Universe Today

Image credit: Hubble

A new article published in the journal Nature helps settle a long-time mystery about some of the earliest solid particles in the Universe. By measuring supernova remnant Cassiopeia A with the very precise SCUBA telescope, astronomers were able to detect enormous quantities of cosmic dust below -257 degrees Celsius. Hot dust had been found in the past, but the colder dust was mostly invisible – until now. It appears that supernovae are extremely efficient at producing the dust that later forms planets, rocks, and people.

We have just discovered that some supernovae have bad habits – they belch out huge quantities of smoke, known as cosmic dust. This solves a long-standing mystery over the origin of cosmic dust and suggests that supernovae, which are exploding stars, were responsible for producing the first ever solid particles in the Universe.

The Prime Suspects
Supernovae are the violent explosions of stars occurring at the end of their lives. They occur around every 50 years or so in our Galaxy and there are two main types – Type Ia and II. Type II are the explosions of very massive stars with mass greater than 8 times the mass of the Sun (Msun). These stars are ‘live fast – die young’ using up their hydrogen and helium fuel in only a few million years, thousands of times faster than the Sun burns it’s fuel. When the fuel supply is exhausted the star must burn heavier and heavier elements until, finally, when it can do no more to keep itself alive the inner parts of the star collapse to form a neutron star or Black Hole, and the outer parts are flung off in the cataclysm we call a supernova. The enormous explosion sweeps up the surrounding gas into a shell which shines at X-ray, optical and radio wavelengths, and sends shock waves through the galaxy. Supernovae release more energy in a single instant than the Sun will produce in its whole life-time. If the nearest massive star, Betelgeuse in the constellation Orion, were to go supernova it would (for a short time) be brighter than the full moon.

The Cosmic Smoke-Screen
Interstellar dust consists of tiny particles of solid material floating around in the space between the stars – with sizes typically that of cigarette smoke. It is not the same as the dust we clean up in our houses, and in fact the Earth is a giant lump of cosmic dust! It is responsible for blocking about half of all the light emitted from stars and galaxies and profoundly affects our view of the Universe. This ‘dusty’ cloud has a silver lining though, as the astronomers can `see’ the dust radiating the stolen starlight using special cameras designed to work at longer wavelengths, in the Infra-Red (IR: 10 – 100 microns) and Submillimeter (sub-mm: 0.3 – 1mm) part of the electromagnetic spectrum. One such camera is called SCUBA and it is located on the James Clerk Maxwell Telescope in Hawaii. SCUBA is a UK-built instrument which detects light-waves at sub-mm wavelengths and is able to see dust right out to where the furthest stars and galaxies are found.

Dusty Beginnings
Recent observations with SCUBA have shown that a huge amount of dust exists in galaxies and quasars when the Universe was only 1/10th of its present age, long before the Earth and solar system had formed. The presence of all this dust in the distant Universe has a great impact on what astronomers are able to see with their giant optical telescopes, as it limits the amount of starlight which can escape from a distant galaxy and be seen on Earth.

That there were so many solid particles in Universe at such an early time was a great surprise to astronomers as they had believed that dust was mainly formed in cool winds from red giant stars near the end of their lives. Since it takes a long time for star to reach this stage in its evolution (the Sun will take around 9 billion years) there has simply not been enough time for so much dust to have been made in this way.

‘Dust has been swept under the cosmic carpet – for years astronomers have treated it as a nuisance because of the way it hides the light from the stars. But then we found that there is dust right at the edge of the Universe, in the earliest stars and galaxies, and we realised that we were ignorant of even its basic origin’ explained Dr Dunne.

Supernovae also make large amounts of heavy elements, such as carbon and oxygen, and throw them out into interstellar space. These are the elements which make up our bodies and, since they are also the elements which make up dust grains, supernovae have long been a prime suspect in the mystery of the origin of cosmic dust. As it takes only a few million years for the most massive stars to reach the end of lives and explode as supernovae, they could make dust quickly enough to explain what is seen in the early Universe. However, until this team’s work, only tiny amounts of dust had ever been found in supernovae – leaving astronomers with a smoking gun but no ‘smoke’

Haley Morgan, a PhD student at Cardiff said ‘If supernovae were efficient dust ‘factories’ they would each be producing more than the mass of the Sun in dust.’

