An Unusual Supernova May Be a Missing Link in Stellar Evolution Research

Figure 1: SN 2005cz taken by the Subaru telescope. The supernova is marked by an arrow. On top-right of a supernova is the elliptical host galaxy HGC4589. Copyright: NAOJ, Subaru telescope

Figure 2: A spectrum of SN 2005cz (red, taken by the Subaru telescope) as compared to other supernovae. Spectra of Type Ib supernovae in the late-phases (about half a year and thereafter) are characterized by a strong emission from oxygen (labeled as [OI]). SN 2005cz does not show this feature, while a calcium emission line ([Ca II]) is very strong. [from Nature]

The standard theory of stellar evolution tells us that a life of a star is determined when it is born – the mass at the birth is a main function. Stars whose initial masses are above 8 – 10 solar masses experience a violent death – at the end of their lives, their inner core should suffer from a gravitational collapse, which then leads to the gigantic explosion known as a supernova explosion. This phenomenon is believed to be the origin of "apparently new stars" ("supernovae") which suddenly appear on the night sky (note that there is another channel leading to a supernova explosion, a type Ia supernova, that is a thermonuclear explosion of a white dwarf. In this paper, we hereafter call the core-collapse supernova explosions from massive stars as supernovae). The number of stars in the Universe decreases as a function of their mass: Namely, there are more "less-massive" stars than "more-massive" stars. Therefore, it has been believed that stars that are about 10 solar masses at the birth are the largest population among stars that end their lives as supernovae.

However, a supernova from this lower boundary in the progenitor mass had not been identified. It seemed that all core-collapse supernovae for which the research group estimated the progenitor masses originated from stars whose initial masses were at least 12 solar masses – sometimes above 40 solar masses. The researchers wondered why they did not identify the explosion from the “lower-limit-mass” stars – Is there anything wrong in the theory of stellar evolution? If it is the case, it is a disaster for astronomy: In many fields of astronomy, it has been assumed that these stars are the predominant population of supernovae

The research group observed a peculiar type Ib supernova SN 2005cz, using several telescopes, including 8.2m Subaru Telescope of NAOJ. They found various puzzling properties of this supernova; (1) it showed up in an elliptical galaxy that usually lacks massive stars to become type Ib supernovae, (2) it was faint, reaching only 20% of typical luminosity of other type Ib supernovae, and (3) it faded very quickly. On top of these properties, a late-time spectrum taken by the Subaru telescope at about 200 days after the explosion was most striking. An emission line from oxygen which is the strongest in type Ib supernovae is nearly missing, while there is a very strong emission line from calcium. This property is indeed a unique feature expected for an explosion of a star with about 10 solar masses. The research group concluded that all the peculiarities that SN 2005cz showed can now be understood consistently by this scenario of the explosion of the "least-massive-star". Note: In the same volume, Perets et al., reported observations of SN 2005E which look similar to SN 2005cz in many aspects, and suggested that it is a new type of the explosion that is an explosion within the surface layer of a white-dwarf. Kawabata and collaborators on the other hand argued that the core-collapse explosion scenario is more likely at least for SN 2005cz. Further study is necessary to understand a possible link between these two supernovae.

"Our study has rescued the standard theory of stellar evolution," mentioned Koji Kawabata from Hiroshima University, continuing, "This supernova was faint and gone quickly. This is probably a main reason why we have not got this kind of supernovae before". "This type of supernovae should be intrinsically abundant in the Universe", mentioned Keiichi Maeda, an assistant professor at IPMU, "They are important as origins of various things which we see in the Universe today. For example, these stars are believed to be main contributors of some elements including carbon and nitrogen which are essential ingredient of the life on Earth." Another example includes the diffuse neutrino background in the Universe. IPMU researchers have been trying to detect a signature of past supernovae explosions by looking into the "sum" of neutrino emissions from multiple past supernovae. Supernovae of the type discovered by this research may well be a predominant population here.

Institute for the Physics and Mathematics of the Universe (IPMU)

A research group led by Koji Kawabata of Hiroshima University, Keiichi Maeda, Ken’ichi Nomoto, Masaomi Tanaka of IPMU and their collaborators published their result in Nature (2010 May 20 issue). In the paper, they reported observations of a peculiar type Ib supernova 2005cz, and concluded that this is a supernova whose progenitor mass at its birth was about 10 times the Sun. Such a star represents a boundary between stars that end their lives with the gigantic supernova explosion and those without explosions. Supernovae from stars that were originally about 10 solar mass should occupy a large fraction of supernova explosions taking place in the whole universe. However a supernova whose progenitor mass lies just above the boundary has not been identified. This is a reason why astronomers have been seeking for an explosion in this mass range. This study thus finally provides a solid confirmation on the stellar evolution theory. Having identified properties of the resulting supernova explosion, this study also serves as an important step forward to understand roles of supernovae in evolution of the universe.

Publication：Nature　 2010 May 20 issue
Title："A Massive Star Origin for An Unusual Helium-Rich Supernovae in An Elliptical Galaxy"
Authors：Koji Kawabata (Hiroshima Univ.), Keiichi Maeda (IPMU), Ken’ichi Nomoto (IPMU), Stefan Taubenberger (MPA), Masaomi Tanaka (IPMU), Jinsong Deng (NAOC), Elena Pian (Pisa), Takashi Hattori (NAOJ), Koichi Itagaki (Itagaki Obs.)

---

CONTACTS

For more details

Keiichi Maeda,

IPMU Assistant Professor

e-mail: keiichi.maeda@ipmu.jp


Ken'ichi Nomoto,

IPMU Professor, IPMU Principal Investigator

e-mail: nomoto@astron.s.u-tokyo.ac.jp


Media Contact

Fusae Miyazoe,

IPMU Press Officer

e-mail: press@ipmu.jp

REFERENCES

• Hiroshima University, Astrophysics Center Topic Page
• Subaru Telescope
• Calar Alto Telescope
• Keck Telescope

Cannibalistic Galaxy Bends Light and Reveals its Monstrous Appetite

Gemini Legacy Image: R. Carrasco et al., Gemini Observatory/AURA

Central region of Abell 3827 as imaged using the Gemini Multi-Object Spectrograph on the Gemini South telescope in Chile. The central supermassive galaxy (ESO 146-IG 005) is clearly visible among its cluster companions as well as the remains of at least four nuclei that are being “digested” by the large galaxy. The central galaxy is thought to be the most massive galaxy in our local universe (out to about 1.5 billion light years). The field of view of this image is approximately 5 x 5 arcminutes and is a color composite made from g-, r- and i-band images combined and processed by Travis Rector (University of Alaska Anchorage). The inset (black on white image) is the single g-band image processed to reveal the gravitational lensed background galaxy arcs more clearly. Labeled on the inset are the most visible arcs from the closer background galaxy (z = 0.2 and labeled "A") and an arc from the more distant background galaxy (z = 0.4 and labeled "B"). This composite, labeled image, as well as all individual images, without text/labels, are available at full-resolution with the following links:

composite JPG 569 KB | TIFF 24.9 MB

text-free background image JPG 705 KB | TIFF 22.7 MB

inset image JPG 311 KB | TIFF 8.3 MB

A newly discovered gravitational lens in a relatively nearby galaxy cluster is leading astronomers to conclude that the cluster hosts the most massive galaxy known in our local universe. The study also reaffirms that galactic cannibalism is one reason that this galaxy is so obese, tipping the scales at up to 30 trillion times the mass of our Sun.

