Friday, January 29, 2010

Discovery of New Stellar Streams in the Andromeda Galaxy Shows Galaxy Formation Through Mergers

Figure 1:Illustration of a galactic structure in the edge-on view. A stellar halo has a huge size with a diameter of over 500,000 light years and contains old halo stars and globular clusters.

Figure 2:Traditional view of the Andromeda galaxies, showing only its bright bulge and inner disk and extending out to a projected distance of only about 65,000 light years from the galaxy's center. Image credit: National Astronomical Observatory of Japan.

Figure 3:False-color map of the density of red giant stars in Andromeda, constructed from Subaru/Suprime-Cam images. The stellar density enhancements in Streams E, F, and SW are indicated. The map extends out to a projected distance of 300,000 light years from Andromeda's center. Image credit for star count map: Mikito Tanaka (Tohoku University).

Figure 4:Distribution of line-of-sight velocities of stars in the Stream SW field. The filled portion of the histogram corresponds to Andromeda red giant stars while the open portion corresponds to foreground Milky Way stars. The concentration of red giant stars (at a velocity of -370 kilometers per second) is characteristic of tidal streams. Image credit: Puragra Guhathakurta (University of California, Santa Cruz).+

A team of astronomers from Tohoku University, University of Tokyo, NAOJ, University of California Santa Cruz, and other universities (Note 1) have discovered new stellar streams in a vast region surrounding the disk of Andromeda, in its so-called stellar halo. These stellar or tidal streams (Note 2), which are localized in space and move as a coherent group through the parent galaxy, intensify the density of stars and are remnants of past mergers of relatively small (i.e., dwarf) galaxies. The data from the team's observations using both Subaru's Suprime-Cam for photometry and Keck II's Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) for spectroscopy provided detailed spatial and velocity distributions of the stellar streams and led to this discovery.

Stars spread over the vast reaches of a halo in a big galaxy like the Milky Way or Andromeda Galaxy (Figure 1) are characterized by old age, few elements other than helium and hydrogen (i.e., low metallicities), and high velocities. The exceptional nature of these halo stars, when compared to stars in a galaxy's disk, reflects the early dynamics and chemical evolution of the galaxy when its appearance differed significantly from what we see today. Consequently, the halo provides important insights into the processes involved in the formation and evolution of a massive galaxy. According to the current theory of galaxy formation, we expect a halo to preserve evidence of past galaxy mergers and/or tidal dissolution in the course of halo formation.

Since the merging and dissolution of a dwarf galaxy typically last for a couple of billion years, these events are occasionally seen in a large galaxy. Given the assumption that past merging events are recorded as stellar streams, identification of these stellar substructures in a halo plays a key role in studying the past history of galaxies. The Andromeda Galaxy is an excellent test case for this purpose: it is the nearest, large spiral galaxy similar to our own Milky Way Galaxy (Figure 2) and is close enough for individual stars to be studied in great detail.

Motivated by the scientific significance of examining Andromeda's halo, an international research team led by Mikito Tanaka (Tohoku University) carried out photometric observations of Andromeda's halo fields with V and I bands of Suprime-Cam, a wide field imager mounted on the Subaru Telescope. Rather than spending an enormous number of observation nights mapping its entire halo, the team looked at specific portions of Andromeda's minor axis fields, including the hitherto uncharted north side as well as some fields at the major axis. This survey led to the discovery of two stellar streams to the northwest (Streams E and F: Figure 3) at projected distances of 200,000 and 300,000 light years from Andromeda's center. The study also confirmed a few previously known streams, including the little-studied diffuse stream to the southwest (Stream SW: Figure 3), which lies at a projected distance of 200,000 to 300,000 light years from Andromeda's center.

Another scientific team led by Puragra Guhathakurta (University of California, Santa Cruz) followed up the photometric observations with a spectroscopic survey of several hundred red giant stars in Streams E, F, and SW, using Keck II's 10-meter telescope fitted with DEIMOS. Red giant stars are large, bright stars with low or intermediate mass that are in a late phase of stellar evolution. Because the spectrograph spreads out the light from each star into a spectrum, it allows astronomers to measure the star's velocity and thus distinguish Andromeda red giant stars from foreground stars in the Milky Way. The spectral data confirmed the presence of coherent groups of Andromeda red giant stars moving with a common velocity (Figure 4).

The features of these newly discovered stellar streams in Andromeda are evidence of past galaxy mergers associated with the formation of a stellar halo. The next research step will be to measure in detail the chemical properties of Andromeda's giant red stars within their stellar streams. Mikito Tanaka anticipated the significance of future research by saying, "Further observational surveys of an entire halo region in Andromeda will provide very useful information on galaxy formation, including how many and how massive individual dwarf galaxies as building blocks are and how star formation and chemical evolution proceeded in each dwarf galaxy."

The photometric survey of Andromeda with Subaru's Suprime-Cam was published in a recent ApJ article (Note 3). Findings from the spectroscopic survey with Keck/DEIMOS were presented at the 215th meeting of the American Astronomical Society in Washington, D.C. (Note 4).

Note 1: Mikito Tanaka (Tohoku University, University of Tokyo), Masashi Chiba (Tohoku University), Yutaka Komiyama (NAOJ), Masanori Iye (NAOJ), Puragra Guhathakurta (University of California, Santa Cruz), Jason S. Kalirai (Space Telescope Science Institute), and other collaborators at the University of Virginia, UC Irvine, University of Massachusetts, Yale University, University of Washington, Columbia University, and California Institute of Technology.

Note 2: Stellar streams represent enhancements in the density of stars, localized in space and moving as a coherent group through the parent galaxy. Because tidal forces play a role in their creation, they are also referred to as tidal streams. These substructures in stellar distribution are remnants of past merging events of dwarf galaxies when they fall into a big galaxy like the Milky Way and Andromeda.

