Thursday, April 29, 2010

M82: "Survivor" Black Holes May Be Mid-Sized

Credit Inset: X-ray: NASA/CXC/Tsinghua Univ./H. Feng et al.;
Full-field: X-ray: NASA/CXC/JHU/D.Strickland;
Optical: NASA/ESA/STScI/AURA/The Hubble Heritage Team;
IR: NASA/JPL-Caltech/Univ. of AZ/C. Engelbracht







This composite image of the nearby starburst galaxy M82 shows Chandra X-ray Observatory data in blue, optical data from the Hubble Space Telescope in green and orange, and infrared data from the Spitzer Space Telescope in red. The pullout is a Chandra image that shows the central region of the galaxy and contains two bright X-ray sources -- identified in a labeled version, roll your mouse over the image to view -- of special interest.

New studies with Chandra and ESA's XMM-Newton show that these two sources may be intermediate-mass black holes, with masses in between those of the stellar-mass and supermassive variety. These "survivor" black holes avoided falling into the center of the galaxy and could be examples of the seeds required for the growth of supermassive black holes in galaxies, including the one in the Milky Way.

This is the first case where good evidence for more than one mid-sized black hole exists in a single galaxy. The evidence comes from how their X-ray emission varies over time and analysis of their X-ray brightness and spectra, i.e., the distribution of X-rays with energy.

One of the black holes is located at a projected distance of 290 light years from the center of M82 (labeled with an "x") and its mass is estimated to be between 12,000 and 43,000 times the mass of the Sun. At this close distance, if the black hole was born at the same time as the galaxy and its mass was more than about 30,000 solar masses, it likely would have been pulled into the center of the galaxy. That is, it may have just escaped falling into the supermassive black hole that is presumably located in the center of M82.

The second black hole is located 600 light years in projection away from the center of M82. The best model for this M82 black hole has a mass between 200 and 800 times that of the Sun, and tilted at an angle between 60 and 80 degrees, meaning that the disk is viewed almost side-on. However, because of relativistic effects for a rapidly spinning black hole with this mass, a disk viewed at a high inclination is almost as bright as one viewed at a low inclination (i.e., face-on).

These results are interesting because they may help address the mystery of how supermassive black holes in the centers of galaxies form. M82 is located about 12 million light years from Earth and is the nearest place to us where the conditions are similar to those in the early Universe, with lots of stars forming.

Multiple observations of M82 have been made with Chandra beginning soon after launch. The Chandra data shown here were not used in the new research because the X-ray sources are so bright that some distortion is introduced into the X-ray spectra. To combat this, the pointing of Chandra is changed so that images of the sources are deliberately blurred, producing fewer counts in each pixel.

Fast Facts for M82:

Scale: Inset image is 1.8 arcmin across (about 6,300 light years across)
Category: Normal Galaxies & Starburst Galaxies, Black Holes
Coordinates: (J2000) RA 09h 55m 50.70s | Dec +69° 40' 37.00
Constellation: Ursa Major
Observation Dates: 08/17/2005
Observation Time: 11 days, 22 hours
Obs. IDs: 5644
Color Code: Intensity
Instrument: ACIS
Also Known As: Cigar Galaxy
References: Feng, H. et al. 2010, ApJ 710, L137; Feng, H., Kaaret, P., 2010, ApJ 712, L169
Distance Estimate: About 12 million light years

Cassini and Amateurs Chase Storm on Saturn

Amateur astronomer Christopher Go took this image of the storm on March 13, 2010. The arrow indicates the location of the storm and the red outlines show where Cassini's composite infrared spectrometer gathered data. Image credit: C.Go and NASA/JPL-Caltech/GSFC

Amateur astronomer Anthony Wesley obtained this image of a storm on Saturn from his backyard telescope in Murrumbateman, Australia, on March 22, 2010. He sent it to scientists working with NASA's Cassini spacecraft the next day. Image credit: A. Wesley

With the help of amateur astronomers, the composite infrared spectrometer instrument aboard NASA's Cassini spacecraft has taken its first look at a massive blizzard in Saturn's atmosphere. The instrument collected the most detailed data to date of temperatures and gas distribution in that planet's storms.

The data showed a large, turbulent storm, dredging up loads of material from the deep atmosphere and covering an area at least five times larger than the biggest blizzard in this year's Washington, D.C.-area storm front nicknamed "Snowmageddon."

"We were so excited to get a heads-up from the amateurs," said Gordon Bjoraker, a composite infrared spectrometer team member based at NASA's Goddard Space Flight Center in Greenbelt, Md. Normally, he said, "Data from the storm cell would have been averaged out."

Cassini's radio and plasma wave instrument and imaging cameras have been tracking thunder and lightning storms on Saturn for years in a band around Saturn's mid-latitudes nicknamed "storm alley." But storms can come and go on a time scale of weeks, while Cassini's imaging and spectrometer observations have to be locked in place months in advance.

The radio and plasma wave instrument regularly picks up electrostatic discharges associated with the storms, so team members have been sending periodic tips to amateur astronomers, who can quickly go to their backyard telescopes and try to see the bright convective storm clouds. Amateur astronomers including Anthony Wesley, Trevor Barry and Christopher Go got one of those notices in February and were able to take dozens of pictures over the next several weeks.

In late March, Wesley, an amateur astronomer from Australia who was actually the first person to detect the new dark spot caused by an impact on Jupiter last summer, sent Cassini scientists an e-mail with a picture of the storm.

"I wanted to be sure that images like these were being seen by the Cassini team just in case this was something of interest to be imaged directly by Cassini or the Hubble Space Telescope," Wesley wrote.

Cassini scientists eagerly pored through the images, including a picture of the storm at its peak on March 13 by Go, who lives in the Philippines.

By a stroke of luck, the composite infrared spectrometer happened to be targeting the latitude of the storms. The instrument's scientists knew there could be storms there, but didn't know when they might be active.

Data obtained by the spectrometer on March 25 and 26 showed larger than expected amounts of phosphine, a gas typically found in Saturn's deep atmosphere and an indicator that powerful currents were dredging material upward into the upper troposphere. The spectrometer data also showed another signature of the storm: the tropopause, the dividing line between the serene stratosphere and the lower, churning troposphere, was about 0.5 Kelvin (1 degree Fahrenheit) colder in the storm cell than in neighboring areas.

"A balloonist floating about 100 kilometers down from the bottom of Saturn's calm stratosphere would experience an ammonia-ice blizzard with the intensity of Snowmageddon," said Brigette Hesman, a composite infrared spectrometer team member who is an assistant research scientist at the University of Maryland. "These blizzards appear to be powered by violent storms deeper down - perhaps another 100 to 200 kilometers down - where lightning has been observed and the clouds are made of water and ammonia."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, 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 composite infrared spectrometer team is based at NASA's Goddard Space Flight Center, Greenbelt, Md., where the instrument was built.

