Tuesday, March 31, 2015

Herschel and Planck find missing clue to galaxy cluster formation

Proto-cluster candidates
The Planck all-sky map at submillimetre wavelengths (545 GHz). The band running through the middle corresponds to dust in our Milky Way galaxy. The black dots indicate the location of the proto-cluster candidates identified by Planck and subsequently observed by Herschel. The inset images showcase some of the observations made by Herschel’s SPIRE instrument; the contours represent the density of galaxies.Copyright: ESA and the Planck Collaboration/ H. Dole, D. Guéry & G. Hurier, IAS/University Paris-Sud/CNRS/CNES. Hi-res Image
The history of the Universe
Copyright: ESA

By combining observations of the distant Universe made with ESA’s Herschel and Planck space observatories, cosmologists have discovered what could be the precursors of the vast clusters of galaxies that we see today. 

Galaxies like our Milky Way with its 100 billion stars are usually not found in isolation. In the Universe today, 13.8 billion years after the Big Bang, many are in dense clusters of tens, hundreds or even thousands of galaxies. 

However, these clusters have not always existed, and a key question in modern cosmology is how such massive structures assembled in the early Universe. 

Pinpointing when and how they formed should provide insight into the process of galaxy cluster evolution, including the role played by dark matter in shaping these cosmic metropolises. 

Now, using the combined strengths of Herschel and Planck, astronomers have found objects in the distant Universe, seen at a time when it was only three billion years old, which could be precursors of the clusters seen around us today.   

Planck’s main goal was to provide the most precise map of the relic radiation of the Big Bang, the cosmic microwave background. To do so, it surveyed the entire sky in nine different wavelengths from the far-infrared to radio, in order to eliminate foreground emission from our galaxy and others in the Universe.

But those foreground sources can be important in other fields of astronomy, and it was in Planck’s short wavelength data that scientists were able to identify 234 bright sources with characteristics that suggested they were located in the distant, early Universe. 

Herschel then observed these objects across the far-infrared to submillimetre wavelength range, but with much higher sensitivity and angular resolution.

Herschel revealed that the vast majority of the Planck-detected sources are consistent with dense concentrations of galaxies in the early Universe, vigorously forming new stars.

Each of these young galaxies is seen to be converting gas and dust into stars at a rate of a few hundred to 1500 times the mass of our Sun per year. By comparison, our own Milky Way galaxy today is producing stars at an average rate of just one solar mass per year.

While the astronomers have not yet conclusively established the ages and luminosities of many of these newly discovered distant galaxy concentrations, they are the best candidates yet found for ‘proto-clusters’ – precursors of the large, mature galaxy clusters we see in the Universe today.

“Hints of these kinds of objects had been found earlier in data from Herschel and other telescopes, but the all-sky capability of Planck revealed many more candidates for us to study,” says Hervé Dole of the Institut d’Astrophysique Spatiale, Orsay, lead scientist of the analysis published today in Astronomy & Astrophysics.

“We still have a lot to learn about this new population, requiring further follow-up studies with other observatories. But we believe that they are a missing piece of cosmological structure formation.”

“We are now preparing an extended catalogue of possible proto-clusters detected by Planck, which should help us identify even more of these objects,” adds Ludovic Montier, a CNRS researcher at the Institut de Recherche en Astrophysique et Planétologie, Toulouse, who is the lead scientist of the Planck catalogue of high-redshift source candidates, which is about to be delivered to the community.

“This exciting result was possible thanks to the synergy between Herschel and Planck: rare objects could be identified from the Planck data covering the entire sky, and then Herschel was able to scrutinise them in finer detail,” says ESA’s Herschel Project Scientist, Göran Pilbratt.

“Both space observatories completed their science observations in 2013, but their rich datasets will be exploited for plentiful new insights about the cosmos for years to come.” 

Note for Editors

Planck detected the sky at nine frequencies, from 30 GHz to 857 GHz. The Planck frequencies used to detect the candidate proto-clusters in this study were 857 GHz, 545 GHz and 353 GHz. The follow-up observations made by Herschel’s SPIRE instrument were at 250, 350 and 500 microns. The SPIRE 350 micron and 500 micron bands overlap with Planck’s High Frequency Instrument (HFI) at 857 GHz and 545 GHz.

The Planck Scientific Collaboration consists of all the scientists who have contributed to the development of the mission, and who participate in the scientific exploitation of the data during the proprietary period. These scientists are members of one or more of four consortia: the LFI Consortium, the HFI Consortium, the DK-Planck Consortium and ESA’s Planck Science Office. The two European-led Planck Data Processing Centres are located in Paris, France and Trieste, Italy. The LFI consortium is led by N. Mandolesi, ASI, Italy (deputy PI: M. Bersanelli, Universita’ degli Studi di Milano, Italy), and was responsible for the development and operation of LFI. The HFI consortium is led by J.L. Puget, Institut d’Astrophysique Spatiale in Orsay, France (deputy PI: F. Bouchet, Institut d’Astrophysique de Paris, France), and was responsible for the development and operation of HFI.

For more information, please contact:

Markus Bauer

ESA Science and Robotic Exploration Communication Officer

Tel: +31 71 565 6799; +34 91 8131 199

Mob: +31 61 594 3954


Hervé Dole
Institut d’Astrophysique Spatiale (CNRS & Univ. Paris-Sud) and Institut Universitaire de France Orsay, France

Tel: +33 1 69 85 85 72

Ludovic Montier
Institut de Recherche en Astrophysique et Planétologie (CNRS & Univ. Paul Sabatier Toulouse III), Toulouse, France
Tel: +33 5 61 55 65 51

Jan Tauber

ESA Planck Project Scientist

Tel: +31 71 565 5342

: Jan.Tauber@esa.int

Göran Pilbratt

ESA Herschel Project Scientist

Tel: +31 71 565 3621


Source: ESA

The tumultuous heart of the Large Magellanic Cloud

The tumultuous heart of the Large Magellanic Cloud
Copyright: ESA/NASA/JPL-Caltech/STScI

A scene of jagged fiery peaks, turbulent magma-like clouds and fiercely hot bursts of bright light. Although this may be reminiscent of a raging fire or the heart of a volcano, it actually shows a cold cosmic clump of gas, dust and stars.

