Universe Today

Simulated image that shows the distribution of matter in the Universe. Image credit: MPG. Click to enlarge.
The Virgo consortium, an international group of astrophysicists from the UK, Germany, Japan, Canada and the USA has today (June 2nd) released first results from the largest and most realistic simulation ever of the growth of cosmic structure and the formation of galaxies and quasars. In a paper published in Nature, the Virgo Consortium shows how comparing such simulated data to large observational surveys can reveal the physical processes underlying the build-up of real galaxies and black holes.

The “Millennium Simulation” employed more than 10 billion particles of matter to trace the evolution of the matter distribution in a cubic region of the Universe over 2 billion light-years on a side. It kept the principal supercomputer at the Max Planck Society’s Supercomputing Centre in Garching, Germany occupied for more than a month. By applying sophisticated modelling techniques to the 25 Terabytes (25 million Megabytes) of stored output, Virgo scientists are able to recreate evolutionary histories for the approximately 20 million galaxies which populate this enormous volume and for the supermassive black holes occasionally seen as quasars at their hearts.

Telescopes sensitive to microwaves have been able to image the Universe directly when it was only 400,000 years old. The only structure at that time was weak ripples in an otherwise uniform sea of matter and radiation. Gravitationally driven evolution later turned these ripples into the enormously rich structure we see today. It is this growth which the Millennium Simulation is designed to follow, with the twin goals of checking that this new paradigm for cosmic evolution is indeed consistent with what we see, and of exploring the complex physics which gave rise to galaxies and their central black holes.

Recent advances in cosmology demonstrate that about 70 percent of our Universe currently consists of Dark Energy, a mysterious force field which is causing it to expand ever more rapidly. About one quarter apparently consists of Cold Dark Matter, a new kind of elementary particle not yet directly detected on Earth. Only about 5 percent is made out of the ordinary atomic matter with which we are familiar, most of that consisting of hydrogen and helium. All these components are treated in the Millennium Simulation.

In their Nature article, the Virgo scientists use the Millennium Simulation to study the early growth of black holes. The Sloan Digital Sky Survey (SDSS) has discovered a number of very distant and very bright quasars which appear to host black holes at least a billion times more massive than the Sun at a time when the Universe was less than a tenth its present age.

“Many astronomers thought this impossible to reconcile with the gradual growth of structure predicted by the standard picture”, says Dr Volker Springel (Max Planck Institute for Astrophysics, Garching) the leader of the Millennium project and the first author of the article, “Yet, when we tried out our galaxy and quasar formation modelling we found that a few massive black holes do form early enough to account for these very rare SDSS quasars. Their galaxy hosts first appear in the Millennium data when the Universe is only a few hundred million years old, and by the present day they have become the most massive galaxies at the centres of the biggest galaxy clusters.”

For Prof Carlos Frenk (Institute for Computational Cosmology, University of Durham) the head of Virgo in the UK, the most interesting aspect of the preliminary results is the fact that the Millennium Simulation demonstrates for the first time that the characteristic patterns imprinted on the matter distribution at early epochs and visible directly in the microwave maps, should still be present and should be detectable in the observed distribution of galaxies. “If we can measure the baryon wiggles sufficiently well”, says Prof Frenk, “then they will provide us with a standard measuring rod to characterise the geometry and expansion history of the universe and so to learn about the nature of the Dark Energy.”

“These simulations produce staggering images and represent a significant milestone in our understanding of how the early Universe took shape.” said PPARC’s Chief Executive, Prof Richard Wade. “The Millennium Simulation is a brilliant example of the interaction between theory and experiment in astronomy as the latest observations of astronomical objects can be used to test the predictions of theoretical models of the Universe’s history.”

The most interesting and far-reaching applications of the Millennium Simulation are still to come according to Prof Simon White (Max Planck Institute for Astrophysics) who heads Virgo efforts in Germany. “New observational campaigns are providing us with information of unprecedented precision about the properties of galaxies, black holes and the large-scale structure of our Universe,” he notes. “Our ability to predict the consequences of our theories must reach a matching level of precision if we are to use these surveys effectively to learn about the origin and nature of our world. The Millennium Simulation is a unique tool for this. Our biggest challenge now is to make its power available to astronomers everywhere so that they can insert their own galaxy and quasar formation modelling in order to interpret their own observational surveys.”