‘As massive stars evolve to become supernovae in the blink of an eye by astronomical standards, they could easily explain why the early Universe appears so dusty.’ added Dr Rob Ivison of the Royal Observatory Edinburgh.

Supernova Sleuths
The team from Cardiff and Edinburgh used SCUBA to look for the emission from dust in the remains of a recent supernova. Cassiopeia A is the remnant of a supernova which happened around 320 years ago. It is located in the constellation Cassiopeia, 11,000 light years from Earth and is about 10 light years across. Cas A is the brightest radio source in the sky so it is well studied at many wavelengths from the optical to X-rays. The images below show Cas A in the X-rays, optical, infra-red and radio. The X-rays follow the really hot gas (10 million degrees Kelvin), and the other wavelengths trace material at: 10 thousand degrees (optical), hot dust at 100 K (IR) and high energy electrons (radio).

Although astronomers had been searching for dust in supernova remnants for decades, they had used instruments which could only detect dust that was quite warm, such as that in the ISO infra-red image above. SCUBA has the advantage here because it is able to see dust which is very cold and this is because it works at longer sub-mm wavelengths.

‘In the same way that you can only see an iron poker glowing when it’s been in a fire, you can only see dust with infra-red cameras when it is warmer than about 25 Kelvin, but SCUBA can see it when it’s colder too’ explained Dr Steve Eales, Reader in Astrophysics at Cardiff University.

Cold Hard Evidence
SCUBA found a large amount of dust in the Cas A remnant, 1-4 times more than the mass of the Sun ! This is over 1,000 times more than had been seen before. This means that Cas A was very efficient at creating dust from the elements available. The temperature of the dust is very low, only 18 Kelvin (-257 degrees Celsius), and this is the reason that it had never been seen before. Below are the two sub-mm images of Cas A at 850 and 450 microns taken with SCUBA. You can see that the left image looks a little like the radio one above, and this is because the high energy electrons which make the radio image also emit some of their energy at slightly shorter wavelengths – contaminating the sub mm emission at 850microns. The middle image is at 450 microns where the contamination is much lower, and so most of this emission is from cold dust. If we remove the contamination we get a different picture (right). All the dust is seen in the bottom half of the remnant and the two sub-mm images now look much more similar!
850 microns without radio contamination

‘The puzzle is how the dust can remain so cold when we know that there is gas at over a million degrees present from the X-ray radiation it gives off.’ commented Prof. Mike Edmunds, head of the School of Physics & Astronomy in Cardiff.

The dust also has different properties to the ‘everyday’ kind of dust in the Milky Way and other galaxies – it is better at ‘shining’ in the sub-mm, maybe because it is still very young and relatively pristine. If all supernovae were this efficient at making dust they would be the biggest dust ‘factories’ in the Galaxy. Smoking supernovae provide a solution to the mystery of the huge amounts of dust seen in the early Universe.

‘These observations give us a tantalising glimpse of how the first solid particles in the Universe were created’ said Haley Morgan.

Original Source: Cardiff University News Release

Image credit: ESO

Astronomers from the European Southern Observatory have found a very rare “Einstein ring” gravitational lens, where the light from a distant quasar is warped and magnified by the gravity of a closer galaxy. The two objects are so closely aligned that the image of the quasar forms a ring around the galaxy from our vantage point here on Earth. With careful measurements, the team was able to determine that the quasar is 6.3 billion light-years away, and the galaxy is only 3.5 billion light-years away, making it the closest gravitational lens ever discovered.

Using the ESO 3.6-m telescope at La Silla (Chile), an international team of astronomers [1] has discovered a complex cosmic mirage in the southern constellation Crater (The Cup). This “gravitational lens” system consists of (at least) four images of the same quasar as well as a ring-shaped image of the galaxy in which the quasar resides – known as an “Einstein ring”. The more nearby lensing galaxy that causes this intriguing optical illusion is also well visible.

The team obtained spectra of these objects with the new EMMI camera mounted on the ESO 3.5-m New Technology Telescope (NTT), also at the La Silla observatory. They find that the lensed quasar [2] is located at a distance of 6,300 million light-years (its “redshift” is z = 0.66 [3]) while the lensing elliptical galaxy is rougly halfway between the quasar and us, at a distance of 3,500 million light-years (z = 0.3).

The system has been designated RXS J1131-1231 – it is the closest gravitationally lensed quasar discovered so far.