Note 3: "Structure and Population of the Andromeda Stellar Halo from a Subaru/Suprime-Cam Survey" by Mikito Tanaka, Masashi Chiba, Yutaka Komiyama, Puragra Guhathakurta, Jason S. Kalirai, Masanori Iye, 2010, ApJ, 708, 1168-1203

Note 4: "The SPLASH Survey: Spectroscopy of Newly Discovered Tidal Streams in the Outer Halo of the Andromeda Galaxy" by Puragra Guhathakurta, R. Beaton, J. Bullock, M. Chiba, M. Fardal, M. Geha, K. Gilbert, K. Howley, M. Iye, K. Johnston, J. Kalirai, E. Kirby, Y. Komiyama, S. Majewski, R. Patterson, M. Tanaka, E. Tollerud, SPLASH collaboration, 2010, AAS Meeting #215, #354.01

Astronomer Discover Cool Stars in Nearby Space

UKIRT UKIDSS near infrared image of SDSS1416+13AB (left panel) and the Spitzer+UKIDSS image at mid-infrared wavelengths (right panel). Image: UKIRT / Spitzer

Subaru near infared spectrum of SDSS1416+13B, taken with the IRCS spectrograph. The almost total absence of light at wavelengths between 1.7 and 2.5 microns is apparent. This causes the very blue near infrared colour. Image: Subaru

An international team, led by astronomers at the University of Hertfordshire have discovered what may be the coolest sub-stellar body ever found outside our own solar system. Using the United Kingdom Infrared Telescope (UKIRT) in Hawaii, a discovery has been made of an object which is technically known as a brown dwarf. The team's findings have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

What has excited astronomers are its very peculiar colours, which actually make it appear either very blue or very red, depending on which part of the spectrum is used to look at it.

The object is known as SDSS1416+13B and it is in a wide orbit around a somewhat brighter and warmer brown dwarf, SDSS1416+13A. The brighter member of the pair was detected in visible light by the Sloan Digital Sky Survey. By contrast, SDSS1416+13B is only seen in infrared light. The pair is located between 15 and 50 light years from the solar system, which is quite close in astronomical terms.

"This looks like being the fourth time in three years that the UKIRT has discovered made a record breaking discovery of the coolest known brown dwarf, with an estimated temperature not far above 200 degrees Celsius,” said Dr Philip Lucas at the University of Hertfordshire’s School of Physics, Astronomy and Mathematics.

“We have to be a bit careful about this one because its colours are so different than anything seen before that we don't really understand it yet. Even if it turns out that the low temperature is not quite record breaking, the colours are so extreme that this object will keep a lot of physicists busy trying to explain it.”

SDSS1416+13B was first noticed by Dr Ben Burningham of the University of Hertfordshire as part of a dedicated search for cool brown dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS). The object appeared far bluer at near infrared wavelengths than any brown dwarf seen before. A near infrared spectrum taken with the Japanese Subaru Telescope in Hawaii showed that it is a type of brown dwarf called a T dwarf, which has a lot of methane in its atmosphere, but with peculiar features including a big gap at certain wavelengths.

Dr Burningham soon noticed that a previously observed brighter star (SDSS1416+13A) which appears close by in the UKIDSS discovery image was also a brown dwarf. Team member Dr Sandy Leggett, of the Gemini Observatory, then used the orbiting Spitzer Space Telescope to investigate SDSS1416+13B at longer wavelengths. She measured its colour at mid-infrared wavelengths, which are thought to be the most reliable indicator of temperature, and found that it is the reddest known brown dwarf at these wavelengths by some margin. Comparison with theoretical models of the brown dwarf atmospheres then provided a temperature estimate of about 500 Kelvin (227 degrees Celsius).

"The fact that it is a binary companion to a warmer brown dwarf that also has an unusual spectrum is helping us to fill in some gaps in our understanding", says Dr Burningham. "It seems likely that both brown dwarfs are somewhat poor in heavy elements. This can be explained if they are very old, which also fits with the very low temperature of the faint companion."

Too small to be stars, brown dwarfs have masses smaller than stars but larger than gas giant planets like Jupiter. Due to their low temperature these objects are very faint in visible light, and are detected by their glow at infrared wavelengths. They were originally dubbed "brown dwarfs" long before any were actually discovered, to describe the idea of bodies that were cooler, fainter and redder than red dwarf stars, with the colour brown representing the mix of red and black.


Dr Ben Burningham
University of Hertfordshire
Tel: +44 (0)1707 285179
Mob: +44 (0)7815 122383

Dr Sandy Leggett
Gemini Observatory
Tel: +1 808-974-2604

Dr Philip Lucas
University of Hertfordshire
Tel: +44 (0)1707 286070
Mobile: +44 (0)7951 630957

Dr Andy Adamson
Joint Astronomy Centre
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Prof. Gary Davis
Joint Astronomy Centre
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Dr Robert Massey
Press and Policy Officer
Royal Astronomical Society
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Helene Murphy
Press Office
University of Hertfordshire
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Images are available from:

1. UKIRT UKIDSS near infrared image of SDSS1416+13AB (left panel) and the Spitzer+UKIDSS image at mid-infrared wavelengths (right panel).

2. Subaru near infared spectrum of SDSS1416+13B, taken with the IRCS spectrograph. The almost total absence of light at wavelengths between 1.7 and 2.5 microns is apparent. This causes the very blue near infrared colour.


A preprint of the paper can be found at



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 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. For more information see


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One light year is the distance travelled by light in a year. It corresponds to roughly 10 thousand billion kilometres or 6 thousand billion miles.


