Plenty of dark matter near the Sun

Posted by carsimulator on Thursday, August 9, 2012

The high resolution simulation of the Milky Way used to test the mass-measuring technique. Credit: Dr A. Hobbs.

Astronomers at the University of Zürich, the ETH Zurich, the University of Leicester and NAOC Beijing have found large amounts of invisible "dark matter" near the Sun. Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive "halo" of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations. The authors also find tantalising hints of a new dark matter component in our Galaxy. The team's results will be published in the journal Monthly Notices of the Royal Astronomical Society.

Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s. He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart. At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone. In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighbourhood has remained more mysterious. For decades after Oort's measurement, studies found 3-6 times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The community was left puzzled, generally believing that the observations and analyses simply weren't sensitive enough to perform a reliable measurement.

In this latest study, the authors are much more confident in their measurement and its uncertainties. This is because they used a state-of-the-art simulation of our Galaxy to test their mass-measuring technique before applying it to real data. This threw up a number of surprises. They found that standard techniques used over the past 20 years were biased, always tending to underestimate the amount of dark matter. They then devised a new unbiased technique that recovered the correct answer from the simulated data. Applying their technique to the positions and velocities of thousands of orange K dwarf stars near the Sun, they obtained a new measure of the local dark matter density.

Lead author Silvia Garbari says: "We are 99% confident that there is dark matter near the Sun. In fact, our favoured dark matter density is a little high. There is a 10% chance that this is merely a statistical fluke. But with 90% confidence, we find more dark matter than expected. If future data confirms this high value, the implications are exciting. It could be the first evidence for a "disc" of dark matter in our Galaxy, as recently predicted by theory and numerical simulations of galaxy formation. Or it could be that the dark matter halo of our Galaxy is squashed, boosting the local dark matter density."

Many physicists are placing their bets on dark matter being a new fundamental particle that interacts only very weakly with normal matter -- but strongly enough to be detected in experiments deep underground where confusing cosmic ray events are screened by over a kilometre of solid rock.

An accurate measure of the local dark matter density is vital for such experiments as co-author Prof. George Lake explains: "If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time your finish reading this article. Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of."

Science contacts

Silvia Garbari
Tel: +41 76 211 05 12

Prof. Justin Read
Tel: +41 76 200 5394

Media contact

Robert Massey
Royal Astronomical Society
Mob: +44 (0)794 124 8035

Image and caption

An image from the simulation can be downloaded from:

Caption: The high resolution simulation of the Milky Way used to test the mass-measuring technique. Credit: Dr A. Hobbs

Further information

The new work appears in: "A new determination of the local dark matter density from the kinematics of K dwarfs", Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper is available from

Notes for editors

The Royal Astronomical Society (RAS,, founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

Follow the RAS on Twitter via @royalastrosoc

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The Kepler-INT Survey

Posted by carsimulator

KIS U-g-i colour postage stamp of NGC 6791,
a dense open cluster in the Kepler field [ PNG ]

The first data were obtained in May 2011 and completed coverage of the Kepler field in July 2012. Time is allocated through the multiple national time allocation committees of the Isaac Newton Group of Telescopes, with contributions from the UK (PI Steeghs), Netherlands (PI Groot) and Spain (PI Martín). The raw data is processed by the Cambridge Astronomy Survey Unit (CASU) using the same pipeline as employed in the IPHAS data releases. A key element of the project is rapid public release of the processed data, to maximise exploitation and to guide Kepler GO and DDT proposals. To this end, about half the field (all observations between May-August 2011 that passed our quality control thresholds) has been released as part of our initial data release DR1. We plan to release the remaining data later this year.

