Category: Astronomy

Artist’s impression of the Extragalactic Background Light emission and absorption. Image credit: HESS Collaboration. Click to enlarge
The Universe is filled with a diffuse glow of radiation coming from all the stars and galaxies. This cosmic fog is actually hard to detect because we have much brighter objects nearby that can wash it out; like how the city lights obscure the stars at night. One way to measure this radiation is by using the radiation from quasars, which are extremely bright and distant. The high-energy radiation from the quasars loses energy as it passes through this background radiation, and this can be measured.

All throughout space, a cosmic background light shimmers. Stars, galaxies – all kinds of sources – contribute to it; the light is their leftovers, in fact. Now, astrophysicists have discovered that this light is hardly as intense as anyone had guessed. The researchers used two distant quasars as “probes”, and recorded their gamma spectra using the H.E.S.S. telescopes in Namibia. These spectra turned out to be just a bit reddened; the background light seemed to only lightly obfuscate the quasars’ radiation. These observations do not just shed light on the background light – but on topics as great as the birth and development of galaxies (Nature, April 20, 2006).

Stars, galaxies, quasars, and many other objects contribute to the fog of radiation in the universe. It permeates all of intergalactic space; it is the “leftover” light that all these objects emit. Extragalactic background light – EBL – covers up epochs worth of stellar activity, from the time the first stars were created to the present. Scientists have been trying for a long time to measure this emission. Doing that directly is not easy, however, and extremely inaccurate, because Earth’s atmosphere, the Solar System, and the Milky Way send out radiation which gets in the way of observing weak EBL.

One way out of this problem is observing quasars – the cosmic energy factories which have a huge black hole in their middle. These “gravity traps” swallow up gas around them and spit some of it back as plasma, accelerated to nearly the speed of light. It is radiation bundled out of protons, electrons, and electromagnetic waves. Often, it can be hundreds of times wider than its mother galaxy. If this “quasar spray” heads in the direction of Earth, the radiation can appear quite strong – astronomers call this a “blazar”.

The two objects which H.E.S.S. researchers observed are both blazars. How to use them as probes? They send out very energetic gamma light particles, which lose strength on their way to Earth when they hit EBL photons. This causes the original blazar gamma spectrum to redden – like when the Sun nears the horizon at dusk and the Earth’s atmosphere disperses more of the blue part of the sunlight than the red. The thicker the atmosphere, the redder the sun. Reddening depends on the thickness of the medium. This fact is the key to investigating the composition of EBL.

Luigi Costamante of the Max Planck Institute for Nuclear Physics in Heidelberg says “the main problem is that energy distribution in quasars can take many different forms. Until now, we could not really say whether any observed spectrum looks red because it truly had a strong reddening, or if it was that way from the beginning.”

This problem has been solved thanks to the gamma spectra of two quasars — H 2356-309 and 1ES 1101-232. These objects are more distant than any sources observed until now. The sensitivity of the H.E.S.S. telescope made it possible to investigate them. It turns out that EBL’s intensity is not strong enough to redden quasar light; the spectra are too blue, and contain too many higher-energy gamma rays.

H.E.S.S. data has allowed the scientists to derive the maximum intensity of the diffused light. It is near the lowest limit resulting from the sum of the light of single galaxies visible in an optical telescope. That answers a question that has puzzled astronomers for years: is diffuse light created above all by the radiation from the first stars? The H.E.S.S. results seem to eliminate this possibility. There is also little room for contributions from other sources, like normal galaxies. Looking more closely at intergalactic space gives new perspectives on investigating gamma rays outside our own galaxy.

Original Source: Max Planck Society

The Field of Streams. Image credit: Vasily Belokurov/SDSS-II. Click to enlarge
The Milky Way is continuing to consume entire galaxies, and the evidence is right there in the night sky. After analyzing data from the Sloan Digital Sky Survey, astronomers have found many streams of stars – all that remains from these gobbled up galaxies. As a satellite galaxy merges with the Milky Way, it’s slowly torn apart as it sinks into the galactic halo. Streams of stars are unraveled like a ball of yarn, and these continue to orbit the Milky Way, distinct from the orbital movements of the rest of the stars in our galaxy.

A new map of stars in the Milky Way Galaxy, constructed with data from the Sloan Digital Sky Survey (SDSS-II), reveals a night sky criss-crossed with streams of stars, left behind by satellite galaxies and star clusters spiraling to their deaths.

Analyzing five years of data spanning nearly one-quarter of the sky, Cambridge University (UK) researchers Vasily Belokurov and Daniel Zucker created a dramatic new image of the outer Milky Way, using stellar colors eliminating the redder, nearby stars that would otherwise swamp the view of background structures. They found so many trails of stars in their high contrast image that they named the area the “Field of Streams.”

Satellite galaxies orbiting the Milky Way are literally ripped apart by the tidal forces of our galaxy. As these satellites sink in gravitational quicksand, their stars are torn from them in giant streams that trace their orbital paths — just like meteor streams lie along the paths of defunct comets in the Solar system.