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.

jia-rui.c.cook@jpl.nasa.gov

Nancy Neal Jones/Elizabeth Zubritsky 301-286-0039/301-614-5438
Goddard Space Flight Center, Greenbelt, Md.

nancy.n.jones@nasa.gov/elizabeth.a.zubritsky@nasa.gov

Scientists Say Ice Lurks In Asteroid's Cold Heart

In this artist's concept, a narrow asteroid belt filled with rocks and dusty debris orbits a star similar to our own sun. Image credit: NASA/JPL-Caltech - Larger image

Scientists using a NASA funded telescope have detected water-ice and carbon-based organic compounds on the surface of an asteroid. The cold hard facts of the discovery of the frosty mixture on one of the asteroid belt's largest occupants, suggests that some asteroids, along with their celestial brethren, comets, were the water carriers for a primordial Earth. The research is published in today's issue of the journal Nature.

"For a long time the thinking was that you couldn't find a cup's worth of water in the entire asteroid belt," said Don Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. "Today we know you not only could quench your thirst, but you just might be able to fill up every pool on Earth - and then some."

The discovery is a result of six years of observing asteroid 24 Themis by astronomer Andrew Rivkin of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Rivkin, along with Joshua Emery, of the University of Tennessee in Knoxville, employed the NASA Infrared Telescope Facility to take measurements of the asteroid on seven separate occasions beginning in 2002. Buried in their compiled data was the consistent infrared signature of water ice and carbon-based organic materials.

The study's findings are particularly surprising because it was believed that Themis, orbiting the sun at "only" 479 million kilometers (297 million miles), was too close to the solar system's fiery heat source to carry water ice left over from the solar system's origin 4.6 billion years ago.

Now, the astronomical community knows better. The research could help re-write the book on the solar system's formation and the nature of asteroids.

"This is exciting because it provides us a better understanding about our past - and our possible future," said Yeomans. "This research indicates that not only could asteroids be possible sources of raw materials, but they could be the fueling stations and watering holes for future interplanetary exploration."

Rivkin and Emory's findings were independently confirmed by a team led by Humberto Campins at the University of Central Florida in Orlando.

NASA detects, tracks and characterizes asteroids and comets passing close to Earth using both ground- and space-based telescopes. The Near-Earth Object Observations Program, commonly called "Spaceguard," discovers these objects, characterizes a subset of them, and plots their orbits to determine if any could be potentially hazardous to our planet.

JPL manages the Near-Earth Object Program Office for NASA's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena.

More information about asteroids and near-Earth objects is at: http://www.jpl.nasa.gov/asteroidwatch

Wednesday, April 28, 2010

XMM-Newton releases new edition of cosmic catalogue

One of the teams behind ESA's XMM-Newton X-ray mission has unveiled the latest edition of their 2XMM catalogue. The newest incarnation boasts an additional 42 000 entries, ratcheting up the total to over a quarter of a million X-ray sources. This unprecedented cosmic X-ray library is a valuable resource allowing astronomers to explore the extreme Universe.

The XMM-Newton spacecraft. Credit: ESA

This latest edition of the 2XMM catalogue, an unrivalled storehouse of information on X-ray sources, is the largest ever assembled. It is constructed through the serendipitous data acquired whilst piggy-backing on XMM-Newton's normal observing programme, which is based on competitive bids from the astronomical community. XMM-Newton makes over 600 observations each year but the target object typically only takes up a tiny fraction of the field of view of 30 arc minutes, equivalent to the diameter of the full Moon. Keen not to waste such an opportunity, the images are also searched for additional X-ray sources, with the findings being accumulated in the 2XMM catalogue for future reference. On average, an extra seventy sources additional to the main object of interest are found.

"We are getting 40 000 new detections a year. The vast majority of those, 98% of them, are completely unknown, they've never been detected before in X-rays," said Professor Mike Watson, leader of the XMM-Newton Survey Science Centre (SSC) based at the University of Leicester. The SSC team is responsible for the compilation of the 2XMM catalogue. In the decade since XMM-Newton took to the skies, previous catalogues have been sifted and have yielded several landmark discoveries. "With a serendipitous catalogue surprises are always possible," explains XMM-Newton Project Scientist Norbert Schartel.

An intermediate-mass black hole (shown here above and to the left of the galactic bulge) was discovered in the 2XMM catalogue. Credit: Heidi Sagerud

Amongst the many surprises that have emerged from the 2XMM catalogue is the chance discovery, in 2009, of a variable hyper-luminous X-ray source, 2XMM J011028.1-460421, also known as HLX-1. Weighing in at a hefty 500 solar masses, HLX-1 shows all the signs of being an intermediate-mass black hole, the missing link between stellar-mass and supermassive black holes. While it is accepted that stellar-mass black holes are the remnants of massive stars, the formation process for supermassive black holes is still unclear, although a number of explanations have been proposed, amongst them one scenario that involves mergers of intermediate-mass black holes. This XMM-Newton detection was the first solid evidence that such an intermediate class exists.

A massive galaxy cluster discovered in the 2XMM catalogue.
Credit: ESA XMM-Newton/EPIC (G. Lamer)
Hi-Res [jpg] 846.88 kb

Another serendipitous discovery, made in 2008, was the detection of the most massive galaxy cluster found up to that point in the distant Universe. The heavyweight, known by its catalogue number, 2XMM J083026+524133, tipped the scales at roughly 5 × 1014 solar masses, equivalent to the combined mass of over a thousand large galaxies. The discovery also lent weight to the growing evidence for dark energy, the force attributed to the observed expansion of the Universe. XMM-Newton is particularly well suited to this probing of the early, far-off Universe because of its large collecting area.

A continually enhanced 2XMM catalogue also allows astronomers to compare X-ray data with other large catalogues in other wavebands. In doing so, they can spot classes of objects which either don't show up or behave radically differently at other wavelengths. "One productive approach is simply to look for rare objects. You can do that by finding X-ray sources which have very peculiar properties when you start looking at what you find at other wavelengths," said Watson. Some of the rarest types of Active Galactic Nuclei (AGNs), quasars and X-ray binaries are just some of the unusual objects unmasked by 2XMM catalogue data.
X-ray source catalogues.
Image courtesy of M. Watson, University of Leicester

Passing the milestone of a quarter of a million sources is something that Watson believes will allow astronomers to build on these previous successes. "The catalogue is now big enough that it really does provide a very good solid basis for exploring the cosmic X-ray source population," he said. And far from resting on their laurels the XMM-Newton team hope to continue to increase their catalogue. "As the catalogue gets bigger it has more and more potential. At the current rate we'll reach half a million catalogued X-ray sources in the next five or six years," Watson added.

Schartel also recognises the need to continue with building the catalogue despite a decade of success. "The 2XMM catalogue is still, even after 10 years, producing exciting new results. It is also a key catalogue for future missions to optimise their target choices for future research. So catalogues such as this have a legacy that goes much further into the future than just current research," he said.