A global photometric calibration was derived by placing the KIS magnitudes as close as possible to the KIC photometry. The initial data release catalogue containing around 6 million sources from all the good photometric fields is available for download from the KIS webpage ( as well as via MAST CasJobs (

Colour-colour diagrams are key in selecting rare and exotic targets within the Kepler field. Here we illustrate the location of several key source classes (white dwarfs, CVs, ultra-cool dwarfs and AGNs) in the KIS colour-colour planes. The different types of objects fall within their expected locations in colour-space. The CVs are found in the 'blue' region of the (U – g, g – r) colour-colour diagram and they also stand out as H-alpha emitters in the (r – H-alpha, r – i) diagram. Also, the DA white dwarfs are known to be H-alpha deficit objects due to strong broad Balmer line absorption, which can be seen in the bottom panel of the figure. These have already been used to identify new targets both for Kepler as well as for supporting ground-based observations which include the William Herschel Telescope [ PNG ].


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Astronomers Release the Largest Ever Three-Dimensional Map of the Sky

Posted by carsimulator on Wednesday, August 8, 2012

This is a still image from a video fly-through of the SDSS-III galaxies mapped in Data Release 9. Credit: Yushu Yao and Prabhat (Lawrence Berkeley National Laboratory, NERSC), Miguel A. Aragon (Johns Hopkins University), and the SDSS-III Collaboration.

Cambridge, MA - The Sloan Digital Sky Survey III (SDSS-III) has released the largest three-dimensional map of massive galaxies and distant black holes ever created. The new map pinpoints the locations and distances of over a million galaxies. It covers a total volume of 70 billion cubic light-years.

"We want to map the largest volume of the universe yet, and to use that map to understand how the expansion of the universe is accelerating," said Daniel Eisenstein (Harvard-Smithsonian Center for Astrophysics), the director of SDSS-III.

The map is the centerpiece of Data Release 9 (DR9), which publicly releases the data from the first two years of a six-year survey project. The release includes images of 200 million galaxies and spectra of 1.35 million galaxies. (Spectra take more time to collect than photographs, but provide the crucial third dimension by letting astronomers measure galaxy distances.)

"Our goal is to create a catalog that will be used long after we are done," said Michael Blanton of New York University, who led the team that prepared Data Release 9.

The release includes new data from the ongoing SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), which will measure the positions of massive galaxies up to six billion light-years away, as well as quasars - giant black holes actively feeding on stars and gas - up to 12 billion light-years from Earth.

BOSS is targeting these big, bright galaxies because they live in the same places as other galaxies and they're easy to spot. Mapping these big galaxies thus provides an effective way to make a map of the rest of the galaxies in the universe.

With such a map, scientists can retrace the history of the universe over the last six billion years. With that history, they can get better estimates for how much of the universe is made up of "dark matter" - matter that we can't directly see because it doesn't emit or absorb light - and "dark energy," the even more mysterious force that drives the accelerating expansion of the universe.

"Dark matter and dark energy are two of the greatest mysteries of our time," said David Schlegel of Lawrence Berkeley National Laboratory, the principal investigator of BOSS. "We hope that our new map of the universe can help someone solve the mystery."

This release is being issued jointly with the SDSS-III Collaboration.

All the data are available now on the Data Release 9 website at The new data are being made available to astronomers, as well as students, teachers, and the public. The SkyServer website includes lesson plans for teachers that use DR9 data to teach astronomy and other topics in science, technology, and math. DR9 data will also feature in a new release of the Galaxy Zoo citizen science project, which allows online volunteers to contribute to cutting-edge astronomy research.

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

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics

Jordan Raddick, SDSS-III

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Close Encounter with the Tarantula

Posted by carsimulator on Tuesday, August 7, 2012

Tarantula Nebula
Credit: ESA/Hubble& NASA
Acknowledgement: Judy Schmidt

Turning its 2.4-metre eye to the Tarantula Nebula, the NASA/ESA Hubble Space Telescope has taken this close-up of the outskirts of the main cloud of the Nebula.

The bright wispy structures are the signature of an environment rich in ionised hydrogen gas, called H II by astronomers. In reality these appear red, but the choice of filters and colours of this image, which includes exposures both in visible and infrared light, make the gas appear green.

These regions contain recently formed stars, which emit powerful ultraviolet radiation that ionises the gas around them. These clouds are ephemeral as eventually the stellar winds from the newborn stars and the ionisation process will blow away the clouds, leaving stellar clusters like the Pleiades.