Dominating the Field of Streams image is the enormous, arching stream of the Sagittarius dwarf galaxy. The Sagittarius dwarf was discovered more than a decade ago and other researchers have previously mapped its long tidal stream in other regions of the sky.

But the new SDSS-II data had a remarkable surprise in store.

“The stream appears forked,” said Belokurov. “We are seeing different wraps superimposed on the sky, as the stream goes around the galaxy two or three times.”

Because of the multiple wraps, the observations provide strong new constraints on the dark matter halo of the Milky Way, according to Mike Fellhauer of Cambridge University. “The leading theories of dark matter predict that the Galaxy’s halo should be flattened, like a rugby football. But our simulations only match the forked Sagittarius stream if the inner halo is round, like a soccer ball.”

In addition to the Sagittarius arches, the Field shows faint trails of stars torn from globular clusters, and other rings, trails, and lumps that appear to be the remains of disrupted dwarf galaxies. “There are more streams here than in a river delta,” commented Zucker.

Prominent among these is the Monoceros stream, discovered previously by SDSS-II scientists Heidi Jo Newberg of Rensselaer Polytechnic Institute and Brian Yanny of the Fermi National Accelerator Laboratory. The multiple rings of stars are all that remain from a dwarf satellite that was absorbed by the Milky Way long ago.

Crossing the Field is an enigmatic, new stream of stars extending over 70 degrees on the sky, whose original source remains unknown.

“Some of these ‘murdered’ galaxies have been named,” explained SDSS-II team member Wyn Evans of Cambridge, “but this galactic corpse hasn’t been identified yet. We’re looking for it right now.”

These new discoveries add weight to a picture in which galaxies like the Milky Way are built up from the merging and accretion of smaller galaxies.

“We’ve known about merging for some time,” said Yanny, “but the Field of Streams gives us a striking demonstration of multiple merger events going on the Milky Way galaxy right now. This is happening all over the Universe, as big galaxies grow by tearing up smaller ones into streams.”

These streams also provide new tests of the nature of dark matter itself, according to theorist James Bullock of University of California at Irvine; Bullock was not part of the SDSS team.

“The fact that we can see a ‘Field of Streams’ like this suggests that dark matter particles are very ‘cold’, or slow moving. If the dark matter was made up of ‘warm,’ fast moving particles, we wouldn’t expect these thin streams to hang around long enough for us to find them.”

Original Source: RAS News Release

An artist’s impression of the Milky way galaxy. Image credit: NASA. Click to enlarge
Astronomers have turned up two new companion galaxies to the Milky Way by looking through images in the Sloan Digital Sky Survey. The first is about 640,000 light years away in the constellation Canes Venatici – the most remote satellite galaxy ever discovered. The second is smaller and dimmer, and located in the constellation Bootes. It has a squashed structure because it’s being distorted by the Milky Way’s gravitational tides.

The Sloan Digital Sky Survey (SDSS-II) announced today (May 8) the discoveries of two new, very faint companion galaxies to the Milky Way.

The first was found in the direction of the constellation Canes Venatici (the Hunting Dog) by SDSS-II researcher Daniel Zucker at Cambridge University (UK). His colleague Vasily Belokurov discovered the second in the constellation Bootes (the Herdsman).

“I was poring over the survey’s map of distant stars in the Northern Galactic sky – what we call a Field of Streams — and noticed an overdensity in Canes Venatici,” Zucker explained. “Looking further, it proved to be a previously unknown dwarf galaxy. It’s about 640,000 light years (200 kiloparsecs) from the Sun. This makes it one of the most remote of the Milky Way’s companion galaxies.”

Zucker emailed Belokurov with the news, and, just as discoveries often build upon one another, Belokurov excitedly emailed back a few hours later with the discovery of a new, even fainter dwarf galaxy. The new galaxy in Bootes, which Belokurov called ‘Boo,’ shows a distorted structure that suggests it is being disrupted by the Milky Way’s gravitational tides. “Something really bashed Boo about,” said Belokurov.

Although the dwarf galaxies are in our own cosmic backyard, they are hard to discover because they are so dim. In fact, the new galaxy in Bootes is the faintest galaxy so far discovered, with a total luminosity of only about 100,000 Suns. But because of its distance (640,000 light years) it appears almost invisible to most telescopes. The previous dimness record holder was discovered last year in Ursa Major using SDSS-II data.

New galactic neighbors are exciting in their own right, but the stakes in searches for ultra-faint dwarfs are especially high because of a long-standing conflict between theory and observations. The leading theory of galaxy formation predicts that hundreds of clumps of “cold dark matter” should be orbiting the Milky Way, each one massive enough in principle to host a visible dwarf galaxy. But only about ten dwarf companions have been found to date.

One possibility is that the galaxies in the smaller dark matter clumps are too faint to have appeared in previous searches, but might be detectable in deep surveys like SDSS-II.