Cosmic mirages
The physical principle behind a “gravitational lens” (also known as a “cosmic mirage”) has been known since 1916 as a consequence of Albert Einstein’s Theory of General Relativity. The gravitational field of a massive object curves the local geometry of the Universe, so light rays passing close to the object are bent (like a “straight line” on the surface of the Earth is necessarily curved because of the curvature of the Earth’s surface).

This effect was first observed by astronomers in 1919 during a total solar eclipse. Accurate positional measurements of stars seen in the dark sky near the eclipsed Sun indicated an apparent displacement in the direction opposite to the Sun, about as much as predicted by Einstein’s theory. The effect is due to the gravitational attraction of the stellar photons when they pass near the Sun on their way to us. This was a direct confirmation of an entirely new phenomenon and it represented a milestone in physics.

In the 1930’s, astronomer Fritz Zwicky (1898 – 1974), of Swiss nationality and working at the Mount Wilson Observatory in California, realised that the same effect may also happen far out in space where galaxies and large galaxy clusters may be sufficiently compact and massive to bend the light from even more distant objects. However, it was only five decades later, in 1979, that his ideas were observationally confirmed when the first example of a cosmic mirage was discovered (as two images of the same distant quasar).

Cosmic mirages are generally seen as multiple images of a single quasar [2], lensed by a galaxy located between the quasar and us. The number and the shape of the images of the quasar depends on the relative positions of the quasar, the lensing galaxy and us. Moreover, if the alignment were perfect, we would also see a ring-shaped image around the lensing object. Such “Einstein rings” are very rare, though, and have only been observed in a very few cases.

Another particular interest of the gravitational lensing effect is that it may not only result in double or multiple images of the same object, but also that the brightness of these images increase significantly, just as it happens with an ordinary optical lens. Distant galaxies and galaxy clusters may thereby act as “natural telescopes” which allow us to observe more distant objects that would otherwise have been too faint to be detected with currently available astronomical telescopes.

Image sharpening techniques resolve the cosmic mirage better
A new gravitational lens, designated RXS J1131-1231, was serendipitously discovered in May 2002 by Dominique Sluse, then a PhD student at ESO in Chile, while inspecting quasar images taken with the ESO 3.6-m telescope at the La Silla Observatory. The discovery of this system profited from the good observational conditions prevailing at the time of the observations. From a simple visual inspection of these images, Sluse provisionally concluded that the system had four star-like (the lensed quasar images) and one diffuse (the lensing galaxy) component.

Because of the very small separation between the components, of the order of one arcsecond or less, and the unavoidable “blurring” effect caused by turbulence in the terrestrial atmosphere (“seeing”), the astronomers used sophisticated image-sharpening software to produce higher-resolution images on which precise brightness and positional measurements could then be performed (see also ESO PR 09/97). This so-called “deconvolution” technique makes it possible to visualize this complex system much better and, in particular, to confirm and render more conspicuous the associated Einstein ring, cf. PR Photo 20a/03.

Identification of the source and of the lens
The team of astronomers [1] then used the ESO 3.5-m New Technology Telescope (NTT) at La Silla to obtain spectra of the individual image components of this lensing system. This is imperative because, like human fingerprints, the spectra allow unambiguous identification of the observed objects.

Nevertheless, this is not an easy task because the different images of the cosmic mirage are located very close to each other in the sky and the best possible conditions are needed to obtain clean and well separated spectra. However, the excellent optical quality of the NTT combined with reasonably good seeing conditions (about 0.7 arcsecond) enabled the astronomers to detect the “spectral fingerprints” of both the source and the object acting as a lens, cf. ESO PR Photo 20b/03.

The evaluation of the spectra showed that the background source is a quasar with a redshift of z = 0.66 [3], corresponding to a distance of about 6,300 million light-years. The light from this quasar is lensed by a massive elliptical galaxy with a redshift z=0.3, i.e. at a distance of 3,500 million light-years or about halfway between the quasar and us. It is the nearest gravitationally lensed quasar known to date.

Because of the specific geometry of the lens and the position of the lensing galaxy, it is possible to show that the light from the extended galaxy in which the quasar is located should also be lensed and become visible as a ring-shaped image. That this is indeed the case is demonstrated by PR Photo 20a/03 which clearly shows the presence of such an “Einstein ring”, surrounding the image of the more nearby lensing galaxy.

