Category: Astronomy

The NASA-led Swift mission has measured the distance to two gamma-ray bursts — back to back, from opposite parts of the sky — and both were from over nine billion light years away, unleashed billions of years before the Sun and Earth formed.

These represent the mission’s first direct distance, or redshift, measurements, its latest milestone since being launched in November 2004. The distances were attained with Swift’s Ultraviolet/OpticalTelescope (UVOT).

The Swift science team said that these types of distance measurements will become routine, allowing scientists to create a map to understand where, when and how these brilliant, fleeting bursts of light are created.

“Swift will detect more gamma-ray bursts than any satellite that has come before it, and now will be able to pin down distances to many of these bursts too,” said Dr. Peter Roming, UVOT Lead Scientist at Penn State. “These two aren’t distance record-breakers, but they’re certainly from far out there. The second of the two bursts was bright enough to be seen from Earth with a good backyard telescope.”

Gamma-ray bursts are the most powerful explosions known in the Universe and are thought to signal the birth of a black hole –either through a massive star explosion or through a merger smaller black holes or neutron stars. Several appear each day from our vantage point. They are difficult to detect and study, however, because they occur randomly from any point in the sky and last only a few milliseconds to about a minute.

Swift, with three telescopes, is designed to detect bursts and turn autonomously within seconds to focus its telescopes on the burst afterglow, which can linger for hours to weeks. The UVOT is a joint product of Penn State and the Mullard Space Science Laboratory in England.

Swift detected bursts on March 18 and 19, as indicted in their names: GRB 050318 and GRB 050319. The UVOT team estimated that the redshifts are 1.44 and 3.24, respectively, which corresponds to distances of about 9.2 billion and 11.6 billion light years. (The second estimate reflects a more precise measurement made with the ground-based Nordic Optical Telescope.) Distance measurements are attained through analysis of the burst afterglow.

Swift has detected 24 bursts so far. GRB 050318 was the first burst in which the UVOT detected an afterglow. The lack of afterglow detection is interesting in its own right, Roming said, because it helps scientists understand why some bursts create certain kinds of afterglows, if any. For example, Swift’s X-ray Telescope has detected afterglows from several bursts. The UVOT detected afterglows in GRB 050318 and GRB 050319 in optical light, but not significantly in ultraviolet.

“Every burst is a little different, and when we add them all up we will begin to see the full picture,” said Dr. Keith Mason, the U.K. UVOT Lead at University College London’s Mullard Space Science Laboratory.

Mason said that UVOT distance measurements will become more precise in the upcoming months as new instruments aboard Swift are employed.

Swift is a medium-class explorer mission managed by NASA Goddard Space Flight Center in Greenbelt, Md. Swift is a NASA mission with participation of the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom. It was built in collaboration with national laboratories, universities and international partners, including Penn State; Los Alamos National Laboratory in New Mexico; Sonoma State University in California; the University of Leicester in Leicester, England; the Mullard Space Science Laboratory in Dorking, England; the Brera Observatory of the University of Milan in Italy; and the ASI Science Data Center in Rome, Italy.

More information about each of the Swift-detected gamma-ray bursts, updated every five minutes, is available on the web at: http://grb.sonoma.edu

Original Source: Penn State News Release

A team of European scientists has used Virtual Observatories to compare observations of distant “starburst” galaxies made at radio and X-ray wavelengths. This is the first study to combine the highest resolution and sensitivity radio and X-ray images which penetrate the dust hiding the centres of some of these distant galaxies.

The team focused on galaxies so far away that their radiation took more than six billion years to reach us. The galaxies are seen as they were when they were less than half the age that the Universe is today.

Speaking on Tuesday 5 April at the RAS National Astronomy Meeting in Birmingham, Dr. Anita Richards (Jodrell Bank Observatory, University of Manchester) will explain how the team used the UK?s MERLIN array of radio telescopes and the Very Large Array to investigate how galaxies in the early Universe differ from those nearby.

“The more remote starburst galaxies, so called because of their high rate of star formation, typically produce 1,000 or more solar masses of stars per year – at least 50 times more than the most active star-forming galaxies in the nearby Universe,” said Dr. Richards.

“Each distant starburst region is tens of thousands of light years across, equivalent to about the inner quarter of the Milky Way – also vastly larger than any such regions found in our part of the Universe.”

The radio search took place in an area known as the Hubble Space Telescope Deep Field North – a patch of sky smaller than the full Moon that contains tens of thousands of galaxies.

Apart from Hubble, radio telescope arrays are the only instruments that can see detailed structures within these galaxies. Moreover, only radio or X-ray emissions can penetrate the dense dust in the innermost regions of some of these galaxies.

