if this were to really happen?’.

Yet, this is the moon one more time. Some of us luckily enough saw the live broadcasts. Many subsequent films deliver the message of exploration and accomplishment. This is not new. The newness is the huge IMAX screen and the computer generated imagery. Don’t wait to watch this at home! This needs the big screen and the comical but vital 3D glasses. Experience the dust getting kicked into other face, feel the force of the rocket roaring overhead, lean with the car while zipping through a corner. The medium is what brings this alive again.

Adding to the my experience was one special audience member. Dr. Buzz Aldrin attended the special viewing for participants of the International Lunar Conference being held in Toronto. He generously offered a short question and answer session before and after the screening. Sharing of his memories of walking on the Moon and his thoughts for today, dovetailing with NASA’s recent announcement on returning to the moon, amplify the timely and authentic message of this film.

Our Moon spans our history, our culture and our world view. We need it as a stepping stone to greater things. Though indeed desolate, it has its own beauty and special nature that fascinates and attracts us. The film Magnificent Desolation by Tom Hanks, Gary Goetzman and Mark Cowen puts our Moon before our eyes and under our toes. Take the easy journey to watch this film and prepare for our much more difficult real life journey to return to the Moon.

Review by Mark Mortimer

Magnificent Desolation starts on September 23 at IMAX theatres around the world.

Posted on by Fraser Cain

NASA Wants Rovers That Can Dig Lunar Soil

Artist illustration of future astronauts on the Moon. Image credit: NASA. Click to enlarge.
NASA today announced the Regolith Excavation Challenge, a new Centennial Challenges prize competition that will award $250,000 to the winning team and has the potential to significantly contribute to the nation’s space exploration goals. The competition is in collaboration with the California Space Education and Workforce Institute (CSEWI).

The Regolith Excavation Challenge will award the prize money to the team that can design and build autonomously operating systems to excavate lunar regolith, or “moon dirt,” and deliver it to a collector.

The challenge will be conducted in a “head-to-head” competition format in late 2006 or early 2007 and will require teams to excavate and deliver as much regolith as possible in 30 minutes. A detailed set of rules for the competition will be finalized later this year.

“Excavation of lunar regolith is an important and necessary step toward using the resources on the moon to establish a successful base for life on its surface,” said NASA’s acting Associate Administrator for the Exploration Systems Mission Directorate, Douglas R. Cooke. “The unique physical properties of the lunar regolith make excavation a difficult technical challenge,” he added.

“This challenge continues NASA’s efforts to broaden interest in innovative concepts,” said Brant Sponberg, NASA’s Centennial Challenges program manager. “We hope to see teams from a broad spectrum of technical areas take part in this competition,” he noted.

“CSEWI is pleased to collaborate with NASA and to participate with the Centennial Challenges Regolith Excavation Prize Competition,” said CSEWI Director, the Honorable Andrea Seastrand. “This is a challenge that places all companies, institutions and individuals on a level playing field, thereby widening the doors of opportunity for technology innovators. While welcoming entities with existing NASA relationships, this challenge stimulates and reaches out to the nation’s untapped intellectual capital,” she added.

NASA’s Centennial Challenges program promotes technical innovation through a novel program of prize competitions. It is designed to tap the nation’s ingenuity to make revolutionary advances to support the Vision for Space Exploration and NASA goals. NASA’s Exploration Systems Mission Directorate manages the program.

CSEWI is a charitable, nonprofit corporation. It was formed to create understanding, enthusiasm and appreciation for space enterprise and space technology, and inspire parents, educators and students to engage in space-related education and enrichment activities. The Institute hopes to stimulate greater awareness and understanding of the space enterprise work force and research needs throughout academia, and attract, integrate and retain a robust space work force.

For more information about Centennial Challenges on the Internet, visit:
http://centennialchallenges.nasa.gov

For information about the California Space Education and Workforce Institute on the Internet, visit:
http://www.californiaspaceauthority.org/html/level-one/institute.html

Original Source: NASA News Release

Posted on by Fraser Cain

Brand New Martian Gullies

Before (2002) and after pictures of a new gully on a sand dune on Mars. Image credit: NASA/JPL. Click to enlarge.
New gullies that did not exist in mid-2002 have appeared on a Martian sand dune.