Infrared wavelengths are longer wavelengths than light waves. They are typically measured in microns, also called micrometres. One micron is one millionth of a metre, one 10000th of a centimetre, or one 25000th of an inch.


The world's largest telescope dedicated solely to infrared astronomy, the 3.8-metre (12.5-foot) UK Infrared Telescope (UKIRT) is sited near the summit of Mauna Kea, Hawaii, at an altitude of 4194 metres (13760 feet) above sea level. It is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Science and Technology Facilities Council.

UKIRT's technical innovation and privileged position on the high, dry Mauna Kea site have placed it at the forefront of infrared astronomy since its opening in 1979. UKIRT now spends most of its time surveying the sky for the UKIDSS sky survey but it continues to undertake a variety of smaller science projects covering all areas of astronomy.


The Subaru telescope is Japan's premier optical-infrared telescope operated by the National Astronomical Observatory of Japan. Located on Mauna Kea on the island of Hawaii, the telescope, with an effective aperture of 8.2 m, is also one of the world's largest and most technologically advanced telescopes. Through the open use program astronomers throughout the world have access to Subaru and its excellent image quality.


The Spitzer Space Telescope is a space-borne, cryogenically-cooled infrared observatory capable of studying objects ranging from our Solar System to the distant reaches of the Universe. Spitzer is the final element in NASA's Great Observatories Program, and an important scientific and technical cornerstone of the Astronomical Search for Origins Program.

Thursday, January 28, 2010

e-VLBI reveals missing link between Supernovae-Gamma Ray Burst explosions

The observation was conducted on 6-7 September 2007 for 12 hours at 4.97 GHz with the the European VLBI Network (EVN) at an aggregate bitrate of 256 Mbps. The observation identified an unresolved source. It is therefore considered an e-VLBI detection of a supernova, and further observations are planned. This observation and the resulting publication are only possible because of the rapid response time of e-VLBI.

Credit: Paragi, Z.; Kouveliotou, C.; Garrett, M.A.; Ramirez-Ruiz E.; Langevelde, H.J. van; Szomoru, A.; Argo, M. "e-VLBI detection of SN2007gr. " The Astronomer's Telegram. #1215. (12 September 2007).

An international team of scientists, including several astronomers from the Joint Institute for VLBI in Europe (JIVE) and the Netherlands Institute for Radio Astronomy (ASTRON), both located in Dwingeloo, have observed a supernova with peculiar radio emission. In a paper to be published in the 28 January 2010 issue of Nature, the team led by JIVE's Zsolt Paragi reports, for the first time ever, detection of a relativistic outflow in a Type Ic supernova, thus supporting the link with the even more energetic Gamma Ray Bursts, some of the most energetic explosions in the Universe.

At the end of its life, the central region of a massive star collapses while its outer layer is expelled in a gigantic explosion. This phenomenon is known as a supernova. Supernova SN 2007gr was discovered in 2007 with the Katzman Automatic Imaging Telescope in California, USA. Optical observations showed that it was Type Ic, known to result from the most massive stars. Supernovae are very distant sources, and the radio emissions they produce fade quickly. Therefore, the highest resolution imaging technique, called Very Long Baseline Interferometry (VLBI), is required to receive the extremely faint emission and reveal the details of the explosion process. Because SN 2007gr was located in a relatively nearby galaxy, closer than any other Type Ic supernovae detected in the radio spectrum, it offered a unique opportunity to study this phenomenon.

With the VLBI technique, radio telescopes located up to thousands of kilometers from each other carry out measurements synchronously. Paragi's team exploited the electronic, real-time VLBI (e-VLBI) capabilities of the European VLBI Network (EVN), by which the data are streamed in real-time from the telescopes to the central data processor at JIVE. Rapid analysis of the SN 2007gr data, obtained 22 days after the initial discovery, showed that the source was still visible in the radio spectrum, and confirmed the technical feasibility of radio observations. Based on this result, the team carried out further observations with the EVN and the Green Bank Telescope in West Virginia, USA. For the first time ever, they were able to show mildly relativistic expansion in such a source.

Although it showed peculiar radio properties, SN 2007gr was otherwise a normal Type Ic supernova. It appears that only a small fraction of the matter that was ejected in the explosion reached a velocity at least half the speed of light. According to the emerging picture, this mildly relativistic matter was collimated into a bipolar narrow cone, or jet. The team concludes that it is possible that all, or at least most, Type Ic supernovae produce bipolar jets, but the energy content of these mildly-relativistic outflows varies dramatically, while the total energy of the explosions is much more standard.

"At least a fraction of Type Ic supernovae have been thought for a long time to produce highly collimated relativistic jets," says Paragi. "Our observations support this and provide new clues for the understanding of how supernovae explode, and how some of them may be related to the even more energetic gamma ray bursts."

The Westerbork Synthesis Array Telescope, operated by ASTRON, played an important role in obtaining this result due to its large collecting area, which significantly improved the sensitivity of the VLBI observations. Moreover, it provided an independent measurement of the total flux density, or brightness, of the source.

These observations also showcase how the new e-VLBI services of the EVN empower astronomers to react quickly when transient events occur. "Organizing VLBI observations on a short timescale with the most sensitive radio telescopes on Earth is a challenging task," notes JIVE director Huib Jan van Langevelde. "Using the electronic-VLBI technique eliminates some of the major issues. Moreover, it allows us to produce immediate results necessary for the planning of additional measurements. The scientific outcome from the SN 2007gr observations shows the impact of the technological development in our field in the last few years, which allows highly efficient collaboration between radio telescopes within and even outside of Europe."