The supermassive galaxy is located at the core of the galaxy cluster Abell 3827, which lies some 1.4 billion light-years away. This galaxy and hundreds of its smaller cluster companions are visible in a dramatic new image released by the Gemini Observatory. The image is part of an upcoming paper in The Astrophysical Journal Letters that reports on the study of the massive galaxy using the gravitational lens formed by its core (also visible in the image) to provide new measurements of the galaxy’s extreme mass.

Although this bright galaxy (known as ESO 146-IG 005) dominates the core of Abell 3827, “the magnitude of its appetite has not been fully appreciated,” said Gemini astronomer Rodrigo Carrasco, who is a member of the team that used the 8-meter Gemini South telescope in Chile to study this galaxy and its cluster. The Gemini observations revealed, for the first time, the effects of gravitational lensing near the core of ESO 146-IG 005.

A gravitational lens is created when a massive object (in this case the core of the super-massive galaxy) distorts its local space. Light from a background galaxy (in this case two galaxies) that is passing by appears deflected from its original path. From our perspective, we see the background galaxies’ light reshaped as a ring-like structure and arcs around the lensing object. These arcs from both galaxies are clearly visible in the new Gemini images.

“The gravitational lens we discovered allowed us to estimate for the first time the mass of this monster galaxy very accurately. The inferred mass is a factor of 10 bigger than previous estimates derived from X-ray,” said Carrasco. “Assuming our model is correct, this is by far the most massive galaxy known in our local universe.”

The exceptional galaxy was not simply born massive; it has grown by consuming its companions in perhaps the most extreme example of ongoing “galaxy cannibalism” known. “This unabashed cannibal is something of a messy eater, with the partially digested remains of at least four smaller galaxies still visible near its center,” said team member Michael West, astronomer at the European Southern Observatory who first observed this system more than a decade ago and says that he was immediately struck by the complex morphology of this giant cannibal galaxy (see West’s Astronomy Picture of the Day August 31, 1998). “Eventually this galaxy will grow even bigger judging by the number of nearby galaxies already within its gravitational grasp.”

These observations yield important insight into the process of galaxy growth, especially of elliptical galaxies; these galaxies do not appear to acquire their full mass quickly in the early universe, but instead show significant growth through mergers and cannibalism at later times, after many of their stars have formed. The resulting galaxies, such as this one can be extremely massive.

The Gemini observations were made using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini South telescope in Chile. Follow-up spectroscopic observations used the same instrument to confirm the distances (redshifts) of the two background galaxies whose light is diverted by the massive galaxy. These two galaxies were found to lie at about 2.7 and 5.1 billion light-years away (z= 0.2 and 0.4 respectively).

In addition to R. Carrasco and M. West, the team includes Gemini astronomers P. Gomez, H. Lee, R. Diaz, J. Turner, B. Miller, M. Bergmann, and T. Verdugo (University of Valparaiso, Chile). Complete results appear in Carrasco et al. “Strong Gravitational Lensing by the Super-Massive cD Galaxy in Abell 3827”,The Astrophysical Journal Letters 715, L1, 2010.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawai'i (Gemini North) and the other telescope at Cerro Pachón in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Science and Technology Facilities Council (STFC), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.

Media Contacts:

• Peter Michaud
  Gemini Observatory, Hilo, HI
  Email: pmichaud"at"gemini.edu
  Cell: (808) 936-6643
  Desk: (808) 974-2510

• Antonieta Garcia
  Gemini Observatory, La Serena, Chile
  Email: agarcia"at"gemini.edu
  Phone (Desk): 56-51-205628

Science Contact:

• Rodrigo Carrasco
  Gemini Observatory, La Serena, Chile
  Email: rcarrasco"at"gemini.edu
  Phone (Desk): 56-51-205607

Two Peas in an Irregular Pod

New evidence from NASA's Spitzer Space Telescope is showing that tight-knit twin stars might be triggered to form by asymmetrical envelopes like the ones shown in this image. Image credit: NASA/JPL-Caltech/J.Tobin (Univ. of Michigan). Larger image

How Binary Stars May Form

Our sun may be an only child, but most of the stars in the galaxy are actually twins. The sibling stars circle around each other at varying distances, bound by the hands of gravity.

How twin stars form is an ongoing question in astronomy. Do they start out like fraternal twins developing from two separate clouds, or "eggs”? Or do they begin life in one cloud that splits into two, like identical twins born from one egg? Astronomers generally believe that widely spaced twin, or binary, stars grow from two separate clouds, while the closer-knit binary stars start out from one cloud. But how this latter process works has not been clear.

New observations from NASA's Spitzer Space Telescope are acting like sonograms to reveal the early birth process of snug twin stars. The infrared telescope can see the structure of the dense, dusty envelopes surrounding newborn stars in remarkable detail. These envelopes are like wombs feeding stars growing inside -- the material falls onto disks spinning around the stars, and then is pulled farther inward by the fattening stars.

The Spitzer pictures reveal blob-like, asymmetrical envelopes for nearly all of 20 objects studied. According to astronomers, such irregularities might trigger binary stars to form.

"We see asymmetries in the dense material around these proto-stars on scales only a few times larger than the size of the solar system. This means that the disks around them will be fed unevenly, possibly enhancing fragmentation of the disk and triggering binary star formation," said John Tobin of the University of Michigan, Ann Arbor, lead author of a recent paper in the Astrophysical Journal.

All stars, whether they are twins or not, form from collapsing envelopes, or clumps, of gas and dust. The clumps continue to shrink under the force of gravity, until enough pressure is exerted to fuse atoms together and create an explosion of energy.

Theorists have run computer simulations in the past to show that irregular-shaped envelopes may cause the closer twin stars to form. Material falling inward would be concentrated in clumps, not evenly spread out, seeding the formation of two stars instead of one. But, until now, observational evidence for this scenario was inconclusive.

