INTEGRAL helps unravel the tumultuous recent history of the solar neighbourhood

Just like archaeologists, who rely on radioactive carbon to date the organic remains from past epochs, astronomers have exploited the radioactive decay of an isotope of aluminium to estimate the age of stars in the nearby Scorpius-Centaurus association, the closest group of young and massive stars to the Sun. The new observations, performed in gamma rays by ESA's INTEGRAL observatory, provide evidence for recent ejections of matter from massive stars that took place only a few million years ago in our cosmic neighbourhood.

A common technique used in archaeology to establish the age of fossils and other organic samples from the past consists of measuring how much of a particular isotope of carbon, namely carbon-¹⁴ (¹⁴C), they contain. This radioactive isotope decays into the element nitrogen on a time scale of a few thousand years, hence the amount of it remaining in these ancient fossils is a strong indicator of the epoch from which they date. An analogous method, based on the radioactive decay of an unstable isotope of aluminium, has been recently exploited by astronomers to probe and assess the age of the Scorpius-Centaurus association, the closest group of very young and massive stars. Stellar age estimates can be then used to investigate how nearby massive stars have shaped our local region of the Milky Way.

The radioactive decay process of ²⁶Al. Credit: ESA

This dating procedure is possible because aluminium is one of the elements synthesised by massive stars during their late evolutionary stages, and its abundance in a stellar complex such as the Scorpius-Centaurus association varies strongly with time. One isotope of this element, namely aluminium-26 (²⁶Al), is radioactive and decays with an exponential lifetime of about one million years. The decay process results in a stable isotope of the element magnesium (²⁶Mg) and a number of by-products, including an extremely energetic photon observable in gamma rays at an energy of about 1.8 MeV.

“Conveniently for astronomers, the decay of ²⁶Al involves a similar time scale to that spanned by the life time of massive stars, which is of the order of a few million years,” explains Roland Diehl from the Max-Planck Institute for Extraterrestrial Physics in Germany, who led a recent study targeting the gamma-ray emission from this isotope in the Scorpius-Centaurus association. “As its decay time is 'just right', measuring the abundance of ²⁶Al is an excellent tool to trace the presence of young and massive stars, and it allows us to directly estimate their age,” he adds.

COMPTEL all-sky image of ²⁶Al gamma rays.
Image courtesy of Plüschke et al. 2001.

Earlier observations, conducted in the 1990s with the COMPTEL instrument on NASA's Compton Gamma-Ray Observatory, revealed for the first time the emission of ²⁶Al across the entire sky. Subsequent data collected by ESA's INTEGRAL mission confirmed these results, probing the global properties of this isotope throughout the plane of the Milky Way thanks to INTEGRAL's improved spectral resolution.

“At the characteristic energy of the ²⁶Al line, INTEGRAL has a spectral resolution over 60 times better than COMPTEL's, enabling us to study the intensity and shape of this line across the Galaxy in much greater detail,” comments Chris Winkler, INTEGRAL Project Scientist. “The data, gathered over five years, are so deep that it is now possible to isolate the contribution due to an individual, nearby stellar complex from the overall galactic ²⁶Al emission,” adds Winkler.

The evolution of the abundance of ²⁶Al in a stellar group.
Image courtesy of R. Voss.

The data analysed by Diehl's team focussed on the Scorpius-Centaurus association, which is located at a distance of about 100–150 parsec from the Sun, and revealed robust evidence for recent massive star formation therein. “The gamma-ray data show that the stars in the Upper Scorpius subgroup of the Scorpius-Centaurus association are only about 5 million years old,” notes co-author Thomas Preibisch from the University Observatory Munich, also in Germany. “This is a direct estimate, in contrast to other procedures used to evaluate the ages of stars, which rely heavily on stellar evolution models. The very good agreement between these independent dating methods is an extremely reassuring result,” adds Preibisch.

Via stellar winds and supernova explosions, the stars in the Scorpius-Centaurus association are currently enriching the surrounding interstellar medium with heavy elements, including aluminium, and from the shape of the emission line of ²⁶Al it is possible to constrain the kinematics of such ejecta. “By investigating the details of these outflows of radioactive gas, streaming at velocities of about 100 km/s towards the Sun, we are starting to unravel the recent history of massive star formation in the Solar System's vicinity and its implications on our own cosmic environment,” comments Diehl.

The new INTEGRAL data also allowed the astronomers to refine the estimate of the total content of ²⁶Al in the Milky Way, which is lower by about 20 per cent than previous estimates. This is a critical step that is required to validate our understanding of the star formation and nucleosynthesis processes in our Galaxy and to predict the expected rate of supernova explosions.

Notes for editors:

The study is based on observations performed with the gamma-ray spectrometer, SPI, on board INTEGRAL. The data have been gathered in the energy range between 1785–1826 keV, which embraces the ²⁶ line energy at 1808.63 keV, in bins of 0.5 keV. At this energy, the spectral resolution of SPI is 3 keV; the line positioning precision of 0.5 keV corresponds to a velocity resolution of about 75 km/s.

The data have been collected during about five years of INTEGRAL observations, between February 2003 and November 2007, with a total exposure of 61 million seconds, corresponding to almost 17,000 hours.

The half-life of a radioactive substance is the time required for half the nuclei to decay; for ²⁶Al this quantity is ~700,000 years. Another quantity used to measure the time scale of radioactive decay is the exponential lifetime, or the time required for the number of nuclei to decrease by a factor of e (Euler's number); the exponential lifetime of ²⁶Al is ~1 million years. As a comparison, the half-life of ¹⁴C , used for archaeological dating, is ~5700 years, and the exponential lifetime is ~8200 years.

Contacts

Roland Diehl
Max-Planck Institute for Extraterrestrial Physics
Garching, Germany
Phone: +49-89-30000-3850
Email: rod@mpe.mpg.de

Thomas Preibisch
University Observatory Munich, Germany
Phone: +49-89-2180-6016
Email: preibisch@usm.uni-muenchen.de

Chris Winkler
INTEGRAL Project Scientist
Research and Scientific Support Department
Directorate of Science and Robotic Exploration
ESA, The Netherlands
Phone: +31-71-565-3591
Email: cwinkler@rssd.esa.int

Stripes Are Back in Season on Jupiter

This image is a composite of three color images taken on Nov. 18, 2010, by the Gemini North telescope in Hawaii. The composite image shows a belt that had previously vanished in Jupiter's atmosphere is now reappearing. Image credit: NASA/JPL/UH/NIRI/Gemini. Larger image

A false-color composite image of Jupiter and its South Equatorial Belt shows an unusually bright spot, or outbreak, where winds are lofting particles to high altitudes in this image made from data obtained by the W.M. Keck telescope on Nov. 11, 2010. Image credit: NASA/JPL-Caltech/W. M. Keck Observatory.

This image of Jupiter is a composite of three color images taken on Nov. 16, 2010, by NASA's Infrared Telescope Facility. The particles lofted by the initial outbreak are easily identified in green as high altitude particles at the upper right, with a second outbreak to the lower left. Image credit: NASA/JPL-Caltech/IRTF.