For further details please contact:

Mike Watson, XMM-Newton Survey Scientist
Department of Physics & Astronomy,
University of Leicester, United Kingdom
Email:
mgwstar.le.ac.uk

Norbert Schartel, ESA XMM-Newton Project Scientist
European Space Astronomy Centre, Spain
Email:
Norbert.Schartelsciops.esa.int

Notes for editors

This latest release of the 2XMM catalogue, referred to as 2XMMi-DR3, contains source detections drawn from 4953 observations made with XMM-Newton's European Photon Imaging Cameras (EPIC) between February 2000 and October 2009. The catalogue contains 353 191 X-ray source detections which relate to 262 902 unique X-ray sources, making it the largest collection of X-ray objects to date. The total area covered on the sky by the combined observation fields is ~800 square degrees.

The XMM-Newton Survey Science Centre, led by Professor Mike Watson at the University of Leicester, is a consortium of the following institutions:

University of Leicester, United Kingdom
Mullard Space Science Laboratory, University College London, United Kingdom
Institute of Astronomy, Cambridge, United Kingdom
Max-Planck Institut für extraterrestrische Physik, Garching, Germany
Astrophysikalisches Institut, Potsdam, Germany
Service d'Astrophysique, CEA/DSM/Dapnia, Saclay, France
Centre d'Etude Spatiale des Rayonnements, Toulouse, France
Observatoire Astronomique de Strasbourg, France
Instituto de Fisica de Cantabria, Santander, Spain
Osservatorio Astronomico di Brera, Milan, Italy

Monday, April 26, 2010

Research Illuminates the Shape of Dark Matter's Distribution

Figure 1: A Subaru Suprime-Cam image for one of the clusters used in the analysis, A2390 (2.7 billion light years from Earth). The purple hue shows the dark matter distribution measured by the gravitational lensing effect on distant galaxies (typically 8 billion light years from Earth), with the darker color indicating the denser dark matter concentration. It shows that the dark matter distribution is elongated along the northwest-southeast direction.

Figure 2: An illustration of the measurement of the dark matter distribution using gravitational lensing. Color contours indicate the density of dark matter, with redder color marking higher density. Black ellipses show the distortion pattern of background galaxies; positions of distant galaxies are systematically distorted into the shapes shown by black ellipses due to the gravitational lensing effect. (In practice, background galaxies have their own shapes and orientations, and hence we average many galaxies' shapes to extract distortions by gravitational lensing). Left and right panels show spherical and elliptical distributions of dark matter respectively. The difference in the distortion patterns suggests that one can measure the shape of the dark matter distribution from two-dimensional lensing distortion patterns.

Figure 3: The number distribution of the "ellipticity" of the dark matter distribution in 18 clusters of galaxies, obtained from the team's analysis of the gravitational lensing effect seen in Subaru images. Zero ellipticity means that the distribution is spherical, while a larger value of ellipticity indicates a more flattened distribution. The measurement peaks at around 0.5 (corresponding the 2:1 ratio of major to minor axes of the ellipse), suggesting that the flattened shape of the dark matter distribution is detected at a high level of significance. The black line shows a theoretical prediction based on the standard collisionless, cold dark matter model, made in 2002 by Yipeng Jing (Shanghai Astronomical Observatory) and Yasushi Suto (University of Tokyo). This demonstrates that the observed distribution agrees well with the theoretical prediction.

The nature of dark matter is still unknown and is currently a central problem in modern astronomy and physics. Dark matter is dark in a couple of ways. It is undetectable to visible light and has escaped detection at all electromagnetic wavelengths. Because it is invisible, its existence has to be inferred from its gravitational effect on other celestial objects as well as from theoretical models. Indirect evidence has established its relative abundance in our universe-probably five times greater than visible matter-in addition to its significance for understanding galaxy formation. For example, a considerable amount of dark matter probably sustains the structure of galaxies, because the gravitational force of visible matter cannot bind its member stars. The scientific challenge is how to study the nature of dark matter. Astronomers seek ways to use their observations to solve this puzzle.

One approach to a solution is to make detailed measurements of the spatial distribution of dark matter and then compare the data to predictions drawn from theoretical models. Both aspects of this approach have their difficulties. How can the distribution of dark matter be measured? What are plausible assumptions to include in models of dark matter?

A team of astronomers led by Masamune Oguri at the National Astronomical Observatory of Japan and Masahiro Takada at University of Tokyo decided to use gravitational lensing to measure and analyze the distribution of dark matter. Gravitational lensing provides a unique opportunity to explore dark matter distributions by measuring the distances that light travels from distant to foreground objects. Einstein's general theory of relativity predicts that light from a distant object will bend around a massive object in the foreground, e.g., a cluster of galaxies or a concentration of dark matter. By measuring the distortion pattern of many distant galaxies, it is possible to infer the mass(es) of the object(s) in the foreground. Since the technique does not rely on assumptions about the visibility of the matter bending the light, gravitational lensing can be a powerful probe of dark matter.

The team fine-tuned their research by observing 20 massive clusters of galaxies with the Subaru Telescope's Prime Focus Camera (Suprime-Cam). Clusters of galaxies are ideal sites for studying the distribution of dark matter, because they contain thousands of galaxies and are known to accompany a large amount of dark matter. The superb light-collecting power and excellent image quality of the Subaru Telescope gave the researchers an extra advantage. By using Suprime-Cam at prime focus, they could capture objects in a particularly wide field-of-view.

Observations with Suprime-Cam yielded wide-field images of 20 massive clusters of galaxies (typically located at 3 billion light years from Earth), which the team then used to measure and analyze dark matter distributions (Figure 1). From their detailed analysis of gravitational lensing effects in the images, the team obtained clear evidence that the distribution of dark matter in the clusters has, on average, an extremely flattened shape rather than a simple spherical contour (Figure 2 and 3). The measured degree of the flattening is quite large, corresponding to 2:1 in terms of the ratio of major to minor axes of the ellipse. This finding represents the first direct and clear detection of flattening in the dark matter distribution with the use of gravitational lensing.

In addition to the promise of using gravitational lensing for exploring the nature of dark matter, this research contributes to the theoretical modeling of dark matter. Detailed comparisons of the team's findings with theoretical model predictions of the distribution of dark matter show that the observed degree of the flattening is in excellent agreement with theoretical expectations.

Theoretical predictions for dark matter distributions in clusters of galaxies are dependent on what kind of dark matter model is assumed. This research strongly supports the prevailing model, which begins with the assumption that dark matter consists of weakly interacting massive particles that are relics of the Big Bang. These particles are assumed to be "cold", i.e., thermal motions of the particles are negligibly small. According to this scenario, clusters of galaxies are dynamically young objects that form through the merging of many small objects. This theory predicts that the dark matter distribution in clusters of galaxies would be non-spherical, reflecting a large-scale structure of dark matter filaments (i.e, ribbons of cold material, see also this release). Since the team's findings confirm a non-spherical distribution, they demonstrate the feasibility of exploring the nature of dark matter via flattening in the dark matter distribution.