Tobin and his team initially did not set out to test this theory. They were studying the effects of jets and outflows on envelopes around young stars when they happened to notice that almost all the envelopes were asymmetrical. This led them to investigate further -- 17 of 20 envelopes examined were shaped like blobs instead of spheres. The remaining three envelopes were not as irregular as the others, but not perfectly round either. Many of the envelopes were already known to contain embryonic twin stars – possibly caused by the irregular envelopes.

"We were really surprised by the prevalence of asymmetrical envelope structures," said Tobin. "And because we know that most stars are binary, these asymmetries could be indicative of how they form."

Spitzer was able to catch such detailed views of these stellar eggs because it has highly sensitive infrared vision, which can detect the faint infrared glow from our Milky Way galaxy itself. The dusty envelopes around the young stars block background light from the Milky Way, creating the appearance of a shadow in images from Spitzer.

"Traditionally, these envelopes have been observed by looking at longer infrared wavelengths where the cold dust is glowing. However, those observations generally have much lower resolution than the Spitzer images," said Tobin.

Further study of these envelopes, examining the velocity of the material falling onto the forming stars using radio-wavelength telescopes, is already in progress. While the researchers may not yet be able to look at a picture of a stellar envelope and declare "It's twins," their work is offering important clues to help solve the mystery of how twin stars are born.

Other authors of this study include Lee Hartmann of the University of Michigan, Ann Arbor; and Hsin-Fang Chiang and Leslie Looney of the University of Illinois, Urbana-Champaign. The observations were made before Spitzer ran out its liquid coolant in May 2009, beginning 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 the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

A Completely Grown-Up Galaxy in the Young Universe

Figure: The false-color background image is composed from images taken with the Subaru telescope's Suprime-Cam instrument (B and z' images for the blue and green channels, respectively) and WIRCAM on the Canada-France-Hawaii telescope (Ks filter for the red channel). The field size is 6 arc-minutes by 4 arc-minutes. The small square is centered on the target galaxy which is at a distance of 10 billion light years. The spectra taken with MOIRCS on Subaru is shown on the background. A two-dimensional spectrum is shown above and a one dimensional spectrum is shown with a gray line. The heavy black line represents the smoothed gray line. Red line represents a model spectrum, which is in good agreement with the observed spectrum. Strong absorption lines detected and used to measure the line broadenings are indicated with black arrows which come from Hydrogen, Calcium, and CH radicals (noted as "G-band"). Masks have been added to the brightest stars in the background images.

The central wavelength of B, z', and Ks filters are 440 nm, 900 nm, and 2200 nm, respectively.

An international team of astronomers led by Dr. Masato Onodera at the Commissariat a l'Energie Atomique in France has used the Subaru Telescope to take infrared spectra of a very distant, unusually bright, and massive elliptical galaxy. This galaxy is 10 billion light-years from Earth and was observed at a time when the Universe was only about one-quarter of its current age. Paradoxically, and in contrast with some previous studies, this galaxy appears to be similar to its cousins in the local Universe. Its size appears to be normal for its mass, and its velocity dispersion (about 300 km per second) is consistent with its large size. This research deepens the puzzle as to how and why some elliptical galaxies seem to reach their full size very early in the evolution of the Universe while other, very compact ones increase in volume a hundredfold over time.

Giant elliptical galaxies are the Universe’s most massive galaxies near to Earth. They have a regular, oval shape and lack the disk typical of spiral galaxies such as our own Milky Way. Using large telescopes, astronomers have identified elliptical galaxies over ten times more massive than the Milky Way and as far away from Earth as 10 billion light years. Observing the light from such distant elliptical galaxies permits the direct study of how they looked shortly after their formation and opens a window to exploring the past of the Universe.

Five years ago, extremely deep images from the Hubble Space Telescope (HST) suggested that distant elliptical galaxies may be twice to five times smaller than nearby, local elliptical galaxies of the same mass. If these findings were accurate, then the densities of the more distant elliptical galaxies are 10 to 100 times higher than that of the local ones. Since then, experts have debated how these very compact galaxies could expand over the intervening 10 billion years so that they matched the size of their local counterparts. Many questioned whether the measurements of the size of the distant elliptical galaxies were accurate. Could some measurement error or bias account for their apparently small size?

To address this question, an international team of astronomers led by Dr. Masato Onodera at the Commissariat a l'Energie Atomique in France turned to one of the world’s largest censuses of the distant Universe, the Cosmic Evolution Survey (COSMOS), to find new candidates for the most massive, distant, giant elliptical galaxies. They looked for objects with a unique “fingerprint” of visible and near-infrared light as measured by Subaru’s Prime Focus Camera (Suprime-Cam) and the Canada-France-Hawaii Telescope’s Wide-field Infrared Camera (WIRCam). Finally, they used the COSMOS team’s unique database of high-resolution Hubble Space Telescope images to pick objects that had similar shapes to local elliptical galaxies of the same mass. This final sample of objects was selected for further observations at Subaru.

Onodera’s team decided to use a different measurement—velocity dispersion of stars—to differentiate the evolutionary status of a distant elliptical galaxy. Velocity dispersion refers to the range of velocities of stars or galaxies in a cluster and is a way of determining the mass of objects within this spread. The smaller the size of a galaxy of a given mass, the faster the stars have to move in order to balance the pull of gravity. Measurements of the broadening of spectral lines in the galaxy’s spectrum can indicate the speed of the stars and enable derivation of the galaxy’s mass by combining measures of its size and velocity.

However, strong spectral lines appropriate for making these measurements of distant galaxies exist in the near infrared range of the spectrum, beyond visible light, where observations are particularly difficult. Equipped with its Multi-Object Infrared Camera and Spectrograph (MOIRCS), the Subaru Telescope was particularly well-suited for revealing these spectral lines, because it can capture infrared light from multiple objects in a wide field of view, providing images and spectroscopic measurements of their composition.

This is a relatively new way to measure the mass of distant galaxies. The first measurement of this kind as published only recently and revealed a velocity dispersion (over 500 km per second) of an elliptical galaxy that is consistent with its probable small size but has no counterpart among local galaxies. Using the same technique, Onodera’s team found an elliptical galaxy (ID 254025) with a smaller velocity dispersion (over 300 km per second) that is consistent with its large size (about 19,000 light years). These results provide evidence that large, fully-grown galaxies coexist with very compact ones in the early development of the Universe, a few billion years after the Big Bang.

The mystery of how different elliptical galaxies form and develop remains. Onodera’s team is now turning to the problem of determining the relative proportion of these two extreme types of elliptical galaxies as a function of cosmic time. Further observations with Subaru Telescope’s MOIRCS lie ahead to help solve this puzzle.