The subject of this image, from ESA’s Herschel Space Observatory and NASA’s Spitzer Space Telescope, is the irregularly shaped Large Magellanic Cloud (LMC), one of the nearest galaxies to our own.

The dark, orange-tinted patches throughout the galaxy are plumes of murky dust. The hints of deep red and green mark areas of particularly cool dust, with white and blue tones highlighting hot regions of furious star formation. These pale pockets of gas are heated by the very stars they are creating, which push hot winds out into their surroundings.

To make this scene even more uninviting, the LMC is also home to a giant cosmic spider – the Tarantula Nebula. This hot cloud of gas and dust is easily visible as the brightest region in this image, located towards the lower left of the frame. This nebula is very well studied, for example by the NASA/ESA Hubble Space Telescope, which last year produced a stunning infrared mosaic showing the celestial creepy-crawly in great detail.

This is one of the reasons astronomers like to explore the LMC; it is close enough to us that we can pick out individual nebulas – including the Tarantula – and study how stars form, evolve and die in other galaxies. The LMC is populated by a mix of old and young stars, many of which are lined up along the galaxy’s central ‘bar’, which slants from the bottom left to the top right of this image.

ESA’s Herschel and NASA’s Spitzer are both space telescopes that explore the Universe in infrared light. The LMC looks quite different – and much more serene – in visible light, instead resembling a scattering of pale stars with occasional plumes of pink and purple.

The data making up this image are from Herschel’s Spectral and Photometric Imaging Receiver (SPIRE) and Photoconductor Array Camera and Spectrometer (PACS), and Spitzer’s Multiband Imaging Photometer (MIPS).

This image was previously published by NASA/JPL.

As Stars Form, Magnetic Fields Influence Regions Big and Small

The new study, which the journal Nature is publishing online on March 30th, probed the Cat's Paw Nebula, also known as NGC 6334. This nebula contains about 200,000 suns' worth of material that is coalescing to form new stars, some with up to 30 to 40 times as much mass as our sun. It is located 5,500 light-years from Earth in the constellation Scorpius.

The team painstakingly measured the orientation of magnetic fields within the Cat's Paw. "We found that the magnetic field direction is quite well preserved from large to small scales, implying that self-gravity and cloud turbulence are not able to significantly alter the field direction," said lead author Hua-bai Li (The Chinese University of Hong Kong), who conducted the high-resolution observations while a post-doctoral fellow at the Harvard-Smithsonian Center for Astrophysics (CfA).

"Even though they're much weaker than Earth's magnetic field, these cosmic magnetic fields have an important effect in regulating how stars form," added Smithsonian co-author T.K. Sridharan (CfA).

The team observed polarized light coming from dust within the nebula using several facilities, including the Smithsonian's Submillimeter Array. "The SMA's unique capability to measure polarization at high angular resolution allowed access to the magnetic fields at the smallest spatial scales," said SMA director Ray Blundell (CfA).

"The SMA has made significant contributions in this field which continues with this work," said Smithsonian co-author Qizhou Zhang (CfA).

Because dust grains align themselves with the magnetic field, the researchers were able to use dust emission to measure the field's geometry. They found that the magnetic fields tended to line up in the same direction, even though the relative size scales they examined were different by orders of magnitude. The magnetic fields only became misaligned on the smallest scales in cases where strong feedback from newly formed stars created other motions.

This work represents the first time magnetic fields in a single region have been measured at so many different scales. It also has interesting implications for the history of our galaxy.

When a molecular cloud collapses to form stars, magnetic fields hinder the process. As a result, only a fraction of the cloud's material is incorporated into stars. The rest gets dispersed into space, where it is available to make new generations of stars. Thanks to magnetic fields, the star-forming process is more drawn out.

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:

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics

Monday, March 30, 2015

An Efficient Star Making Galaxy

The dwarf galaxy NGC 5253 as seen in the optical with Hubble and in molecular gas with the Submillimeter Array (in red). The bright central region appears to making new stars with an efficiency ten times greater than that in the Milky Way, perhaps the result of the infalling CO gas streamer seen to the left.  Credit:  Nature; NASA HST; SMA

New stars regularly appear in the night sky as the gas and dust in giant interstellar clouds gradually coalesce under the influence of gravity. The process of making stars, however, is inefficient, and (at least in present-day galaxies) there are copious amounts of material that don't make it into stars. For the Milky Way, the efficiency overall (as measured by the mass in stars compared to the total mass of the galaxy) is about 5%; in clouds with turbulent gas motions this value can be even lower. The low efficiency is a critical parameter in galaxy evolution, and is one reason why stars are still forming nearly fourteen billion years after the Big Bang. Another consequence is seen in the production of star clusters. A low efficiency that produces stars gradually does not easily produce star clusters, because the new stars can drift away from the diffuse cloud. The existence of ancient massive bound star clusters (globular clusters) in the Milky Way, therefore, suggests that when they formed early in galactic history, star formation efficiencies were higher.

A local dwarf galaxy, NGC 5253, has a young star cluster that provides an example of highly efficient star formation. CfA astronomer Jun-Hui Zhao and his colleagues used the Submillimeter Array (SMA) to study the molecular gas (carbon monoxide, CO) at the center of this galaxy in a source called "Cloud D". Usually astronomers use the intensity of the CO radiation to estimate the total gas mass, but this can be a misleading measure since it requires knowing the relative amount of CO to the total material. The team instead used the motions of the gas to infer the total mass present; they used the amount of ultraviolet light to determine the number of stars. The scientists report that their technique is a much more reliable way of measuring the star formation rate.

The astronomers, writing in the latest issue of Nature, find that when they apply their method to the hot, dense and dusty Cloud D, they find a star-formation efficiency exceeding 50%. They note that their SMA images show a streamer of molecular gas falling into the galaxy toward this cloud, and they argue that this infalling material (about two million solar masses of gas) could compress the cloud and thereby induce the dramatic star formation efficiency seen. The new paper also suggests that a similar kind of infall and compression mechanism might have enabled comparably higher star formation rates at earlier times in cosmic history.