Zsolt Paragi, Senior Support Scientist
Joint Institute for VLBI in Europe
office: +31 (0)521 596536
mobile: +31 06-12787218

Huib Jan van Langevelde, Director
Joint Institute for VLBI in Europe
office: +31 (0)521 596515 / (0)521 596524
mobile: +31 06-21201419

Michael A. Garrett, Director
Netherlands Institute for Radio Astronomy
office: +31 (0)521 596126
mobile: +31 06-21201417

Kristine Yun, Public Outreach Officer
Joint Institute for VLBI in Europe
office: +31 (0)521 596543
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Femke Boekhorst, PR & Communications Officer
Netherlands Institute for Radio Astronomy
office: +31 (0)521 596204

Wednesday, January 27, 2010

Astronomers Find Rare Beast by New Means

Core-collapse supernova explosion
expelling nearly-spherical debris shell.

"Engine-driven" supernova explosion
with accretion disk and high-velocity jets.

For the first time, astronomers have found a supernova explosion with properties similiar to a gamma-ray burst, but without seeing any gamma rays from it. The discovery, using the National Science Foundation's Very Large Array (VLA) radio telescope, promises, the scientists say, to point the way toward locating many more examples of these mysterious explosions.

"We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites," said Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics.

The telltale clue came when the radio observations showed material expelled from the supernova explosion, dubbed SN2009bb, at speeds approaching that of light. This characterized the supernova, first seen last March, as the type thought to produce one kind of gamma-ray burst.

"It is remarkable that very low-energy radiation, radio waves, can signal a very high-energy event," said Roger Chevalier of the University of Virginia.

When the nuclear fusion reactions at the cores of very massive stars no longer can provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star's material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a "core-collapse supernova" as the cause of one kind of gamma-ray burst.

Not all supernovae of this type, however, produce gamma-ray bursts. "Only about one out of a hundred do this," according to Soderberg.

In the more-common type of such a supernova, the explosion blasts the star's material outward in a roughly-spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.

The superfast speeds in these rare blasts, astronomers say, are caused by an "engine" in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.

"This is the only way we know that a supernova explosion could accelerate material to such speeds," Soderberg said.

Until now, no such "engine-driven" supernova had been found any way other than by detecting gamma rays emitted by it.

"Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough. With the new capabilities of the Expanded VLA coming soon, we believe we'll find more in the future through radio observations than with gamma-ray satellites," Soderberg said.

Why didn't anyone see gamma rays from this explosion? "We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen," Soderberg said. In that case, finding such blasts through radio observations will allow scientists to discover a much larger percentage of them in the future.

"Another possibility," Soderberg adds, "is that the gamma rays were 'smothered' as they tried to escape the star. This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites."

One important question the scientists hope to answer is just what causes the difference between the "ordinary" and the "engine-driven" core-collapse supernovae. "There must be some rare physical property that separates the stars that produce the 'engine-driven' blasts from their more-normal cousins," Soderberg said. "We'd like to find out what that property is."

One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen. However, Soderberg points out, that does not seem to be the case for this supernova.

Soderberg and Chevalier worked with Alak Ray and Sayan Chakrabarti of the Tata Institute of Fundamental Research in India; Poonam Chandra of the Royal Military College of Canada; and a large group of collaborators at the Harvard-Smithsonian Center for Astrophysics. The scientists reported their findings in the January 28 issue of the journal Nature.

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


Dave Finley, Public Information Officer
Socorro, NM
(575) 835-7302

Black Hole Hunters Set New Distance Record

The black hole inside NGC 300 X-1 (artist’s impression)

NGC 300 X-1 in the spiral galaxy NGC 300

NGC 300 X-1 in the spiral galaxy NGC 300

The surroundings of NGC 300

Artist’s impression

Zoom in onto the stellar black hole NGC 300 X-1

Artist’s impression

Astronomers using ESO’s Very Large Telescope have detected, in another galaxy, a stellar-mass black hole much farther away than any other previously known. With a mass above fifteen times that of the Sun, this is also the second most massive stellar-mass black hole ever found. It is entwined with a star that will soon become a black hole itself.

The stellar-mass black holes [1] found in the Milky Way weigh up to ten times the mass of the Sun and are certainly not be taken lightly, but, outside our own galaxy, they may just be minor-league players, since astronomers have found another black hole with a mass over fifteen times the mass of the Sun. This is one of only three such objects found so far.

The newly announced black hole lies in a spiral galaxy called NGC 300, six million light-years from Earth. “This is the most distant stellar-mass black hole ever weighed, and it’s the first one we’ve seen outside our own galactic neighbourhood, the Local Group,” says Paul Crowther, Professor of Astrophysics at the University of Sheffield and lead author of the paper reporting the study. The black hole’s curious partner is a Wolf–Rayet star, which also has a mass of about twenty times as much as the Sun. Wolf–Rayet stars are near the end of their lives and expel most of their outer layers into their surroundings before exploding as supernovae, with their cores imploding to form black holes.

In 2007, an X-ray instrument aboard NASA’s Swift observatory scrutinised the surroundings of the brightest X-ray source in NGC 300 discovered earlier with the European Space Agency’s XMM-Newton X-ray observatory. “We recorded periodic, extremely intense X-ray emission, a clue that a black hole might be lurking in the area,” explains team member Stefania Carpano from ESA.

Thanks to new observations performed with the FORS2 instrument mounted on ESO’s Very Large Telescope, astronomers have confirmed their earlier hunch. The new data show that the black hole and the Wolf–Rayet star dance around each other in a diabolic waltz, with a period of about 32 hours. The astronomers also found that the black hole is stripping matter away from the star as they orbit each other.

“This is indeed a very ‘intimate’ couple,” notes collaborator Robin Barnard. “How such a tightly bound system has been formed is still a mystery.”