“It’s like panning for gold. Our view of the sky is enormous, and we’re looking for very small clumps of stars,” explained Cambridge University astronomer Wyn Evans, a member of the SDSS-II research team. Added collaborator Mark Wilkinson: “Finding and studying these small galaxies is really important. From their structure and their motions, we can learn about the properties of dark matter, as well as measure the mass and the gravity field of the Milky Way”.

The new discoveries are part of the SEGUE project (Sloan Extension for Galactic Understanding and Exploration), one of the three component surveys of SDSS-II. SEGUE will probe the structure and stellar make-up of the Milky Way Galaxy in unprecedented detail.

“I’m confident there are more dwarf galaxies out there and SEGUE will find them,” said Heidi Newberg of Rensselaer Polytechnic Institute, co-chair of SEGUE.

Original Source: RAS News Release

The Galaxy NGC 7424 as imaged by Gemini. Image credit: Gemini South GMOS. Click to enlarge
When a supernova was discovered in December 2001, astronomers immediately tagged it as a Type II – when a gigantic star runs out of fuel and explodes. But then the tell tale hydrogen surrounding it disappeared, and astronomers had to re-classify it as a Type I supernova – when a white dwarf steals matter from a companion. Astronomers using the Gemini telescope in Chile think they’ve solved the mystery. They found a companion star left behind when the supernova exploded; this was providing the hydrogen, and masking the original supernova.

Using the Gemini South telescope in Chile, Australian astronomers have found a predicted “companion” star left behind when its partner exploded as a very unusual supernova. The presence of the companion explains why the supernova, which started off looking like one kind of exploding star, seemed to change its identity after a few weeks.

The Gemini observations were originally intended to be reconnaissance for later imaging with the Hubble Space Telescope. “But the Gemini data were so good we got our answer straight away,” said lead investigator, Dr. Stuart Ryder of the Anglo-Australian Observatory (AAO).

Renowned Australian supernova hunter Bob Evans first spotted supernova 2001ig in December 2001. It lies in the outskirts of a spiral galaxy NGC 7424, which is about 37 million light-years away in the southern constellation of Grus (the Crane).

The supernova was monitored over the next month by optical telescopes in Chile. Supernovae are classified according to the features in their optical spectra. SN2001ig initially showed the telltale signs of hydrogen, which had it tagged as a Type II supernova, but the hydrogen later disappeared, which put it into the Type I category.

But how could a supernova change its type? Only a handful of such supernovae, classified as “Type IIb” to indicate their curious change of identity, have ever been seen. Only one (called SN 1993J) was closer than SN 2001ig.

Astronomers studying SN1993J had suggested an explanation: the supernova’s progenitor had a companion star that stripped material off the star before it exploded. This would leave only a little hydrogen on the progenitor-so little that it could disappear from the supernova spectrum within a few weeks.

A decade later observations with the orbiting Hubble Space Telescope and one of the Keck telescopes in Hawaii confirmed that SN 1993J did indeed have a companion. Ryder and colleagues wondered if SN2001ig might have had a companion as well.

Soon after SN2001ig was discovered, Ryder and his colleagues began monitoring it with a radio telescope, the CSIRO (Commonwealth Scientific and Industrial Research Organisation) Australia Telescope Compact Array in eastern Australia. The radio emission did not fall off smoothly over time but instead showed regular bumps and dips. This suggested that the material in space around the star that exploded-which must have been shed late in its life-was unusually lumpy.

Although the lumps might have represented matter periodically shed from the convulsing star, their spacing was such that another explanation seemed more likely: that they were generated by a companion in an eccentric orbit. As it orbited, the companion would have swept material shed by the progenitor into a spiral (pinwheel) pattern, with denser lumps at the point in the orbit-periastron-where the two stars approached most closely.

Such spirals have been imaged around hot, massive stars called Wolf-Rayet stars by Dr Peter Tuthill of the University of Sydney, using the Keck telescopes. The bumps in the radio light-curve of SN2001ig were spaced in a way consistent with the curvature of one of the spirals Tuthill has imaged.

“Stellar evolution theory suggests that a Wolf-Rayet star with a massive companion could produce this unusual kind of supernova,” said Ryder.

If the supernova progenitor had a companion, it might be visible when the supernova debris had cleared. So the astronomers put in a request to observe with the GMOS (Gemini Multi-Object Spectrograph) camera on the 8-meter Gemini South telescope.

When the time came to observe, the “seeing conditions” (stability of the atmosphere) were excellent. Just an hour and a half was needed to image the supernova field-and reveal a yellow-green point-like object at the location of the supernova explosion.

“We believe this is the companion,” said Ryder. “It’s too red to be a patch of ionized hydrogen, and too blue to be part of the supernova remnant itself.”

The companion has a mass of between 10 and 18 times that of the Sun. The astronomers hope to use GMOS again in coming months to get a spectrum of the companion, to refine this estimate.