PASADENA, Calif. - New NASA images support findings that one of Jupiter's stripes that "disappeared" last spring is now showing signs of a comeback. These new observations will help scientists better understand the interaction between Jupiter's winds and cloud chemistry.

Earlier this year, amateur astronomers noticed that a longstanding dark-brown stripe, known as the South Equatorial Belt, just south of Jupiter's equator, had turned white. In early November, amateur astronomer Christopher Go of Cebu City, Philippines, saw an unusually bright spot in the white area that was once the dark stripe. This phenomenon piqued the interest of scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and elsewhere.

After follow-up observations in Hawaii with NASA's Infrared Telescope Facility, the W.M. Keck Observatory and the Gemini Observatory telescope, scientists now believe the vanished dark stripe is making a comeback.

First-glimpse images of the re-appearing stripe are online at: http://www.nasa.gov/topics/solarsystem/features/jupiter20101124-i.html.

"The reason Jupiter seemed to 'lose' this band - camouflaging itself among the surrounding white bands - is that the usual downwelling winds that are dry and keep the region clear of clouds died down," said Glenn Orton, a research scientist at JPL. "One of the things we were looking for in the infrared was evidence that the darker material emerging to the west of the bright spot was actually the start of clearing in the cloud deck, and that is precisely what we saw."

This white cloud deck is made up of white ammonia ice. When the white clouds float at a higher altitude, they obscure the missing brown material, which floats at a lower altitude. Every few decades or so, the South Equatorial Belt turns completely white for perhaps one to three years, an event that has puzzled scientists for decades. This extreme change in appearance has only been seen with the South Equatorial Belt, making it unique to Jupiter and the entire solar system.

The white band wasn't the only change on the big, gaseous planet. At the same time, Jupiter's Great Red Spot became a darker red color. Orton said the color of the spot - a giant storm on Jupiter that is three times the size of Earth and a century or more old - will likely brighten a bit again as the South Equatorial Belt makes its comeback.

The South Equatorial Belt underwent a slight brightening, known as a "fade," just as NASA's New Horizons spacecraft was flying by on its way to Pluto in 2007. Then there was a rapid "revival" of its usual dark color three to four months later. The last full fade and revival was a double-header event, starting with a fade in 1989, revival in 1990, then another fade and revival in 1993. Similar fades and revivals have been captured visually and photographically back to the early 20th century, and they are likely to be a long-term phenomenon in Jupiter's atmosphere.

Scientists are particularly interested in observing this latest event because it's the first time they've been able to use modern instruments to determine the details of the chemical and dynamical changes of this phenomenon. Observing this event carefully may help to refine the scientific questions to be posed by NASA's Juno spacecraft, due to arrive at Jupiter in 2016, and a larger, proposed mission to orbit Jupiter and explore its satellite Europa after 2020.

The event also signifies another close collaboration between professional and amateur astronomers. The amateurs, located worldwide, are often well equipped with instrumentation and are able to track the rapid developments of planets in the solar system. These amateurs are collaborating with professionals to pursue further studies of the changes that are of great value to scientists and researchers everywhere.

"I was fortunate to catch the outburst," said Christopher Go, referring to the first signs that the band was coming back. "I had a meeting that evening and it went late. I caught the outburst just in time as it was rising. Had I imaged earlier, I would not have caught it," he said. Go, who also conducts in the physics department at the University of San Carlos, Cebu City, Philippines, witnessed the disappearance of the stripe earlier this year, and in 2007 he was the first to catch the stripe's return. "I was able to catch it early this time around because I knew exactly what to look for."

NASA's Exoplanet Science Institute at the California Institute of Technology in Pasadena manages time allocation on the Keck telescope for NASA. Caltech manages JPL for NASA.

For more information about NASA and agency programs, visit: http://www.nasa.gov/home

Priscilla Vega/Jia-Rui Cook 818-354-1357/354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
priscilla.r.vega@jpl.nasa.gov / Jia-Rui.C.Cook@jpl.nasa.gov

Astronomers Probe 'Sandbar' Between Islands of Galaxies

This diagram shows an unusual galaxy with bent jets (see inset)
Image credit: NASA/JPL-Caltech.
Full image and caption - See diagram

Astronomers have caught sight of an unusual galaxy that has illuminated new details about a celestial "sandbar" connecting two massive islands of galaxies. The research was conducted in part with NASA's Spitzer Space Telescope.

These "sandbars," or filaments, are known to span vast distances between galaxy clusters and form a lattice-like structure known as the cosmic web. Though immense, these filaments are difficult to see and study in detail. Two years ago, Spitzer's infrared eyes revealed that one such intergalactic filament containing star-forming galaxies ran between the galaxy clusters called Abell 1763 and Abell 1770.

Now these observations have been bolstered by the discovery, inside this same filament, of a galaxy that has a rare boomerang shape and unusual light emissions. Hot gas is sweeping the wandering galaxy into this shape as it passes through the filament, presenting a new way to gauge the filament's particle density. Researchers hope that other such galaxies with oddly curved profiles could serve as signposts for the faint threads, which in turn signify regions ripe for forming stars.

"These filaments are integral to the evolution of galaxy clusters -- among the biggest gravitationally bound objects in the universe -- as well as the creation of new generations of stars," said Louise Edwards, a postdoctoral researcher at the California Institute of Technology in Pasadena, and lead author of a study detailing the findings in the Dec. 1 issue of the Astrophysical Journal Letters. Her collaborators are Dario Fadda, also at Caltech, and Dave Frayer from the National Science Foundation's National Radio Astronomy Observatory, based in Charlottesville, Virginia.

Blowing in the cosmic breeze

Astronomers spotted the bent galaxy about 11 million light-years away from the center of the galaxy cluster Abell 1763 during follow-up observations with the WIYN Observatory near Tucson, Ariz., and radio-wave observations by the Very Large Array near Socorro, N.M. The WIYN Observatory is named after the consortium that owns and operates it, which includes the University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatories.

The galaxy has an unusual ratio of radio to infrared light, as measured by the Very Large Array and Spitzer, making it stand out like a beacon. This is due in part to the galaxy having twin jets of material spewing in opposite directions from a supermassive black hole at its center. These jets have puffed out into giant lobes of material that emit a tremendous amount of radio waves.

Edwards and her colleagues noticed that these lobes appear to be bent back and away from the galaxy's trajectory through the filament. This bow shape, the astronomers reasoned, is due to particles in the filament pushing on the gas and dust in the lobes.

By measuring the angle of the arced lobes, Edwards' team calculated the pressure exerted by the filaments' particles and then determined the density of the medium. The method is somewhat like looking at streamers on a kite soaring overhead to judge the wind strength and the thickness of the air.

According to the data, the density inside this filament is indeed about 100 times the average density of the universe. This value agrees with that obtained in a previous X-ray study of filaments and also nicely matches predictions of supercomputer simulations.

Interconnected superclusters

Galaxies tend to bunch together as great islands in the void of space, called galaxy clusters. These galaxy groupings themselves often keep company with other clusters in "superclusters" that loom as gargantuan, gravitationally associated walls of galaxies. These structures evolved from denser patches of material as the universe rapidly expanded after the Big Bang, some 13.7 billion years ago.