Only one other system of this type has previously been seen, but other systems comprising a black hole and a companion star are not unknown to astronomers. Based on these systems, the astronomers see a connection between black hole mass and galactic chemistry. “We have noticed that the most massive black holes tend to be found in smaller galaxies that contain less ‘heavy’ chemical elements,” says Crowther [2]. “Bigger galaxies that are richer in heavy elements, such as the Milky Way, only succeed in producing black holes with smaller masses.” Astronomers believe that a higher concentration of heavy chemical elements influences how a massive star evolves, increasing how much matter it sheds, resulting in a smaller black hole when the remnant finally collapses.

In less than a million years, it will be the Wolf–Rayet star’s turn to go supernova and become a black hole. “If the system survives this second explosion, the two black holes will merge, emitting copious amounts of energy in the form of gravitational waves as they combine [3],” concludes Crowther. However, it will take some few billion years until the actual merger, far longer than human timescales. “Our study does however show that such systems might exist, and those that have already evolved into a binary black hole might be detected by probes of gravitational waves, such as LIGO or Virgo [4].”

[1] Stellar-mass black holes are the extremely dense, final remnants of the collapse of very massive stars. These black holes have masses up to around twenty times the mass of the Sun, as opposed to supermassive black holes, found in the centre of most galaxies, which can weigh a million to a billion times as much as the Sun. So far, around 20 stellar-mass black holes have been found.

[2] In astronomy, heavy chemical elements, or “metals”, are any chemical elements heavier than helium.

[3] Predicted by Einstein’s theory of general relativity, gravitational waves are ripples in the fabric of space and time. Significant gravitational waves are generated whenever there are extreme variations of strong gravitational fields with time, such as during the merger of two black holes. The detection of gravitational waves, never directly observed to date, is one of the major challenges for the next few decades.

[4] The LIGO and Virgo experiments have the goal of detecting gravitational waves using sensitive interferometers in Italy and the United States.

More information

This research was presented in a letter to appear in the Monthly Notices of the Royal Astronomical Society (NGC 300 X-1 is a Wolf–Rayet/Black Hole binary, P.A. Crowther et al.).

The team is composed of Paul Crowther and Vik Dhillon (University of Sheffield, UK), Robin Barnard and Simon Clark (The Open University, UK), and Stefania Carpano and Andy Pollock (ESAC, Madrid, Spain).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory, and VISTA, the largest survey telescope in the world. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Research paper


Paul Crowther
University of Sheffield, UK
Tel: +44-114 222 4291

Stefania Carpano
The Netherlands
Tel: +31-71-5654827

Caught in the act: a merging binary QSO

The NOAO i band image of the binary quasar on the right, with the Chandra X-ray image on the left. The X-ray image is contoured and those contours overlaid on the i-band image.

It has been assumed for some time that binary supermassive black holes (SMBH) should be common in the universe, given that galaxies regularly interact and merge and that most, if not all, galaxies contain a SMBH. Such a SMBH will only be detected as a quasar when it is accreting matter. And galaxy merging is a leading proposal to trigger such accretion. Now the first luminous, spatially resolved binary quasar that clearly inhabits an interacting/merging galaxy pair has been reported. ( SDSS J1254+0846: A Binary Quasar Caught in the Act of Merging, Green et. al., Ap. J accepted Jan. 2010 ). The unique properties of this system allow detailed numerical simulations to create plausible scenarios for the histories of both the host galaxies and the SMBH that inhabit them.

The first spectrum confirming this binary QSO was taken by A. Myers at the KPNO Mayall 4-meter using R-C Spectrograph on Feb 12, 2008. Subsequent Chandra/NOAO observations (P. Green, P.I.) with the MOSAIC imager on the Mayall 4m image obtained March 18, 2009 (Barkhouse, Myers observing) revealed the existence of the tidal arms in the host galaxy,seen in the figure above. Additional deeper imaging and spectroscopy with Magellan/IMACS were used by the authors to determine the properties and history of this merger.

Sunday, January 24, 2010

The First of Many Asteroid Finds for WISE

The red dot at the center of this image is the first near-Earth asteroid discovered by NASA's Wide-Field Infrared Survey Explorer, or WISE. Image credit: NASA/JPL-Caltech/UCLA

NASA's Wide-field Infrared Survey Explorer, or WISE, has spotted its first never-before-seen near-Earth asteroid, the first of hundreds it is expected to find during its mission to map the whole sky in infrared light.

The near-Earth object, designated 2010 AB78, was discovered by WISE Jan. 12. After the mission's sophisticated software picked out the moving object against a background of stationary stars, researchers followed up and confirmed the discovery with the University of Hawaii's 2.2-meter (88-inch) visible-light telescope near the summit of Mauna Kea.

The asteroid is currently about 158 million kilometers (98 million miles) from Earth. It is estimated to be roughly 1 kilometer (0.6 miles) in diameter and circles the sun in an elliptical orbit tilted to the plane of our solar system. The object comes as close to the sun as Earth, but because of its tilted orbit, it is not thought to pass near our planet. This asteroid does not pose any foreseeable impact threat to Earth, but scientists will continue to monitor it.

WISE, which began its all-sky survey on Jan. 14, is expected to find about 100-thousand previously undiscovered asteroids in the Main Belt between Mars and Jupiter, and hundreds of new near-Earth asteroids. It will also spot millions of new stars and galaxies.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the WISE for NASA's Science Mission Directorate, Washington. The principal investigator, Edward Wright, is at UCLA. The mission was competitively selected under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA. The ground-based observations are partly supported by the National Science Foundation.

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

Friday, January 22, 2010

West Virginia Student Discovers New Pulsar

Credit: NASA

Shay Bloxton

A West Virginia high-school student has discovered a new pulsar, using data from the giant Robert C. Byrd Green Bank Telescope (GBT).