Micro lensing within macro lensing ?
The particular configuration of the individual lensed images observed in this system has enabled the astronomers to produce a detailed model of the system. From this, they can then make predictions about the relative brightness of the various lensed images.

Somewhat unexpectedly, they found that the predicted brightnesses of the three brightest star-like images of the quasar are not in agreement with the observed ones – one of them turns out to be one magnitude (that is, a factor of 2.5) brighter than expected. This prediction does not call into question General Relativity but suggests that another effect is at work in this system.

The hypothesis advanced by the team is that one of the images is subject to “microlensing”. This effect is of the same nature as the cosmic mirage – multiple amplified images of the object are formed – but in this case, additional light-ray deflection is caused by a single star (or several stars) within the lensing galaxy. The result is that there are additional (unresolved) images of the quasar within one of the macro-lensed images.

The outcome is an “over-amplification” of this particular image. Whether this is really so will soon be tested by means of new observations of this gravitational lens system with the ESO Very Large Telescope (VLT) at Paranal (Chile) and also with the Very Large Array (VLA) radio observatory in New Mexico (USA).

Outlook
Until now, 62 multiple-imaged quasars have been discovered, in most cases showing 2 or 4 images of the same quasar. The presence of elongated images of the quasar and, in particular, of ring-like images is often observed at radio wavelengths. However, this remains a rare phenomenon in the optical domain – only four such systems have been imaged by optical/infrared telecopes until now.

The complex and comparatively bright system RXS J1131-1231 now discovered is a unique astrophysical laboratory. Its rare characteristics (e.g., brightness, presence of a ring-shaped image, small redshift, X-ray and radio emission, visible lens, …) will now enable the astronomers to study the properties of the lensing galaxy, including its stellar content, structure and mass distribution in great detail, and to probe the source morphology. These studies will use new observations which are currently being obtained with the VLT at Paranal, with the VLA radio interferometer in New Mexico and with the Hubble Space Telescope.
More information

The research described in this press release is presented in a Letter to the Editor, soon to appear in the European professional journal Astronomy & Astrophysics (“A quadruply imaged quasar with an optical Einstein ring candidate : 1RXS J113155.4-123155”, by Dominique Sluse et al.).

More information on gravitational lensing and on this research group can also be found at the URL : http://www.astro.ulg.ac.be/GRech/AEOS/.

Notes
[1]: The team consists of Dominique Sluse, Damien Hutsem?kers, and Thodori Nakos (ESO and Institut d’Astrophysique et de G?ophysique de l’Universit? de Li?ge – IAGL), Jean-Fran?ois Claeskens, Fr?d?ric Courbin, Christophe Jean, and Jean Surdej (IAGL), Malvina Billeres (ESO), and Sergiy Khmil (Astronomical Observatory of Shevchentko University).

[2]: Quasars are particularly active galaxies, the centres of which emit prodigious amounts of energy and energetic particles. It is believed that they harbour a massive black hole at their centre and that the energy is produced when surrounding matter falls into this black hole. This type of object was first discovered in 1963 by the Dutch-American astronomer Maarten Schmidt at the Palomar Observatory (California, USA) and the name refers to their “star-like” appearance on the images obtained at that time.

[3]: In astronomy, the “redshift” denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. Since the redshift of a cosmological object increases with distance, the observed redshift of a remote galaxy also provides an estimate of its distance.

Original Source: ESO News Release

Image credit: ANU

An Australian astronomer has discovered 20 galaxies that contain mostly gas, rather than stars – revising the definition of “galaxy”. These galaxies are giant discs of gas, tens of thousands of light-years across, and contain the mass of billions of sun, but for some reason their hydrogen hasn’t coalesced into stars like regular galaxies. The discovery of these gas galaxies will help astronomers better understand what it takes for a galaxy to form.

Any dictionary will tell you that a galaxy is a vast collection of stars, floating deep in space. But this definition may need revision following new research by an ANU graduate student who has discovered galaxies that consist mostly of gas, rather than stars.

In research to be presented to the General Assembly of the International Astronomical Union in Sydney today, Brad Warren will reveal his discovery of twenty gassy galaxies, which have very few stars.

?When you look for gas [in these galaxies] the signal just booms in,? Mr Warren said. ?But when you look for stars, all you see is a barely recognisable smudge.?

The galaxies are vast discs of hydrogen, tens of thousands of light years across, weighing more than a billion suns, with a tiny number of barely visible stars in their centre.