That’s just one of the surprising discoveries that have resulted from the extended life of NASA’s Mars Global Surveyor, which this month began its ninth year in orbit around Mars. Boulders tumbling down a Martian slope left tracks that weren’t there two years ago. New impact craters formed since the 1970s suggest changes to age-estimating models. And for three Mars summers in a row, deposits of frozen carbon dioxide near Mars’ south pole have shrunk from the previous year’s size, suggesting a climate change in progress.

“Our prime mission ended in early 2001, but many of the most important findings have come since then, and even bigger ones might lie ahead,” said Tom Thorpe, project manager for Mars Global Surveyor at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. The orbiter is healthy and may be able to continue studying Mars for five to 10 more years, he said.

Mars years are nearly twice as long as Earth years. The orbiter’s longevity has enabled monitoring of year-to-year patterns on Mars, such as seasonal dust storms and changes in the polar caps. “Mars is an active planet, and over a range of timescales changes occur, even in the surface,” said Dr. Michael Malin of Malin Space Science Systems, San Diego, principal investigator for the Mars Orbiter Camera on Mars Global Surveyor.

“To see new gullies and other changes in Mars surface features on a time span of a few years presents us with a more active, dynamic planet than many suspected before Mars Global Surveyor got there,” said Michael Meyer, Mars Exploration Program chief scientist, NASA Headquarters, Washington.

Two gullies appear in an April 2005 image of a sand-dune slope where they did not exist in July 2002. The Mars Orbiter Camera team has found many sites on Mars with fresh-looking gullies, and checked back at more than 100 gullied sites for possible changes between imaging dates, but this is the first such find. Some gullies, on slopes of large sand dunes, might have formed when frozen carbon dioxide, trapped by windblown sand during winter, vaporized rapidly in spring, releasing gas that made the sand flow as a gully-carving fluid.

At another site, more than a dozen boulders left tracks when they rolled down a hill sometime between the taking of images in November 2003 and December 2004. It is possible that they were set in motion by strong wind or by a “marsquake,” Malin said.

Some changes are slower than expected. Studies suggest new impact craters might appear at only about one-fifth the pace assumed previously, Malin said. That pace is important because crater counts are used to estimate the ages of Mars surfaces.

The camera has recorded seasonal patterns of clouds and dust within the atmosphere over the entire planet. In addition, other instruments on Mars Global Surveyor have provided information about atmospheric changes and year-to-year patterns on Mars as the mission has persisted. Daily mapping of dust abundance in Mars’ atmosphere by the Thermal Emission Spectrometer has shown dust over large areas during three Mars southern hemisphere summers in a row. However, the extent and duration of dust storms varied from year to year.

Mars Global Surveyor was launched Nov. 7, 1996; entered orbit around Mars Sept. 12, 1997; and returned the first Mars data from its science instruments Sept. 15, 1997. Beyond its own investigations, the orbiter provides support for other Mars missions, such as landing-site evaluations, atmospheric monitoring, communication relay and imaging of hardware on the surface. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. JPL’s industrial partner is Lockheed Martin Space Systems, Denver, which built and operates the spacecraft.

For newly released images on the Internet, visit: http://www.nasa.gov/vision/universe/solarsystem/mgs-092005-images.html and http://www.msss.com/mars_images/moc/2005/09/20/ .

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Binary Star Baby Picture

The centre of this infrared image shows the higher mass primary star (pink) and its lower mass companion. Image credit: CfA. Click to enlarge.
Newborn stars are difficult to photograph. They tend to hide in the nebulous stellar nurseries where they formed, enshrouded by thick layers of dust. Now, Smithsonian astronomer T.K. Sridharan (Harvard-Smithsonian Center for Astrophysics) and his colleagues have photographed a pair of stellar twins in infrared light, which penetrates the dust. And these babies are whoppers, weighing several times the mass of the Sun.

Moreover, Sridharan’s images reveal a circumstellar disk surrounding the more massive of the two stars. The presence of a disk suggests that massive, multiple-star systems form the same way as the Sun, by gradually accreting material from a gaseous disk.

“This system is the youngest massive binary ever to be directly imaged – only about 100,000 years old,” said Sridharan.