The two main sources of radio waves and X-rays are star formation and emissions from Active Galactic Nuclei (AGN) that are generated when material is sucked into a massive black hole and ejected in jets. The team found about twice as many starbursts as AGN, where these could be distinguished in radio images.

The UK AstroGrid and the European AVO ? parts of the international Virtual Observatory – were used to find counterparts for the radio sources from a variety of other data held by archives and observatories around the world. In this way it was discovered that 50 distant X-ray sources with measured redshifts had also been detected by the Chandra space observatory.

Virtual Observatory tools made it easy to calculate the intrinsic brightness of the sources, corrected for distance and redshift. However, the team found that there was no obvious relationship between radio and X-ray luminosity. This was a surprise since there is such a link in most local starburst galaxies.

Some of the faintest radio sources were found to emit the most X-rays and vice versa – suggesting that two separate mechanisms within each galaxy were generating powerful emissions at opposite extremes of the spectrum.

Members of the European Virtual Observatory team had earlier used the Chandra X-ray data and Hubble images to find 47 AGN in the Hubble Deep Field North. These appeared to be seen sideways on, so that the dusty torus surrounding the black hole blocked all but the most energetic X-rays from emerging in our direction.

“Astonishingly, only 4 of these looked like AGN in the radio observations,” said Richards. “10 had radio emissions characteristic of starbursts, 4 could not be classified, and the rest went undetected by radio telescopes.”

The 10 super-starburst/AGN hybrids tended to be at a higher redshift ? indicating that they are much further away from Earth than the rest of the radio galaxies. Over half of them were among the enigmatic ?SCUBA sources?. These objects are very bright at wavelengths just under a millimetre, probably as a result of dust being strongly heated by violent star formation, but almost invisible to most other instruments.

“We concluded that, not only were these young galaxies undergoing much more violent and extended star formation than we see today, but they were simultaneously feeding active, supermassive black holes responsible for the X-ray emission,” said Richards.

“One clue to the origin of this phenomenon is that the Hubble Space Telescope often reveals two or more distorted galaxies associated with these sources, suggesting that galaxy interactions were commoner when the Universe was young. The ensuing collisions of gas and dust clouds trigger star formation and also feed the central black hole.

“Modern starburst galaxies are not only slower at star formation, but mostly have much quieter AGN, if any. This is not surprising as the super-starbursts must run out of fuel quite quickly (by cosmological standards), when all the available material has either turned into stars or fallen into the black hole.”

Original Source: RAS News Release

Data from ISO, the infrared observatory of the European Space Agency (ESA), have provided the first direct evidence that shock waves generated by galaxy collisions excite the gas from which new stars will form. The result also provides important clues on how the birth of the first stars was triggered and speeded up in the early Universe.

By observing our galaxy and others, scientists have long concluded that the explosion of massive stars like supernovae generates shock waves and ?winds? that travel through and excite the surrounding gas clouds. This process triggers the collapse of nearby gas that eventually leads to the birth of new stars, like a domino effect.

The signature of this process is the radiation emitted by molecular hydrogen. When hydrogen molecules are ?excited? by the energy of a nearby explosion, they emit a distinctive type of radiation that can be detected in the infrared.

This type of radiation is also observed in places where galaxies have collided with one another and the formation of new stars goes at a very high rate. So far, however, there was no clear picture of what happens in the time between the collision of two galaxies and the birth of the first new stars.

The missing link has now been found by a team of German astronomers that have analysed ISO data of the galaxy pair nicknamed the ?Antennae? (NGC 4038/4039). These two galaxies, located 60 million light-years away in the constellation ?Corvus? (the Crow), are currently at an early stage of encounter. The scientists noticed that the overlapping region of the two colliding galaxies is very rich in molecular hydrogen, which is in an excited state.

In particular, the radiation from molecular hydrogen is evenly strong in the northern and southern areas of the overlap region. Much to the team?s surprise, however, there are too few supernova explosions or regions of intense star formation there to explain the observed molecular hydrogen emission. So, the excitation of the molecular hydrogen must be the signature of that observationally rare pre-star birth phase in which hydrogen is excited by the mechanical energy produced in the collision and transported by shock waves. In other words, these results provide the first direct evidence of the missing link between gas collision and the birth of the first stars. The team estimates that when the gas will collapse to form new stars, during the next million years, the Antennae galaxy will become at least two times brighter in the infrared.

The astronomers believe that star formation induced by shocks may have played a role in the evolution of proto-galaxies in the first thousand million years of life of our Universe. Shock waves produced through the collision of proto-galaxies may have triggered the condensation process and speeded-up the birth of the very first stars. These objects, made up of only hydrogen and helium, would otherwise have taken much longer to form, since light elements such as hydrogen and helium take a long time to cool down and condense into a proto-star. Shock waves from the first cloud collisions may have been the helping hand.