The clumps and threads of this primordial matter eventually cooled, and some of it has condensed into the galaxies we see today. The leftover gas is strewn in filaments between galaxy clusters. Much of it is still quite hot -- about one million degrees Celsius (1.8 million degrees Fahrenheit) -- and blazes in high-energy X-rays that permeate galaxy clusters. Filaments are therefore best detected in X-ray light, and one direct density reading of the strands has previously been obtained in this band of frequencies.

But the X-ray-emitting gas in filaments is much more diffuse and weak than in clusters, just as submerged sandbars are extremely hard to spot at sea compared to islands poking above the water. Therefore, obtaining quality observations of filaments is time-consuming with current space observatories.

The technique by Edwards and her colleagues, which uses radio frequencies that can reach a host of ground-based telescopes, points to an easier way to probe the interiors of galaxy-cluster filaments. Instead of laboring to find subtle X-rays clues, astronomers could trust these arced "lighthouse" galaxies to indicate just where cosmic filaments lie.

Knowing how much material these filaments contain and how they interact with galaxy clusters will be very important for understanding the overall evolution of the universe, Edwards said.

The Spitzer observations were made before it ran out of its liquid coolant in May 2009 and began its warm mission.

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

Written by Adam Hadhazy

Media Contact:
Whitney Clavin (818) 354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

Pulsating Star Mystery Solved

PR Image eso1046a
Artist’s impression of the remarkable double star OGLE-LMC-CEP0227

PR Image eso1046b

Wide-field view of part of the Large Magellanic Cloud
and the remarkable double star OGLE-LMC-CEP0227

By discovering the first double star where a pulsating Cepheid variable and another star pass in front of one another, an international team of astronomers has solved a decades-old mystery. The rare alignment of the orbits of the two stars in the double star system has allowed a measurement of the Cepheid mass with unprecedented accuracy. Up to now astronomers had two incompatible theoretical predictions of Cepheid masses. The new result shows that the prediction from stellar pulsation theory is spot on, while the prediction from stellar evolution theory is at odds with the new observations.

The new results, from a team led by Grzegorz Pietrzyński (Universidad de Concepción, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), appear in the 25 November 2010 edition of the journal Nature.

Grzegorz Pietrzyński introduces this remarkable result: “By using the HARPS instrument on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile, along with other telescopes, we have measured the mass of a Cepheid with an accuracy far greater than any earlier estimates. This new result allows us to immediately see which of the two competing theories predicting the masses of Cepheids is correct.”

Classical Cepheid Variables, usually called just Cepheids, are unstable stars that are larger and much brighter than the Sun [1]. They expand and contract in a regular way, taking anything from a few days to months to complete the cycle. The time taken to brighten and grow fainter again is longer for stars that are more luminous and shorter for the dimmer ones. This remarkably precise relationship makes the study of Cepheids one of the most effective ways to measure the distances to nearby galaxies and from there to map out the scale of the whole Universe [2].

Unfortunately, despite their importance, Cepheids are not fully understood. Predictions of their masses derived from the theory of pulsating stars are 20–30% less than predictions from the theory of the evolution of stars. This embarrassing discrepancy has been known since the 1960s.

To resolve this mystery, astronomers needed to find a double star containing a Cepheid where the orbit happened to be seen edge-on from Earth. In these cases, known as eclipsing binaries, the brightness of the two stars dims as one component passes in front of the other, and again when it passes behind the other star. In such pairs astronomers can determine the masses of the stars to high accuracy [3]. Unfortunately neither Cepheids nor eclipsing binaries are common, so the chance of finding such an unusual pair seemed very low. None are known in the Milky Way.

Wolfgang Gieren, another member of the team, takes up the story: “Very recently we actually found the double star system we had hoped for among the stars of the Large Magellanic Cloud. It contains a Cepheid variable star pulsating every 3.8 days. The other star is slightly bigger and cooler, and the two stars orbit each other in 310 days. The true binary nature of the object was immediately confirmed when we observed it with the HARPS spectrograph on La Silla.”

The observers carefully measured the brightness variations of this rare object, known as OGLE-LMC-CEP0227 [4], as the two stars orbited and passed in front of one another. They also used HARPS and other spectrographs to measure the motions of the stars towards and away from the Earth — both the orbital motion of both stars and the in-and-out motion of the surface of the Cepheid as it swelled and contracted.

This very complete and detailed data allowed the observers to determine the orbital motion, sizes and masses of the two stars with very high accuracy — far surpassing what had been done before for a Cepheid. The mass of the Cepheid is now known to about 1% and agrees exactly with predictions from the theory of stellar pulsation. However, the larger mass predicted by stellar evolution theory was shown to be significantly in error.

The much-improved mass estimate is only one outcome of this work, and the team hopes to find other examples of these remarkably useful pairs of stars to exploit the method further. They also believe that from such binary systems they will eventually be able to pin down the distance to the Large Magellanic Cloud to 1%, which would mean an extremely important improvement of the cosmic distance scale.

Notes

[1] The first Cepheid variables were spotted in the 18th century and the brightest ones can easily be seen to vary from night to night with the unaided eye. They take their name from the star Delta Cephei in the constellation of Cepheus (the King), which was first seen to vary by John Goodricke in England in 1784. Remarkably, Goodricke was also the first to explain the light variations of another kind of variable star, eclipsing binaries. In this case two stars are in orbit around each other and pass in front of each other for part of their orbits and so the total brightness of the pair drops. The very rare object studied by the current team is both a Cepheid and an eclipsing binary. Classical Cepheids are massive stars, distinct from similar pulsating stars of lower mass that do not share the same evolutionary history.

[2] The period luminosity relation for Cepheids, discovered by Henrietta Leavitt in 1908, was used by Edwin Hubble to make the first estimates of the distance to what we now know to be galaxies. More recently Cepheids have been observed with the Hubble Space Telescope and with the ESO VLT on Paranal to make highly accurate distance estimates to many nearby galaxies.

[3] In particular, astronomers can determine the masses of the stars to high accuracy if both stars happen to have a similar brightness and therefore the spectral lines belonging to each of the two stars can be seen in the observed spectrum of the two stars together, as is the case for this object. This allows the accurate measurement of the motions of both stars towards and away from Earth as they orbit, using the Doppler effect.

[4] The name OGLE-LMC-CEP0227 arises because the star was first discovered to be a variable during the OGLE search for gravitational microlensing. More details about OGLE are available at: http://ogle.astrouw.edu.pl/.

More information

This research was presented in a paper to appear in the journal Nature on 25 November 2010.

The team is composed of G. Pietrzyński (Universidad de Concepción, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), I. B. Thompson (Carnegie Observatories, USA), W. Gieren (Universidad de Concepción, Chile), D. Graczyk (Universidad de Concepción, Chile), G. Bono (INAF-Osservatorio Astronomico di Roma, Universita’ di Roma, Italy), A. Udalski (Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), I. Soszyński (Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), D. Minniti (Pontificia Universidad Católica de Chile) and B. Pilecki (Universidad de Concepción, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland).

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”.