"Highly Efficient Star Formation in NGC 5253 Possibly from Stream-Fed Accretion," J. L. Turner, S. C. Beck, D. J. Benford, S. M. Consiglio, P. T. P. Ho, A. Kovacs, D. S. Meier & J.-H. Zhao, Nature 519, 331, 2015

Don’t Blink: A Light Show in a Dynamic Stellar Nursery

The region of Re50 and Re50N observed in 2006 with SuprimeCam at the Subaru telescope, and in 2014 with the Gemini Multi-Object Spectrograph (GMOS) at the Gemini South telescope. A [SII] filter was used for both images. The seeing was in both cases 0.5 arcsec. Each image is about 3 arcmin wide. North is up and east is left. 

Changes in the universe don’t often happen on human timescales.

In the cosmic “blink of an eye,” astronomers have detected rapid changes in brightness and appearance of a restless stellar nursery in Orion. The luminous cloud of gas, going by the designation Re50, first appeared about half a century ago in the constellation of Orion. Now, astronomers using the Gemini South telescope, and other telescopes around the world, have discovered that the chaotic caldron has once again brightened further. According to team member Bo Reipurth, of the University of Hawaii’s Institute for Astronomy, “This most recent brightening, happened, I believe in 2014, when unfortunately we weren’t able to look since Orion was in the Sun’s glare.” Reipurth adds that areas of stellar birth, in this case called a Class I protostar, are extremely dynamic places and change on human timescales, “… while we missed the initial brightening event, we can still study the changes going on and learn a lot about what’s happening.” Based on the team’s observations, they conclude that curtains of obscuring material are likely casting patterns of illumination and shadows onto the molecular cloud that envelopes the nursery, “…which gives us a spectacular stellar light show!” says Reipurth. 
Learn more in the team’s paper, which is accepted for publication in The Astrophysical Journal, at: http://arxiv.org/abs/1503.04241


The luminous Class I protostar HBC 494, embedded in the Orion A cloud, is associated with a pair of reflection nebulae, Re50 and Re50N, which appeared sometime between 1955 and 1979. We have found that a dramatic brightening of Re50N has taken place sometime between 2006 and 2014. This could result if the embedded source is undergoing a FUor eruption. However, the near-infrared spectrum shows a featureless very red continuum, in contrast to the strong CO bandhead absorption displayed by FUors. Such heavy veiling, and the high luminosity of the protostar, is indicative of strong accretion but seemingly not in the manner of typical FUors. We favor the alternative explanation that the major brightening of Re50N and the simultaneous fading of Re50 is caused by curtains of obscuring material that cast patterns of illumination and shadows across the surface of the molecular cloud. This is likely occurring as an outflow cavity surrounding the embedded protostar breaks through to the surface of the molecular cloud. Several Herbig-Haro objects are found in the region. 

Sunday, March 29, 2015

Chemical fingerprints of ancient supernovae found Carnegie Institution of Washington

The Sculptor dwarf galaxy composed from data from the Digitized Sky Survey 2, courtesy of ESO/Digitized Sky Survey 2. 
A larger version is available here.

Pasadena, CA— A Carnegie-based search of nearby galaxies for their oldest stars has uncovered two stars in the Sculptor dwarf galaxy that were born shortly after the galaxy formed, approximately 13 billion years ago. The unusual chemical content of the stars may have originated in a single supernova explosion from the first generation of Sculptor stars. The team, which includes Carnegie’s Josh Simon, Ian Thompson, and Stephen Shectman, will publish their work in The Astrophysical Journal on Thursday.

The Sculptor dwarf is a small galaxy that orbits around our own Milky Way, just as the Moon orbits around the Earth. Large galaxies like the Milky Way can contain several hundred billion stars, but Sculptor is home to just a few million. Because Sculptor’s stars are all located the same distance away from us, their ages can be determined by studying the pattern of their colors and brightnesses. This technique tells astronomers that Sculptor, like many dwarf galaxies, stopped evolving long ago. While the Milky Way has been forming stars throughout the universe’s 14 billion year existence, Sculptor’s youngest stars are 7 billion years old. Dwarf galaxies thus provide scientists an opportunity to see what galaxies looked like in the early epochs of the universe.

Stars in all galaxies are born out of collapsing clouds of dust and gas. Only a few million years after they begin burning, the most-massive of these stars explode in titanic blasts called supernovae. These explosions seed the surrounding gas with the elements that were manufactured by the stars during their lifetimes. Those elements are then incorporated into the formation of the next generation of stars. Generally this process is cyclical, with each generation of stars contributing more elements to the raw material from which the next set of stars will be formed.

Astronomers hoping to learn about the first stages of galaxy formation after the Big Bang can use the chemical composition of stars to help them unravel the histories of our own and nearby galaxies. Elements heavier than hydrogen, helium, and lithium can only be produced by stars. The more stars a galaxy forms, the more enriched in heavy elements it becomes.

Thus, young stars contain larger amounts of heavy elements produced by dozens or hundreds of supernovae, while the oldest stars have a very simple chemical makeup with few of the heavy elements. Typically, stars are characterized by how much iron they contain, because iron is a relatively common element and is almost always the easiest for astronomers to detect.

The team—also including Heather Jacobson and Anna Frebel of the Massachusetts Institute of Technology, as well as former Carnegie postdoc Josh Adams—studied five stars in Sculptor, measuring the abundance of 15 elements in each one. The two most-primitive stars have less than half as much magnesium and calcium as would be expected based on their iron content and just 10 percent as much silicon as similar stars in other galaxies.

“The only way to explain the shortage of magnesium, calcium, and silicon in these stars is if their heavy elements were made by fewer than four supernovae, and those supernovae need to have been a rare kind of explosion,” explained Simon.

The astronomers concluded that these two primitive stars were probably formed from a gas cloud that had been seeded with heavy elements made by just one previously exploded star. This parent star is thought to be one of the very first stars ever formed in Sculptor.

“Most likely, we are seeing the leftover traces of just a single supernova,” added Jacobson.

“These stars are giving us an unprecedented view of the earliest history of another galaxy,” Frebel said.

This work was supported in part by the National Science Foundation. It made use of NASA’s Astrophysics Data System Bibliographic services. Data were gathered with the 6.5-meter Magellan telescopes at Carnegie’s Las Campanas Observatory in the Atacama desert of Chile.

Source: Carnagie Science (Carnegie Institution of Washington)

Saturday, March 28, 2015

Astronomers Upgrade Their Cosmic Light Bulbs

A new study analyzes several sites where dead stars once exploded
Image credit: SDSS
› Full image and caption

The brilliant explosions of dead stars have been used for years to illuminate the far-flung reaches of our cosmos. The explosions, called Type Ia supernovae, allow astronomers to measure the distances to galaxies and measure the ever-increasing rate at which our universe is stretching apart.