Located in the Large Magellanic Cloud, one of our neighbouring galaxies, and situated at a distance of 170 000 light-years away from Earth, the Tarantula Nebula is the brightest known nebula in the Local Group of galaxies. It is also the largest (around 650 light-years across) and most active star-forming region known in our group of galaxies, containing numerous clouds of dust and gas and two bright star clusters. A recent Hubble image shows a large part of the nebula immediately adjacent to this field of view.

The cluster at the Tarantula nebula’s centre is relatively young and very bright. While it is outside the field of view of this image, the energy from it is responsible for most of the brightness of the Nebula, including the part we see here. The nebula is in fact so luminous that if it were located within 1000 light-years from Earth, it would cast shadows on our planet.

The Tarantula Nebula was host to the closest supernova ever detected since the invention of the telescope, supernova 1987A, which was visible to the naked eye.

The image was produced by Hubble’s Advanced Camera for Surveys, and has a field of view of approximately 3.3 by 3.3 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Judy Schmidt. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results will be published soon.

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Subaru Telescope Reveals 3D Structure of Supernovae

Posted by carsimulator on Monday, August 6, 2012

A research group led by Dr. Masaomi Tanaka (National Astronomical Observatory of Japan), Dr. Koji Kawabata (Hiroshima University), Dr. Takashi Hattori (National Astronomical Observatory of Japan), and Dr. Keiichi Maeda (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe) used the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope to conduct observations that revealed a clumpy 3D structure of supernovae (Figure 1). This finding supports a clumpy 3D scenario of supernovae explosions rather than the widely accepted bipolar explosion scenario. It advances our understanding of how supernovae explode, a process that has been a persistent mystery.

Figure 1: Schematic drawing of the 3D structure (left) and the image of SN 2009mi (right) captured with FOCAS on the Subaru Telescope. This supernova was discovered in the galaxy IC 2151 (in the direction of the constellation Lepas, about 100 million light years away) by Berto Monard in South Africa. (Credit: NAOJ)

The Mystery of Supernovae

Stars heavier than eight solar masses will end their lives with a brilliant explosion called a "supernova". A supernova ejects elements synthesized within its star that are heavier than hydrogen and helium, the main elements of the primeval Universe. The ejection of these heavier elements into interstellar space has enriched the chemical composition of the Universe.

Despite its important role in the evolution of the Universe, the process of how supernovae explosions occur has been unclear. Based on recent numerical simulations, researchers agree that supernovae would not succeed as one-dimensional, spherical events and that multi-dimensional effects are important for understanding their occurrence. Scientists have proposed two main scenarios to explain how supernovae explosions occur: (1) a bipolar explosion facilitated by rotation, and (2) a clumpy 3D explosion driven by convection. However, scientists have not known which scenario is more plausible, because they have not actually observed the shape of supernovae.

Seeing the "Shape" of Supernovae by Polarization

Although it would seem easy to see the shape of supernovae by simply taking a picture of them, observing them is really a very challenging task. Since most supernovae occur in galaxies millions or hundreds of million light years away, they only look like a point, even though they expand at a speed of 10,000 km/s.

The current research team used a special method of detection to reveal the shape of supernovae; they measured so-called "polarization", which supplies information about the direction of vibrating electromagnetic waves. They performed numerical simulations for emissions from supernovae and found clearly different polarization patterns for clumpy and bipolar explosions. An object shows various angles of polarization in a clumpy explosion while it shows a single angle of polarization in a bipolar explosion (Figure 2).