For an unknown reason, they have not transformed their rich source of hydrogen gas into masses of stars like their brilliant, twinkling counterparts.

?Hydrogen is the most common element in the Universe and it forms the building blocks for stars,? Mr Warren said.

?Most galaxies, like our own Milky Way, have transformed most of their gas into stars but the galaxies we have discovered have held back and we are not sure why.

?Discovering this missing link will give us important insights into how, when and why galaxies, such as our own, formed.?

Although the existence of gassy galaxies has been documented in the past, it is the first time they have been discovered with such prominent discrepancies between the amount of hydrogen gas and stars.

?This research throws up a further challenge in the ongoing quest to discover the secrets of the Universe,? Mr Warren said.

Mr Warren, from the Research School of Astronomy and Astrophysics, collaborated with fellow ANU researcher, Dr Helmut Jerjen, and Dr Baerbel Koribalski, from CSIRO?s Australia Telescope National facility.

The team used three of Australia?s most powerful telescopes for their research – the Parkes Radio Telescope; the Australia Telescope Compact Array near Narrabri and the University?s 2.3 metre telescope at Siding Spring Observatory, Coonabarabran.

Original Source: ANU News Release

Image credit: UW-Madison

A new telescope lodged in the ice of Antarctica has completed the first map of the high-energy neutrino sky. AMANDA II consists of 677 glass detectors in the shape of a cylinder sunk into the Antarctic ice at a depth greater than 500 metres. It actually looks down, through the entire Earth to view the Northern sky for neutrinos, which move at high velocity and pass through almost all matter unhindered. AMANDA II has discovered neutrinos with 100 times the energy of any produced in laboratory experiments on Earth.

A novel telescope that uses the Antarctic ice sheet as its window to the cosmos has produced the first map of the high-energy neutrino sky.

The map, unveiled for astronomers here today (July 15) at a meeting of the International Astronomical Union, provides astronomers with their first tantalizing glimpse of very high-energy neutrinos, ghostly particles that are believed to emanate from some of the most violent events in the universe – crashing black holes, gamma ray bursts, and the violent cores of distant galaxies.

“This is the first data with a neutrino telescope with realistic discovery potential,” says Francis Halzen, a University of Wisconsin-Madison professor of physics, of the map compiled using AMANDA II, a one-of-a-kind telescope built with support from the National Science Foundation (NSF) and composed of arrays of light-gathering detectors buried in ice 1.5 kilometers beneath the South Pole. “To date, this is the most sensitive way ever to look at the high-energy neutrino sky,” he says.

The ability to detect high-energy neutrinos and trace them back to their points of origin remains one of the most important quests of modern astrophysics.

Because cosmic neutrinos are invisible, uncharged and have almost no mass, they are next to impossible to detect. Unlike photons, the particles that make up visible light, and other kinds of radiation, neutrinos can pass unimpeded through planets, stars, the vast magnetic fields of interstellar space and even entire galaxies. That quality – which makes them very hard to detect – is also their greatest asset because the information they harbor about cosmologically distant and otherwise unobservable events remains intact.

The map produced by AMANDA II is preliminary, Halzen emphasizes, and represents only one year of data gathered by the icebound telescope. Using two more years of data already harvested with AMANDA II, Halzen and his colleagues will next define the structure of the sky map and sort out potential signals from statistical fluctuations in the present map to confirm or disprove them.

The significance of the map, according to Halzen, is that it proves the detector works. “It establishes the performance of the technology,” he says, “and it shows that we have reached the same sensitivity as telescopes used to detect gamma rays in the same high-energy region” of the electromagnetic spectrum. Roughly equal signals are expected from objects that accelerate cosmic rays, whose origins remain unknown nearly a century after their discovery.

Sunk deep into the Antarctic ice, the AMANDA II (Antarctic Muon and Neutrino Detector Array) Telescope is designed to look not up, but down, through the Earth to the sky in the Northern Hemisphere. The telescope consists of 677 glass optical modules, each the size of a bowling ball, arrayed on 19 cables set deep in the ice with the help of high-pressure hot-water drills. The array transforms a cylinder of ice 500 meters in height and 120 meters in diameter into a particle detector.

The glass modules work like light bulbs in reverse. They detect and capture faint and fleeting streaks of light created when, on occasion, neutrinos crash into ice atoms inside or near the detector. The subatomic wrecks create muons, another species of subatomic particle that, conveniently, leaves an ephemeral wake of blue light in the deep Antarctic ice. The streak of light matches the path of the neutrino and points back to its point of origin.