Shay Bloxton, 15, a participant in a project in which students analyze data from the radio telescope, spotted evidence of the pulsar on October 15. Bloxton, along with NRAO astronomers observed the object again one month later. The new observation confirmed that the object is a pulsar, a rotating, superdense neutron star. Bloxton is a sophomore at Nicholas County High School in Summersville, West Virginia.

"I was very excited when I found out I had actually made a discovery," Bloxton said. She went to Green Bank in November to participate in the follow-up observation. She termed that visit "a great experience."

"It also helped me learn a lot about how observations with the GBT are actually done," she added.

The project in which she participated, called the Pulsar Search Collaboratory (PSC), is a joint project of the National Radio Astronomy Observatory (NRAO) and West Virginia University, funded by a grant from the National Science Foundation.

Pulsars are known for their lighthouse-like beams of radio waves that sweep through space as the neutron star rotates, creating a pulse as the beam sweeps by the Earth. First discovered in 1967, pulsars serve as valuable natural "laboratories" for physicists studying exotic states of matter, quantum mechanics and General Relativity. The GBT, dedicated in 2000, has become one of the world's leading tools for discovering and studying pulsars.

The PSC, led by NRAO Education Officer Sue Ann Heatherly and Project Director Rachel Rosen, includes training for teachers and student leaders, and provides parcels of data from the GBT to student teams. The project involves teachers and students in helping astronomers analyze data from 1500 hours of observing with the GBT. The 120 terabytes of data were produced by 70,000 individual pointings of the giant, 17-million-pound telescope. Some 300 hours of the observing data were reserved for analysis by student teams.

The student teams use analysis software to reveal evidence of pulsars. Each portion of the data is analyzed by multiple teams. In addition to learning to use the analysis software, the student teams also must learn to recognize man-made radio interference that contaminates the data. The project will continue through 2011. Teachers interested in participating in the program can learn more at this link.

For Bloxton, the pulsar discovery may be only her first in a scientific career. "Participating in the PSC has definitely encouraged me to pursue my dream of being an astrophysicist," she said, adding that she hopes to attend West Virginia University to study astrophysics.

Late last year, another West Virginia student, from South Harrison High School, Lucas Bolyard, discovered a pulsar-like object called a rotating radio transient. His discovery also came through participation in the PSC.

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


Dr. Rachel Rosen, PSC Project Director
(304) 456-2385

Thursday, January 21, 2010

ESO 137-001: Two Tails to Tell

Credit X-ray: NASA/CXC/UVa/M. Sun, et al;
H-alpha/Optical: SOAR (UVa/NOAO /UNC/CNPq-Brazil)/M.Sun et al.

Two spectacular tails of X-ray emission have been seen trailing behind a galaxy using the Chandra X-ray Observatory. A composite image of the galaxy cluster Abell 3627 shows X-rays from Chandra in blue, optical emission in yellow and emission from hydrogen light -- known to astronomers as "H-alpha" -- in red. The optical and H-alpha data were obtained with the Southern Astrophysical Research (SOAR) Telescope in Chile.

At the front of the tail is the galaxy ESO 137-001. The brighter of the two tails has been seen before and extends for about 260,000 light years. The detection of the second, fainter tail, however, was a surprise to the scientists.

The X-ray tails were created when cool gas from ESO 137-001 (with a temperature of about ten degrees above absolute zero) was stripped by hot gas (about 100 million degrees) as it travels towards the center of the galaxy cluster Abell 3627. What astronomers observe with Chandra is essentially the evaporation of the cold gas, which glows at a temperature of about 10 million degrees. Evidence of gas with temperatures between 100 and 1,000 degrees Kelvin in the tail was also found with the Spitzer Space Telescope.

Galaxy clusters are collections of hundreds or even thousands of galaxies held together by gravity that are enveloped in hot gas. The two-pronged tail in this system may have formed because gas has been stripped from the two major spiral arms in ESO 137-001. The stripping of gas is thought to have a significant effect on galaxy evolution, removing cold gas from the galaxy, shutting down the formation of new stars in the galaxy, and changing the appearance of inner spiral arms and bulges because of the effects of star formation.

The H-alpha data shows evidence for star formation in the tails -- the first unambiguous evidence that star formation can occur when cold gas is stripped out of galaxies as they fall through clusters. The Chandra data also reveal an excess of luminous X-ray point sources around the X-ray tails. Some of them are considered to be young massive binary stars associated with nearby young star clusters, giving more evidence of star formation in the tails. The implication is that a large portion of stars between cluster galaxies can be formed in situ.

The X-ray data also reveal that there is little change in temperature of the hot gas in the tails, and also little change in width of the tails with distance from ESO 137-001. Both of these features present challenges to scientists doing simulations of the galaxy tails.

Fast Facts for ESO 137-001:

Scale: Image is 5 arcmin across
Category: Groups & Clusters of Galaxies
Coordinates: (J2000) RA 16h 13m 25.59s | Dec -60° 45' 43.10
Constellation: Norma
Observation Date: 6/12/2008
Observation Time: 39 hours
Obs. ID: 9518
Color Code: X-ray (Blue); Optical (Yellow); H-alpha (Red)
Instrument: ACIS
References: Sun, M., et al, 2010, ApJ 708 946
Distance Estimate: About 230 million light years

Wednesday, January 20, 2010

On the Trail of a Cosmic Cat

The Cat's Paw Nebula

Around the Cat's Paw Nebula

Zooming into the Cat's Paw Nebula

Panning across the Cat’s Paw Nebula

ESO has just released a stunning new image of the vast cloud known as the Cat’s Paw Nebula or NGC 6334. This complex region of gas and dust, where numerous massive stars are born, lies near the heart of the Milky Way galaxy, and is heavily obscured by intervening dust clouds.