Binary companions could explain much of the diversity seen in supernovae, Ryder suggests. “We’ve been able to show the chameleon-like behaviour of SN2001ig has a surprisingly simple explanation,” he said.

This is only the second time a companion star to a Type IIb supernova has been imaged, and the first time the imaging has been done from the ground.

A paper on the observations, “A post-mortem investigation of the Type IIb supernova 2001ig”, co-authored by Ryder, University of Tasmania graduate student Clair Murrowood and former AAO astronomer Dr Raylee Stathakis, was published online in Monthly Notices of the Royal Astronomical Society on May 2. It is also available HERE.

Original Source: Gemini Observatory

XMM-Newton slew survey of the Vela supernova remnant. Image credit: ESA. Click to enlarge
For most of its time, ESA’s XMM-Newton observatory is staring intently at a single object. But astronomers have figured out how to use the time the observatory spends turning from object to object – called “slewing”. Over the past 4 years, the observatory has actually imaged 25% of the sky in this way. A newly released sky survey contains this “spare time” data, which includes thousands of objects, many of which were previously unknown.

For the past four years, while ESA’s XMM-Newton X-ray observatory has been slewing between different targets ready for the next observation, it has kept its cameras open and used this spare time to quietly look at the heavens. The result is a ‘free-of-charge’ mission spin-off ? a survey that has now covered an impressive 25 percent of the sky.

The rapid slewing of the satellite across the sky means that a star or a galaxy passes in the field of view of the telescope for ten seconds only. However, the great collecting area of the XMM-Newton mirrors, coupled with the efficiency of its image sensors, is allowing thousands of sources to be detected.

Furthermore, XMM-Newton can pinpoint the position of X-rays coming from the sky with a resolution far superior to that available for most previous all-sky surveys. This is sufficient to allow the source of these X-rays to be found in many cases.

By comparing XMM-Newton survey’s data with those obtained over a decade ago by the international ROSAT mission, which also performed an all-sky survey, scientists can now check the long-term stability, or the evolution, of about two thousand objects in the sky.

An initial look shows that some sources have changed their brightness level by an incredible amount. The most extreme of these are variable stars and more surprisingly galaxies, whose unusual volatility may be due to large quantities of matter being consumed by an otherwise dormant central black hole.

The slew survey is particularly sensitive to active galactic nuclei (AGN) – galaxies with an unusually bright nucleus ? which can be traced out to a distance of ten thousand million light years.

While most stars and galaxies look like points in the sky, about 15 percent of the sources catalogued by XMM-Newton have an extended X-ray emission. Most of these are clusters of galaxies – gigantic conglomerations of galaxies which trap hot gas that emit X-rays over scales of a million light years.

Eighty-one of these clusters are already famous from earlier work but many other clusters, previously unknown, appear in this new XMM-Newton sky catalogue.

Scientists hope that the newly detected sources of this kind also include very distant clusters which are highly luminous in X-rays, as these objects are invaluable for investigating the evolution of the Universe. Follow-up observations by large optical telescopes are now needed to determine the distances of the individual galaxies in the newly discovered clusters.

Using traditional pointed observations, it takes huge amounts of telescope-time to image very large sky features, such as old supernova remnants, in their entirety. The slewing mechanism provides a very efficient method of mapping these objects, and several have been imaged including the 20 000 year-old Vela supernova remnant, which occupies a sky area 150 times larger than the full moon.

Extraordinarily bright, low-mass X-ray binary systems of stars (called ‘LMXB’) ? either powered by matter pulled from a normal star, or exploding onto the surface of a neutron star, or being consumed by a black hole – are observed with sufficient sensitivity to record their detailed light spectrum. Passes across these intense X-ray sources can help astronomers to understand the long-term physics of the interaction between the two stars of the binary system.

Many areas of astronomy are expected to be influenced by the XMM-Newton sky survey. Today, 3 May 2006, the XMM-Newton scientist have released a part of the catalogue resulting from the initial processing of the highest quality data obtained so far.

Such data correspond to a sky coverage of about 15 percent, and include more than 2700 very bright sources and a further 2000 sources of lower significance. Currently, about 55 percent of the catalogue entries have been identified with known stars, galaxies, quasars and clusters of galaxies.

A faster turn-around of slew-data processing is now planned to catch interesting transient (or temporary) targets in the act, before they have a chance to fade. This will give access to rare, energetic events, which only a sensitive wide-angle survey such as XMM-Newton’s can achieve.

It is planned to continually update the catalogue as XMM-Newton charts its way through the stars. This will cover at least 80 percent of the sky, leaving a tremendous legacy for the future.

Original Source: ESA Portal

The surface patterns for different torsional modes. Image credit: Max Planck. Click to enlarge
A massive explosion on the surface of a neutron star gave astronomers an opportunity to peer inside its surface, similar to how geologists understand the structure of the Earth beneath our feet. The explosion jolted the neutron star, and set it ringing like a bell. The vibrations then passed through layers of different density – slushy or solid – changing the X-rays streaming off. Astronomers calculated that it has a thicker crust approximately 1.6 km (1 mile) deep, matching theoretical estimates.