Original Source: ESA News Release

Large spiral galaxies such as our own Milky Way are like huge sprawling continents in space. Like any continent, such galaxies should have many smaller islands lying off the coast. Current models of galaxy formation suggest that galactic continents should have more neighboring islands than actually seen with telescopes. Now one more island has been added to the Milky Way’s contingent and this one is small enough to map well against predictions. Other dwarfs – like the one recently discovered in Ursa Major – are likely to follow.

Located 300 thousand plus light-years away in the direction of the Big Dipper, the recently discovered Ursa Major (UMa) dwarf galaxy has roughly one-tenth the surface brightness of the next smallest Milky Way dwarf (located in Sextans). Like the Sextans dwarf, the UMa dwarf is spherical in shape (galaxy type dSph) and is in some ways similar to globular clusters which are also found in association with large spiral galaxies.

According to Beth Willman of New York University – principal investigator of a team of 15 astronomers studying data returned by the Sloan Digital Sky Survey (SDSS), “Ursa Major appears to be old and metal poor, like all of the other known Milky Way dwarf spheroidal companions. However, it may be 10 times fainter than the faintest known Milky Way satellite. We are in the process of obtaining more detailed observations that will provide a more detailed picture of UMa’s properties, which we will then compare with the other known satellites.

Beth goes on to explain, “UMa was detected as part of a systematic survey for Milky Way companions. It was detected as a slight statistical fluctuation in the number of red stars in that region of the sky.”

All galaxies and globular clusters include a wide range of stellar types in their makeup. These range from young, massive, short-lived, intensely bright blue-giants, through longer-lived, modestly massive, mostly middle-aged fainter yellow stars such as our Sun, to old, moderately bright, but hugely swollen red-giants similar to Scorpio’s Antares and Orion’s Betelguese. When it comes to finding nearby dwarf galaxies – such as the UMa dwarf – it is this last group of stars that are of especial interest. Red-giants are bright enough to be detected, identified spectroscopically, and counted using automated sky-surveying telescopes such as the SDSS in New Mexico – even from small satellite galaxies located several hundreds of thousands of light years away.

Once data from SDSS is available, teams such as Beth’s can analyze it for high-concentrations of red-giants in small regions of the sky. Their presence can indicate an unsupected dwarf galaxy or a globular cluster. Spectrographic information is used by teams such as Beth’s to filter out fainter – but far closer – red stars within the Milky Way itself. Finally a more detailed view of the study can be made using higher sensitivity instruments at other observatories.

Once data showed that a UMa dwarf galaxy might exist, the 2.5 meter wide-field camera of the Isaac Newton Telescope in the Canary Islands helped determine its general appearance. Images from the Newton Telescope plus data from SDSS was combined to verify the nature of the study as a spheroidal galaxy and not simply a rogue globular cluster – such as the Intergalactic Wanderer (NGC 2419) in Lynx located at a similar distance in space.

Although smaller dwarf galaxies have absolute magnitudes similar to the brightest globular clusters, one important difference between large globulars and small dwarfs lies in their size. The UMa dwarf is roughly ten times as large as the largest globulars known. And much of its mass is likely to be non-stellar “dark matter” – while nearly all the mass in a globular cluster is packed into stars. Since it’s large, but not very luminous, the team has tagged UMa as a dwarf galaxy.

From a cosmological perspective, satellite galaxies such as the Ursa Major dSph play an important role in explaining the formation of large, intermediate, and smaller scale structure throughout the Universe. On the largest scales, spiral galaxies (such as our Milky Way and the Great Galaxy of Andromeda) are known to dwell in extended groups of galaxies called groups and clusters. Our own group (the Local Group) is small in mass and extent while its two largest members, though large by spiral galaxy standards, are quite modest in comparison to the largest galaxies known to astronomers (the giant ellipticals). The very largest scales of galactic formation in the Universe include thousands of large galaxies while our own local group has but several dozen members. On the very smallest scales, the Milky Way and its retinue, which include the two irregular Magellanic Clouds plus now ten dwarf sphericals, make up a single gravitationally bound contingent. Because of this, astronomers have an opportunity to explore the smallest possible building blocks of extragalactic structure.

In their paper entitled “A new Milky Way Dwarf Galaxy In Ursa Major” Beth and her team go on to say, “UMa was detected very close to our detection limits. Numerous other dwarfs with properties similar to or fainter than the Ursa Major dSph may thus exist around the Milky Way… it is reasonable to expect that 8-9 additional dwarfs brighter than our detection limits still remain undiscovered over the entire sky. If true, that number would preclude (galactic formation) models that do not predict the presence of many ultra-faint dwarfs.”