Because it provides the first glimpse of the high-energy neutrino sky, the map will be of intense interest to astronomers because, says Halzen, “we still have no clue how cosmic rays are accelerated or where they come from.”

The fact that AMANDA II has now identified neutrinos up to one hundred times the energy of the particles produced by the most powerful earthbound accelerators raises the prospect that some of them may be kick-started on their long journeys by some of the most supremely energetic events in the cosmos. The ability to routinely detect high-energy neutrinos will provide astronomers not only with a lens to study such bizarre phenomena as colliding black holes, but with a means to gain direct access to unedited information from events that occurred hundreds of millions or billions of light years away and eons ago.

“This map could hold the first evidence of a cosmic accelerator,” Halzen says. “But we are not there yet.”

The hunt for sources of cosmic neutrinos will get a boost as the AMANDA II Telescope grows in size as new strings of detectors are added. Plans call for the telescope to grow to a cubic kilometer of instrumented ice. The new telescope, to be known as IceCube, will make scouring the skies for cosmic neutrino sources highly efficient.

“We will be sensitive to the most pessimistic theoretical predictions,” Halzen says. “Remember, we are looking for sources, and even if we discover something now, our sensitivity is such that we would see, at best, on the order of 10 neutrinos a year. That’s not good enough.”

Original Source: WISC News Release

Image credit: ESO

A team of European and Chilean astronomers have discovered several large clusters of galaxies at a distance of 8 billion light years which should provide insights into the structure and evolution of the Universe. The galaxy clusters were discovered by combining images from the ESA’s XMM-Newton space telescope and the ESO’s Very Large Telescope. Galaxy clusters aren’t spread evenly, but appear strung through the Universe like a web, and so far it seems like the shape of these clusters hasn’t changed since the Universe was very young..

Using the ESA XMM-Newton satellite, a team of European and Chilean astronomers [2] has obtained the world’s deepest “wide-field” X-ray image of the cosmos to date. This penetrating view, when complemented with observations by some of the largest and most efficient ground-based optical telescopes, including the ESO Very Large Telescope (VLT), has resulted in the discovery of several large clusters of galaxies.

These early results from an ambitious research programme are extremely promising and pave the way for a very comprehensive and thorough census of clusters of galaxies at various epochs. Relying on the foremost astronomical technology and with an unequalled observational efficiency, this project is set to provide new insights into the structure and evolution of the distant Universe.

The universal web
Unlike grains of sand on a beach, matter is not uniformly spread throughout the Universe. Instead, it is concentrated into galaxies which themselves congregate into clusters (and even clusters of clusters). These clusters are “strung” throughout the Universe in a web-like structure, cf. ESO PR 11/01.

Our Galaxy, the Milky Way, for example, belongs to the so-called Local Group which also comprises “Messier 31”, the Andromeda Galaxy. The Local Group contains about 30 galaxies and measures a few million light-years across. Other clusters are much larger. The Coma cluster contains thousands of galaxies and measures more than 20 million light-years. Another well known example is the Virgo cluster, covering no less than 10 degrees on the sky !

Clusters of galaxies are the most massive bound structures in the Universe. They have masses of the order of one thousand million million times the mass of our Sun. Their three-dimensional space distribution and number density change with cosmic time and provide information about the main cosmological parameters in a unique way.

About one fifth of the optically invisible mass of a cluster is in the form of a diffuse hot gas in between the galaxies. This gas has a temperature of the order of several tens of million degrees and a density of the order of one atom per liter. At such high temperatures, it produces powerful X-ray emission.

Observing this intergalactic gas and not just the individual galaxies is like seeing the buildings of a city in daytime, not just the lighted windows at night. This is why clusters of galaxies are best discovered using X-ray satellites.

Using previous X-ray satellites, astronomers have performed limited studies of the large-scale structure of the nearby Universe. However, they so far lacked the instruments to extend the search to large volumes of the distant Universe.

The XMM-Newton wide-field observations
Marguerite Pierre (CEA Saclay, France), with a European/Chilean team of astronomers known as the XMM-LSS consortium [2], used the large field-of-view and the high sensitivity of ESA’s X-ray observatory XMM-Newton to search for remote clusters of galaxies and map out their distribution in space. They could see back about 7,000 million years to a cosmological era when the Universe was about half its present size and age, when clusters of galaxies were more tightly packed.