Links

Research letter to Nature link

Contacts

Grzegorz Pietrzyński
Universidad de Concepción
Chile
Tel: +56 41 220 7268
Cell: +56 9 6245 4545
Email: pietrzyn@astrouw.edu.pl

Wolfgang Gieren
Universidad de Concepción
Chile
Tel: +56 41 220 3103
Cell: +56 9 8242 8925
Email: wgieren@astro-udec.cl

Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

UKIRT Instrumental in Discovery of the First Methane Dwarf Orbiting a Dying Star

An artist's impression of the binary as it might appear from a point in space near the methane dwarf. From here the distant white dwarf would appear as a bright star, only gently illuminating its cooling companion. Credit: Andrew McDonagh. Full size image (JPG, 762 KB)

An international team of astronomers using the United Kingdom Infrared Telescope (UKIRT) and the Gemini Observatory on Mauna Kea have discovered a unique and exotic star system with a very cool methane-rich brown dwarf (T dwarf) and a dying white dwarf stellar remnant in orbit around each other. The system is a 'Rosetta Stone' for T dwarfs, giving scientists the first good handle on their masses and ages.

This system is the first of its type to be found. The two stars are low in mass and have a weak mutual gravitational attraction as they are separated by about a light year or 2.5 trillion km, which is about one quarter of a light year. Despite the frailty of this system, it has stayed together for billions of years, but its stars are cooling down to a dark demise. The system is about 5 billion years old and about 160 light years away from us in the constellation of Virgo.

Methane dwarfs are on the boundary between stars and planets with temperatures typically less than 1000 degrees Celsius (in comparison the Sun's surface is at 5500 degrees Celsius). Methane is a fragile molecule destroyed at warmer temperatures, so is only seen in very cool stars and giant planets like Jupiter. Neither giant planets nor T dwarfs are hot enough for the hydrogen fusion that powers the Sun to take place, so that they simply cool and fade over time. This new T dwarf has a temperature of about 1300 Kelvin (= 1030 Celsius = 1900 Fahrenheit) and a mass of about 70 Jupiters.

White dwarfs are the end state of stars similar to and including the Sun. Once such stars have exhausted the available hydrogen fuel in their cores, they expel most of their outer layers into space forming a remnant planetary nebula and leaving behind a small, dense, hot, but cooling core or white dwarf. For our Sun this process will begin about 5 billion years in the future.

"By the time our Sun 'dies' and becomes a white dwarf itself, the methane dwarf will have cooled to around room temperature, and the white dwarf will be as cool as the methane dwarf was at the start of its life", comments team leader Dr Avril Day-Jones from the Universidad de Chile.

In the newly-discovered binary, the remnant nebula has long since dissipated and all that is left is the cooling white dwarf and methane dwarf pair. This binary system is providing a crucial test of the physics of ultra-cool stellar atmospheres because the white dwarf lets us establish the age of both objects. By comparison it determines properties of the methane dwarf such as its mass, making it a kind of 'Rosetta Stone' for similar stars with complex, hazy ultra-cool atmospheres.

The two stars are today separated by at least 2.5 trillion km, but would have been closer in the past before the white dwarf was formed. Once the star that formed the white dwarf reached the end of its life and expelled its outer layers, the loss of mass weakened the gravitational pull between the stars, causing the methane dwarf to spiral outwards to create the gravitationally fragile system that we see today. But we know from the current age of the white dwarf that this system has survived for several billion years. So the new discovery shows that despite their fragility, such binaries are able to remain united even in the maelstrom of the galactic disk.

This system was discovered by an international team led by Dr Avril Day-Jones from the Universidad de Chile, with astronomers from the University of Hertfordshire (UK), and the University of Montreal (Canada). The methane dwarf was identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) as part of the Large Area Survey's T-Dwarf Programme to identify the coolest objects in the galaxy. Its temperature and spectrum were measured by the Gemini North Telescope's NIRI Spectrometer in Hawaii.

The team found that the methane dwarf shares its motion across the sky with a nearby blue object catalogued as LSPM 1459+0857. They studied the blue object using the world's largest optical telescope, the European Southern Observatory's Very Large Telescope (VLT) in Chile. The new VLT observations revealed the blue object to be a cool white dwarf and companion to the methane dwarf. The objects were thus renamed LSPM 1459+0857 A and B.

"Binary systems like this provide vital information and allow us to better understand ultra-cool atmospheres and the very low-mass dwarfs and planets they enshroud" said Dr David Pinfield of the University of Hertfordshire. "The fact that these binaries survive intact for billions of years means that we could find many more lurking out there in the future."

The team, led by Dr Avril Day-Jones of the Universidad de Chile and including Dr David Pinfield of the University of Hertfordshire, will publish their results in the journal Monthly Notices of the Royal Astronomical Society.

Issued by: Inge Heyer, Public Information Officer
Joint Astronomy Centre
Email: outreach@jach.hawaii.edu
Desk: +1 808 969 6524

Notes and Contact

Black Hole True Power Revealed

A composite image showing the position of the 'miniature galaxy' S26 in the galaxy NGC 7793. The image of S26 is a radio image, made with a CSIRO telescope while the image of the galaxy is made from combined X-ray and optical data. Image credit - Soria et al / CSIRO / ATCA; NGC 7793 - NASA, ESO and NOAO.

Following a study of what is in effect a miniature galaxy buried inside a normal-sized one - like a Russian doll - astronomers using a CSIRO telescope have concluded that massive black holes are more powerful than we thought.

An international team of astronomers led by Dr Manfred Pakull at the University of Strasbourg in France has discovered a 'microquasar' - a small black hole, weighing only as much as a star, that shoots jets of radio-emitting particles into space.

Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13M light-years away in the Southern constellation of Sculptor.

Earlier this year Pakull and colleagues observed S26 with optical and X-ray telescopes (the European Southern Observatory's Very Large Telescope and NASA's Chandra space telescope).

Now they have made new observations with CSIRO's Compact Array radio telescope near Narrabri, NSW. These show that S26 is a near-perfect analogue of the much larger 'radio galaxies' and 'radio quasars'. Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13M light-years away in the Southern constellation of Sculptor.

Powerful radio galaxies and quasars are almost extinct today, but they dominated the early Universe, billions of years ago, like cosmic dinosaurs. They contain big black holes, billions of times more massive than the Sun, and shoot out huge radio jets that can stretch millions of light-years into space.

Astronomers have been working for decades to understand how these black holes form their giant jets, and how much of the black hole's energy those jets transmit to the gas they travel through. That gas is the raw material for forming stars, and the effects of jets on star-formation have been hotly debated.

"Measuring the power of black hole jets, and therefore their heating effect, is usually very difficult," said co-author Roberto Soria (University College London), who carried out the radio observations.

"With this unusual object, a bonsai radio quasar in our own backyard, we have a unique opportunity to study the energetics of the jets."

Using their combined optical, X-ray and radio data, the scientists were able to determine how much of the jet's energy went into heating the gas around it, and how much went into making the jet glow at radio wavelengths.

They concluded that only about a thousandth of the energy went into creating the radio glow.

"This suggests that in bigger galaxies too the jets are about a thousand times more powerful than we'd estimate from their radio glow alone," said Dr Tasso Tzioumis of CSIRO Astronomy and Space Science.