This study is a part of the Local Cluster Substructure Survey (LoCuSS), an international project carrying out a systematic study of galaxy clusters by combining the Subaru data with a wide range of data sets from radio, infrared, optical and X-ray telescopes. The goal of the project is to reveal new aspects of cluster physics and cosmology. The results presented here are one of its initial achievements. More detailed explorations of dark matter in clusters of galaxies are planned using a wide-field survey with the Hyper Suprime-Cam, the next generation prime-focus camera on the Subaru Telescope. These future studies will greatly improve our understanding of galaxy clusters and, hopefully, the properties of dark matter.

The results of this research will be published in Monthly Notices of the Royal Astronomical Society.

The List of Authors

Masamune Oguri (National Astronomical Observatory of Japan)
Masahiro Takada (IPMU, University of Tokyo, Japan)
Nobuhiro Okabe (Institute of Astronomy & Astrophysics, Academia Sinica, Taiwan)
Graham P. Smith (University of Birmingham, UK)

M81's "Halo" Sheds Light on Galaxy Formation

Figure : Visible light image of spiral galaxy M81 taken by Suprime-Cam.
Object: northern half of Spiral Galaxy M81, distance 12 million light years, toward the constellation Ursa Major
Telescope: Subaru Telescope (effective aperture 8.2 m), primary focus
Instrument: Suprime-Cam (Subaru Prime Focus Camera)
Filters: V, i'
Color composite: Blue (V), Green (V and i' averaged), Red (i')
Observation date: January 8, 2005 UT
Exposure time: 105 minutes for V and 72 minutes for i'
Field of view: approximately 34 arcminutes x 27 arcminutes (60 arcminutes = 1 degree)


Observations with Subaru Telescope's Prime Focus Camera (Suprime-Cam) have revealed an extended structure of the spiral galaxy Messier 81 (M81) that may hold a key to understanding the formation of galaxies. This structure could be M81's halo. Until now, ground-based telescopes have only observed individual stars in the haloes around the Milky Way and Andromeda Galaxies. Differences in M81's extended structure from the Milky Way's halo may point to variations in the formation histories of spiral galaxies.

M81 is one of the largest galaxies in the M81 Group, a group of 34 galaxies located toward the constellation Ursa Major. At 11.7 million light years from Earth, it is one of the closest groups to the Local group, the group of galaxies that includes our own Milky Way. Thanks to its proximity and similarity to the Milky Way, M81 provides an excellent laboratory for testing galaxy formation models.

The most prominent of these models predicts that galaxies are built up from the merging and accretion of many smaller galaxies that orbit within their gravitational sphere of influence. This chaotic, bottom-up growth leaves behind a halo of stars around massive spirals like the Milky Way. Do the findings about M81's extended structure, possibly its halo, support this view?

True to its promise as an effective tool for the study of galaxy evolution, Subaru's telescope has provided data to address this question. The enormous light-gathering power of Subaru Telescopes's 8.2 meter primary mirror and the wide field-of-view of its Suprime-Cam enabled the telescope to provide evidence for a faint, extended structural component beyond M81's bright optical disk. It probed into space over one-hundred times darker than the night sky and imperceptible to the naked eye. The telescope spotted individual stars and gathered enough of them to identify M81's extended component and analyze its physical properties.

The results defy exact classification of the extended structure as a halo. Although the spatial distribution of its stars resembles the Milky Way's halo, M81's "halo" differs from the Milky Way's in other respects. Measurements of the total light from all of its stars and analysis of their colors point to estimates that M81's "halo" could be several times brighter and contain more processed materials, nearly twice as much mass in the form of metals (all elements heavier than helium), than the Milky Way's halo.

These differences prompt some fascinating questions. Do we need to expand our definition of a halo? Does this structure have a very different formation history than the Milky Way's halo? Did these differences arise because M81 cannibalized more or different kinds of small galaxies in the past than the Milky Way did? Regardless of the answers to these queries, the results of this research contribute to the growing body of evidence that the outer structures of apparently similar galaxies are much more important and complex than astronomers have previously thought.

Planck highlights the complexity of star formation

An active star-formation region in the Orion Nebula, as seen By Planck. This image covers a region of 13x13 degrees. It is a three-colour combination constructed from three of Planck's nine frequency channels: 30, 353 and 857 GHz. Credits: ESA/LFI & HFI Consortia

Download the individual channels:


HI-RES JPEG (Size: 257 kb)

New images from ESA’s Planck space observatory reveal the forces driving star formation and give astronomers a way to understand the complex physics that shape the dust and gas in our Galaxy.

Star formation takes place hidden behind veils of dust but that doesn’t mean we can’t see through them. Where optical telescopes see only black space, Planck’s microwave eyes reveal myriad glowing structures of dust and gas. Now, Planck has used this ability to probe two relatively nearby star-forming regions in our Galaxy.
The Orion region is a cradle of star formation, some 1500 light-years away. It is famous for the Orion Nebula, which can be seen by the naked eye as a faint smudge of pink.

A low activity, star-formation region in the constellation Perseus, as seen with Planck. This image covers a region of 30x30 degrees. It is a three-colour combination constructed from three of Planck's nine frequency channels: 30, 353 and 857 GHz. Credits: ESA/LFI & HFI Consortia

Download the individual channels:


HI-RES JPEG (Size: 605 kb)

The first image covers much of the constellation of Orion. The nebula is the bright spot to the lower centre. The bright spot to the right of centre is around the Horsehead Nebula, so called because at high magnifications a pillar of dust resembles a horse’s head.

The giant red arc of Barnard’s Loop is thought to be the blast wave from a star that blew up inside the region about two million years ago. The bubble it created is now about 300 light-years across.

In contrast to Orion, the Perseus region is a less vigorous star-forming area but, as Planck shows in the other image, there is still plenty going on.

The images both show three physical processes taking place in the dust and gas of the interstellar medium. Planck can show us each process separately. At the lowest frequencies, Planck maps emission caused by high-speed electrons interacting with the Galaxy’s magnetic fields. An additional diffuse component comes from spinning dust particles emitting at these frequencies.

The region of sky covered by the Planck images is shown on a view of half the sky as seen in visible and infrared light. The smaller patch corresponds to Orion and the larger to Perseus. Credits: ESA/LFI & HFI Consortia/STScI DSS - HI-RES JPEG (Size: 450 kb)

At intermediate wavelengths of a few millimetres, the emission is from gas heated by newly formed hot stars.

At still higher frequencies, Planck maps the meagre heat given out by extremely cold dust. This can reveal the coldest cores in the clouds, which are approaching the final stages of collapse, before they are reborn as fully-fledged stars. The stars then disperse the surrounding clouds.