Tracking down the clusters is a painstaking, multi-step process, requiring both space and ground-based telescopes. Indeed, from X-ray images with XMM, it was possible to select several tens of cluster candidate objects, identified as areas of enhanced X-radiation (cf PR Photo 19b/03).

But having candidates is not enough ! They must be confirmed and further studied with ground-based telescopes. In tandem with XMM-Newton, Pierre uses the very-wide-field imager attached to the 4-m Canada-France-Hawaii Telescope, on Mauna Kea, Hawaii, to take an optical snapshot of the same region of space. A tailor-made computer programme then combs the XMM-Newton data looking for concentrations of X-rays that suggest large, extended structures. These are the clusters and represent only about 10% of the detected X-ray sources. The others are mostly distant active galaxies.

Back to the Ground
When the programme finds a cluster, it zooms in on that region and converts the XMM-Newton data into a contour map of X-ray intensity, which is then superimposed upon the CFHT optical image (PR Photo 19c/03). The astronomers use this to check if anything is visible within the area of extented X-ray emission.

If something is seen, the work then shifts to one of the world’s prime optical/infrared telescopes, the European Southern Observatory’s Very Large Telescope (VLT) at Paranal (Chile). By means of the FORS multi-mode instruments, the astronomers zoom-in on the individual galaxies in the field, taking spectral measurements that reveal their overall characteristics, in particular their redshift and hence, distance.

Cluster galaxies have similar distances and these measurement ultimately provide, by averaging, the cluster’s distance as well as the velocity dispersion in the cluster. The FORS instruments are among the most efficient and versatile for this type of work, taking on the average spectra of 30 galaxies at a time.

The first spectroscopic observations dedicated to the identification and redshift measurement of the XMM-LSS galaxy clusters took place during three nights in the fall of 2002.

As of March 2003, there were only 5 known clusters in the literature at such a large redshift with enough spectroscopically measured redshifts to allow an estimate of the velocity dispersion. But the VLT allowed obtaining the dispersion in a distant cluster in 2 hours only, raising great expectations for future work.

700 spectra…
Marguerite Pierre is extremely content : Weather and working conditions at the VLT were optimal. In three nights only, 12 cluster fields were observed, yielding no less than 700 spectra of galaxies. The overall strategy proved very successful. The high observing efficiency of the VLT and FORS support our plan to perform follow-up studies of large numbers of distant clusters with relatively little observing time. This represents a most substantial increase in efficiency compared to former searches.

The present research programme has begun well, clearly demonstrating the feasibility of this new multi-telescope approach and its very high efficiency. And Marguerite Pierre and her colleagues are already seeing the first tantalising results: it seems to confirm that the number of clusters 7,000 million years ago is little different from that of today. This particular behaviour is predicted by models of the Universe that expand forever, driving the galaxy clusters further and further apart.

Equally important, this multi-wavelength, multi-telescope approach developed by the XMM-LSS consortium to locate clusters of galaxies also constitutes a decisive next step in the fertile synergy between space and ground-based observatories and is therefore a basic building block of the forthcoming Virtual Observatory.

More information
This work is based on two papers to be published in the professional astronomy journal, Astronomy and Astrophysics (The XMM-LSS survey : I. Scientific motivations, design and first results by Marguerite Pierre et al., astro-ph/0305191 and The XMM-LSS survey : II. First high redshift galaxy clusters: relaxed and collapsing systems by Ivan Valtchanov et al., astro-ph/0305192).

Dr. M. Pierre will give an invited talk on this subject at the IAU Symposium 216 – Maps of the Cosmos – this Thursday July 17, 2003 during the IAU General Assembly 2003 in Sydney, Australia.

Notes
[1]: This a coordinated ESO/ESA release.

[2]: The XMM-LSS consortium is led by the Service d’Astrophysique du CEA (France) and consists of institutes from the UK, Ireland, Denmark, The Netherlands, Belgium, France, Italy, Germany, Spain and Chile. The homepage of the XMM-LSS project can be found at http://vela.astro.ulg.ac.be/themes/spatial/xmm/LSS/index_e.html

[3]: In astronomy, the “redshift” denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. Since the redshift of a cosmological object increases with distance, the observed redshift of a remote galaxy also provides an estimate of its distance.

Original Source: ESO News Release