Sridharan and his colleagues studied an object known as IRAS 20126+4104, located more than 5,000 light-years away in the constellation Cygnus the Swan. IRAS 20126+4104 was suspected of harboring a binary star because outflows from the region wobbled back and forth like a spinning top. The wobble hinted at the gravitational tug of an unseen companion.

On several exceptionally clear and steady nights, the researchers were able to take highly detailed infrared images of this object using the UKIRT telescope on Mauna Kea, Hawaii. Those images revealed not one but two stars, as well as a dark dust lane where the inner parts of the disk, known from previous radio-wavelength observations, appeared nearly edge-on in silhouette.

“Many people have seen the iconic Hubble Space Telescope images of circumstellar disks around low-mass stars. This image is the equivalent for high-mass stars,” said Sridharan.

Between them the two stars weigh more than 10 times the mass of the Sun. Sridharan calculates that the surrounding disk contains at least one-tenth of a solar mass, which is enough material to make 100 Jupiter-sized worlds. The disk may be even more massive. It extends outward for at least 850 astronomical units, or 80 billion miles (more than 20 times the distance to Pluto). Interestingly, the smaller companion star currently is located at the same distance from the primary star, hinting that the companion’s gravity may play a role in limiting the outer reaches of the disk.

Sridharan said that the next step in studying this intriguing twin system is to get higher-resolution observations using adaptive optics or interferometry. Such data will yield a better estimate of the companion’s mass and a detailed profile of the disk.

“We are currently following several leads to investigate this star system, so stay tuned,” Sridharan added.

Sridharan’s co-authors are S.J. Williams and G.A. Fuller of UMIST (Manchester, UK). This research was published in the Sept. 20, 2005, issue of The Astrophysical Journal Letters and is available online at http://arxiv.org/abs/astro-ph/0508342.

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

Original Source: Harvard CfA News Release

Posted on by Fraser Cain

Halo of Blue Stars Around a Black Hole

Artist illustration of the heart of galaxy M31. Image credit: NASA. Click to enlarge.
Astronomers using NASA’s Hubble Space Telescope have identified the source of a mysterious blue light surrounding a supermassive black hole in our neighboring Andromeda Galaxy (M31). Though the light has puzzled astronomers for more than a decade, the new discovery makes the story even more mysterious.

The blue light is coming from a disk of hot, young stars. These stars are whipping around the black hole in much the same way as planets in our solar system are revolving around the Sun. Astronomers are perplexed about how the pancake-shaped disk of stars could form so close to a giant black hole. In such a hostile environment, the black hole’s tidal forces should tear matter apart, making it difficult for gas and dust to collapse and form stars. The observations, astronomers say, may provide clues to the activities in the cores of more distant galaxies.

By finding the disk of stars, astronomers also have collected what they say is ironclad evidence for the existence of the monster black hole. The evidence has helped astronomers rule out all alternative theories for the dark mass in Andromeda’s core, which scientists have long suspected was a black hole.

“Seeing these stars is like watching a magician pulling a rabbit out of a hat. You know it happened but you don’t know how it happened,” said Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona. He and a team of astronomers, led by Ralf Bender of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and John Kormendy of the University of Texas in Austin, made the Hubble observations. The team’s results will be published in the Sept. 20, 2005 issue of the Astrophysical Journal.

Hubble Probes Strange Blue Light
Astronomer Ivan King of the University of Washington and colleagues first spotted the strange blue light in 1995 with the Hubble telescope. He thought the light might have come from a single, bright blue star or perhaps from a more exotic energetic process. Three years later, Lauer and Sandra Faber of the University of California at Santa Cruz used Hubble again to study the blue light. Their observations indicated that the blue light was a cluster of blue stars.

Now, new spectroscopic observations by Hubble’s Space Telescope Imaging Spectrograph (STIS) reveal that the blue light consists of more than 400 stars that formed in a burst of activity about 200 million years ago. The stars are tightly packed in a disk that is only a light-year across. The disk is nested inside an elliptical ring of older, cooler, redder stars, which was seen in previous Hubble observations.