A US-German team of scientists from the Max Planck Institute for Astrophysics and NASA have used NASA’s Rossi X-ray Timing Explorer to estimate the depth of the crust on a neutron star, the densest object known in the universe. The crust, they say, is approximately 1.6 kilometres deep and so tightly packed that a teaspoon of this material would weigh about 10 million tonnes on Earth.

This measurement, the first of its kind, came courtesy of a massive explosion on a neutron star in December 2004. Vibrations from the explosion revealed details about the star’s composition. The technique is analogous to seismology, the study of seismic waves from earthquakes and explosions, which reveal the structure of the Earth’s crust and interior.

This new seismology technique provides a way to probe a neutron star’s interior, a place of great mystery and speculation. Pressure and density are so intense here that the core might harbour exotic particles thought to have existed only at the moment of the Big Bang.

Dr Anna Watts, of the Max Planck Institute for Astrophysics in Garching, carried out this research in collaboration with Dr. Tod Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

“We think this explosion, the biggest of its kind ever observed, really jolted the star and literally started it ringing like a bell,” said Strohmayer. “The vibrations created in the explosion, although faint, provide very specific clues about what these bizarre objects are made of. Just like a bell, a neutron star’s ring depends on how waves pass through layers of differing density, either slushy or solid.”

A neutron star is the core remains of a star once several times more massive than the sun. A neutron star contains about 1.4 solar masses of material crammed into a sphere only about 20 kilometres across. The two scientists examined a neutron star named SGR 1806-20, which is situated about 40,000 light years from Earth in the constellation Sagittarius. The object is in a subclass of highly magnetic neutron stars called magnetars.

On December 27, 2004, the surface of SGR 1806-20 experienced an unprecedented explosion, the brightest event ever seen from beyond our solar system. The explosion, called a hyperflare, was caused by a sudden change in the star’s powerful magnetic field that cracked the crust, likely producing a massive starquake. The event was detected by many space observatories, including the Rossi Explorer, which observed the X-ray light emitted.

Strohmayer and Watts think that the oscillations are evidence of global torsional vibrations within the star’s crust. These vibrations are analogous to the S-waves observed during terrestrial earthquakes, like a wave moving through a rope. Their study, building on observations of vibrations from this source by Dr. GianLuca Israel of Italy’s National Institute of Astrophysics, found several new frequencies during the hyperflare.

Watts and Strohmayer subsequently confirmed their measurements using NASA’s Ramaty High Energy Solar Spectroscopic Imager, a solar observatory that also recorded the hyperflare, and found the first evidence for a high-frequency oscillation at 625 Hz, indicative of waves traversing the crust vertically.

The abundance of frequencies – similar to a chord, as opposed to a single note – enabled the scientists to estimate the depth of the neutron star crust. This is based on a comparison of frequencies from waves travelling around the star’s crust and from those travelling radially through it. The diameter of a neutron star is uncertain, but based on the estimate of about 20 kilometres across, the crust would be about 1.6 kilometres deep. This figure, based on the observed frequencies, is in line with theoretical estimates.

Starquake seismology holds great promise for determining many neutron star properties. Strohmayer and Watts have analyzed archived Rossi data from a dimmer 1998 magnetar hyperflare (from SGR 1900+14) and found telltale oscillations here too, although not strong enough to determine the crust thickness.

A larger neutron star explosion detected in X-rays might reveal deeper secrets, such as the nature of matter at the star’s core. One exciting possibility is that the core might contain free quarks. Quarks are the building blocks of protons and neutrons, and under normal conditions are always tightly bound together. Finding evidence for free quarks would aid in understanding the true nature of matter and energy. Laboratories on Earth, including massive particle accelerators, cannot generate the energies needed to reveal free quarks.

“Neutron stars are great laboratories for the study of extreme physics,” said Watts. “We’d love to be able to crack one open, but since that’s probably not going to happen, observing the effects of a magnetar hyperflare on a neutron star is perhaps the next best thing.”

Original Source: Max Planck Society

The cores of the two galaxies NGC 2207 and IC 2163. Image credit: NASA. Click to enlarge
The two “eyes” in this photograph are actually the cores of two merging galaxies; as viewed by NASA’s Spitzer Space Telescope. The galaxies are called NGC 2207 and IC 2163, and the surrounding material is their twisted spiral arms. Dotted along these arms are knotted clusters of newborn stars, created when the two galaxies smashed into each other. The pair is located 140 million light-years away in the Canis Major constellation, and they’ll eventually become a single galaxy in another 500 million years.

A pair of dancing galaxies appears dressed for a cosmic masquerade in a new image from NASA’s Spitzer Space Telescope.

The infrared picture shows what looks like two icy blue eyes staring through an elaborate, swirling red mask. These “eyes” are actually the cores of two merging galaxies, called NGC 2207 and IC 2163, which recently met and began to twirl around each other.