The paper appeared as “A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies” (Onodera et al. 2010, Astrophysical Journal Letters, Volume 715, pp. L6-L11).

This project was partly funded by the Agence Nationale de la Recherche, grant number ANR-07-BLAN-0228.

Team Members

Masato Onodera, Emanuele Daddi, Raphael Gobat (CEA/Saclay, France), Nobuo Arimoto, Naoyuki Tamura, Yoshihiko Yamada (National Astronomical Observatory of Japan, Japan), Michele Cappellari (University of Oxford, UK), Chiara Mancini, Alvio Renzini (Osservatorio Astronomico di Padova, Italy), Henry J. McCracken (Institut d’Astrophysique de Paris, France), Peter Capak, Nick Scoville (California Institute of Technology, USA), Marcella Carollo, Simon Lilly (ETH Zurich, Switzerland), Andrea Cimatti (Universita di Bologna, Italy), Mauro Giavalisco (University of Massachusetts, USA), Olivier Ilbert (Laboratoire d’Astrophysique de Marseille, France), Xu Kong (University of Science and Technology of China, China), Kentaro Motohara (University of Tokyo, Japan), Kouji Ohta (Kyoto University, Japan), Dave. B. Sanders (University of Hawaii, USA), Yoshiaki Taniguchi (Ehime University, Japan)

Links

Host institute of the principal investigator CEA/Saclay(France) and the releases posted there: in French, and in English

Hubble Finds Star Eating a Planet

About this image: This is an artist's concept of the exoplanet WASP-12b. It is the hottest known planet in the Milky Way galaxy, and potentially the shortest lived. The planet is only 2 million miles from its sunlike parent star — a fraction of Earth's distance from the Sun. Gravitational tidal forces from the star stretch the planet into an egg shape. The planet is so hot that it has puffed up to the point where its outer atmosphere spills onto the star. An accretion bridge streams toward the star and material is smeared into a swirling disk. The planet may be completely devoured by the star in 10 million years. The planet is too far away for the Hubble Space Telescope to photograph, but this interpretation is based in part on analysis of Hubble spectroscopic and photometric data. Credit: NASA, ESA, and G. Bacon (STScI) - STScI-PRC10-15

The hottest known planet in the Milky Way galaxy may also be its shortest-lived world. The doomed planet is being eaten by its parent star, according to observations made by a new instrument on NASA's Hubble Space Telescope, the Cosmic Origins Spectrograph (COS). The planet may only have another 10 million years left before it is completely devoured.

The planet, called WASP-12b, is so close to its sunlike star that it is superheated to nearly 2,800 degrees Fahrenheit and stretched into a football shape by enormous tidal forces. The atmosphere has ballooned to nearly three times Jupiter's radius and is spilling material onto the star. The planet is 40 percent more massive than Jupiter.

This effect of matter exchange between two stellar objects is commonly seen in close binary star systems, but this is the first time it has been seen so clearly for a planet.

"We see a huge cloud of material around the planet, which is escaping and will be captured by the star. We have identified chemical elements never before seen on planets outside our own solar system," says team leader Carole Haswell of The Open University in Great Britain.

Haswell and her science team's results were published in the May 10, 2010 issue of The Astrophysical Journal Letters.

A theoretical paper published in the science journal Nature last February by Shu-lin Li of the Department of Astronomy at the Peking University, Beijing, first predicted that the planet's surface would be distorted by the star's gravity, and that gravitational tidal forces make the interior so hot that it greatly expands the planet's outer atmosphere. Now Hubble has confirmed this prediction.

WASP-12 is a yellow dwarf star located approximately 600 light-years away in the winter constellation Auriga. The exoplanet was discovered by the United Kingdom's Wide Area Search for Planets (WASP) in 2008. The automated survey looks for the periodic dimming of stars from planets passing in front of them, an effect called transiting. The hot planet is so close to the star it completes an orbit in 1.1 days.

The unprecedented ultraviolet (UV) sensitivity of COS enabled measurements of the dimming of the parent star's light as the planet passed in front of the star. These UV spectral observations showed that absorption lines from aluminum, tin, manganese, among other elements, became more pronounced as the planet transited the star, meaning that these elements exist in the planet's atmosphere as well as the star's. The fact the COS could detect these features on a planet offers strong evidence that the planet's atmosphere is greatly extended because it is so hot.

The UV spectroscopy was also used to calculate a light curve to precisely show just how much of the star's light is blocked out during transit. The depth of the light curve allowed the COS team to accurately calculate the planet's radius. They found that the UV-absorbing exosphere is much more extended than that of a normal planet that is 1.4 times Jupiter's mass. It is so extended that the planet's radius exceeds its Roche lobe, the gravitational boundary beyond which material would be lost forever from the planet's atmosphere.

CONTACT

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Carole Haswell
The Open University, Great Britain, UK
011-44-190-865-3396
c.a.haswell@open.ac.uk

How a supernova obtains its shape

Fig. 1: Three-dimensional explosion simulation about 0.5 seconds after core bounce. The bluish, nearly transparent surface is the shock front with an average radius of 1900 km. Copyright: Max Planck Institute for Astrophysics

Fig. 2: These snap-shots show the outward mixing of certain elements in the supernova explosion from two different viewing directions, 350 seconds after core bounce in the upper two panels and after 9000 seconds in the lower two panels, when the shock has broken out of the stellar surface. The surfaces denote the radially outermost locations of carbon (green), oxygen (red), and nickel (blue) with a constant mass fraction. Copyright: Max Planck Institute for Astrophysics

Fig. 3: The Cassiopeia A nebula is the gaseous remnant of a supernova explosion whose light reached the Earth around the year 1680. The asymmetries and filamentary structure of this expanding cloud of stellar debris are a consequence of the clumping and mixing processes that also played a role in Supernova 1987A and that were simulated for the first time in all three dimensions by the team at the Max Planck Institute for Astrophysics. Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech/Steward/O.Krause et al.

Researchers of the Max Planck Institute for Astrophysics in Garching managed for the first time to reproduce the asymmetries and fast-moving iron clumps of observed supernovae by complex computer simulations in all three dimensions. To this end they successfully followed the outburst in their models consistently from milliseconds after the onset of the blast to the demise of the star several hours later. (Astrophysical Journal, 10 May 2010)

Massive stars end their lives in gigantic explosions, so called supernovae, and can become — for a short time — brighter than a whole galaxy, which is made up of billions of stars. Although supernovae have been studied theoretically by computer models for several decades, the physical processes happening during these blasts are so complex that astrophysicists until now can simulate only parts of the process and so far only in one or two dimensions. Researches at the Max Planck Institute for Astrophysics in Garching have now carried out the first fully three-dimensional computer simulations of a core collapse supernova over a timescale of hours after the initiation of the blast. They thus could answer the question how initial asymmetries, which emerge deep in the dense core during the very early stages of the explosion, fold themselves into inhomogeneities observable during the supernova blast.