The delicate balance between cloud collapse and dispersion regulates the number of stars that the Galaxy makes. Planck will advance our understanding of this interplay hugely, because, for the first time, it provides data on several major emission mechanisms in one go.

Planck’s primary mission is to observe the entire sky at microwave wavelengths in order to map the variations in the ancient radiation given out by the Big Bang. Thus, it cannot help but observe the Milky Way as it rotates and sweeps its electronic detectors across the night sky.

Friday, April 23, 2010

High-speed plasma jets: origin uncovered

For more than a decade, mysterious, high-speed plasma jets have been observed in space, downstream of the Earth's bow shock. The underlying formation mechanism for these jets has now been unveiled, thanks to data collected by the four ESA Cluster satellites. This study also suggests that such mechanisms may be relevant to other astrophysical shocks.

Image 1 Orbit of the Cluster satellites on 17 March 2007. Credit: ESA

The four Cluster satellites orbit the Earth in a pyramidal configuration along a nominal polar orbit of 4 × 19.6 Earth radii (1 Earth radius = 6380 km). The image above depicts the configuration on 17 March 2007.

In the summer of 2000 the four Cluster satellites embarked on the first three-dimensional mapping of the Earth's magnetosphere. During the second extension of the mission (2005-2009), Cluster explored regions of the magnetosphere not originally targeted, thanks to the natural evolution of their nominal polar orbit.

On 17 March 2007, the four satellites were travelling along an orbit that crossed the magnetospheric region closest to the Sun, known as the subsolar magnetopause. Shortly after 17:00 UT, they exited the terrestrial magnetosphere. Five and a half hours later, they crossed the Earth's bow shock at a distance of close to 100 000 km from Earth. The bow shock is the boundary where the permanent flow of solar particles - the solar wind - is decelerated, typically from 500 km/s to 100-300 km/s.

"Between 17:00 and 20:00 UT, instruments on the Cluster satellites observed several high-speed jets of plasma behind the bow shock with a speed close to 500 km/s," says Heli Hietala, a PhD student at the University of Helsinki, Finland, and lead author of the study that was published in the 11 December 2009 issue of Physical Review Letters.

"The speed of these jets was very close to the incoming solar wind speed (~530 km/s) as if, locally, the bow shock was not able to slow down the solar wind as it normally would," adds Hietala.

In a detailed analysis, Hietala and co-authors convincingly argue that these jets are due to passing ripples that travel along the Earth's bow shock. Such a shock ripple produces two main effects (see animated illustration):

Image 2 Producing high-speed plasma jets. Credit: ESA
(A larger version of this animation is available here.)

Image 3 Cluster detects high-speed jets. Credit: ESA

Top panel: total plasma speed measured by one of the Cluster satellites (C1) on 17 March 2007 from 18:13 to 18:17 UT. The plasma regions crossed by the satellite on its way out of the magnetosphere are colour coded as follows: yellow (magnetosphere), green (magnetosheath), pink (2nd shock) and purple (jet). The lower panel displays the angle (α) between the upstream plasma velocity and the normal to the bow shock, located upstream of the second shock. The calculation is not expected to be valid at the edges of the jet where the shock is weak and hence α is shown for the centre only.

First, an indentation – caused by the ripple - induces a higher-density region downstream of the shock. Second, it locally changes the angle between the normal of the shock and the velocity direction of the solar wind. When this angle is large, for example on the borders of the ripple, the solar wind goes through the bow shock experiencing only minor effects. In particular, it is hardly slowed down and continues at high-speed through the magnetosheath. This induces the creation of a second (local) shock downstream of the nominal shock – this was observed by the Cluster satellites just before 18:15 UT (see Image 3). The higher density together with the high speed leads to a jet with very high dynamic pressure.

"The mechanism that we have proposed to generate such high-speed jets is not only in agreement with these Cluster measurements but with all those reported so far," notes Tiera Laitinen, a co-author of the study and post-doctoral researcher at the Swedish Institute of Space Physics, Uppsala, Sweden, at the time of the study.

Uncovering this mechanism was possible because the Cluster satellites were in the right place at the right time to observe these transient jets. In addition, having several satellites operating together provided additional clues. For instance, the distance between two of the satellites was only 950 km. Since their observations were very similar, this immediately provides a lower limit to the spatial scale of the jet, of approximately 1000 km. The three-dimensional structure of the ripple analysed in this paper is the subject of an on-going study.

Image 4 Features of the heliosphere. Credit: NASA

The Solar System is immersed in a protective bubble, known as the heliosphere. This is caused by the solar wind, the expanding plasma of charged atoms and electrons emitted by the Sun, which excludes the local interstellar medium from the area encompassing the Sun and the planets.

The termination shock marks the region where the solar wind is quickly decelerated to subsonic speeds.

The heliosphere is bordered by a layer called the heliopause. Here, the pressure from the solar wind balances that of the interstellar medium.

The bow shock marks the region where the interstellar medium slows down as it impacts the heliosphere.

Moreover, this result may apply not only to the Earth's bow shock but to other astrophysical shocks. As Voyager 1 and 2 crossed the heliospheric termination shock (Image 4), in 2004 and 2007 respectively, their observations also revealed a rippled shock even though high-speed jets have not been observed. In an astrophysical context, such jets can act as seeds for magnetic field amplification and particle acceleration on the downstream side of supernova blast waves. Some of these blast waves are triggered by the death of massive stars, which end their lifetime as neutron stars.

"Explaining a phenomenon that has been observed for more than 10 years but whose origin was unknown is exciting. This result is an unexpected science nugget from this mission," says Philippe Escoubet, ESA Cluster Mission Manager.

Related publication

Hietala, H., T. V. Laitinen, K. Andréeová, R. Vainio, A. Vaivads, M. Palmroth, T. Pulkkinen, H. Koskinen, E. A. Lucek, and H. Rème, Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock, Physical Review Letters, 103, 245001, 2009.
DOI: 10.1103/PhysRevLett.103.245001

Notes for editors

Cluster is the first space mission able to study, in three dimensions, the natural physical processes occurring within and in the near vicinity of the Earth's magnetosphere. It is a project of international collaboration between ESA and NASA. Launched in 2000, Cluster is composed of four identical spacecraft orbiting the Earth in a pyramidal configuration, along a nominal polar orbit of 4 × 19.6 Earth radii (1 Earth radius = 6380 km). Cluster's payload consists of state-of-the-art plasma instrumentation to measure electric and magnetic fields over a wide frequency range, and key physical parameters characterizing electrons and ions from energies of nearly 0 eV to a few MeV. The science operations are coordinated by the Joint Science Operations Centre (JSOC), at the Rutherford Appleton Laboratory, United Kingdom, and implemented by ESA's European Space Operations Centre (ESOC), in Darmstadt, Germany.