The “mask” is made up of the galaxies’ twisted spiral arms. Dotted along the arms, like strings of decorative pearls, are dusty clusters of newborn stars. This is the first time that clusters of this type, called “beads on a string” by astronomers, have been seen in NGC 2207 and IC 2163.

“This is the most elaborate case of beading we’ve seen in galaxies,” said Dr. Debra Elmegreen of Vassar College in Poughkeepsie, N.Y. “They are evenly spaced and sized along the arms of both galaxies.”

Elmegreen is lead author of a paper describing the Spitzer observations in the May 1 issue of the Astrophysical Journal.

Astronomers say the beads were formed when the galactic duo first met. “The galaxies shook each other, causing gas and dust to move around and collect into pockets dense enough to collapse gravitationally,” said Dr. Kartik Sheth of NASA’s Spitzer Science Center at the California Institute of Technology in Pasadena. Once this material condensed into thick bead-like clouds, stars of various sizes began to pop up within them.

Spitzer’s infrared camera was able to see the dusty clouds for the first time because they glow with infrared light. The hot, young stars housed inside the clouds heat up the dust, which then radiates at infrared wavelengths. This dust is false-colored red in the image, while stars are represented in blue.

The Spitzer data also reveal an unusually bright bead adorning the left side of the “mask.” This dazzling orb is so packed full of dusty materials that it accounts for five percent of the total infrared light coming from both galaxies. Elmegreen’s team thinks the central stars in this dense cluster might have merged to become a black hole.

Visible-light images of the galaxies show stars located inside the beads, but the beads themselves are invisible. In those pictures, the galaxies look more like a set of owl-like eyes with “feathers” of scattered stars.

NGC 2207 and IC 2163 are located 140 million light-years away in the Canis Major constellation. The two galaxies will meld into one in about 500 million years, bringing their masquerade days to an end.

Other authors of this research include Bruce Elmegreen of IBM Watson Research Center, Yorktown Heights, N.Y., Michele Kaufman of Ohio State University, Columbus; Curt Struck of Iowa State, Ames; Magnus Thomasson of Onsala Space Observatory, Sweden; and Elias Brinks of the University of Hertfordshire, United Kingdom.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech. Spitzer’s infrared array camera was built by NASA’s Goddard Space Flight Center, Greenbelt, Md. The instrument’s principal investigator is Dr. Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics.

Original Source: Spitzer Space Telescope

Fossil galaxy cluster as observed by XMM-Newton. Image credit: ESA. Click to enlarge
Galaxies start small, but grow over time as they merge with other galaxies. After a while, however, the nearby space runs out of galaxies to merge with. All that’s left is one large galaxy called a fossil group, which sits inside an even larger halo of dark matter. Astronomers are puzzled at how these fossil groups are able to form rapidly – some shouldn’t be able to do it in the lifetime of the Universe. New observations from Chandra and ESA’s XMM-Newton observatories have provided new clues about how these clusters collapse and form.

Taking advantage of the high sensitivity of ESA’s XMM-Newton and the sharp vision of NASA’s Chandra X-Ray space observatories, astronomers have studied the behaviour of massive fossil galaxy clusters, trying to find out how they find the time to form.

Many galaxies reside in galaxy groups, where they experience close encounters with their neighbours and interact gravitationally with the dark matter – mass which permeates the whole intergalactic space but is not directly visible because it doesn’t emit radiation.

These interactions cause large galaxies to spiral slowly towards the centre of the group, where they can merge to form a single giant central galaxy, which progressively swallows all its neighbours.

If this process runs to completion, and no new galaxies fall into the group, then the result is an object dubbed a ‘fossil group’, in which almost all the stars are collected into a single giant galaxy, which sits at the centre of a group-sized dark matter halo. The presence of this halo can be inferred from the presence of extensive hot gas, which fills the gravitational potential wells of many groups and emits X-rays.

A group of international astronomers studied in detail the physical features of the most massive and hot known fossil group, with the main aim to solve a puzzle and understand the formation of massive fossils. In fact, according to simple theoretical models, they simply could not have formed in the time available to them!

The fossil group investigated, called ‘RX J1416.4+2315’, is dominated by a single elliptical galaxy located one and a half thousand million light years away from us, and it is 500 thousand million times more luminous than the Sun.

The XMM-Newton and Chandra X-ray observations, combined with optical and infrared analyses, revealed that group sits within a hot gas halo extending over three million light years and heated to a temperature of 50 million degrees, mainly due to shock heating as a result of gravitational collapse.

Such a high temperature, about as twice as the previously estimated values, is usually characteristic of galaxy clusters. Another interesting feature of the whole cluster system is its large mass, reaching over 300 trillion solar masses. Only about two percent of it in the form of stars in galaxies, and 15 percent in the form of hot gas emitting X-rays. The major contributor to the mass of the system is the invisible dark matter, which gravitationally binds the other components.