While the great energy of the outburst makes these stellar explosions visible far out into the Universe, they are relatively rare. In a galaxy of the size of our Milky Way, on average only one supernova will occur in 50 years. About twenty years ago, a supernova could be seen even with the naked eye: SN 1987A in the Tarantula Nebula in the Large Magellanic Cloud, our neighbouring galaxy. This relative closeness — ”only“ about 170 000 light years away — allowed many detailed observations in different wavelength bands over weeks and even months. SN 1987A turned out to be a core-collapse supernova, a so-called Type II event. It occurs when a massive star, which is at least nine times heavier than the sun, has burned almost all its fuel. The fusion engine in the centre of the star begins to stutter, triggering an internal collapse and thus a violent explosion of the entire star. In the case of SN 1987A the star had about 20 solar masses at its birth.

SN 1987A is probably the best studied supernova and it is still a great challenge to develop and refine models of what was happening inside the dying star to produce its emission of radiation. One of the astonishing and unexpected discoveries in SN 1987A and many subsequent supernovae was the fact that nickel and iron — heavy elements that are formed near the centre of the explosion — are mixed outward in big clumps into the hydrogen shell of the disrupted star. Nickel bullets were observed to propagate at velocities of thousands of kilometres per second, much faster than the surrounding hydrogen and much faster than predicted by simple hydrodynamic calculations in one dimension (1D), i.e., only studying the radial profile from the centre outwards.

In fact, it turned out that the brightness evolution (the so-called light curve) of SN 1987A and of similar core-collapse supernovae can only be understood if large amounts of heavy core material (in particular radioactive nickel) are mixed outwards into the stellar envelope, and light elements (hydrogen and helium from the envelope) are carried inwards to the core.

The details of supernova explosions are very difficult to simulate, not only because of the complexity of the physical processes involved but also because of the duration and range of scales — from hundreds of metres near the centre to tens of millions of kilometres near the stellar surface — that need to be resolved in ultimately three- dimensional (3D) computer models. Previously conducted simulations in two dimensions (2D, i.e., with the assumption of axial symmetry) indeed showed that the spherical shell structure of the progenitor star is destroyed during the supernova blast and large-scale mixing takes place. But the real world is three-dimensional and not all observational aspects can be reproduced by 2D models.

The new computer models of the team at the Max Planck Institute for Astrophysics now simulate for the first time the complete burst in all three dimensions, from the first milliseconds after the explosion is triggered in the core to a time three hours later, when the shock breaks out of the progenitor star. ”We found substantial deviations in our 3D models compared to previous work in 2D,“ says Nicolay Hammer, the lead author of the paper, ”especially the growth of instabilities and the propagation of clumps differ. These are not just minor variations; this effect determines the long-time evolution and ultimately the extent of mixing and observable appearance of core- collapse supernovae.“

In the 3D-simulations, metal-rich clumps have much higher velocities than in the 2D case. These ”bullets“ expand much more rapidly, overtaking material from the outer layers. ”With a simple analytic model we could demonstrate that the different geometry of the bullets, toroidal versus quasi-spherical, can explain the differences observed in our simulations,“ explains co-author Thomas Janka. ”While we think that the differences between the 2D- and 3D-models that we found are probably generic, many features will depend strongly on the structure of the progenitor star, the overall energy and the initial asymmetry of the blast.“

”We hope that our models, in comparison to observations, will help us to understand how stellar explosions start and what causes them“, adds Ewald Müller, the third author of the paper. Investigating a wider variety of progenitor stars and initial conditions will therefore be the focus of future simulation work. In particular, a detailed model that reproduces all observational features of SN 1987A still remains a challenge.

Original publication

N.J. Hammer, H.-Th. Janka, E. Müller, "Three-dimensional simulations of mixing instabilities in supernova explosions", The Astrophysical Journal 714 (2010) 1371-1385

Links

Computer simulation of the first 500 milliseconds (Source: Leonhard Scheck, Max Planck Institute for Astrophysics)

Computer simulation up to 9000 seconds (Visualisation: Markus Rampp, Rechenzentrum Garching)

Contact

Dr. Hannelore Hämmerle
Press Officer
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone: +49 89 30000-3980
E-Mail: hhaemmerlempa-garching.mpg.de

Demise of a star under surprising circumstances

Fig.1: The environment of SN 2005 E. The image to the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right. Note the remote location of the supernova (marked by the arrow) with respect to its host, about 750 000 light years from the galaxy nucleus. Credits: SDSS, Lick Observatory

Supernovae, gigantic stellar explosions, are not only used as cosmic yardsticks by cosmologists, they are also important chemical element factories in our Universe. So far, astrophysicists know of two physical processes giving rise to these bursts: one is the core collapse of a massive star at the end of its lifetime, the other the thermonuclear detonation of an old white dwarf star. An international team of researchers, including scientists from the Max Planck Institute for Astrophysics, have now identified a third type of these stellar explosions, arising from a helium-rich, old stellar system. (Nature, 20 May 2010)

Depending on certain chemical elements identified in the light of supernovae, these stellar explosions are classified as Type Ia, Ib, Ic or Type II. As the light curves of Type Ia supernovae are very characteristic and uniform, astronomers use them as ”standard“ candles in extragalactic astronomy to determine the distance to their host galaxies. These supernovae are thought to arise when a white dwarf star, the burnt-out remnant of a normal star such as our Sun, approaches the so-called Chandrasekhar limit by accreting material from a binary companion. The dense core of mainly carbon and oxygen then ignites and releases so much energy that the star explodes as a supernova.

The other process leading to a supernova explosion is the gravitational collapse of the core of massive, short-lived stars at the end of their lifetimes. Astronomers believe that these are observed as Type Ib/c or Type II supernovae, which are associated with young stellar populations. Most of the stellar material is ejected due to the enormous amounts of energy released in the explosion, leaving behind a remnant with only a fraction of the initial mass of the star.

In January 2005, a faint supernova (SN 2005E) appeared in the halo of the nearby galaxy NGC 1032, and an international team of astronomers collected observations of this supernova from telescopes around the world. Surprisingly, the measurements of the chemical composition and amount of material expelled in the burst fit neither of the two known explosion mechanisms. The lack of any recent star formation activity near the supernova location and the very small mass ejected in the explosion (only about one third of the mass of the Sun) do not agree with an exploding giant star, i.e. a core collapse origin. The alternative, an exploding old white dwarf star that had a long time to travel from its star formation birthplace out to the halo, does not agree with the observations either, as the light spectrum indicates a different chemical composition. The material expelled by the supernova contains a higher fraction of calcium and titanium than any supernova observed so far. These elements are produced in nuclear reactions involving helium rather than the carbon and oxygen found in the centre of white dwarf stars.