Contact

Heli Hietala
Division of Geophysics and Astronomy, Department of Physics, University of Helsinki, Finland
Email:
heli.hietalahelsinki.fi

Web story author and co-editor

Arnaud Masson
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email:
Arnaud.Massonesa.int
Phone: +31-71-565-5634

Web story co-editors

Philippe Escoubet
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email:
Philippe.Escoubetesa.int
Phone: +31-71-565-4564

Matt Taylor
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email:
Matthew.Tayloresa.int
Phone: +31-71-565-8009

Starry-Eyed Hubble Celebrates 20 Years of Awe and Discovery

HH 901, HH 902
Credit: NASA, ESA,
and M. Livio and the Hubble 20th Anniversary Team (STScI)

This craggy fantasy mountaintop enshrouded by wispy clouds looks like a bizarre landscape from Tolkien's "The Lord of the Rings" or a Dr. Seuss book, depending on your imagination. The NASA Hubble Space Telescope image, which is even more dramatic than fiction, captures the chaotic activity atop a three-light-year-tall pillar of gas and dust that is being eaten away by the brilliant light from nearby bright stars. The pillar is also being assaulted from within, as infant stars buried inside it fire off jets of gas that can be seen streaming from towering peaks.

This turbulent cosmic pinnacle lies within a tempestuous stellar nursery called the Carina Nebula, located 7,500 light-years away in the southern constellation Carina. The image celebrates the 20th anniversary of Hubble's launch and deployment into an orbit around Earth.

Scorching radiation and fast winds (streams of charged particles) from super-hot newborn stars in the nebula are shaping and compressing the pillar, causing new stars to form within it. Streamers of hot ionized gas can be seen flowing off the ridges of the structure, and wispy veils of gas and dust, illuminated by starlight, float around its towering peaks. The denser parts of the pillar are resisting being eroded by radiation much like a towering butte in Utah's Monument Valley withstands erosion by water and wind.

Nestled inside this dense mountain are fledgling stars. Long streamers of gas can be seen shooting in opposite directions off the pedestal at the top of the image. Another pair of jets is visible at another peak near the center of the image. These jets (known as HH 901 and HH 902, respectively) are the signpost for new star birth. The jets are launched by swirling disks around the young stars, which allow material to slowly accrete onto the stars' surfaces.

Hubble's Wide Field Camera 3 observed the pillar on Feb. 1-2, 2010. The colors in this composite image correspond to the glow of oxygen (blue), hydrogen and nitrogen (green), and sulfur (red).

Compass and Scale Image for UVIS/IR/Details
Credit: NASA, ESA,
and M. Livio and the Hubble 20th Anniversary Team (STScI)

NASA's best-recognized, longest-lived, and most prolific space observatory zooms past a threshold of 20 years of operation this month. On April 24, 1990, the space shuttle and crew of STS-31 were launched to deploy the Hubble Space Telescope into a low Earth orbit. What followed was one of the most remarkable sagas of the space age. Hubble's unprecedented capabilities made it one of the most powerful science instruments ever conceived by humans, and certainly the one most embraced by the public. Hubble discoveries revolutionized nearly all areas of current astronomical research, from planetary science to cosmology. And, its pictures were unmistakably out of this world.

At times Hubble's starry odyssey played out like a space soap opera, with broken equipment, a bleary-eyed primary mirror, and even a space shuttle rescue/repair mission cancellation. But the ingenuity and dedication of Hubble scientists, engineers, and NASA astronauts have allowed the observatory to rebound time and time again. Its crisp vision continues to challenge scientists with exciting new surprises and to enthrall the public with ever more evocative color images.

NASA and the Space Telescope Science Institute (STScI) are celebrating Hubble's journey of exploration with a stunning new picture, online educational activities, an opportunity for people to explore galaxies as armchair scientists, and an opportunity for astronomy enthusiasts to send in their own personal greetings to Hubble for posterity.

NASA is releasing today a brand new Hubble photo of a small portion of one of the largest seen star-birth regions in the galaxy, the Carina Nebula. Towers of cool hydrogen laced with dust rise from the wall of the nebula. The scene is reminiscent of Hubble's classic "Pillars of Creation" photo from 1995, but is even more striking in appearance. The image captures the top of a three-light-year-tall pillar of gas and dust that is being eaten away by the brilliant light from nearby bright stars. The pillar is also being pushed apart from within, as infant stars buried inside it fire off jets of gas that can be seen streaming from towering peaks like arrows sailing through the air.

Hubble fans worldwide are being invited to share the ways the telescope has affected them. They can send an e-mail, post a Facebook message, use the Twitter hashtag #hst20, or send a cell phone text message. Or, they can visit the "Messages to Hubble" page on http://hubblesite.org, type in their entry, and read selections from other messages that have been received. Fan messages will be stored in the Hubble data archive along with the telescope's many terabytes of science data. Someday, future researchers will be able to read these messages and understand how Hubble had such an impact on the world.

The public will also have an opportunity to be at-home scientists by helping astronomers sort out the thousands of galaxies seen in a deep Hubble observation. STScI is partnering with the Galaxy Zoo consortium of scientists to launch an Internet-based astronomy project (http://hubble.galaxyzoo.org) where amateur astronomers can peruse and sort galaxies from Hubble's deepest view of the universe into their classic shapes: spiral, elliptical, and irregular. Dividing the galaxies into categories will allow astronomers to study how they relate to one another and provide clues that might help scientists understand how they formed.

For students, STScI is opening an education portal called "Celebrating Hubble's 20th Anniversary" (http://amazing-space.stsci.edu/hubble_20/). It offers links to "fun facts" and trivia about Hubble, a news story that chronicles the Earth-orbiting observatory's life and discoveries, and the IMAX "Hubble 3D" educator guide. An anniversary poster containing Hubble's "hall-of-fame" images, including the Eagle Nebula and Saturn, is also being offered with downloadable classroom activity information.

To date, Hubble has looked at over 30,000 celestial objects and amassed over one-half million pictures in its archive. The last heroic astronaut servicing mission to Hubble in May 2009 made it 100 times more powerful than when it was launched. In addition to its irreplaceable scientific importance, Hubble brings cosmic wonders into millions of homes and schools every day. For the past 20 years the public has become co-explorers with this wondrous observatory.

CONTACT

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514

villard@stsci.edu

Mario Livio
Space Telescope Science Institute, Baltimore, Md.
410-338-4439

mlivio@stsci.edu

Thursday, April 22, 2010

NASA's New Eye on the Sun Delivers Stunning First Images

Photograph taken by the Atmospheric Imaging Assembly (AIA) immediately after its CCD cameras cooled on March 30, 2010. In this representative-color image, red shows emission from ionized helium (He II) at a temperature of 140,000 Fahrenheit (80,000 Kelvin), while green shows ionized iron (Fe IX) at a temperature of 1,800,000 F (1,000,000K). The extent of the He II loop is equivalent to 30 Earth diameters. Credit: NASA

Cambridge, MA - NASA's recently launched Solar Dynamics Observatory, or SDO, is returning early images that confirm an unprecedented new capability for scientists to better understand our sun's dynamic processes. These solar activities affect everything on Earth.