According to calculations, a fossil cluster as massive as RX J1416.4+2315 would have not had the time to form during the whole age of the universe. The key process in the formation of such fossil groups is the process known as ‘dynamical friction’, whereby a large galaxy loses its orbital energy to the surrounding dark matter. This process is less effective when galaxies are moving more quickly, which they do in massive ‘clusters’ of galaxies.

This, in principle, sets an upper limit to the size and mass of fossil groups. The exact limits are, however, still unknown since the geometry and mass distribution of groups may differ from that assumed in simple theoretical models.

“Simple models to describe the dynamical friction assume that the merging galaxies move along circular orbits around the centre of the cluster mass”, says Habib Khosroshahi from the University of Birmingham (UK), first author of the results. “Instead, if we assume that galaxies fall towards the centre of the developing cluster in an asymmetric way, such as along a filament, the dynamic friction and so the cluster formation process may occur in a shorter time scale,” he continues. Such a hypothesis is supported by the highly elongated X-ray emission we observed in RX J1416.4+2315, to sustain the idea of a collapse along a dominant filament.”

The optical brightness of the central dominant galaxy in this fossil is similar to that of brightest galaxies in large clusters (called ‘BCGs’). According to the astronomers, this implies that such galaxies could have originated in fossil groups around which the cluster builds up later. This offers an alternative mechanism for the formation of BCGs compared to the existing scenarios in which BCGs form within clusters during or after the cluster collapse.

“The study of massive fossil groups such as RX J1416.4+2315 is important to test our understanding of the formation of structure in the universe,” adds Khosroshahi. “Cosmological simulations are underway which attempt to reproduce the properties we observe, in order to understand how these extreme systems develop,” he concludes.

Original Source: ESA News Release

A scientifically accurate model of Beta Pictoris and its disk. Image credit: NAOJ. Click to enlarge
The disks of gas and dust that surround newborn stars are known as proto-planetary disks; which are thought to be regions where planets will eventually form. These disks disappear as the stars mature, but some stars can still be seen with a cloud of material around them called debris disks. One of the most famous of these is the disk surrounding Beta Pictoris, located only 60 light years away.

Planets form in disks of gas and dust that surround new born stars. Such disks are called proto-planetary disks. The dust in these disks become rocky planets like Earth and the inner cores of giant gas planets like Saturn. This dust is also a repository of elements that form the basis of life.

Proto-planetary disks disappear as stars mature, but many stars have what are called debris disks. Astronomers hypothesize that once objects such as asteroids and comets are born from the proto-planetary disk, collisions among them can produce a secondary dust disk.

The most well-known example of such dust disks is the one surrounding the second brightest star in the constellation Pictor, meaning “painter’s easel”. This star, known as Beta Pictoris or Beta Pic, is a very close neighbor of the Sun, only sixty light years away, and therefore easy to study in great detail.

Beta Pic is twice as bright as the Sun, but the light from the disk is much fainter. Astronomers Smith and Terrile were the first to detect this faint light in 1984, by blocking the light from the star itself using a technique called coronagraphy. Since then, many astronomers have observed the Beta Pic disk using ever better instruments and ground and space-based telescopes to understand in detail the birth place of planets, and hence life.

A team of astronomers from the National Astronomical Observatory of Japan, Nagoya University and Hokkaido University combined several technologies for the first time to obtain an infrared polarization image of the Beta Pic disk with better resolution and higher contrast than ever before: a large aperture telescope (the Subaru telescope, with its large 8.2 meter primary mirror), adaptive optics technology, and a coronagraphic imager capable of taking images of light with different polarizations (Subaru’s Coronagraphic Imager with Adaptive Optics,CIAO).

A large aperture telescope, especially with Subaru’s great imaging quality, allows faint light to be seen at high resolution. Adaptive optics technology reduces Earth’s atmosphere’s distorting effects on light, allowing higher resolution observations. Coronagraphy is a technique for blocking light from a bright object such as a star, to see fainter objects near it, such as planets and dust surrounding a star. By observing polarized light, reflected light can be distinguished from light coming directly from its original source. Polarization also contains information about the size, shape, and alignment of dust reflecting light.

With this combination of technologies, the team succeeded in observing Beta Pic in infrared light two micrometers in wavelength at a resolution of a fifth of an arcsecond. This resolution corresponds to being able to see an individual grain of rice from one mile away or a mustard seed from a kilometer away. Achieving this resolution represents a huge improvement over comparable previous polarimetric observations from the 1990’s that had only resolutions of about one and a half arcseconds.

The new results strongly suggest that Beta Pic’s disk contains planetesimals, asteroid or comet-like objects, that collide to generate dust that reflects starlight.

The polarization of the light reflected from the disk can reveal the physical properties of the disk such as composition, size, and distribution. An image of all the two micrometer wavelength light shows the long thin structure of the disk seen nearly edge on. The polarization of the light shows that ten percent of the two micrometer light is polarized. The pattern of polarization indicates that the light is a reflection of light that originated from the central star.