Computer models have now shown that the supernova most likely occurred in an interacting system of two close white dwarf stars, where the helium shell of one white dwarf is drawn onto the other one. ”Once the receiving star has accumulated a certain amount, the helium starts to burn explosively,“ explains Paolo Mazzali, (Max Planck Institute for Astrophysics) who performed the calculations together with David Arnett (University of Arizona). ”The unique processes producing certain chemical elements in these explosions could solve some of the puzzles related to chemical enrichment. This could, for example, be the main source of titanium.“

The supernova SN 2005E might be only one of a new subset of dim supernovae arising from this distinct physical class of explosions. Several similar supernova events have been identified in evolved elliptical galaxies, whose light curves, environments and ejected mass are best described by the helium detonation process.

”When we observed SN 2005E it soon became clear that we were seeing a new type of supernova,“ says Hagai Perets (Weizmann Institute, now at the Center for Astrophysics, Harvard University), the lead observer. ”As these kinds of supernovae are relatively faint, they are difficult to detect. But if they are actually not all that rare, they might provide an answer to some fundamental physics puzzles about the production of chemical elements in the universe.“

Unusual supernovae are a speciality of this astronomer team. Only a few months ago they reported the first confirmed observation of another very peculiar type of supernova, which does not leave behind any remnant. Depending on their mass, stars end their lives as white dwarfs, neutron stars or black holes. Extremely massive stars, however, might disappear completely in the supernova explosion at the end of their lifetime. In these so-called pair-instability supernovae, energetic light particles are converted into electron-positron pairs, which cannot counteract the gravitational collapse. The violent contraction triggers a nuclear explosion that rips the star apart completely. The astronomers identified such a supernova, SN 2007bi, in a nearby dwarf galaxy, and published their findings in the journal Nature in December 2009.

Original publications

H.B. Perets, A. Gal-Yam, P. Mazzali et al., "A new type of stellar explosion from a helium rich progenitor", Nature, Vol. xxx, 20 May 2010

A. Gal-Yam, P. Mazzali, E. O. Ofek, et al., "Supernova 2007bi was a pair-instability supernova explosion", Nature, Vol. 462, p. 624-627, 3 December 2009

Contact

Dr. Hannelore Hämmerle
Press Officer
Max Planck Institute for Astrophysics
Phone: +49 89 30000-3980
E-Mail: hhaemmerlempa-garching.mpg.de

Dr. Paolo Mazzali
Max Planck Institute for Astrophysics
Scuola Normale Superiore and INAF Observatory, Italy
Phone: +49 89 30000-2221
E-Mail: pmazzalimpa-garching.mpg.de

Clear New View of a Classic Spiral

PR Image eso1020a

The classic spiral Messier 83 seen in the infrared with HAWK-I

PR Image eso1020b

Highlights of the HAWK-I infrared image of Messier 83

PR Image eso1020c

An infrared/visible comparison view of Messier 83

PR Image eso1020d

The sky around Messier 83

PR Video eso1020a

Zooming in on the HAWK-I infrared view of Messier 83

PR Video eso1020b

Panning across Messier 83 in the infrared

PR Video eso1020c

Visible/infrared cross-fades of Messier 83

ESO is releasing a beautiful image of the nearby galaxy Messier 83 taken by the HAWK-I instrument on ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile. The picture shows the galaxy in infrared light and demonstrates the impressive power of the camera to create one of the sharpest and most detailed pictures of Messier 83 ever taken from the ground.

The galaxy Messier 83 (eso0825) is located about 15 million light-years away in the constellation of Hydra (the Sea Serpent). It spans over 40 000 light-years, only 40 percent the size of the Milky Way, but in many ways is quite similar to our home galaxy, both in its spiral shape and the presence of a bar of stars across its centre. Messier 83 is famous among astronomers for its many supernovae: vast explosions that end the lives of some stars. Over the last century, six supernovae have been observed in Messier 83 — a record number that is matched by only one other galaxy. Even without supernovae, Messier 83 is one of the brightest nearby galaxies, visible using just binoculars.

Messier 83 has been observed in the infrared part of the spectrum using HAWK-I [1], a powerful camera on ESO’s Very Large Telescope (VLT). When viewed in infrared light most of the obscuring dust that hides much of Messier 83 becomes transparent. The brightly lit gas around hot young stars in the spiral arms is also less prominent in infrared pictures. As a result much more of the structure of the galaxy and the vast hordes of its constituent stars can be seen. This clear view is important for astronomers looking for clusters of young stars, especially those hidden in dusty regions of the galaxy. Studying such star clusters was one of the main scientific goals of these observations [2]. When compared to earlier images, the acute vision of HAWK-I reveals far more stars within the galaxy.

The combination of the huge mirror of the VLT, the large field of view and great sensitivity of the camera, and the superb observing conditions at ESO’s Paranal Observatory makes HAWK-I one of the most powerful near-infrared imagers in the world. Astronomers are eagerly queuing up for the chance to use the camera, which began operation in 2007 (eso0736), and to get some of the best ground-based infrared images ever of the night sky.

Notes

[1] HAWK-I stands for High-Acuity Wide-field K-band Imager. More technical details about the camera can be found in an earlier press release (eso0736).

[2] The data used to prepare this image were acquired by a team led by Mark Gieles (University of Cambridge) and Yuri Beletsky (ESO). Mischa Schirmer (University of Bonn) performed the challenging data processing.

More information

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

Contacts

Richard Hook
ESO
Garching, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org

Cassini Double Play: Enceladus and Titan

On the left, Saturn's moon Enceladus is backlit by the sun, showing the fountain-like sources of the fine spray of material that towers over the south polar region. On the right, is a composite image of Titan. Image credit: NASA/JPL/SSI and NASA/JPL/University of Arizona.

About a month and a half after its last double flyby, NASA's Cassini spacecraft will be turning another double play this week, visiting the geyser moon Enceladus and the hazy moon Titan. The alignment of the moons means that Cassini can catch glimpses of these two contrasting worlds within less than 48 hours, with no maneuver in between.

Cassini will make its closest approach to Enceladus late at night on May 17 Pacific time, which is in the early hours of May 18 UTC. The spacecraft will pass within about 435 kilometers (270 miles) of the moon's surface.