The astronomers also used STIS to measure the velocities of those stars. They obtained the stars’ speeds by calculating how much their light waves are stretched and compressed as they travel around the black hole. Under the black hole’s gravitational grip, the stars are traveling very fast: 2.2 million miles an hour (3.6 million kilometers an hour, or 1,000 kilometers a second). They are moving so fast that it would take them 40 seconds to circle the Earth and six minutes to arrive at the Moon. The fastest stars complete an orbit in 100 years.

Andromeda’s active core probably made similar disks of stars in the past and may continue to make them.

“The blue stars in the disk are so short-lived that it is unlikely in the long 12-billion-year history of Andromeda that such a short-lived disk would appear now,” Lauer said. “That’s why we think that the mechanism that formed this disk of stars probably formed other stellar disks in the past and will trigger them again in the future. We still don’t know, however, how such a disk could form in the first place. It still remains an enigma.”

The astronomers credit Hubble’s superb vision for finding the disk.

“Only Hubble has the resolution in blue light to observe this disk,” said team member Richard Green of the National Optical Astronomy Observatory in Tucson. “It is so small and so distinct from the surrounding red stars that we were able to use it to probe into the very dynamical heart of Andromeda. These observations were taken by the members of our team that built STIS. We designed its visible channel specifically to seize such an opportunity ? to measure starlight closer to a black hole than in any other galaxy outside our own.”

Solid Evidence for a Monster Black Hole
In addition to the discovery of the disk of stars, the astronomers used this uniquely close look at Andromeda to prove unambiguously that the galaxy hosts a central black hole. In 1988, in independent ground-based studies, John Kormendy and the team of Alan Dressler and Douglas Richstone discovered a central dark object in Andromeda that they believed was a supermassive black hole. This was the first strong case for what are now 40 detections of black holes, most of them made by Hubble. Those observations, however, did not definitively rule out other, very exotic, and far less likely, alternatives.

“There are compelling reasons to believe that these are supermassive black holes,” Kormendy said. “But extreme claims require extraordinarily strong evidence. We have to be sure that these are black holes and not dark clusters of dead stars.”

The STIS observations of Andromeda are so precise that astronomers have eliminated all other possibilities for what the central, dark object could be. They also calculated that the black hole’s mass is 140 million Suns, which is three times more massive than once thought.

So far, dark clusters have definitively been ruled out in only two galaxies, NGC 4258 and our galaxy, the Milky Way. “These two galaxies give us unambiguous proof that black holes exist,” Kormendy added. “But both are special cases ? NGC 4258 contains a disk of water masers that we observe with radio telescopes, and our galactic center is so close that we can follow individual stellar orbits. Andromeda is the first galaxy in which we can exclude all exotic alternatives to a black hole using Hubble and using the same techniques by which we find almost all supermassive black holes.”

“Studying black holes always was a primary mission of Hubble,” Kormendy said. “Nailing the black hole in Andromeda is without a doubt an important part of its legacy. It makes us much more confidant that the other central dark objects detected in galaxies are black holes, too.”

“Now that we have proven that the black hole is at the center of the disk of blue stars, the formation of these stars becomes hard to understand,” Bender added. “Gas that might form stars must spin around the black hole so quickly ? and so much more quickly near the black hole than farther out ? that star formation looks almost impossible. But the stars are there.”

A Galaxy’s Active Core
The black hole and the disk of stars are not the only pieces of architecture in Andromeda’s core. A team led by Lauer and Faber used Hubble in 1993 to discover that the galaxy appears to have a double cluster of stars at its center. This finding was a surprise, because two clusters should merge into one in only a few hundred thousand years. Scott Tremaine of Princeton University solved this problem by suggesting that the “double nucleus” was actually a ring of old, red stars. The ring looked like two star clusters because astronomers were only seeing the stars on the opposite ends of the ring. The ring is about five light-years from the black hole and its surrounding disk of blue stars. The disk and the ring are tilted at the same angle as viewed from Earth, suggesting that they may be related.

Although astronomers are surprised to find a blue disk of stars swirling around a supermassive black hole, they also say the puzzling architecture may not be that unusual.

“The dynamics within the core of this neighboring galaxy may be more common than we think,” Lauer explained. “Our own Milky Way apparently has even younger stars close to its own black hole. It seems unlikely that only the closest two big galaxies should have this odd activity. So this behavior may not be the exception but the rule. And we have found other galaxies that have a double nucleus.”

Original Source: Hubble News Release