Original Source: PPARC News Release

VLBA image of quasar 3C 273, with its long jet blasting out. Image credit: NRAO. Click to enlarge.
When a pair of researchers aimed the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope toward a famous quasar, they sought evidence to support a popular theory for why the superfast jets of particles streaming from quasars are confined to narrow streams. Instead, they got a surprise that “may send the theorists back to the drawing boards,” according to one of the astronomers.

“We did find the evidence we were looking for, but we also found an additional piece of evidence that seems to contradict it,” said Robert Zavala, an astronomer at the U.S. Naval Observatory’s Flagstaff, Arizona, station. Zavala and Greg Taylor, of the National Radio Astronomy Observatory and the Kavli Institute of Particle Astrophysics and Cosmology, presented their findings to the American Astronomical Society’s meeting in Minneapolis, Minnesota.

Quasars are generally thought to be supermassive black holes at the cores of galaxies, the black hole surrounded by a spinning disk of material being drawn inexorably into the black hole’s gravitational maw. Through processes still not well understood, powerful jets of particles are propelled outward at speeds nearly that of light. A popular theoretical model says that magnetic-field lines in the spinning disk are twisted tightly together and confine the fast-moving particles into narrow “jets” streaming from the poles of the disk.

In 1993, Stanford University and Kavli Institute astrophysicist Roger Blandford suggested that such a twisted magnetic field would produce a distinct pattern in the alignment, or polarization, of radio waves coming from the jets. Zavala and Taylor used the VLBA, capable of producing the most detailed images of any telescope in astronomy, to seek evidence of Blandford’s predicted pattern in a well-known quasar called 3C 273.

“We saw exactly what Blandford predicted, supporting the idea of a twisted magnetic field. However, we also saw another pattern that is not explained by such a field,” Zavala said.

In technical terms, the twisted magnetic field should cause a steady change, or gradient, in the amount by which the alignment (polarization) of the radio waves is rotated as one looks across the width of the jet. That gradient showed up in the VLBA observations. However, with a twisted magnetic field, the percentage of the waves that are similarly aligned, or polarized, should be at its greatest at the center of the jet and decrease steadily toward the edges. Instead, the observations showed the percentage of polarization increasing toward the edges.

That means, the astronomers say, there either is something wrong with the twisted-magnetic-field model or its effects are washed out by interactions between the jet and the interstellar medium that it is drilling through. “Either way, the theorists have to get to work to figure out how this can happen,” Zavala said.

When notified of the new results, Blandford said, “these observations are good enough to warrant further development of the theory.”

3C 273 is one of the most famous quasars in astronomy, and was the first to be recognized as a very distant object in 1963. Caltech astronomer Maarten Schmidt was working on a brief scientific article about 3C273 on the afternoon of February 5 that year when he suddenly recognized a pattern in the object’s visible-light spectrum that allowed an immediate calculation of its distance. He later wrote that “I was stunned by this development…” Just minutes later, he said, he met his colleague Jesse Greenstein, who was studying another quasar, in a hallway. In a matter of another few minutes, they found that the second one also was quite distant. 3C 273 is about two billion light-years from Earth in the constellation Virgo, and is visible in moderate-sized amateur telescopes.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in space. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.

“The extremely sharp radio ‘vision’ of the VLBA was absolutely necessary to do this work,” Zavala explained. “We used the highest radio frequencies at which we could detect 3C273’s jet to maximize the detail we could get, and this effort paid off with great science,” he added.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Galaxy Cluster Abell 2218 distorting the light from several more distant galaxies. Image credit: ESO. Click to enlarge.
Fifty years after his death, Albert Einstein’s work still provides new tools for understanding our universe. An international team of astronomers has now used a phenomenon first predicted by Einstein in 1936, called gravitational lensing, to determine the shape of stars. This phenomenon, due to the effect of gravity on light rays, led to the development of gravitational optics techniques, among them gravitational microlensing. It is the first time that this well-known technique has been used to determine the shape of a star.