An analysis of how the brightness of the disk changes with distance from the central shows a gradual decrease in brightness with a small oscillation. The slight oscillation in brightness corresponds to variations in the density of the disk. The most likely explanation is that denser regions correspond to where planetesimals are colliding. Similar structures have been seen closer to the star in earlier observations at longer wavelengths using Subaru’s COoled Mid-Infrared Camera and Spectrograph (COMICS) and other instruments.

A similar analysis of how the amount of polarization changes with distance from the star shows a decrease in polarization at a distance of one hundred astronomical units (an astronomical unit is the distance between Earth and the Sun). This corresponds to a location where the brightness also decreases, suggesting that at this distance from the star there are fewer planetesimals.

As the team investigated models of the Beta Pic disk that can explain both the new and old observations, they found that the dust in Beta Pic’s disk is more than ten times larger than typical grains of interstellar dust. Beta Pics dust disk is probably made of micrometer sized loose clumps of dust and ice like miniscule bacteria-size dust bunnies.

Together, these results provide very strong evidence that the disk surrounding Beta Pic is generated by the formation and collision of planetesimals. The level of detail of this new information solidifies our understanding of the environment in which planets form and develop.

Motohide Tamura who leads the team says “few people have been able to study the birth place of planets by observing polarized light with a large telescope. Our results show that this is a very rewarding approach. We plan on extending our research to other disks, to get a comprehensive picture of how dust transforms into planets.”

These results were published in the April 20, 2006, edition of the Astrophysical Journal.

Team Members: Motohide Tamura, Hiroshi Suto, Lyu Abe (NAOJ), Misato Fukagawa (Nagoya University, California Institute of Technology), Hiroshi Kimura, Tetsuo Yamamoto (Hokkaido University)

This research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan through a Grant-in-Aid for Scientific Research on Priority Areas for the “Development of Extra-solar Planetary Science.”

Original Source: NAOJ News Release

Massive star forming region as seen by ISO. Image credit: ESA. Click to enlarge
Astronomers have used ESA’s Infrared Space Observatory to peer at the biggest stars being born. These stars form from the collapse of enormous clouds of gas, and can shine more than 100,000 times as bright as our own Sun. The images were captured as a bonus, taken while the space observatory was slowly turning from one target to another. A team of astronomers built up a vast web of images comprised from 10,000 of these telescope repositionings, and then identified potential star forming regions from the data.

Scientists have secured their first look at the birth of monstrous stars that shine 100 000 times more brightly than the Sun, thanks to ESA’s Infrared Space Observatory (ISO).

The discovery allows astronomers to begin investigating why only some regions of space promote the growth of these massive stars.

Space is littered with giant clouds of gas. Occasionally, regions within these clouds collapse to form stars. “One of the major questions in the field of study is why do some clouds produce high- and low-mass stars, whilst others form only low-mass stars?” asks Oliver Krause, Max-Planck-Institut fur Astronomie, Heidelberg and Steward Observatory, Arizona.

The conditions necessary to form high-mass stars are difficult to deduce because such stellar monsters form far away and are shrouded behind curtains of dust. Only long wavelengths of infrared radiation can escape from these obscuring cocoons and reveal the low temperature dust cores that mark the sites of star formation. This radiation is exactly what ISO’s ISOPHOT far-infrared camera has collected.

Stephan Birkmann, Oliver Krause and Dietrich Lemke, all of the Max-Planck-Institut f?r Astronomie, Heidelberg, used ISOPHOT’s data to zero-in on two intensely cold and dense cores, each containing enough matter to form at least one massive star. “This opens up a new era for the observations of the early details of high-mass star formation,” says Krause.

The data was collected in the ISOPHOT Serendipity Survey (ISOSS), a clever study pioneered by Lemke. He realised that when ISO was turning from one celestial object to another, valuable observing time was being lost. He organised for ISOPHOT’s far-infrared camera to continuously record during such slews and beam this data to Earth.

During the ISO mission, which lasted for two and a half years during 1995?98, the spacecraft made around 10 000 slews, providing a web of data across the sky for the previously unexplored window of infrared emission at 170 micrometres. This wavelength is 310 times longer than optical radiation and reveals cold dust down to just 10K (-263 degrees Celsius). A catalogue was produced of the cold sites in the survey.

Birkmann and his colleagues investigated this catalogue and found fifty potential places of high-mass stellar birth. A campaign of follow-up observations using ground-based telescopes revealed that object ISOSS J18364-0221 was in fact two cold dense cores that looked suspiciously like those associated with the birth of low-mass stars, but containing much more mass.

The first core is at 16.5 Kelvin (?256.5 degrees Celsius). It contains seventy-five times the mass of the Sun and shows signs of gravitational collapse. The second one is around 12K (?261 degrees Celsius) and contains 280 solar masses. The team are currently studying the other potential sites.

Original Source: ESA Portal