"That means that black hole jets can be both more powerful and more efficient than we thought, and that their heating effect on the galaxies they live in can be stronger."

The study was made possible by a recent upgrade to the Compact Array, which can now do work of this kind five times faster than before.

Publication: Roberto Soria, Manfred W. Pakull, Jess W. Broderick, Stephane Corbel, and Christian Motch. "Radio lobes and X-ray hotspots in the microquasar S26." In press in Monthly Notices of the Royal Astronomical Society. Available online on the MNRAS website and at http://arxiv.org/abs/1008.0394.

The Sun Steals Comets from Other Stars

A cluster of stars forming in the Orion nebula. According to Hal Levison's research, these stars could be swapping comets. [more]. Credit: NASA, JPL-Caltech, J. Stauffer (SSC/Caltech)

The next time you thrill at the sight of a comet blazing across the night sky, consider this: it's a stolen pleasure. You're enjoying the spectacle at the expense of a distant star.

Sophisticated computer simulations run by researchers at the Southwest Research Institute (SWRI) have exposed the crime.

"If the results are right, our Sun snatched comets from neighboring stars' back yards," says SWRI scientist Hal Levison. And he believes this kind of thievery accounts for most of the comets in the Oort Cloud at the edge of our solar system.

"We know that stars form in clusters. The Sun was born within a huge community of other stars that formed in the same gas cloud. In that birth cluster, the stars were close enough together to pull comets away from each other via gravity. It's like neighborhood children playing in each others' back yards. It's hard to imagine it not happening."

According to this "thief" model, comets accompanied the nearest star when the birth cluster blew apart. The Sun made off with quite a treasure – the Oort Cloud, which was swarming with comets from all over the "neighborhood."

The Oort cloud is an immense cloud of comets orbiting the Sun far beyond Pluto. It is named after mid-20th century Dutch astronomer Jan Oort, who first proposed such a cloud to explain the origin of comets sometimes seen falling into the inner solar system. Although no confirmed direct observations of the Oort cloud have been made, most astronomers believe that it is the source of all long-period and Halley-type comets.

An artist's concept of the Oort cloud. Note that the distance scale is logarithmic. Compared to the size of planetary orbits, the Oort cloud is very far away. Indeed, the estimated size of the Oort cloud, 10^5 AU, is approximately 1 light year. If the Sun passed within 2 light years of another sun-like star, the stars' Oort clouds would overlap and their comets would intermingle. Image credit: ESO. [more]

Could this comet rock-star have been stolen from another stellar system? No one knows. Read more about Comet Hartley 2 here.

The standard model of comet production asserts that our Sun came by these comets honestly.

"That model says the comets are dregs of our own solar system's planetary formation and that our planets gravitationally booted them to huge distances, populating the cloud. But we believe this kind of scenario happened in all the solar systems before the birth cluster dispersed."

Otherwise, says Levison, the numbers just don't add up.

"The standard model can't produce anywhere near the number of comets we see [falling in from the Oort Cloud]. The Sun's sibling stars had to have contributed some comets to the mix."

Comets in the Oort Cloud are typically 1 or 2 miles across, and they're so far away that estimating their numbers is no easy task. But Levison and his team say that, based on observations, that there should be something like 400 billion comets there. The "domestic" model of comet formation can account for a population of only about 6 billion.

"That's a pretty anemic Oort Cloud, and a huge discrepancy – too huge to be explained by mistakes in the estimates. There's no way we could be that far off, so there has to be something wrong with the model itself."

He points to the cometary orbits as evidence.

"These comets are in very odd orbits – highly eccentric long-period orbits that take them far from our Sun, into remote regions of space. So they couldn't have been born in orbit around the Sun. They had to have formed close to other stars and then been hijacked here."

This means comets can tell us not only about the early history of the Sun – but also about the history of other stars.

This means comets can tell us not only about the early history of the Sun – but also about the history of other stars.

"We can study the orbits of comets and put their chemistry into the context of where and around which star they formed. It's intriguing to think we got some of our 'stuff' from distant stars. We're kin."

Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA

Early universe was a liquid: The ALICE experiment announces first results from lead nuclei collisions at the LHC

Real lead-lead collision in ALICE
Credit: CERN

Real lead-lead collision in ALICE inner detector
Credit: CERN

Another Real lead-lead collision in ALICE inner detector
Credit: CERN

In an experiment to collide lead nuclei together at CERN’s Large Hadron Collider physicists from the ALICE detector team including researchers from the University of Birmingham have discovered that the very early Universe was not only very hot and dense but behaved like a hot liquid.

By accelerating and smashing together lead nuclei at the highest possible energies, the ALICE experiment has generated incredibly hot and dense sub-atomic fireballs, recreating the conditions that existed in the first few microseconds after the Big Bang. Scientists claim that these mini big bangs create temperatures of over ten trillion degrees.

At these temperatures normal matter is expected to melt into an exotic, primordial ‘soup’ known as quark-gluon plasma. These first results from lead collisions have already ruled out a number of theoretical physics models, including ones predicting that the quark-gluon plasma created at these energies would behave like a gas.

Although previous research in the USA at lower energies, indicated that the hot fire balls produced in nuclei collisions behaved like a liquid, many expected the quark-gluon plasma to behave like a gas at these much higher energies.

Scientists from the University of Birmingham’s School of Physics and Astronomy are playing a key role in this new phase of the LHC’s programme which comes after seven months of successfully colliding protons at high energies. Dr David Evans, from the University of Birmingham’s School of Physics and Astronomy, and UK lead investigator at ALICE experiment, said: “Although it is very early days we are already learning more about the early Universe.”

He continues: “These first results would seem to suggest that the Universe would have behaved like a super-hot liquid immediately after the Big Bang.”

The team has also discovered that more sub-atomic particles are produced in these head-on collisions than some theoretical models previously suggested. The fireballs resulting from the collision only lasts a short time, but when the ‘soup’ cools down, the researchers are able to see thousands of particles radiating out from the fireball. It is in this debris that they are able to draw conclusions about the soup’s behaviour.

This research is funded by the Science and Technology Facilities Council (STFC).

Notes to Editors

Two papers detailing this research have been submitted for publication and posted on:
http://xxx.lanl.gov/abs/1011.3914 and http://xxx.lanl.gov/abs/1011.3916

Images and captions

Pictures of lead collisions and the ALICE detector can be found at:
http://epweb2.ph.bham.ac.uk/user/evans/lead2010
and
http://aliceinfo.cern.ch/Public/Welcome.html
Images should be credited to CERN unless otherwise stated.

Further information
Kate Chapple, Press Officer, University of Birmingham, Tel: 0121 414 2772 or 07789 921164.

The ALICE Experiment
Physicists working on the ALICE experiment will study the properties, still largely unknown, of the state of matter called a quark-gluon plasma. This will help them understand more about the strong force and how it governs matter; the nature of the confinement of quarks – why quarks are confined in matter, such as protons; and how the Strong Force generates 98% of the mass of protons and neutrons. The ALICE detector is placed in the LHC ring, some 300 feet (100 metres) underground, is 52 feet (16 metres) high, 85 feet (26 metres) long and weighs about 10,000 tons.