Most of the stars in the sky are point-like, making it very difficult to evaluate their shape. Recent progress in optical interferometry has made it possible to measure the shape of a few stars. In June 2003, for instance, the star Achernar (Alpha Eridani) was found to be the flattest star ever seen, using observations from the Very Large Telescope Interferometer (see ESO Press Release for details about this discovery). Until now, only a few measurements of stellar shape have been reported, partly due to the difficulty of carrying such measurements. It is important, however, to obtain further accurate determinations of stellar shape, as such measurements help to test theoretical stellar models.

For the first time, an international team of astronomers [1], led by N.J. Rattenbury (from Jodrell Bank Observatory, UK), applied gravitational lensing techniques to determine the shape of a star. These techniques rely on the gravitational bending of light rays. If light coming from a bright source passes close to a foreground massive object, the light rays will be bent, and the image of the bright source will be altered. If the foreground massive object (the ‘lens’) is point-like and perfectly aligned with the Earth and the bright source, the altered image as seen from the Earth will be a ring shape, the so-called ‘Einstein ring’. However, most real cases differ from this ideal situation, and the observed image is altered in a more complicated way. The image below shows an example of gravitational lensing by a massive galaxy cluster.

Gravitational microlensing, as used by Rattenbury and his colleagues, also relies on the deflection of light rays by gravity. Gravitational microlensing is the term used to describe gravitational lensing events where the lens is not massive enough to produce resolvable images of the background source. The effect can still be detected as the distorted images of the source are brighter than the unlensed source. The observable effect of gravitational microlensing is therefore a temporary apparent magnification of the background source. In some cases, the microlensing effect may increase the brightness of the background source by a factor of up to 1000. As already pointed out by Einstein, the alignments required for the microlensing effect to be observed are rare. Moreover, as all stars are in motion, the effect is transitory and non-repeating. Microlensing events occur over timescales from weeks to months, and require long-term surveys to be detected. Such survey programs have existed since the 1990s. Today, two survey teams are operating: a Japan/New Zealand collaboration known as MOA (Microlensing Observations in Astrophysics) and a Polish/Princeton collaboration known as OGLE (Optical Gravitational Lens Experiment). The MOA team observes from New Zealand and the OGLE team from Chile. They are supported by two follow-up networks, MicroFUN and PLANET/RoboNET, that operate about a dozen telescopes around the globe.

The microlensing technique has been applied to search for dark matter around our Milky Way and other galaxies. This technique has also been used to detect planets orbiting around other stars. For the first time, Rattenbury and his colleagues were able to determine the shape of a star using this technique. The microlensing event that was used was detected in July 2002 by the MOA group. The event is named MOA 2002-BLG-33 (hereafter MOA-33). Combining the observations of this event by five ground-based telescopes together with HST images, Rattenbury and his colleagues performed a new analysis of this event.

The lens of event MOA-33 was a binary star, and such binary lens systems produce microlensing lightcurves that can provide much information about both the source and lens systems. The particular geometry of the observer, lens and source systems during the MOA-33 microlensing event meant that the observed time-dependent magnification of the source star was very sensitive to the actual shape of the source itself. The shape of the source star in microlensing events is usually assumed to be spherical. Introducing parameters describing the shape of the source star into the analysis allowed the shape of the source star to be determined.

Rattenbury and his colleagues estimated the MOA-33 background star to be slightly elongated, with a ratio between the polar and equatorial radius of 1.02 -0.02/+0.04. However, given the uncertainties of the measurement, a circular shape of the star cannot be completely excluded. The figure below compares the shape of the MOA-33 background star with those recently measured for Altair and Achernar. While both Altair and Achernar are only a few parsecs from the Earth, the MOA-33 background star is a more distant star (about 5000 parsecs from the Earth). Indeed, interferometric techniques can only be applied to bright (thus nearby) stars. On the contrary, the microlensing technique makes it possible to determine the shape of much more distant stars. Indeed, there is currently no alternative technique to measure the shape of distant stars.