But these tools aren't perfect. In the cosmic hardware store of our universe, improvements are ongoing. In a new report, appearing March 27 in the journal Science, astronomers identify the best, top-of-the-line Type Ia supernovae for measuring cosmic distances, pushing other, more clunky tools to the back of the shelf.

Using archived data from NASA's Galaxy Evolution Explorer (GALEX), scientists show that a particular class of Type Ia supernovae that occur near youthful stars can improve these measurements with a precision of more than two times that achieved before.

"We have discovered a population of Type Ia supernovae whose light output depends very precisely on how quickly they fade, making it possible to measure very exact distances to them," said Patrick Kelly of the University of California, Berkeley, lead author of the new study. "These supernovae are found close to populations of bright, hot young stars."

The findings will help light the way to understanding dark energy, one of the greatest mysteries in the field of cosmology, the study of the origin and development of the universe. Dark energy is the leading culprit behind the baffling acceleration of our cosmos, a phenomenon discovered in 1998. The acceleration was uncovered when astronomers observed that galaxies are pulling away from each other at increasing speeds. 

The key to measuring this acceleration -- and thus the nature of dark energy -- lies with Type Ia supernovae, which work much like light bulbs strung across space. Imagine lining up 60-watt light bulbs across a field and standing at one end. The farthest light bulb wouldn't appear as bright as the closest one due to its distance. Since you know how bright the light bulb inherently is, you can use the extent of its dimming to figure out the distance.

Type Ia supernovae, also referred to as "standard candles," work in a similar way because they consistently shine with about the same amount of light. While the process that leads to these explosions is still not clear, they occur when the burnt-out core of a star, called a white dwarf, blasts apart in a regular way, briefly lighting up the host galaxy.

However, the explosions aren't always precisely uniform. They can differ considerably depending on various factors, which appear to be connected to the environments and histories of the exploding stars. It's as if our 60-watt bulbs sometimes give off 55 watts of light, skewing distance measurements.

Kelly and his team investigated the reliability of these tools by analyzing the surroundings of nearly 100 previous Type Ia explosions. They used data from GALEX, which detects ultraviolet light. Populations of hot, young stars in galaxies will shine brightly with ultraviolet light, so GALEX can distinguish between young and older star-forming communities.

The results showed that the Type Ia supernovae affiliated with the hot, young stars were significantly more reliable at indicating distances than their counterparts.

"These explosions are likely the result of youthful white dwarfs," said Kelly.

By focusing on this particular brand of Type Ia tools, astronomers will be able to, in the future, make even sharper measurements of the size and scale of our universe. According to the science team, this class of tools could work at distances up to six billion light-years away, and perhaps farther.

"GALEX surveyed the entire sky, allowing past and future eruptions of these high-quality standard candles to be identified easily," said Don Neill, a member of the GALEX team at the California Institute of Technology in Pasadena, not affiliated with the study. "Any improvement in the standard candles will have a direct impact on theories of dark energy, allowing us to home in on this mysterious force propelling the acceleration of the universe."

Caltech led the Galaxy Evolution Explorer mission and was responsible for science operations and data analysis. The mission ended in 2013 after more than a decade of scanning the skies in ultraviolet light. NASA's Jet Propulsion Laboratory in Pasadena, California, managed the mission and built the science instrument. The mission was developed under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Maryland. Researchers sponsored by Yonsei University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France collaborated on this mission. ?

Graphics and additional information about the Galaxy Evolution Explorer are online at:  http://www.nasa.gov/galex - http://www.galex.caltech.edu

Media Contact

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, Calif.


Friday, March 27, 2015

A galaxy on the edge

Credit: ESA/Hubble & NASA

This NASA/ESA Hubble Space Telescope image shows an edge-on view of the spiral galaxy NGC 5023. Due to its orientation we cannot appreciate its spiral arms, but we can admire the elegant profile of its disc. The galaxy lies over 30 million light-years away from us.

NGC 5023 is part of the M51 group of galaxies. The brightest galaxy in this group is Messier 51, the Whirlpool Galaxy, which has been captured by Hubble many times. NGC 5023 is less fond of the limelight and seems rather unsociable in comparison — it is relatively isolated from the other galaxies in the group.

Astronomers are particularly interested in the vertical structure of discs like these. By analysing the structure above and below the central plane of the galaxy they can make progress in understanding galaxy evolution. Astronomers are able to analyse the distribution of different types of stars within the galaxy and their properties, in particular how well evolved they are on the Hertzsprung–Russell Diagram — a scatter graph of stars that shows their evolution.

NGC 5023 is one of six edge-on spiral galaxies observed as part of a study using Hubble’s Advanced Camera for Surveys. They study this vertical distribution and find a trend which suggests that heating of the disc plays an important role in producing the stars seen away from the plane of the galaxy.

In fact, NGC 5023 is pretty popular when it comes to astronomers, despite its unsociable behaviour. The galaxy is also one of 14 disc galaxies that are part of the GHOSTS survey — a survey which uses Hubble data to study galaxy halos, outer discs and star clusters. It is the largest study to date of star populations in the outskirts of disc galaxies.

The incredible sharp sight of Hubble has allowed scientist to count more than 30 000 individual bright stars in this image. This is only a small fraction of the several billion stars that this galaxy contains, but the others are too faint to detect individually even with Hubble.

Hubble and Chandra Discover Dark Matter Is Not as Sticky as Once Thought

Six Cluster Collisions, with Dark-Matter Maps (Hubble and Chandra — Annotated)
The clusters shown here are, from left to right and top to bottom: MACS J0416.1-2403, MACS J0152.5-2852, MACS J0717.5+3745, Abell 370, Abell 2744, and ZwCl 1358+62.

Science Credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland; University of Edinburgh, UK), R. Massey (Durham University, UK), T. Kitching (University College London, UK), and A. Taylor and E. Tittley (University of Edinburgh, UK).  Image Credit: NASA, ESA, STScI, and CXC.   

Astronomers using observations from NASA's Hubble Space Telescope and Chandra X-ray Observatory have found that dark matter does not slow down when colliding with each other. This means that it interacts with itself even less than previously thought. Researchers say this finding narrows down the options for what this mysterious substance might be.