The ALICE Collaboration consists of around 1000 physicists and engineers from about 100 institutes in 30 countries. The UK group consists of eight physicists and engineers and seven PhD students from the University of Birmingham. It plays a vital role in the design and construction of the central trigger electronics (the ALICE Brain) and corresponding software. In addition, the UK group is making an important contribution to the analysis of ALICE data.

During collisions of lead nuclei, ALICE will record data to disk at a rate of 1.2 GBytes (two CDs) every second and will write over two PBytes (two million GBytes) of data to disk; this is equivalent to more than three million CDs (or a stack of CDs (without boxes) several miles high). To process these data, ALICE will need 50,000 top-of-the-range PCs, from all over the world, running 24 hours a day.

ALICE utilises state-of-the-art technology including high precision systems for the detection and tracking of subatomic particles, ultra-miniaturised systems for the processing of electronic signals, and a worldwide distribution network of the computing resources for data analysis (the GRID). Many of these technological developments have direct implications to everyday life such as medical imaging, microelectronics and information technology.

CERN

CERN is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works.

University of Birmingham

The University of Birmingham is a truly vibrant, global community and an internationally-renowned institution. Its work brings people from across the world to Birmingham, including researchers and teachers and more than four thousand international students from nearly 150 different countries.

Science and Technology Facilities Council
The Science and Technology Facilities Council ensures the UK retains its leading place on the world stage by delivering world-class science; accessing and hosting international facilities; developing innovative technologies; and increasing the socio-economic impact of its research through effective knowledge exchange partnerships.

The Council has a broad science portfolio including Astronomy, Particle Physics, Particle Astrophysics, Nuclear Physics, Space Science, Synchrotron Radiation, Neutron Sources and High Power Lasers. In addition the Council manages and operates three internationally renowned laboratories:

- The Rutherford Appleton Laboratory, Oxfordshire
- The Daresbury Laboratory, Cheshire
- The UK Astronomy Technology Centre, Edinburgh

The Council gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), the Institute Laue Langevin (ILL), European Synchrotron Radiation Facility (ESRF) and the European Southern Observatory (ESO). It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Chile, and in the UK LOFAR and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank Observatory. (www.stfc.ac.uk)

More information on the previous, lower energy results can be found at:
http://www.bnl.gov/rhic/news2/news.asp?a=1074&t=pr

Spitzer Reveals a Buried Explosion Sparked by a Galactic Train Wreck

Infrared Glow of Stardust Galaxy II ZW 096
Credit:NASA/JPL-Caltech/STScI/H. Inami (SSC/Caltech)

These images show how a brilliant burst of star formation (red glow, right image) is revealed in infrared observations from NASA's Spitzer Space Telescope. The collision of two spiral galaxies, has triggered this luminous starburst, the brightest ever seen taking place far away from the centers, or nuclei, of merging galaxies.

The merging galaxies, known collectively as II Zw 096, can be clearly seen in the image from NASA's Hubble Space Telescope (left). This image combines light spanning the far-ultraviolet through the near-infrared. The real action in this galactic train wreck is barely hinted at in the red speckles near the middle of it all.

The booming blast of star formation only jumps out when Spitzer's mid-infrared view, represented in red, is folded into the mix (right). This tiny region may be as small as 700 light-years across - just a tiny portion of the full 50,000 light-year extent of II Zw 096 - yet it blasts out 80 percent of the infrared light from this galactic tumult. The surrounding shroud of dust renders the stars here nearly invisible in other wavelengths of light.

Researchers were surprised to see such a brilliant infrared glow in an area so far offset from the center of the spiral galaxy. Starbursts are often found crammed into the very centers of merging galaxies, but this is the brightest starburst ever seen outside a galaxy's nucleus. Based on Spitzer data, researchers estimate the starburst is cranking out stars at the breakneck pace of around 100 solar masses, or masses of our Sun, per year.

The Hubble image (left) represents ultraviolet light at a wavelength of 0.15 microns as blue, visible light at 0.44 microns as cyan, and near infrared light at 0.9 microns as red.

In the combined image (right) Hubble's far-ultraviolet and visible light at wavelengths of 0.15 and 0.44 microns is shown as blue, and the near infrared light at 0.9 microns is cyan. Spitzer's infrared light at 4.5 microns is represented by orange, and the mid-infrared light at 8.0 and 24 microns is red.

Astronomers using NASA's Spitzer Space Telescope have found a stunning burst of star formation that beams out as much infrared light as an entire galaxy. The collision of two spiral galaxies has triggered this explosion, which is cloaked by dust that renders its stars nearly invisible in other wavelengths of light.

The starburst newly revealed by Spitzer stands as the most luminous ever seen taking place away from the centers, or nuclei, of merging parent galaxies. It blazes ten times brighter than the nearby Universe's previous most famous "off-nuclear starburst" that gleams in another galactic smashup known as the Antennae Galaxy.

The new findings show that galaxy mergers can pack a real star-making wallop far from the respective galactic centers, where star-forming dust and gases typically pool.

"This discovery proves that merging galaxies can generate powerful starbursts outside of the centers of the parent galaxies," says Hanae Inami, first author of a paper detailing the results in the July issue of The Astronomical Journal. Inami is a graduate student at The Graduate University for Advanced Studies in Japan and the Spitzer Science Center at the California Institute of Technology. She adds: "The infrared light emission of the starburst dominates its host galaxy and rivals that of the most luminous galaxies we see that are relatively close to our home, the Milky Way."

"No matter how you slice it, this starburst is one of the most luminous objects in the local Universe," agrees Lee Armus, second author of the paper and a senior research astronomer also at the Spitzer Science Center.

A dazzling galactic dust-up

Inami, Armus and their colleagues spotted the buried starburst with Spitzer in the interacting galaxies known as II Zw 096. This galactic train wreck - located around 500 million light years away in the constellation Delphinus (the Dolphin) - will continue to unfold for a few hundred million years. Gravitational forces have already dissolved the once-pinwheel shape of one of II Zw 096's pair of merging galaxies.

The ultra-bright starburst region spans 700 light-years or so - just a tiny portion of II Zw 096, which streams across some 50,000 to 60,000 light-years - yet it blasts out 80 percent of the infrared light from this galactic tumult. Based on Spitzer data, researchers estimate the starburst is cranking out stars at the breakneck pace of around 100 solar masses, or masses of our Sun, per year.

The prodigious energy output of this starburst in a decentralized location as revealed in the infrared has surprised the Spitzer researchers. The new observations go to show how the notion of a cosmic object's nature can change tremendously when viewed at different wavelengths of light. In this way, the shapes and dynamics of distant, harder-to-study galactic mergers could turn out to be a good deal more complex than current observations over a narrow range of wavelengths imply.

"Most of the far-infrared emission in II Zw 096, and hence most of the power, is coming from a region that is not associated with the centers of the merging galaxies," Inami explains. "This suggests that the appearances and interactions of distant, early galaxies during epochs when mergers were much more common than today in the Universe might be more complicated than we think."

A fleeting, perhaps prophetic vista?

In galaxy mergers, individual stars rarely slam into one another because of the vast distances separating them; even in the comparatively crowded central hubs of spiral galaxies, trillions of kilometers still often yawn between the stars.