Few objects in the sky have been as well named as the Cat’s Paw Nebula, a glowing gas cloud resembling the gigantic pawprint of a celestial cat out on an errand across the Universe. British astronomer John Herschel first recorded NGC 6334 in 1837 during his stay in South Africa. Despite using one of the largest telescopes in the world at the time, Herschel seems to have only noted the brightest part of the cloud, seen here towards the lower left.

NGC 6334 lies about 5500 light-years away in the direction of the constellation Scorpius (the Scorpion) and covers an area on the sky slightly larger than the full Moon. The whole gas cloud is about 50 light-years across. The nebula appears red because its blue and green light are scattered and absorbed more efficiently by material between the nebula and Earth. The red light comes predominantly from hydrogen gas glowing under the intense glare of hot young stars.

NGC 6334 is one of the most active nurseries of massive stars in our galaxy and has been extensively studied by astronomers. The nebula conceals freshly minted brilliant blue stars — each nearly ten times the mass of our Sun and born in the last few million years. The region is also home to many baby stars that are buried deep in the dust, making them difficult to study. In total, the Cat’s Paw Nebula could contain several tens of thousands of stars.

Particularly striking is the red, intricate bubble in the lower right part of the image. This is most likely either a star expelling large amount of matter at high speed as it nears the end of its life or the remnant of a star that already has exploded.

This new portrait of the Cat’s Paw Nebula was created from images taken with the Wide Field Imager (WFI) instrument at the 2.2-metre MPG/ESO telescope at the La Silla Observatory in Chile, combining images taken through blue, green and red filters, as well as a special filter designed to let through the light of glowing hydrogen.

More information

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.


Henri Boffin
ESO La Silla-Paranal/E-ELT Press Officer
Garching, Germany
Tel: +49 89 3200 6222

Tuesday, January 19, 2010

Weak Lensing Gains Strength

Visible-light images from the Hubble Space Telescope populate this tiny section of the full two-square-degree Cosmic Evolution Survey (COSMOS), which combines data in many wavelengths from space and ground-based telescopes around the world. COSMOS was the basis of a new extension of the mass-luminosity relation for weak lensing studies.

Berkeley, CA — Weak gravitational lensing is a uniquely promising way to learn how much dark matter there is in the Universe and how its distribution has evolved since the distant past. New work by a team led by a cosmologist from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory has made major progress in extending the use of gravitational lensing to the study of much older and smaller structures than was previously possible.

Until recently, weak lensing had been limited to calculating the total mass of relatively nearby groups and clusters of galaxies. Their total mass includes both ordinary, visible matter like stars and dust – what astronomers call “baryonic” matter – plus the much more massive invisible concentrations of dark matter that form groups and clusters by pulling galaxies together.

Astronomers were able to establish an important scaling relationship for nearby clusters between their total masses, determined by gravitational lensing, and the brightness of their x-ray emissions, an indication of the mass of the ordinary matter alone. A new study in the Astrophysical Journal (ApJ) now continues this important relationship to distant objects.

“We’ve been able to extend measurements of mass to much smaller structures, which existed much earlier in the history of the Universe,” says Alexie Leauthaud, a Chamberlain Fellow in Berkeley Lab’s Physics Division and first author of the ApJ study. “This helps us gain a better understanding of the relationship between the normal matter in dense structures, which are seen through the x-ray luminosity, and the total dark-matter mass of these structures, as measured by the weak lensing.” Leauthaud is a member of the Berkeley Center for Cosmological Physics (BCCP) at UC Berkeley and Berkeley Lab.

Mass as a lens

Gravitational lensing occurs because mass curves the space around it, bending the paths along which rays of light travel: the more mass (and the closer to the center of mass), the more space bends, and the more the image of a distant object is displaced and distorted. Thus measuring distortion, or “shear,” is key to measuring the mass of the lensing object.

At least this is so for “strong” lensing. A very massive object or collection of objects, like a nearby galaxy cluster and the invisible dark matter that encloses it, distorts the apparent shape and position of bright objects beyond it so much that the distant images are bent and may even be smeared into rings around the foreground cluster. The visible distortion is a direct measure of the mass of the lens and points to its center.

A spectacular example of strong gravitational lensing is the nearby galaxy cluster Abell 2218 (top), in which the visible distortion of individual background galaxies can be used to measure the mass of the lensing structure. The weak lensing of fainter and more distant structures must be detected by statistical averaging (bottom). (Abell 2218 image by NASA, weak lensing simulation by Bhuvnesh Jain, Uroš Seljak, and Simon White)

Weak lensing works the same way, except that the shear is too subtle to be seen directly. Most of the apparent shear isn’t distortion at all – a galaxy has its own distinct shape, and we often see it from an angle that makes it look elongated. Apparent shear may also be due to the telescope, the detector, or the atmosphere.

Nevertheless, faint additional distortions in a collection of distant galaxies can be calculated statistically, and the average shear due to the lensing of some massive object in front of them can be computed. Yet to calculate the lens’s mass from average shear, one needs to know its center.

“The problem with low-mass, high-redshift clusters is that it is difficult to determine which exact galaxy lies at the center of the cluster,” says Leauthaud. “That’s where x-rays help. The x-ray luminosity from a galaxy cluster can be used to find its center very accurately.”

The hot intracluster medium of gas or plasma that fills almost all galaxy clusters emits x-rays, making x-ray emission a convenient way to find distant galaxy structures in the night sky. But how does this emission help find the center of mass in a galaxy cluster? For the same reason that dark matter is dark.