The main scientific goal at Enceladus will be to watch the sun play peekaboo behind the water-rich plume emanating from the moon's south polar region. Scientists using the ultraviolet imaging spectrograph will be able to use the flickering light to measure whether there is molecular nitrogen in the plume. Ammonia has already been detected in the plume and scientists know heat can decompose ammonia into nitrogen molecules. Determining the amount of molecular nitrogen in the plume will give scientists clues about thermal processing in the moon's interior.

The second of Cassini's two flybys is an encounter with Titan. The closest approach will take place in the late evening May 19 Pacific time, which is in the early hours of May 20 UTC. The spacecraft will fly to within 1,400 kilometers (750 miles) of the surface.

Cassini will primarily be doing radio science during this pass to detect the subtle variations in the gravitational tug on the spacecraft by Titan, which is 25 percent larger in volume than the planet Mercury. Analyzing the data will help scientists learn whether Titan has a liquid ocean under its surface and get a better picture of its internal structure. The composite infrared spectrometer will also get its southernmost pass for thermal data to fill out its temperature map of the smoggy moon.

Cassini has made four previous double flybys and one more is planned in the years ahead.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

More information on the Enceladus flyby, dubbed "E10," is available at: http://saturn.jpl.nasa.gov/mission/flybys/enceladus20100518/

More information on the Titan flyby, dubbed "T68," is available at: http://saturn.jpl.nasa.gov/mission/flybys/titan20100520/

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

Asteroid Caught Marching Across Tadpole Nebula

This image from WISE shows the Tadpole nebula.

Image credit: NASA/JPL-Caltech/UCLA

Full image and caption

A new infrared image from NASA's Wide-field Infrared Survey Explorer, or WISE, showcases the Tadpole nebula, a star-forming hub in the Auriga constellation about 12,000 light-years from Earth. As WISE scanned the sky, capturing this mosaic of stitched-together frames, it happened to catch an asteroid in our solar system passing by. The asteroid, called 1719 Jens, left tracks across the image, seen as a line of yellow-green dots in the boxes near center. A second asteroid was also observed cruising by, as highlighted in the boxes near the upper left (the larger boxes are blown-up versions of the smaller ones).

But that's not all that WISE caught in this busy image -- two satellites orbiting above WISE (highlighted in the ovals) streak through the image, appearing as faint green trails. The apparent motion of asteroids is slower than satellites because asteroids are much more distant, and thus appear as dots that move from one WISE frame to the next, rather than streaks in a single frame.

This Tadpole region is chock full of stars as young as only a million years old -- infants in stellar terms -- and masses over 10 times that of our sun. It is called the Tadpole nebula because the masses of hot, young stars are blasting out ultraviolet radiation that has etched the gas into two tadpole-shaped pillars, called Sim 129 and Sim 130. These "tadpoles" appear as the yellow squiggles near the center of the frame. The knotted regions at their heads are likely to contain new young stars. WISE's infrared vision is helping to ferret out hidden stars such as these.

The 1719 Jens asteroid, discovered in 1950, orbits in the main asteroid belt between Mars and Jupiter. The space rock, which has a diameter of 19 kilometers (12 miles), rotates every 5.9 hours and orbits the sun every 4.3 years.

Twenty-five frames of the region, taken at all four of the wavelengths detected by WISE, were combined into this one image. The space telescope caught 1719 Jens in 11 successive frames. Infrared light of 3.4 microns is color-coded blue: 4.6-micron light is cyan; 12-micron-light is green; and 22-micron light is red.

WISE is an all-sky survey, snapping pictures of the whole sky, including everything from asteroids to stars to powerful, distant galaxies.

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

More information is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu

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

Ancient City of Galaxies Looks Surprisingly Modern

A surprisingly large collections of galaxies (red dots in center) stands out at a remarkably large distance in this composite image combining infrared and visible-light observations. NASA's Spitzer Space Telescope contributed to the infrared component of the observations, while shorter-wavelength infrared and visible data are provided by Japan's Subaru telescope atop Mauna Kea, Hawaii. Image credit: NASA/JPL-Caltech/Subaru. Full image and caption

Astronomers are a bit like archeologists as they dig back through space and time searching for remnants of the early universe. In a recent deep excavation, courtesy of NASA's Spitzer Space Telescope, astronomers unearthed what may be the most distant, primitive cluster of galaxies ever found.

In a twist, however, this apparent ancestor to today's "big cities" of grouped galaxies looks shockingly modern. Called CLG J02182-05102, the ancient cluster is dominated by old, red and massive galaxies, typical of present-day clusters. For example, it is similar to a young version of the Coma Cluster of today, which has had billions of more years to develop.

"We are seeing something already aged and red like a younger version of the Coma Cluster from a distant, bygone era," said Casey Papovich, lead author of a new study and an assistant professor of physics and astronomy at Texas A&M University in College Station.

Papovich added, "it is as though we dug an archeological site in Rome and found pieces of modern Rome in amongst the ruins."

ClG J02182-05102 might have indeed been ahead of its time. Just as Rome was the world's biggest city more than 2,000 years ago with a population of about a million residents - a figure not again matched until the early 1800s in London - so too was this galactic grouping an advanced civilization for so early an era in the developing universe.

Galaxy clusters are the largest gravitationally bound structures in the universe and are thought to have formed piecemeal over cosmic time. For now, ClG J02182-05102 is the only known galactic grouping so far away in the past, and studying it will help researchers understand the overall history of how galaxies congregate and evolve.

A Cosmic Archeological Expedition

In their hunt for rare ancient cities in the early universe, Papovich and his team started with the largest extragalactic survey ever made. Called the Spitzer Wide-area InfraRed Extragalactic (SWIRE) survey, it observed a huge portion of the sky that could contain 250 full moons.

Because more light gathered means more information, the researchers looked at a cosmic region within this giant starscape that had also been studied by other instruments. These additional observations came from a survey combining light from Japan's Subaru telescope - housed atop Mauna Kea, Hawaii - and the European Space Agency's orbiting XMM-Newton telescope. The United Kingdom Infra-Red Telescope, also in Hawaii, provided infrared data along with another set of Spitzer observations called the Public Ultra Deep Sky survey.

When all these data were compiled, Spitzer's infrared observations made dozens of distant galaxies jump out. "We would not have found this object without Spitzer because there is very little optical light coming from this group of galaxies," said Papovich.

His team then obtained time on the Magellan telescope in Chile to study the faint light coming from CLG J02182-05102's least-dim galaxies. This light allowed the astronomers to archeologically date the candidate cluster to 9.6 billion years ago.