This technique, however, requires very specific (and rare) geometrical configurations. From statistical considerations, the team estimated that about 0.1% of all detected microlensing events will have the required configurations. About 1000 microlensing events are observed every year. They should become even more numerous in the near future. The MOA group is presently commissioning a new Japan-supplied 1.8m wide-field telescope that will detect events at an increased rate. Also, a US led group is considering plans for a space-based mission called Microlensing Planet Finder. This is being designed to provide a census of all types of planets within the Galaxy. As a by-product, it would also detect events like MOA-33 and provide information on the shapes of stars.

Original Source: Jodrell Bank Observatory

Spitzer view of the Carina Nebula, a well known nebula containing newborn stars in the Milky Way. Image credit: Spitzer. Click to enlarge.
The saga of how a few monstrous stars spawned a diverse community of additional stars is told in a new image from NASA’s Spitzer Space Telescope.

The striking picture reveals an eclectic mix of embryonic stars living in the tattered neighborhood of one of the most famous massive stars in our Milky Way galaxy, Eta Carinae. Astronomers say that radiation and winds from Eta Carinae and its massive siblings ripped apart the surrounding cloud of gas and dust, shocking the new stars into being.

“We knew that stars were forming in this region before, but Spitzer has shown us that the whole environment is swarming with embryonic stars of an unprecedented multitude of different masses and ages,” said Dr. Robert Gehrz, University of Minnesota, Twin Cities, a member of the team that made the Spitzer observations.

The results were presented yesterday at the 206th meeting of the American Astronomical Society in Minneapolis by Dr. Nathan Smith, lead investigator of the Spitzer findings, University of Colorado, Boulder.

Previous visible-light images of this region, called the Carina Nebula, show cloudy finger-like pillars of dust, all pointing toward Eta Carinae at the center. Spitzer’s infrared eyes cut through much of this dust to expose incubating stars embedded inside the pillars, as well as new star-studded pillars never before seen.

Eta Carinae, located 10,000 light-years from Earth, was once the second brightest star in the sky. It is so massive, more than 100 times the mass of our Sun, it can barely hold itself together. Over the years, it has brightened and faded as material has shot away from its surface. Some astronomers think Eta Carinae might die in a supernova blast within our lifetime.

Eta Carinae’s home, the Carina Nebula, is also quite big, stretching across 200 light-years of space. This colossal cloud of gas and dust not only gave birth to Eta Carinae, but also to a handful of slightly less massive sibling stars. When massive stars like these are born, they rapidly begin to shred to pieces the very cloud that nurtured them, forcing gas and dust to clump together and collapse into new stars. The process continues to spread outward, triggering successive generations of fewer and fewer stars. Our own Sun may have grown up in a similar environment.

The new Spitzer image offers astronomers a detailed “family tree” of the Carina Nebula. At the top of the hierarchy are the grandparents, Eta Carinae and its siblings, and below them are the generations of progeny of different sizes and ages.

“Now we have a controlled experiment for understanding how one giant gas and dust cloud can produce such a wide variety of stars,” said Gehrz.

The false colors in the Spitzer picture correspond to different infrared wavelengths. Red represents dust features and green shows hot gas. Embryonic stars are yellow or white and foreground stars are blue. Eta Carinae itself lies just off the top of image. It is too bright for infrared telescopes to observe.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech. Spitzer’s infrared array camera, which took the picture of the Carina Nebula, was built by NASA Goddard Space Flight Center, Greenbelt, Md.; its development was led by Dr. Giovanni Fazio, Smithsonian Astrophysical Observatory, Cambridge, Mass.

Additional information about the Spitzer Space Telescope is available at: http://www.spitzer.caltech.edu/spitzer.

Original Source: Spitzer News Release

One small corner of the massive Andromeda galaxy (M31). Image credit: Subaru. Click to enlarge.
The lovely Andromeda galaxy appeared as a warm fuzzy blob to the ancients. To modern astronomers millennia later, it appeared as an excellent opportunity to better understand the universe. In the latter regard, our nearest galactic neighbor is a gift that keeps on giving.