Dark matter is a transparent form of matter that makes up most of the mass in the universe. Because dark matter does not reflect, absorb, or emit light, it can only be traced indirectly, such as by measuring how it warps space through gravitational lensing, where the light from distant sources is magnified and distorted by the gravitational effects of dark matter.

The two space observatories were used to study how dark matter in clusters of galaxies behaves when the clusters collide. Hubble was used to map the post-collision distribution of stars and dark matter, which was traced through its gravitational lensing effects on background light. Chandra was used to see the X-ray emission from the colliding gas. The results will be published in the journal Science on March 27.

"Dark matter is an enigma we have long sought to unravel," said John Grunsfeld, assistant administrator of NASA's Science Mission Directorate in Washington. "With the combined capabilities of these great observatories, both in extended mission, we are ever closer to understanding this cosmic phenomenon."

To learn more about dark matter, researchers can study it in a way similar to experiments on visible matter — by watching what happens when it bumps into celestial objects. An excellent natural laboratory for this analysis can be found in collisions between galaxy clusters.

Galaxy clusters are made of three main ingredients: galaxies, clouds of gas, and dark matter. During collisions, the clouds of gas enveloping the galaxies crash into each other and slow down or stop. The galaxies are much less affected by the drag from the gas and, because of the huge gaps between the stars within them, do not have a slowing effect on each other.

"We know how gas and galaxies react to these cosmic crashes and where they emerge from the wreckage. Comparing how dark matter behaves can help us to narrow down what it actually is," explained David Harvey of the École Polytechnique Fédérale de Lausanne, Switzerland, lead author of the new study.

Harvey and his team used data from Hubble and Chandra to study 72 large cluster collisions. The collisions happened at different times, and are seen from different angles — some from the side, and others head-on.

The team found that, like the galaxies, the dark matter continued straight through the violent collisions without slowing down relative to the galaxies. Because galaxies pass through unimpeded, if astronomers observe a separation between the distribution of the galaxies and the dark matter then they know it has slowed down. If the dark matter does slow, it will drag and lie somewhere between the galaxies and the gas, which tells researchers how much it has interacted.

The leading theory is that dark matter particles spread throughout the galaxy clusters do not frequently bump into each other. The reason the dark matter doesn't slow down is because not only does it not interact with visible particles, it also infrequently interacts with other dark matter. The team has measured this "self-interaction" and found it occurs even less frequently than previously thought.

"A previous study had seen similar behavior in the Bullet Cluster," said team member Richard Massey of Durham University, U.K. "But it's difficult to interpret what you're seeing if you have just one example. Each collision takes hundreds of millions of years, so in a human lifetime we only get to see one freeze-frame from a single camera angle. Now that we have studied so many more collisions, we can start to piece together the full movie and better understand what is going on."

By finding that dark matter interacts with itself even less than previously thought, the team has successfully narrowed down the properties of dark matter. Particle physics theorists now have a smaller set of unknowns to work with when building their models.

"It is unclear how much we expect dark matter to interact with itself because dark matter is already going against everything we know, said Harvey. "We know from previous observations that it must interact with itself reasonably weakly, however this study has now placed it below that of two protons interacting with one another — which is one theory for dark matter." Harvey said that the results suggest that dark matter is unlikely to be only a kind of dark proton. If dark matter scattered like protons do with one another (electrostatically) it would have been detected. "This challenges the idea that there exists 'dark photons,' the dark matter equivalent of photons," he said.

Dark matter could potentially have rich and complex properties, and there are still several other types of interactions to study. These latest results rule out interactions that create a strong frictional force, causing dark matter to slow down during collisions. Other possible interactions could make dark matter particles bounce off each other like billiard balls, causing dark matter particles to be ejected from the clouds by collisions or for dark matter blobs to change shape. The team will be studying these next.

To further increase the number of collisions that can be studied, the team is also looking to study collisions involving individual galaxies, which are much more common.

"There are still several viable candidates for dark matter, so the game is not over, but we are getting nearer to an answer," concludes Harvey. "These 'astronomically large' particle colliders are finally letting us gimpse the dark world all around us but just out of reach."

Ray Villard
Space Telescope Science Institute, Baltimore, Md.

Felicia Chou
NASA Headquarters, Washington, D.C.

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.

Georgia Bladon
ESA/Hubble, Garching, Germany

Richard Massey
Durham University, Durham, UK
David Harvey
EPFL, Lausanne, Switzerland;
University of Edinburgh, Edinburgh, UK

Source: HubbleSite

Thursday, March 26, 2015

Best View Yet of Dusty Cloud Passing Galactic Centre Black Hole

The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way

The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way (annotated)

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The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way
The dusty cloud G2 passes the supermassive black hole at the centre of the Milky Way

VLT observations confirm that G2 survived close approach and is a compact object

The best observations so far of the dusty gas cloud G2 confirm that it made its closest approach to the supermassive black hole at the centre of the Milky Way in May 2014 and has survived the experience. The new result from ESO’s Very Large Telescope shows that the object appears not to have been significantly stretched and that it is very compact. It is most likely to be a young star with a massive core that is still accreting material. The black hole itself has not yet shown any increase in activity.

A supermassive black hole with a mass four million times that of the Sun lies at the heart of the Milky Way galaxy. It is orbited by a small group of bright stars and, in addition, an enigmatic dusty cloud, known as G2, has been tracked on its fall towards the black hole over the last few years. Closest approach, known as peribothron, was predicted to be in May 2014.

The great tidal forces in this region of very strong gravity were expected to tear the cloud apart and disperse it along its orbit. Some of this material would feed the black hole and lead to sudden flaring and other evidence of the monster enjoying a rare meal. To study these unique events, the region at the galactic centre has been very carefully observed over the last few years by many teams using large telescopes around the world.

A team led by Andreas Eckart (University of Cologne, Germany) has observed the region using ESO’s Very Large Telescope (VLT) [1] over many years, including new observations during the critical period from February to September 2014, just before and after the peribothron event in May 2014. These new observations are consistent with earlier ones made using the Keck Telescope on Hawaii [2].
The images of infrared light coming from glowing hydrogen show that the cloud was compact both before and after its closest approach, as it swung around the black hole.