Some of the images from the spacecraft show never-before-seen detail of material streaming outward and away from sunspots. Others show extreme close-ups of activity on the sun's surface. The spacecraft also has made the first high-resolution measurements of solar flares in a broad range of extreme ultraviolet wavelengths.

"These initial images show a dynamic sun that I had never seen in more than 40 years of solar research," said Richard Fisher, director of the Heliophysics Division at NASA Headquarters in Washington. "SDO will change our understanding of the sun and its processes, which affect our lives and society. This mission will have a huge impact on science, similar to the impact of the Hubble Space Telescope on modern astrophysics."

Launched on Feb. 11, 2010, SDO is the most advanced spacecraft ever designed to study the sun. During its five-year mission, it will examine the sun's magnetic field and also provide a better understanding of the role the sun plays in Earth's atmospheric chemistry and climate.

The observatory carries three state-of the-art instruments for conducting solar research: the Atmospheric Imaging Assembly (AIA), the Extreme Ultraviolet Variability Experiment (EVE), and the Helioseismic and Magnetic Imager (HMI). These three instruments observe the sun simultaneously, performing the entire range of measurements necessary to understand solar variations.

The Smithsonian Astrophysical Observatory (SAO) is a major partner in the AIA, which is a group of four telescopes that photograph the sun in 10 different wavelength bands, or colors, once every 10 seconds. Its images will help astronomers link changes in the sun's surface to interior changes. SAO built the four telescope assemblies and participates as a full partner in the scientific analysis activities.

"Everything about the AIA images is cleaner and better than anything we've had before. The mirrors are better, the cameras are better and the amount of data available is better. It all combines to give us a view of the corona that we've never had before," said SAO astrophysicist Leon Golub, a co-investigator on the AIA.

SDO is the first mission of NASA's Living with a Star Program, or LWS, and the crown jewel in a fleet of NASA missions that study our sun and space environment. The goal of LWS is to develop the scientific understanding necessary to address those aspects of the connected sun-Earth system that directly affect our lives and society.

This press release has been adapted from text issued by NASA.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.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462

daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463

cpulliam@cfa.harvard.edu

Wednesday, April 21, 2010

'This Planet Tastes Funny,' According to Spitzer Telescope

The planet illustrated here, called GJ 436b
An unusual, methane-free world is partially eclipsed by its star in this artist's concept. NASA's Spitzer Space Telescope has found evidence that a hot, Neptune-sized planet orbiting a star beyond our sun lacks methane -- an ingredient common to many planets in our own solar system.

These plots from NASA's Spitzer Space Telescope show light from a distant planet, GJ 436b, and its star, as measured at six different infrared wavelengths. Astronomers use telescopes like Spitzer to measure the direct light of distant worlds, called exoplanets, and learn more about chemicals in their atmospheres.

The technique involves measuring light from an exoplanet and its star before, during and after the planet circles behind the star. (The technique only works for those planets that happen to cross behind and in front of their stars as seen from our point of view on Earth.) When the planet disappears behind the star, the total light observed drops, as seen by the dips in these light curves. This same measurement is repeated at different wavelengths of light. In this graph, the different wavelengths are on the vertical axis, and time on the horizontal axis. Those dips in the total light tell astronomers exactly how much light is coming from the planet itself.

As the data demonstrate, the amount of light coming off a planet changes with different wavelengths. The differences are due to the temperature of a planet as well as its chemical makeup. In this case, astronomers were able to show that GJ 436b lacks the common planetary ingredient of methane.

PASADENA, Calif. - NASA's Spitzer Space Telescope has discovered something odd about a distant planet -- it lacks methane, an ingredient common to many of the planets in our solar system.

"It's a big puzzle," said Kevin Stevenson, a planetary sciences graduate student at the University of Central Florida in Orlando, lead author of a study appearing tomorrow, April 22 in the journal Nature. "Models tell us that the carbon in this planet should be in the form of methane. Theorists are going to be quite busy trying to figure this one out."

The discovery brings astronomers one step closer to probing the atmospheres of distant planets the size of Earth. The methane-free planet, called GJ 436b, is about the size of Neptune, making it the smallest distant planet that any telescope has successfully "tasted," or analyzed. Eventually, a larger space telescope could use the same kind of technique to search smaller, Earth-like worlds for methane and other chemical signs of life, such as water, oxygen and carbon dioxide.

"Ultimately, we want to find biosignatures on a small, rocky world. Oxygen, especially with even a little methane, would tell us that we humans might not be alone," said Stevenson.

"In this case, we expected to find methane not because of the presence of life, but because of the planet's chemistry. This type of planet should have cooked up methane. It's like dipping bread into beaten eggs, frying it, and getting oatmeal in the end," said Joseph Harrington of the University of Central Florida, the principal investigator of the research.

Methane is present on our life-bearing planet, manufactured primarily by microbes living in cows and soaking in waterlogged rice fields. All of the giant planets in our solar system have methane too, despite their lack of cows. Neptune is blue because of this chemical, which absorbs red light. Methane is a common ingredient of relatively cool bodies, including "failed" stars, which are called brown dwarfs.

In fact, any world with the common atmospheric mix of hydrogen, carbon and oxygen, and a temperature up to 1,000 Kelvin (1,340 degrees Fahrenheit) is expected to have a large amount of methane and a small amount of carbon monoxide. The carbon should "prefer" to be in the form of methane at these temperatures.

At 800 Kelvin (or 980 degrees Fahrenheit), GJ 436b is supposed to have abundant methane and little carbon monoxide. Spitzer observations have shown the opposite. The space telescope has captured the planet's light in six infrared wavelengths, showing evidence for carbon monoxide but not methane.

"We're scratching our heads," said Harrington. "But what this does tell us is that there is room for improvement in our models. Now we have actual data on faraway planets that will teach us what's really going on in their atmospheres."

GJ 436b is located 33 light-years away in the constellation Leo, the Lion. It rides in a tight, 2.64-day orbit around its small star, an "M-dwarf" much cooler than our sun. The planet transits, or crosses in front of, its star as viewed from Earth.

Spitzer was able to detect the faint glow of GJ 436b by watching it slip behind its star, an event called a secondary eclipse. As the planet disappears, the total light observed from the star system drops -- this drop is then measured to find the brightness of the planet at various wavelengths. The technique, first pioneered by Spitzer in 2005, has since been used to measure atmospheric components of several Jupiter-sized exoplanets, the so-called "hot Jupiters," and now the Neptune-sized GJ 436b.