Scott Chapman, from the California Institute of Technology, and Rodrigo Ibata, from the Observatoire Astronomique de Strasbourg in France, have led a team of astronomers in a project to map out the detailed motions of stars in the outskirts of the Andromeda galaxy. Their recent observations with the Keck telescopes show that the tenuous sprinkle of stars extending outward from the galaxy are actually part of the main disk itself. This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated.

At the annual summer meeting of the American Astronomical Society today, Chapman will outline the evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, astronomers looking at the visible evidence thought Andromeda was about 70,000 to 80,000 light-years across. Andromeda itself is about 2 million light-years from Earth.

The new dimensional measure is based on the motions of about 3,000 of the stars some distance from the disk that were once thought to be merely the “halo” of stars in the region and not part of the disk itself. By taking very careful measurements of the “radial velocities,” the researchers were able to determine precisely how each star was moving in relation to the galaxy.

The results showed that the outlying stars are sitting in the plane of the Andromeda disk itself and, moreover, are moving at a velocity that shows them to be in orbit around the center of the galaxy. In essence, this means that the disk of stars is vastly larger than previously known.

Further, the researchers have determined that the nature of the “inhomogeneous rotating disk”-in other words, the clumpy and blobby outer fringes of the disk-shows that Andromeda must be the result of satellite galaxies long ago slamming together. If that were not the case, the stars would be more evenly spaced.

Ibata says, “This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies. You just don’t get giant rotating disks from the accretion of small galaxy fragments.”

The current results, which are the subject of two papers already available and a third yet to be published, are made possible by technological advances in astrophysics. In this case, the Keck/DEIMOS multi-object spectrograph affixed to the Keck II Telescope possesses the mirror size and light-gathering capacity to image stars that are very faint, as well as the spectrographic sensitivity to obtain highly accurate radial velocities.

A spectrograph is necessary for the work because the motion of stars in a faraway galaxy can only be detected within reasonable human time spans by inferring whether the star is moving toward us or away from us. This can be accomplished because the light comes toward us in discrete frequencies due to the elements that make up the star.

If the star is moving toward us, then the light tends to cram together, so to speak, making the light higher in frequency and “bluer.” If the star is moving away from us, the light has more breathing room and becomes lower in frequency and “redder.”

If stars on one side of Andromeda appear to be coming toward us, while stars on the opposite side appear to be going away from us, then the stars can be assumed to orbit the central object.

The extended stellar disk has gone undetected in the past because stars that appear in the region of the disk could not be known to be a part of the disk until their motions were calculated. In addition, the inhomogeneous “fuzz” that makes up the extended disk does not look like a disk, but rather appears to be a fragmented, messy halo built up from many previous galaxies’ crashing into Andromeda, and it was assumed that stars in this region would be going every which way.

“Finding all these stars in an orderly rotation was the last explanation anyone would think of,” says Chapman.

On the flip side, finding that the bulk of the complex structure in Andromeda’s outer region is rotating with the disk is a blessing for studying the true underlying stellar halo of the galaxy. Using this new information, the researchers have been able to carefully measure the random motions of stars in the stellar halo, probing its mass and the form of the elusive dark matter that surrounds it.

Although the main work was done at the Keck Observatory, the original images that posed the possibility of an extended disk were taken with the Isaac Newton Telescope’s Wide-Field Camera. The telescope, located in the Canary Islands, is intended for surveys, and in the case of this study, served well as a companion instrument.

Chapman says that further work will be needed to determine whether the extended disk is merely a quirk of the Andromeda galaxy, or is perhaps typical of other galaxies.

The main paper with which today’s AAS news conference is concerned will be published this year in The Astrophysical Journal with the title “On the Accretion Origin of a Vast Extended Stellar Disk Around the Andromeda Galaxy.” In addition to Chapman and Ibata, the other authors are Annette Ferguson, University of Edinburgh; Geraint Lewis, University of Sydney; Mike Irwin, Cambridge University; and Nial Tanvir, University of Hertfordshire.

Original Source: Caltech News Release