Why dark matter is dark

Except through gravitation, dark matter does not interact (or interacts only very weakly) with itself or with ordinary matter. Indeed, that’s why it’s dark: to emit light it would have to interact via the electromagnetic force.

With ordinary matter, electromagnetism affects everything from chemistry to luminosity to electric and magnetic fields and even the pressure of stellar winds; thus electromagnetism plays an important role in determining the arrangement of ordinary matter, which is often irregular.

Because electromagnetism plays no role in the distribution of dark matter, however, dark matter forms large, smooth, spherical clumps, usually filled by ordinary galaxies plus hot gas or plasma, which it has trapped and retained solely through gravitation.

“Gas density follows the dark matter density, and because x-ray emission scales as the square of the gas density, the x-ray light shines very strongly in the core of the structure,” Leauthaud explains. “So x-rays are an excellent way to determine the center of even a distant, fuzzy galaxy cluster.”

“Basically the more mass, the more heat,” says Jean-Paul Kneib, a lead author of the ApJ paper from the Laboratory of Astrophysics of Marseilles (LAM) and France’s National Center for Scientific Research (CNRS). “But the plasma is baryonic matter, which is only a small part of the total mass of the cluster. While the x-radiation tells you something about the total mass, you need to get the scaling just right.”

Visible matter follows an underlying dark matter scaffolding. At left, blue indicates the mass of stars in galaxies in a given area, yellow the number of galaxies, and red the sources of brightest x-ray emission. Contours at right are the distribution of dark matter, from gravitational lensing. (Richard Massey et al, Nature 2007.

To pin down the scaling relation between x-ray brightness and total mass, Leauthaud and her colleagues first used x-ray luminosity to identify the center of mass of 206 galaxy groups and clusters, including numerous faint, distant clusters listed in the Hubble Space Telescope’s Cosmic Evolution Survey (COSMOS), which is curated by Nick Scoville of the California Institute of Technology, an author of the ApJ paper.

X-ray imaging came from the European Space Agency’s XMM-Newton satellite and from NASA’s Chandra satellite, whose principal investigator is Martin Elvis of the Harvard-Smithsonian Center for Astrophysics, an author of the ApJ paper. Elvis says, “I never thought our Chandra data would enable such a great measurement. In fact I was astonished when Alexie first showed me the results. It’s quite a tour de force of analysis, and really convincing.”

The X-ray analysis itself was performed by Alexis Finoguenov of the Max Planck Institute for Extraterrestrial Physics and the University of Maryland, one of the paper’s lead authors. Knowing the centers of mass from analysis of x-ray emission, the researchers could now use weak lensing to estimate the total mass of the distant groups and clusters with greater accuracy than ever before.

Finally they calculated the mass-luminosity relation for the new collections of groups and clusters and found that it was consistent with previous relations established by surveys of much closer structures – including some studied with strong gravitational lensing. Within calculable uncertainty, the relation follows the same straight slope from nearby galaxy clusters to distant ones; a simple, consistent scaling factor relates a group or cluster’s total mass to its x-ray brightness, or “baryonic tracer.”

“By confirming the mass-luminosity relation and extending it to high redshifts,” Leauthaud says, “we have taken a small step in the right direction toward using weak lensing as a powerful tool to measure the evolution of structure.”

In the beginning

The origin of galaxies can be traced back to slight differences in the density of the hot, liquid-like early universe; traces of these differences can still be seen as minute temperature differences in the cosmic microwave background (CMB).

“The variations we observe in the ancient microwave sky represent the imprints that developed over time into the cosmic dark-matter scaffolding for the galaxies we see today,” says BCCP director and UC Berkeley physics professor George Smoot of Berkeley Lab’s Physics Division, who shared the 2006 Nobel Prize in Physics for measuring anisotropies in the CMB and is one of the authors of the ApJ paper. “It is very exciting that we can actually measure with gravitational lensing how the dark matter has collapsed and evolved since the beginning.”

Dark matter shapes visible matter in a way that reflects the nature of dark energy. How galaxies are distributed in a Universe with no dark energy (left) would differ measurably from one in which dark energy is significant (right).

One goal in studying the evolution of structure is to understand dark matter itself, and how it interacts with the ordinary matter we can see. Another goal is to learn more about dark energy, the mysterious something that is pushing matter apart and causing the Universe to expand at an accelerating rate. Is dark energy constant, or is it dynamic? Or is it unreal, merely an illusion caused by a limitation in Einstein’s General Theory of Relativity?

The tools provided by the extended mass-luminosity relationship will do much to answer these questions about the opposing roles of gravity and dark energy in the once and future shape of the Universe.

“A Weak Lensing Study of X-Ray Groups in the COSMOS Survey: Form and Evolution of the Mass-Luminosity Relation,” by Alexie Leauthaud, Alexis Finoguenov, Jean-Paul Kneib, James E. Taylor, Richard Massey, Jason Rhodes, Olivier Ilbert, Kevin Bundy, Jeremy Tinker, Matthew R. George, Peter Capak, Anton M. Koekemoer, David E. Johnston, Yu-Ying Zhang, Nico Cappelluti, Richard S. Ellis, Martin Elvis, Catherine Heymans, Oliver Le Fèvre, Simon Lilly, Henry J. McCracken, Yannick Mellier, Alexandre Réfrégier, Mara Salvato, Nick Scoville, George Smoot, Masayuki Tanaka, Ludovic Van Waerbeke, and Melody Wolk, appears in the Astrophysical Journal and is available online to subscribers.

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

Additional Information

More about
weak gravitational lensing

More about the COSMOS survey

Alexie Leauthaud demonstrates how dark matter causes gravitational lensing

Scientific contact: Alexie Leauthaud