"The Spitzer technique is being pushed to smaller, cooler planets more like our Earth than the previously studied hot Jupiters," said Charles Beichman, director of NASA's Exoplanet Science Institute at NASA's Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena, Calif. "In coming years, we can expect that a space telescope could characterize the atmosphere of a rocky planet a few times the size of the Earth. Such a planet might show signposts of life."

This research was performed before Spitzer ran out of its liquid coolant in May 2009, officially beginning its "warm" mission.

Other authors include: Sarah Nymeyer, William C. Bowman, Ryan A. Hardy and Nate B. Lust from the University of Central Florida; Nikku Madhusudhan and Sara Seager of the Massachusetts Institute of Technology, Cambridge; Drake Deming of NASA's Goddard Space Flight Center, Greenbelt, Md.; and Emily Rauscher of Columbia University, New York.


Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.

whitney.clavin@jpl.nasa.gov

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .

Searching for Dark Energy with the Whole World’s Supernova Dataset

Two views of one of the six new distant supernovae in the Supernova Cosmology Project's just-released Union2 survey, which among other refinements compares ground-based infrared observations (in this case by Japan's Subaru Telescope on Mauna Kea) with follow-up observations by the Hubble Space Telescope.

Narrower constraints from the newest analysis aren’t quite narrow enough

The international Supernova Cosmology Project (SCP), based at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, has announced the Union2 compilation of hundreds of Type Ia supernovae, the largest collection ever of high-quality data from numerous surveys. Analysis of the new compilation significantly narrows the possible values that dark energy might take—but not enough to decide among fundamentally different theories of its nature.

“We’ve used the world’s best-yet dataset of Type Ia supernovae to determine the world’s best-yet constraints on dark energy,” says Saul Perlmutter, leader of the SCP. “We’ve tightened in on dark energy out to redshifts of one”—when the universe was only about six billion years old, less than half its present age—“but while at lower redshifts the values are perfectly consistent with a cosmological constant, the most important questions remain.”

That’s because possible values of dark energy from supernovae data become increasingly uncertain at redshifts greater than one-half, the range where dark energy’s effects on the expansion of the universe are most apparent as we look farther back in time. Says Perlmutter of the widening error bars at higher redshifts, “Right now, you could drive a truck through them.”

As its name implies, the cosmological constant fills space with constant pressure, counteracting the mutual gravitational attraction of all the matter in the universe; it is often identified with the energy of the vacuum. If indeed dark energy turns out to be the cosmological constant, however, even more questions will arise.

“There is a huge discrepancy between the theoretical prediction for vacuum energy and what we measure as dark energy,” says Rahman Amanullah, who led SCP’s Union2 analysis; Amanullah is presently with the Oskar Klein Center at Stockholm University and was a postdoctoral fellow in Berkeley Lab’s Physics Division from 2006 to 2008. “If it turns out in the future that dark energy is consistent with a cosmological constant also at early times of the universe, it will be an enormous challenge to explain this at a fundamental theoretical level.”

A major group of competing theories posit a dynamical form of dark energy that varies in time. Choosing among theories means comparing what they predict about the dark energy equation of state, a value written w. While the new analysis has detected no change in w, there is much room for possibly significant differences in w with increasing redshift (written z).

“Most dark-energy theories are not far from the cosmological constant at z less than one,” Perlmutter says. “We’re looking for deviations in w at high z, but there the values are very poorly constrained.”

In their new analysis to be published in the Astrophysical Journal, the Supernova Cosmology Project reports on the addition of several well-measured, very distant supernovae to the Union2 compilation. The paper is now available online at http://arxiv4.library.cornell.edu/abs/1004.1711.

Dark energy fills the universe, but what is it?

Dark energy was discovered in the late 1990s by the Supernova Cosmology Project and the competing High-Z Supernova Search Team, both using distant Type Ia supernovae as “standard candles” to measure the expansion history of the universe. To their surprise, both teams found that expansion is not slowing due to gravity but accelerating.

Other methods for measuring the history of cosmic expansion have been developed, including baryon acoustic oscillation and weak gravitational lensing, but supernovae remain the most advanced technique. Indeed, in the years since dark energy was discovered using only a few dozen Type Ia supernovae, many new searches have been mounted with ground-based telescopes and the Hubble Space Telescope; many hundreds of Type Ia’s have been discovered; techniques for measuring and comparing them have continually improved.

In 2008 the SCP, led by the work of team member Marek Kowalski of the Humboldt University of Berlin, created a way to cross-correlate and analyze datasets from different surveys made with different instruments, resulting in the SCP’s first Union compilation. In 2009 a number of new surveys were added.

The inclusion of six new high-redshift supernovae found by the SCP in 2001, including two with z greater than one, is the first in a series of very high-redshift additions to the Union2 compilation now being announced, and brings the current number of supernovae in the whole compilation to 557.

“Even with the world’s premier astronomical observatories, obtaining good quality, time-critical data of supernovae that are beyond a redshift of one is a difficult task,” says SCP member Chris Lidman of the Anglo-Australian Observatory near Sydney, a major contributor to the analysis. “It requires close collaboration between astronomers who are spread over several continents and several time zones. Good team work is essential.”

Union2 has not only added many new supernovae to the Union compilation but has refined the methods of analysis and in some cases improved the observations. The latest high-z supernovae in Union2 include the most distant supernovae for which ground-based near-infrared observations are available, a valuable opportunity to compare ground-based and Hubble Space Telescope observations of very distant supernovae.

Type Ia supernovae are the best standard candles ever found for measuring cosmic distances because the great majority are so bright and so similar in brightness. Light-curve fitting is the basic method for standardizing what variations in brightness remain: supernova light curves (their rising and falling brightness over time) are compared and uniformly adjusted to yield comparative intrinsic brightness. The light curves of all the hundreds of supernova in the Union2 collection have been consistently reanalyzed.

The upshot of these efforts is improved handling of systematic errors and improved constraints on the value of the dark energy equation of state with increasing redshift, although with greater uncertainty at very high redshifts. When combined with data from cosmic microwave background and baryon oscillation surveys, the “best fit cosmology” remains the so-called Lambda Cold Dark Matter model, or ΛCDM.

ΛCDM has become the standard model of our universe, which began with a big bang, underwent a brief period of inflation, and has continued to expand, although at first retarded by the mutual gravitational attraction of matter. As matter spread and grew less dense, dark energy overcame gravity, and expansion has been accelerating ever since.

To learn just what dark energy is, however, will first require scientists to capture many more supernovae at high redshifts and thoroughly study their light curves and spectra. This can’t be done with telescopes on the ground or even by heavily subscribed space telescopes. Learning the nature of what makes up three-quarters of the density of our universe will require a dedicated observatory in space.

This work was supported in part by the U.S. Department of Energy’s Office of Science.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our website at http://www.lbl.gov.

Paul Preuss 510-486-6249 paul_preuss@lbl.gov