With these observations, Papovich and his team confirmed that seven of CLG J02182-05102's galaxies have nearly the same distance, suggesting they are part of a grouping of about 60 galaxies. Whether or not this association of galaxies fully qualifies as a gravitationally bound cluster will rely on further observations. Furthermore, the definition of a "cluster" itself remains unsettled, somewhat like the blurry distinctions between a city and a town, made trickier still given the limited light that makes it to our telescopes from these relics.

The Rise and Fall of CLG J02182-05102

For now, CLG J02182-05102 stands out as a greatly over-dense region of galaxies - a metropolis in a land of isolated villages. At its center regions loom red, monster galaxies containing about 10 times as many stars as our Milky Way galaxy. This puts them on par with the most mammoth galaxies in the nearby universe, which have grown fat through repeated mergers with other galaxies. These big galaxies are so uncharacteristic of those in the early universe that in some sense it is like finding modern skyscrapers in ancient Rome.

The Papovich et al paper was accepted for publication in the Astrophysical Journal on April 21, 2010. A subsequent study by Masayuki Tanaka of the Institute for the Physics and Mathematics of the Universe in Japan confirmed the discovery, and the work was the subject of a news release on May 10, 2010.

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 the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

Whitney Clavin (818) 354-4673

Jet Propulsion Laboratory

whitney.clavin@jpl.nasa.gov

Written by Adam Hadhazy

X-ray discovery points to location of missing matter

Satellite: XMM-Newton
Depicts: Artist's impression of WHIM in the Sculptor Wall
Copyright: Spectrum: NASA/CXC/Univ. of California Irvine/T. Fang.
Illustration: CXC/M. Weiss

Using observations with ESA's XMM-Newton and NASA's Chandra X-ray Observatory, astronomers have announced a robust detection of a vast reservoir of intergalactic gas about 400 million light years from Earth. This discovery is the strongest evidence yet that the 'missing matter' in the nearby Universe is located in an enormous web of hot, diffuse gas.

This missing matter - which is different from dark matter - is composed of baryons, the particles, such as protons and electrons, that are found on the Earth, in stars, gas, galaxies, and so on. A variety of measurements of distant gas clouds and galaxies have provided a good estimate of the amount of this 'normal matter' present when the Universe was only a few thousand million years old. However, an inventory of the much older, nearby Universe has turned up only about half as much normal matter, an embarrassingly large shortfall.

The mystery then is where does this missing matter reside in the nearby Universe? This latest work supports predictions that it is mostly found in a web of hot, diffuse gas known as the Warm-Hot Intergalactic Medium (WHIM). Scientists think the WHIM is material left over after the formation of galaxies, which was later enriched by elements blown out of galaxies.

"Evidence for the WHIM is really difficult to find because this stuff is so diffuse and easy to see right through," said Taotao Fang of the University of California at Irvine and lead author of the latest study. "This differs from many areas of astronomy where we struggle to see through obscuring material."

To look for the WHIM, the researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. This AGN, which is about two thousand million light years away, generates immense amounts of X-ray light as it pulls matter inwards.

Lying along the line of sight to this AGN, at a distance of about 400 million light years, is the so-called Sculptor Wall. This 'wall', which is a large diffuse structure stretching across tens of millions of light years, contains thousands of galaxies and potentially a significant reservoir of the WHIM if the theoretical simulations are correct. The WHIM in the wall should absorb some of the X-rays from the AGN as they make their journey across intergalactic space to Earth.

Using new data from Chandra and previous observations with both Chandra and XMM-Newton, absorption of X-rays by oxygen atoms in the WHIM has clearly been detected by Fang and his colleagues. The characteristics of the absorption are consistent with the distance of the Sculptor Wall as well as the predicted temperature and density of the WHIM. This result gives scientists confidence that the WHIM will also be found in other large-scale structures.

Several previous claimed detections of the hot component of the WHIM have been controversial because the detections had been made with only one X-ray telescope and the statistical significance of many of the results had been questioned.

"Having good detections of the WHIM with two different telescopes is really a big deal," said co-author David Buote, also from the University of California at Irvine. "This gives us a lot of confidence that we have truly found this missing matter."

In addition to having corroborating data from both Chandra and XMM-Newton, the new study also removes another uncertainty from previous claims. Because the distance of the Sculptor Wall is already known, the statistical significance of the absorption detection is greatly enhanced over previous 'blind' searches. These earlier searches attempted to find the WHIM by observing bright AGN at random directions on the sky, in the hope that their line of sight intersects a previously undiscovered large-scale structure.

Confirmed detections of the WHIM have been made difficult because of its extremely low density. Using observations and simulations, scientists calculate the WHIM has a density equivalent to only 6 protons per cubic metre. For comparison, the interstellar medium - the very diffuse gas in between stars in our Galaxy - typically has about a million hydrogen atoms per cubic meter.

"Evidence for the WHIM has even been much harder to find than evidence for dark matter, which is invisible and can only be detected indirectly," said Fang.

There have been important detections of possible WHIM in the nearby Universe with relatively low temperatures of about 100 000 degrees using ultraviolet observations and relatively high temperature WHIM of about 10 million degrees using observations of X-ray emission in galaxy clusters. However, these are expected to account for only a relatively small fraction of the WHIM. The X-ray absorption studies reported here probe temperatures of about a million degrees where most of the WHIM is predicted to be found.

Related publication

Fang, T., et al. 2010, "Confirmation of X-ray absorption by warm-hot intergalactic medium in the sculptor wall", The Astrophysical Journal, Vol. 714, Number 2, pp. 1715-1724. DOI: 10.1088/0004-637X/714/2/1715

Notes for editors

The XMM-Newton spacecraft is controlled by the European Space Operations Centre (ESOC, Darmstadt, Germany) using ground stations at Perth (Australia), Kourou (French Guiana) and Sanitago (Chile). The XMM-Newton Science Operations Centre situated at ESAC in Villafranca, Spain, manages observation requests and receives XMM-Newton data. The XMM-Newton Survey Science Centre (SSC), at Leicester University, UK, processes and correlates all XMM-Newton observations with existing sky data held elsewhere in the world.

NASA's Marshall Space Flight Center in Huntsville, Alabama, USA, manages the Chandra program for NASA's Science Mission Directorate in Washington, D.C., USA. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Massachusetts, USA.

Contacts

Janet Anderson
NASA Marshall Space Flight Center, Alabama, USA
Phone: +1-256-544-6162
Email: janet.l.anderson@nasa.gov

Megan Watzke
Chandra X-ray Center, Cambridge, Mass., USA
Phone: +1-617-496-7998
Email: mwatzke@cfa.harvard.edu

Norbert Schartel, ESA XMM-Newton Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Email: Norbert.Schartel@esa.int