But giant, diffuse clouds of gas and dust in galaxies do crash together - passing through each other somewhat like ocean waves - and in turn spur the gravitational collapse of dense pockets of matter into new stars. These young, hot stars shine intensely in the energetic ultraviolet part of the spectrum. In the case of II Zw 096, however, a thick shroud of gas and dust still surrounds this stellar brood. The blanket of material absorbs the stars' light and re-radiates it in the lower-energy, infrared wavelengths that gleam clear through the dust to Spitzer's camera.

Astronomers were lucky to capture this transient phase in the evolution of the starburst and of the daughter galaxy that will eventually coalesce out of the collision. "Spitzer has allowed us to see the fireworks before all the gas and dust has cleared away, giving us a preview of the exciting new galaxy being built under the blanket," Inami says.

Merging galaxies such as II Zw 096 also offer a sneak peek at the fate of our Milky Way in some 4.5 billion years when it is expected to plow into its nearest large galactic neighbor, the Andromeda Galaxy. Off-nuclear starbursts such as that in II Zw 096 and the Antennae Galaxy could occur in the vicinity of our Solar System, perhaps, which is located about two-thirds of the way out from the Milky Way's glowing, bulging center.

"This kind of dramatic thing happening in II Zw 096 could happen to the Milky Way and Andromeda when they meet in the far future," says Inami.

By Adam Hadhazy

Why Do the Ionized Gas Clouds Stream Out from Galaxies?

Figure 1：Ionized Hydrogen Gas Clouds Ejected from a Galaxy in the Coma Cluster. Key to Figure：The figure is a pseudo-color composite of blue (B-band), green (R-band), and red (H-alpha-band). The red color shows regions with H-alpha emission without stellar light. The white bar at bottom-right represents 10 kpc (33 thousand light years). The image size is 115 x 271 arcsec².

Figure 2：Ionized Hydrogen Gas Clouds Ejected from Galaxies in the Coma Cluster. The colors in the image and the scale of the white bar are the same as those noted for Figure 1. The sizes of the fields are 145 x 87, 121 x 83, and 180 x 96 arcsec² for (a), (b) and (c), respectively.

Additional figures：The differences between the H-alpha-band and R-band images in Figure 1 and 2 (a)-(c) are as follows: The white structure (enclosed with a green contour) shows the region where the H-alpha emission is strong. The red line shows a central bright part of each parent galaxy. The parent galaxy appears as a black color because hydrogen in the atmosphere of stars in the galaxy absorbs the light at the wavelength of H-alpha.

Using the Subaru Prime Focus Camera (Suprime-Cam) in their observations of the Coma Cluster, researchers from the National Astronomical Observatory of Japan (NAOJ), Hiroshima University, the University of Tokyo, and other institutes have discovered 14 galaxies accompanied by extended, ionized hydrogen (Note 1) clouds. The discovery marks the first time that scientists have 1) detected many galaxies with extended ionized hydrogen gas clouds in a cluster and 2) investigated their spatial and velocity distribution as well as the characteristics of their parent galaxies. The observations captured images of this cluster of galaxies during a critical moment of galaxy evolution and contribute to an understanding of how such clouds may have formed.

A cluster of galaxies is an aggregate of a few to hundreds or even thousands of galaxies. Scientists know that more elliptical (E) and lenticular (S0) galaxies exist more often in cores of clusters of galaxies than in less dense environments. The elliptical and lenticular galaxies are called as "quiescent galaxies" because they show no star formation activity. Meanwhile, spiral galaxies such as our Galaxy are still undergoing star formations, and they are likely to reside in less populated regions. These attributes of clusters raise a number of important questions about the evolution of galaxies: "What kind of mechanisms shape these variety of galaxies occurring in different environments?" and "Why do clusters of galaxies contain many galaxies that do not form stars?" The current research provides observational evidence that addresses these issues.

The team focused their observations on the Coma Cluster, a large cluster of more than 3,000 galaxies and one of the nearest (about 300 billion light years away) clusters to our Galaxy. Past observations had found several extended ionized hydrogen clouds associated with galaxies in the cluster. This group of scientists concentrated on examining these clouds and used a special filter in their observations to catch a specific spectral line (the H-alpha line) (Note 2) created by ionized hydrogen at a particular wavelength. Consequently, they detected 14 galaxies with extended ionized hydrogen clouds, examples of which are shown in Figures 1 and 2.

Most of the ionized hydrogen gas appears as if it was ejected from the galaxy. Follow up observations with Subaru's Faint Object Camera and Spectrograph (FOCAS) confirmed that some of the gas clouds have a recession velocity (Note 3) comparable to that of adjacent galaxies. Therefore, the scientists infer that the overlap between the gas and the galaxy did not occur by chance but resulted from the gas streaming out of the galaxy.

A more detailed investigation of the ionized hydrogen clouds and their "parent galaxies" reveals that most of the parent galaxies are currently or were recently forming stars. In addition, most parent galaxies have a relatively large velocity difference (more than 1000 km/s) when compared with the average recession velocity of the Coma Cluster. These observational results suggest that the extended ionized hydrogen gas was probably stripped from the parent galaxies by either 1) interaction with the hot gas of the cluster or 2) by the tidal force of the cluster produced when the parent galaxies are trapped by the gravity of the cluster and fall onto the cluster. This scenario predicts a difference in star formation between low and high mass galaxies. Low mass galaxies that lose all of their gas from stripping cease star formation, while higher mass galaxies retain their gas and continue to form stars. The correlation between mass and star-forming activity derived from the observations in the team's research confirms the prediction.

In summary, this study has clarified some of the specific conditions under which extended ionized hydrogen clouds were formed as well as the relationship between the conditions and characteristics of the parent galaxies. Nevertheless, questions remain. How is the stripped gas ionized, and how does it retain the H-alpha emission? The most distant ionized gas cloud lies 300,000 light years from the parent galaxy, and it would take 100 million years or more for that cloud to travel this distance. Since the brightness of the H-alpha emission of the distant clouds is comparable to the clouds near the parent galaxy, the energy to maintain the H-alpha emission must have somehow persisted for more than 100 million years. How these H-alpha emitting structures endure this long remains a mystery. What is going on in the cluster!?

The research group will conduct further spectroscopic observations to help solve this puzzle. They plan to estimate the temperature and density of several parts of the ionized hydrogen clouds and to tackle the question of how the galaxy and gas are evolving in the nearby cluster of galaxies.

NOTES:
The research on which this article is based will be published in the December 2010 issue of the Astronomical Journal (140:1814-1829): "A Dozen New Galaxies Caught in the Act: Gas Stripping and Extended Emission Line Regions in the Coma Cluster" by M. Yagi et al.