Figure 2: Schematic drawing of the polarization patterns. If a supernova has a clumpy geometry, the polarization has various angles (left), but if it has a bipolar geometry, the polarization has a single angle (right). (Credit: NAOJ)

Based on hypotheses derived from their simulations, the group used the Subaru Telescope's Faint Object Camera and Spectrograph (FOCAS) to conduct polarimetric observations of nearby supernovae; such observations measure the intensity and direction of polarization. Because the researchers did not know when the supernovae would appear, they could not assign an observing time in advance. Fortunately, Subaru Telescope has a Target of Opportunity (ToO) mode that overcomes this difficulty and enables a dynamic allocation of the observing time. Thanks to this mode, the team succeeded in conducting polarimetric observations of two so-called "stripped-envelope supernovae" (SN 2009mi and SN 2009jf), which do not have hydrogen surrounding them and are the best targets for studying explosion geometry.

Revealing a 3D Structure

The team detected the polarization from the two supernovae, which clearly indicated that the supernovae are not usually round. They also found that each supernova had various angles of polarization, a finding consistent with the scenario of a clumpy 3D explosion (Figure 3).

When the team added the two new supernovae to the ones from previous observations, they had a total of six stripped-envelope supernovae, five of which showed the signature of clumpy 3D geometry. The research showed that the clumpy 3D shape is common in supernovae. Although the bipolar explosion scenario is widely accepted, the findings of this research support the clumpy 3D scenario of supernovae explosions. Convective motion in the explosions could account for this clumpy shape. This result serves as a catalyst to further understand how supernovae explosions occur.

Figure 3: Observed polarization around a calcium absorption line of SN 2009jf as a function of velocity caused by expanding motion (Doppler velocity). It shows that the polarization angle changes with wavelength. (Credit: NAOJ)

Research Group
  • Masaomi Tanaka (National Astronomical Observatory of Japan [NAOJ])
  • Koji S. Kawabata (Hiroshima University, Japan)
  • Takashi Hattori (NAOJ)
  • Paolo A. Mazzali (Max Planck Institute for Astrophysics, Germany)
  • Kentaro Aoki (NAOJ)
  • Masanori Iye (NAOJ)
  • Keiichi Maeda (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe, Japan)
  • Kenichi Nomoto (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe, Japan)
  • Elena Pian (Scuola Normale Superiore, Italy)
  • Toshiyuki Sasaki (NAOJ)
  • Masayuki Yamanaka (Kyoto University, Japan)

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The Spectral Energy Distribution of Protostars

Posted by carsimulator on Sunday, August 5, 2012

A simulated image of what a young star looks like in the far infrared. A dark disk is seen edge-on and outflow material is also apparent. A new paper compares models with these simulations to measure their accuracy. Credit: Offner et al., 2012.

Stars form when gravitational forces coalesce the gas and dust in interstellar clouds until the material forms clumps dense enough to become stars. Precisely how this happens, however, is still very uncertain. The infall of matter is probably not symmetric, it may be inhibited by the pressure of very hot radiation around the young stellar embryo, or perhaps it is constrained in other ways. These processes enable surrounding material to develop into disks around the stars, and it in turn can evolve into planets. The differences in the conditions are important to our understanding of the formation of our solar system because planets like the Earth are built from just such material that does not make it into the star.

Observing these various processes directly is difficult. Young stars are embedded in obscuring dust, and moreover they are generally far enough away that the imaging ability of instruments is unable to distinguish a star from its disk or surrounding cloud except in a few cases. Instead of relying on pictures, astronomers measure the total energy emitted by the star and its environment across wavelengths from the optical through several decades of infrared into the submillimeter. The so-called "spectral energy distribution (SED)" samples the stellar nursery's emission at wavelengths where the bulk of its energy lies - in the optical from the star and in infrared bands from dust in the disk or surroundings, with each band highlighting different temperature material. A range of modern space telescopes including the Spitzer Space Telescope and the Herschel Space Observatory have been used to collect these data.

Astronomers use models to reconstruct from the measured SED the detailed physical processes underway. CfA astronomer Tom Robitaille (who recently left CfA) was one of the most successful people modeling young stellar objects, and his codes have been widely used for about six years. In the new issue of The Astrophysical Journal, he and CfA astronomer Stella Offner, together with three colleagues, ask the question: How accurate are the models?