As well as providing very sharp images, the SINFONI instrument on the VLT also splits the light into its component infrared colours and hence allows the velocity of the cloud to be estimated [3]. Before closest approach, the cloud was found to be travelling away from the Earth at about ten million kilometres/hour and, after swinging around the black hole, it was measured to be approaching the Earth at about twelve million kilometres/hour.

Florian Peissker, a PhD student at the University of Cologne in Germany, who did much of the observing, says: “Being at the telescope and seeing the data arriving in real time was a fascinating experience,” and Monica Valencia-S., a post-doctoral researcher also at the University of Cologne, who then worked on the challenging data processing adds: “It was amazing to see that the glow from the dusty cloud stayed compact before and after the close approach to the black hole.”

Although earlier observations had suggested that the G2 object was being stretched, the new observations did not show evidence that the cloud had become significantly smeared out, either by becoming visibly extended, or by showing a larger spread of velocities.

In addition to the observations with the SINFONI instrument the team has also made a long series of measurements of the polarisation of the light coming from the supermassive black hole region using the NACO instrument on the VLT. These, the best such observations so far, reveal that the behaviour of the material being accreted onto the black hole is very stable, and — so far — has not been disrupted by the arrival of material from the G2 cloud.

The resilience of the dusty cloud to the extreme gravitational tidal effects so close to the black hole strongly suggest that it surrounds a dense object with a massive core, rather than being a free-floating cloud. This is also supported by the lack, so far, of evidence that the central monster is being fed with material, which would lead to flaring and increased activity.

Andreas Eckart sums up the new results: “We looked at all the recent data and in particular the period in 2014 when the closest approach to the black hole took place. We cannot confirm any significant stretching of the source. It certainly does not behave like a coreless dust cloud. We think it must be a dust-shrouded young star.”

[1] These are very difficult observations as the region is hidden behind thick dust clouds, requiring observations in infrared light. And, in addition, the events occur very close to the black hole, requiring adaptive optics to get sharp enough images. The team used the SINFONI instrument on ESO’s Very Large Telescope and also monitored the behaviour of the central black hole region in polarised light using the NACO instrument.

[2] The VLT observations are both sharper (because they are made at shorter wavelengths) and also have additional measurements of velocity from SINFONI and polarisation measurement using the NACO instrument.

[3] Because the dusty cloud is moving relative to Earth — away from Earth before closest approach to the black hole and towards Earth afterwards — the Doppler shift changes the observed wavelength of light. These changes in wavelength can be measured using a sensitive spectrograph such as the SINFONI instrument on the VLT. It can also be used to measure the spread of velocities of the material, which would be expected if the cloud was extended along its orbit to a significant extent, as had previously been reported.

More Information

This research was presented in a paper “Monitoring the Dusty S-Cluster Object (DSO/G2) on its Orbit towards the Galactic Center Black Hole” by M. Valencia-S. et al. in the journal Astrophysical Journal Letters.

The team is composed of M. Valencia-S. (Physikalisches Institut der Universität zu Köln, Germany), A. Eckart (Universität zu Köln; Max-Planck-Institut für Radioastronomie, Bonn, Germany [MPIfR]), M. Zajacek (Universität zu Köln; MPIfR; Astronomical Institute of the Academy of Sciences Prague, Czech Republic), F. Peissker (Universität zu Köln), M. Parsa (Universität zu Köln), N. Grosso (Observatoire Astronomique de Strasbourg, France), E. Mossoux (Observatoire Astronomique de Strasbourg), D. Porquet (Observatoire Astronomique de Strasbourg), B. Jalali (Universität zu Köln), V. Karas (Astronomical Institute of the Academy of Sciences Prague), S. Yazici (Universität zu Köln), B. Shahzamanian (Universität zu Köln), N. Sabha (Universität zu Köln), R. Saalfeld (Universität zu Köln), S. Smajic (Universität zu Köln), R. Grellmann (Universität zu Köln), L. Moser (Universität zu Köln), M. Horrobin (Universität zu Köln), A. Borkar (Universität zu Köln), M. García-Marín (Universität zu Köln), M. Dovciak (Astronomical Institute of the Academy of Sciences Prague), D. Kunneriath (Astronomical Institute of the Academy of Sciences Prague), G. D. Karssen (Universität zu Köln), M. Bursa (Astronomical Institute of the Academy of Sciences Prague), C. Straubmeier (Universität zu Köln) and H. Bushouse (Space Telescope Science Institute, Baltimore, Maryland, USA).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. 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 two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.



Andreas Eckart
University of Cologne
Cologne, Germany

Monica Valencia-S.
University of Cologne
Cologne, Germany

Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591

Source: ESO

Black hole winds pull the plug on star formation

Black-hole wind sweeping away galactic gas
Copyright ESA/ATG medialab
JPG (1.32 MB) - TIFF (9.46MB)

Black-hole wind
Copyright ESA/ATG medialab

Galactic merger hosting a supermassive black hole
Copyright: NASA/SDSS/S. Veilleux

Galactic outflow
Copyright: ESA/ATG medialab

Astronomers using ESA’s Herschel space observatory have found that the winds blowing from a huge black hole are sweeping away its host galaxy’s reservoir of raw star-building material. 

Found at the hearts of most galaxies, supermassive black holes are extremely dense and compact objects with masses between millions and billions of times that of our Sun. 

Many are relatively passive, like the one sitting at the centre of our Milky Way. However, some of them are devouring their surroundings with a great appetite. 

These active black holes not only feed on nearby gas but also expel some of it as powerful winds and jets. Astronomers have long suspected these outflows to be responsible for draining galaxies of their interstellar gas, in particular the gas molecules from which stars are born. 

This could eventually affect a galaxy’s star-forming activity, slowing it down or possibly quenching it entirely. 

Until now, it had not been possible to capture a complete view of this process. While astronomers were able to detect winds very close to black holes using X-ray telescopes, and to trace much larger galactic outflows of gas molecules through infrared observations, they had not succeeded at finding both in the same galaxy. 

A new study has changed the scene, detecting winds driven by one particular black hole from the smallest to largest scales.  