Observation Details:

Object：A part of the central region of the Coma Cluster.
Telescope：Subaru Telescope with an effective aperture of 8.2 meters
Focus：Primary
Camera：Subaru Prime Focus Camera (Suprime-Cam)
Filter：Narrow-band H-alpha (671 nm), broadband B (450 nm), broadband R (650nm)
Observation Dates (UT)：2006/04/28, 2007/05/12-15, 2009/05/25-27
Exposure Times: 87.5 min (B), 190 min (R), 450 min (H-alpha)
Orientation: North is up; East is left
Coordinates: RA 13h 00m Dec. +27d 52m (J2000.0)
Constellation: Coma Berenices

Additional notes:

1. Neutral and ionized hydrogen: A hydrogen ion (proton) becomes neutral after combining with a free electron. After the combination, the neutral hydrogen quickly loses its energy until it reaches the minimum energy level ("ground state") of the atom by emitting a number of emission lines, including H-alpha. Because neutral hydrogen at its ground state is much more stable than ionized hydrogen, energy is required to ionize a neutral hydrogen to lead another recombination and H-alpha emission. In order to maintain H-alpha emission for a long time, moderate energy sources are necessary. If the input energy to neutral hydrogen is excessive, the kinetic energy of the free electron produced by ionization of neutral hydrogen increases a lot. Such a strong energy source results in a "totally ionized hot plasma", which does not emit H-alpha, because the free electrons in it have such high energy that they cannot combine with hydrogen ions.

2. H-alpha: One of the emission lines created when a hydrogen ion (proton) combines with a free electron. Its wavelength is 656 nm, and human eyes see it as a deep red color. The H-alpha emission from the Coma Cluster is observed at around 670 nm. The expansion of the universe accounts for the shift in the appearance of the emission to the red end of the visible spectrum ("redshift").

3. Recession velocity: The apparent speed of celestial objects moving away from Earth. The expansion of the universe causes that more distant objects appear to have a larger recession velocity. Generally objects at different distances have different recession velocities.

New evidence for supernova-driven galactic fountains in the Milky Way

Observing the X-ray-bright gas in the halo of the Milky Way, ESA's XMM-Newton has gathered new data which favour a process involving fountains of hot gas in our Galaxy. Such a scenario, with the gas flowing from the galactic disc into the halo where it then condenses into cooler clouds and subsequently falls back to the disc, confirms the importance of supernova explosions in forging the evolution of the interstellar medium and of the entire Galaxy.

The interstellar medium (ISM) in the Milky Way is a complex, dynamical system consisting of gas in different phases, spanning a wide range of densities and temperatures. The interplay among the various phases in the ISM, namely the hot, warm and cold gas, determines the entire history of star formation in our Galaxy, by shaping the birthplaces of stars. The most massive stars, in particular, have a deep influence on the ISM, as they release copious amounts of energy both during their lives and with their eventual dramatic demises in the form of supernova explosions. Understanding the structure and dynamics of the ISM is a key element in figuring out the processes of star formation in the Galaxy, and of the evolution of spiral galaxies in general.

Supernova-driven turbulence in the interstellar medium of the Milky Way contributes to the formation of galactic fountains. Credit: ESA. Illustration of galactic fountains

One phase of the ISM, the hot gas, has very low densities (below 0.01 cm-3) and temperatures as high as a few million Kelvin, hence it is hot enough to emit X-rays. The existence of the hot component of the ISM was first proposed in the 1970s, not long after the new spectral window of X-ray astronomy opened; since then, it has become clear that the hot phase represents an important component of the ISM, as it most directly traces the injection of energy into the ISM from stars and supernovae.

Supernova explosions heating the ISM can drive hot gas out of the disc in so-called galactic fountains, forming a halo of hot gas around the Milky Way. Such a halo was first detected by the ROSAT X-ray telescope in the early 1990s, and similar halos have also been detected around other spiral galaxies. In the galactic fountain scenario, as the gas rises above and below the disc, reaching heights of a few kiloparsecs, it emits radiation and thus becomes cooler. This cooled gas starts to condense into clouds which then fall back into the disc, in a fashion that resembles a fountain: this creates a global circulation of gas in the Galaxy which dynamically connects the disc and the halo. Radio observations of hydrogen gas in our Galaxy show structures that are thought to be superbubbles bursting out of the disc, giving rise to galactic fountains. However, we cannot see the hot gas rising into the halo in these structures, because the X-rays from this hot gas are absorbed by intervening material in the disc.

"Although we cannot directly observe the hot gas rising out of the disc, it has long been suspected that galactic fountains are responsible for the hot gas observed in the Milky Way's halo," explains David Henley from the University of Georgia, in the US, who led a study that provides new evidence supporting a galactic fountain mechanism in our Galaxy. "We have taken spectra of the X-ray-emitting hot gas in the galactic halo and compared them to detailed predictions coming from different models. The galactic fountain scenario turned out to be the one that best describes our data," he adds.

The study relies on a series of spectroscopic observations performed with ESA's X-ray observatory, XMM-Newton, targeting the emission from gas in the galactic halo, which is dominated by highly ionised oxygen atoms in the XMM-Newton energy band. Henley and collaborators compared the data to predictions from three different models that have been put forward to explain the origin of the hot gas in the halo: in one case, the hot gas is accreted from extragalactic material; another model accounts for the heating of the halo gas in terms of individual supernova explosions taking place in the halo itself; finally, a third model relies on supernovae powering the turbulent dynamics of the ISM and producing, among other features, galactic fountains. "The high-quality spectra collected by XMM-Newton were essential in discriminating between the various models, pointing towards the significant contribution of supernova-driven galactic fountains to the X-ray emission of the galactic halo," comments Norbert Schartel, XMM-Newton Project Scientist.

This result shows that galactic fountains are a major player in the mixing and distribution of gas in the ISM, thus confirming previous clues about the crucial role of supernovae in the global evolution of the Milky Way. "There are still some open issues but we feel a step closer to answering the question of the origin of the hot halo gas. Further observations, expanding the extent of the current survey, as well as more detailed simulations on the theoretical front will surely shed new light on this issue," Henley concludes.

Notes for editors:

The study relies on a survey of the soft X-ray background (SXRB) using archival XMM-Newton observations obtained with the EPIC cameras. The survey, presented in a companion publication (Henley & Shelton, 2010, ApJS, 187, 388), consists of 590 measurements of the OVII and OVIII intensities performed at galactic longitude between l=120° and l=240°.

This study employed a sub-sample of the survey, consisting of 26 observations, selected so that the measurements are both at high galactic latitude (|b|>30°) and minimally contaminated by the solar wind charge exchange emission.

The study compared the observed data with predictions coming from three models: a disc galaxy formation model, a model in which the gas is heated by supernova explosions within the halo, and a model which accounts for the heating of the halo in terms of a supernova-driven ISM which gives rise to, among other features, galactic fountains. Further details about these three theoretical scenarios are provided in the paper describing the study (Henley et al., 2010, ApJ, 723, 935).

Related publications:

Henley, D.B., et al., "The origin of the hot gas in the galactic halo: confronting models with XMM-Newton observations", 2010, Astrophysical Journal, volume 723, pages 935-953. DOI: 10.1088/0004-637X/723/1/935

Contacts:

David B. Henley
Department of Physics & Astronomy
University of Georgia
Athens, USA
Email: dbh@physast.uga.edu
Phone: +1-706-542-3913

Norbert Schartel
ESA XMM-Newton Project Scientist
Directorate of Science and Robotic Exploration
ESA, The Netherlands
Email: Norbert.Schartel@esa.int
Phone: +34-91-8131-184