To test them, they use a sophisticated simulation of star formation whose results they use to calculate SEDs which they compare with the models. Of course the simulations, too, make approximations and might be neglecting something important, but they provide an excellent verification for most features such as the importance of the viewing angle in determining the observed SED. Their conclusion: Overall the models are very good at determining a young star's evolutionary state, accretion rate, and stellar mass, but are less good in determining the properties of the disk or envelope. The characteristics of the dust are among the parameters that need to be refined in order to improve the modeling. Their work is continuing, and future research will make it possible to infer even more details of stellar evolution from observations of the spectral energy distribution.

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Supernova progenitor found?

Posted by carsimulator on Friday, August 3, 2012

An optical / infrared / X-ray composite image of the remnant of Tycho's star, a type Ia supernova seen in 1572. Courtesy of Astronomy Picture of the Day. Credit: NASA / CXC / SAO / JPL-Caltech / MPIA / Calar Alto / O. Krause et al. Click for here a large version of this image

Type Ia supernovae are violent stellar explosions. Observations of their brightness are used to determine distances in the universe and have shown scientists that the cosmos is expanding at an accelerating rate. But there is still too little known about the specifics of the processes by which these supernovae form. New research, led by Stella Kafka of the Carnegie Institution for Science in the United States, identifies a star system, prior to explosion, which will possibly become a type Ia supernova. The work will appear in the journal Monthly Notices of the Royal Astronomical Society.

The widely accepted theory is that type Ia supernovae are thermonuclear explosions of a white dwarf star that's part of a binary system—two stars that are physically close and orbit around a common centre of mass. The white dwarf has mass gradually donated to it by its companion. When the white dwarf mass eventually reaches 1.4 times that of the sun, it explodes to produce a type Ia supernova. The crucial questions are: What is the nature of the donor star and how does this white dwarf increase its mass. Also, how would that process affect the properties of the explosion?

With these questions in mind, scientists have been searching for candidate systems that could become type Ia supernovae. There are thousands of possibilities in the candidate pool, none of which have yet been observed to produce an explosion. Recent studies, some of which involved scientists at Carnegie observatories, have identified sodium gas associated with type Ia supernovae. This gas might be ejected from the binary's donor star and linger around the system to be detected after the white dwarf explodes. This provides a clue to the progenitor. Even so, Kafka still compared the search to "looking for a needle in a stellar haystack."

Using data from the DuPont telescope of the Las Campanas observatory in Chile, Kafka and her team—Kent Honeycutt of Indiana University and Bob Williams of the Space Telescope Science Institute— looked at these gas signatures and were able to identify a binary star called QU Carinae as a possible supernova progenitor. It contains a white dwarf, which is accumulating mass from a giant star, and sodium has been detected around the system.

This star belongs to a small category of binaries that are very bright and in which the white dwarf accretes material from its companion at very high rates. Sodium should be produced in the atmosphere of the mass-donor giant star, and it can be ejected from the system via a stellar wind. If the white dwarf of this binary explodes into a supernova, the sodium would be detected with the same sort of signature as those found in other type Ia supernovae.

"We are really excited to have identified such a system," Kafka said. "Understanding these systems, the nature of the two stars, the manner in which mass is exchanged, and their long-term evolution will give us a comprehensive picture on how binaries can create one of the most important explosions in the universe."

Science contact

Stella Kafka
Tel: +1 202 478 8864

Media contacts

Natasha T. Metzler
Science Writer
Carnegie Institution for Science
Tel: +1 202 939 1142

Robert Massey
Royal Astronomical Society
Mob: +44 (0)794 124 8035


A composite X-ray / optical / infrared image of the remnant of Tycho's star, a type Ia supernova seen in 1572, is available at

Further information

The new work appears in the paper "QU Carinae: Supernova Ia in the making?" S. Kafka, K. Honeycutt, B. Williams, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper can be seen at

Notes for editors

This work was funded, in part, by the NASA Astrobiology Institute.

The Carnegie Institution for Science ( is a private, non-profit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

The Royal Astronomical Society (RAS,, founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

Follow the RAS on Twitter via @royalastrosoc

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