“This is the first time that we have seen a supermassive black hole in action, blowing away the galaxy’s reservoir of star-making gas,” explains Francesco Tombesi from NASA’s Goddard Space Flight Center and the University of Maryland, USA, who led the research published this week in Nature

Combining infrared observations from ESA’s Herschel space observatory with new data from the Japanese/US Suzaku X-ray satellite, the astronomers detected the winds close to the central black hole as well as their global effect in pushing galactic gas away in a galaxy known as IRAS F11119+3257. 

The winds start small and fast, gusting at about 25% the speed of light near the black hole and blowing away about the equivalent of one solar mass of gas every year. 

As they progress outwards, the winds slow but sweep up an additional few hundred solar masses of gas molecules per year and push it out of the galaxy. 

This is the first solid proof that black-hole winds are stripping their host galaxies of gas by driving large-scale outflows. 

The new finding supports the view that black holes might ultimately stop stars forming in their host galaxies

“Herschel has already revolutionised our understanding of how stars are born. This new result is now helping us understand why and how star formation in some galaxies can be globally affected and even switched off entirely,” says Göran Pilbratt, Herschel Project Scientist at ESA. 

“The culprit of this cosmic ‘whodunnit’ has been found. As many suspected, a central black hole can power large-scale gas outflows, quenching the formation of stars.” 

Read more about this discovery

More information
“Wind from the black-hole accretion disk driving a molecular outflow in an active galaxy,” by F. Tombesi, et al., is published in the 26 March 2015 issue of the journal Nature

The study is based on observations performed with the Photoconductor Array Camera and Spectrometer (PACS) instrument on ESA’s Herschel space observatory, as well as on data from the Japanese/US Suzaku mission. 

For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799; +34 91 8131 199
Mob: +31 61 594 3954

Francesco Tombesi
X-ray Astrophysics Laboratory
NASA Goddard Space Flight Center
Greenbelt, MD, USA
and Department of Astronomy and CRESST
University of Maryland, MD, USA
Tel: +1 301 405 3615 / +1 301 286 2661

Göran Pilbratt
Herschel Project Scientist
Tel: +31 71 565 3621

Source: ESA/Herschel

Measuring Galaxy Evolution with Globular Clusters

The galaxy NGC 4365 in the Virgo supercluster of galaxies, showing its many globular clusters (most of the more than one hundred dots in this image are actually globular star clusters). A new study of over seven thousand globular clusters around ten Virgo galaxies finds that they are often gathered into distinct groupings whose shapes reflect the formation of the clusters and the histories of the galaxies. Credit: NASA/HST and ESO VLT

Globular clusters are gravitationally bound ensembles of stars, as many as a million stars in some cases, grouped in roughly spherical clusters with diameters as small as only tens of light-years. Globular clusters are typically located in the outer regions (the halos) of galaxies; the Milky Way galaxy has about two hundred globular clusters orbiting it. Astronomers are interested in globular clusters in part because they are home to many of the oldest known stars, but also because of their locations in the halos. Collisions between galaxies are commonplace, and globular clusters may provide fossil evidence of these encounters because they are strongly affected by such interactions. During a collision, a galaxy can grow by absorbing or merging with its neighbor, and some models predict that clusters form during these interactions. Moreover, it is possible that in a merger large numbers of globular clusters originally belonging to a smaller galaxy may be captured by the larger galaxy. In any case, the distribution of globular clusters around a galaxy holds clues to their origins and the history of its host galaxy.

The Virgo Cluster of galaxies, containing between one and two thousand galaxies, is located about fifty-four million light-years away in the direction of the constellation of Virgo. The ten brightest galaxies of the Virgo Cluster alone contain 7053 detected globular clusters. CfA astronomers Raffaele D'Abrusco, Pepi Fabbiano, and Andreas Zezas carefully examined this set of globular clusters looking for information about the history of these galaxies. In a new paper, they report discovering distinctive structures among the globular cluster systems, meaning that the globular clusters around these galaxies are not distributed symmetrically. Their configurations often take shapes ranging from roughly linear to circular, with some more complex shapes as well. The scientists found 229 such structures in this subsample, forty-two of them classified as being medium or large and stretching over as much as seventy-five thousand light-years. The elongated structures tend to be aligned with an axis of the host galaxy, as would be expected if a merger were responsible.

The scientists argue that these structures are indeed the remnants of galaxies that were accreted in the past, and among other things they estimate limits on the masses of these parent galaxies. Computer simulations provide some rough level of agreement. The authors note that with more detailed computations, these structures offer a powerful new tool to advance the study of galaxy evolution.


"Spatial Structures in the Globular Cluster Distribution of the Ten Brightest Virgo Galaxies," R. D'Abrusco, G. Fabbiano, A. Zezas, ApJ 2015 (in press)

Wednesday, March 25, 2015

Unusual Asteroid Suspected of Spinning to Explosion

Active asteroid P/2012 F5 captured by Keck II/DEIMOS in mid-2014. Top panel shows a wide-angle view of the main nucleus and smaller fragments embedded in a long dust trail. Bottom panel shows a close-up view with the trail numerically removed to enhance the visibility of the fragments. Credit: M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory. Hi-res image
Brightness fluctuations of the nucleus of P/2012 F5 during two consecutive rotation cycles. Presented versus time (top panel) and versus the nucleus rotation phase (bottom panel). Credit: M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory. Hi-res image

Mauna Kea, Hawaii – A team led by astronomers from the Jagiellonian University in Krakow, Poland, recently used the W. M. Keck Observatory in Hawaii to observe and measure a rare class of “active asteroids” that spontaneously emit dust and have been confounding scientists for years. The team was able to measure the rotational speed of one of these objects, suggesting the asteroid spun so fast it burst, ejecting dust and newly discovered fragments in a trail behind it. The findings are being published in Astrophysical Journal Letters on March 20, 2015.

Unlike the hundreds of thousands of asteroids in the main belt of our solar system, which move cleanly along their orbits, active asteroids were discovered several years ago mimicking comets with their tails formed by calm, long lasting ice sublimation. 

Then in 2010 a new type of active asteroid was discovered, which ejected dust like a shot without an obvious reason. Scientist gravitated around two possible hypotheses. One states the explosion is a result of a hypervelocity collision with another minor object. The second popular explanation describes it as a consequence of “rotational disruption”, a process of launching dust and fragments by spinning so fast, the large centrifugal forces produced exceed the object’s own gravity, causing it to break apart. Rotational disruption is the expected final state of what is called the YORP effect – a slow evolution of the rotation rate due to asymmetric emission of heat.

To date, astronomers have identified four objects suspected of either collision- or rotation-driven activity. These four freakish asteroids are all very small, at a kilometer or less, which makes them unimaginably faint when viewed from a typical distance of a couple hundred million miles. Despite prior attempts, the tiny size of the objects kept scientists from determining some of the key characteristics that could prove or disprove the theories. 

Until last August, when the team led by Michal Drahus of the Jagiellonian University was awarded time at Keck Observatory. 

“When we pointed Keck II at P/2012 F5 last August, we hoped to measure how fast it rotated and check whether it had sizable fragments. And the data showed us all that,” Drahus said. 

The team discovered at least four fragments of the object, previously established to have impulsively ejected dust in mid-2011. They also measured a very short rotation period of 3.24 hours – fast enough to cause the object impulsively explode. 

“This is really cool because fast rotation has been suspected of catapulting dust and triggering fragmentation of some active asteroids and comets. But up until now we couldn’t fully test this hypothesis as we didn’t know how fast fragmented objects rotate,” Drahus said.

The astronomers calculated the object’s rotation period by measuring small periodic fluctuations in brightness. Such oscillations occur naturally as the irregular nucleus rotates about its spin axis and reflects different amounts of sunlight during a rotation cycle. 

“This is a well-established technique but its application on faint targets is challenging,” said Waclaw Waniak of the Jagiellonian University who processed the Keck Observatory data. “The main difficulty is the brightness must to be probed every few minutes so we don’t have time for long exposures. We needed the huge collecting area of Keck II, which captures a plentiful amount of photons in a very short time.”

The photons were then concentrated in the telescope’s light path and sent to the DEIMOS instrument to produce the data that allowed the scientists to determine P/2012 F5’s nature. While monitoring brightness in the individual 3-minute exposures, scientists also compiled all the data to produce a single ultra-deep image, which revealed the fragments.

The success wouldn’t be possible if the selected target, P/2012 F5, were not an ideal candidate for this study. Alex R. Gibbs discovered the object on March 22, 2012 with the Mount Lemmon 1.5 meter reflector. It was initially classified as a comet, based solely on its “dusty” look. But two independent teams quickly have shown all this dust was emitted in a single pulse about a year before the discovery – something that doesn’t happen to comets. When the dust settled in 2013, another team using the University of Hawaii’s 2.2-meter telescope on Mauna Kea detected a star-like nucleus and suggested a maximum size of 2 kilometers. 

“We suspected that this upper limit was close to the actual size of the object. Consequently, we chose to observe P/2012 F5 because – despite its small size – it appeared to be the largest and easiest to observe active asteroid suspected of rotational disruption,” said Jessica Agarwal of the Max Planck Institute for Solar System Research who chose P/2012 F5 as the subject.

As a result of the study, P/2012 F5 is the first freshly fragmented object in the solar system with a well-determined spin rate, and this spin rate turns out to be the fastest among the active asteroids. A careful analysis made by the team shows that these two features of the object are consistent with the “rotational disruption” scenario. But alternative explanations, such as fragmentation due to an impact, cannot be completely ruled out. 

“There are many faster rotators among asteroids which don’t show signs of a recent mass loss. And there are many hypervelocity impactors straying out there and looking for targets to hit – be it a fast or slow rotator,” Drahus said. 

“We’re indebted to the Caltech Optical Observatories for generously awarding Keck Observatory time for this program,” said Drahus – formerly a NRAO Jansky Fellow at Caltech. “Without the huge collecting area of Keck II’s 10-meter mirror, we wouldn’t be able to achieve our goals so swiftly.”

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems. 

DEIMOS (the DEep Imaging and Multi-Object Spectrograph) boasts the largest field of view (16.7 arcmin by 5 arcmin) of any of the Keck instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Steve JeffersonCommunications Officer
W. M. Keck Observatory


Are the stars singing?

Image from Hubble Space Telescope 
Credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

A chance discovery by a team of researchers has provided experimental evidence that stars may generate sound.

The study of fluids in motion – now known as hydrodynamics – goes back to the Egyptians, so it is not often that new discoveries are made. However when examining the interaction between an ultra-intense laser and a plasma target, the team, which included scientists from the Science and Technology Facilities Council’s (STFC) Central Laser Facility in Oxfordshire, the York Plasma Institute at the University of York, and the Tata Institute of Fundamental Research in Mumbai, India, noticed something unusual.

They realised that in the trillionth of a second after the laser strikes, plasma flowed rapidly from areas of high density to more stagnant regions of low density, in such a way that it created something like a traffic jam.

Plasma piled up at the interface between the high and low density regions, generating a series of pressure pulses: a sound wave.

Dr Alex Robinson from the Plasma Physics Group at STFC’s Central Laser Facility developed a numerical model to generate acoustic waves for the experiment. He said, “It was initially hard to determine the origin of the acoustic signals, but our model produced results that compared favourably with the wavelength shifts observed in the experiment. This showed that we had discovered a new way of generating sound from fluid flows. Similar situations could occur in plasma flowing around stars”

The sound generated was at such a high frequency that it would have left even bats and dolphins struggling. With a frequency of nearly a trillion hertz, the sound generated was not only unexpected, but was also at close to the highest frequency possible in such a material – six million times higher than that which can be heard by any mammal.

Dr John Pasley from the York Plasma Institute said: “One of the few locations in nature where we believe this effect would occur is at the surface of stars. When they are accumulating new material stars could generate sound in a very similar manner to that which we observed in the laboratory – so the stars might be singing – but, since sound cannot propagate through the vacuum of space, no-one can hear them.”

The technique used to observe the sound waves in the lab works very much like a police speed camera. It allows the scientists to very accurately measure how fluid is moving at the point that is struck by the laser on timescales of less than a trillionth of a second.

The research was funded by the Engineering and Physical Sciences Research Council and the Tata Institute of Fundamental Research. It is published in Physical Review Letters.


Marion O’Sullivan
STFC Press Office
Tel: 01235 445627
Mob: 07824 888990

Notes to Editors:  
  • The paper ‘Terahertz acoustics in hot dense laser-plasmas’ by Amitava Adak, A. P. L. Robinson, Prashant Kumar Singh, et al is published in Physical Review Letters.
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