Written by Jeff Barbour

In the March 25th 2005 issue of Science Magazine, the High Energy Stereoscopic System (H.E.S.S.) team of international astrophysicists, including UK astronomers from the University of Durham, report results of a first sensitive survey of the central part of our galaxy in very high energy (VHE) gamma-rays. Included among the new objects discovered are two ‘dark accelerators’ – mysterious objects that are emitting energetic particles, yet apparently have no optical or x-ray counterpart.

This survey reveals a total of eight new sources of VHE gamma-rays in the disc of our Galaxy, essentially doubling the number known at these energies. The results have pushed astronomy into a previously unknown domain, extending our knowledge of the Milky Way in a novel wavelength regime thereby opening a new window on our galaxy.

Gamma-rays are produced in extreme cosmic particle accelerators such as supernova explosions and provide a unique view of the high energy processes at work in the Milky Way. VHE gamma-ray astronomy is still a young field and H.E.S.S. is conducting the first sensitive survey at this energy range, finding previously unknown sources.

Particularly stunning is that two of these new sources discovered by H.E.S.S. have no obvious counterparts in more conventional wavelength bands such as optical and X-ray astronomy. The discovery of VHE gamma-rays from such sources suggests that they may be `dark accelerators’, as Stefan Funk from the Max-Planck Institut in Heidelberg affirms: “These objects seem to only emit radiation in the highest energy bands. We had hoped that with a new instrument like H.E.S.S. we would detect some new sources, but the success we have now exceeds all our expectations.”

Dr Paula Chadwick of the University of Durham adds “Many of the new objects seem to be known categories of sources, such as supernova remnants and pulsar wind nebulae. Data on these objects will help us to understand particle acceleration in our galaxy in more detail; but finding these ‘dark accelerators’ was a surprise. With no counterpart at other wavelengths, they are, for the moment, a complete mystery.”

Cosmic particle accelerators are believed to accelerate charged particles, such as electrons and ions, by acting on these particles with strong shock waves. High-energy gamma rays are secondary products of the cosmic accelerators and are easier to detect because they travel in straight lines from the source, unlike charged particles which are deflected by magnetic fields. The cosmic accelerators are usually visible at other wavelengths as well as VHE gamma rays.

The H.E.S.S. array is ideal for finding these new VHE gamma ray objects, because as well as studying objects seen at other wavelengths that are expected to be sources of very high energy gamma rays, its wide field of view (ten times the diameter of the Moon) means that it can survey the sky and discover previously unknown sources.

Another important discovery is that the new sources appear with a typical size of the order of a tenth of a degree; the H.E.S.S. instrument for the first time provides sufficient resolution and sensitivity to see such structures. Since the objects cluster within a fraction of a degree from the plane of our Galaxy, they are most likely located at a significant distance – several 1000 light years from the sun – which implies that these cosmic particle accelerators extend over a size of light years.

The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of VHE gamma-rays, radiation a million million times more energetic than the visible light. These high energy gamma rays are quite rare – even for relatively strong sources, only about one gamma ray per month hits a square meter at the top of the earth’s atmosphere. Also, since they are absorbed in the atmosphere, a direct detection of a significant number of the rare gamma rays would require a satellite of huge size. The H.E.S.S. telescopes employ a trick – they use the atmosphere as detector medium. When gamma rays are absorbed in the air, they emit short flashes of blue light, named Cherenkov light, lasting a few billionths of a second. This light is collected by the H.E.S.S. telescopes with big mirrors and extremely sensitive cameras and can be used to create images of astronomical objects as they appear in gamma-rays.

The H.E.S.S. telescopes represent several years of construction effort by an international team of more than 100 scientists and engineers from Germany, France, the UK, Ireland, the Czech Republic, Armenia, South Africa and the host country Namibia. The instrument was inaugurated in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab, and its first data have already resulted in a number of important discoveries, including the first astronomical image of a supernova shock wave at the highest gamma-ray energies.

Original Source: PPARC News Release

Image credit: Hubble
University scientists have co-founded an international collaboration that seeks to measure with new precision the mysterious force causing the universe to fly apart. Plans call for the project, named the Dark Energy Survey, to collect data on approximately 300 million galaxies spanning two-thirds of the history of the universe.

The survey could begin making observations as early as the fall of 2009. Although the DES remains more than four years away, more ambitious surveys will take at least a decade to produce results. ?I don?t want to wait that long,? said Joshua Frieman, Professor in Astronomy & Astrophysics and the College.

According to physics accounting methods, dark energy makes up 70 percent of the universe. Dark energy might be a manifestation of Albert Einstein?s cosmological constant, a force that acts at all times and in all places throughout the universe. It might also be a breakdown of Einstein?s theory of gravity on vast scales.

?It essentially requires gravity to be repulsive,? said Wayne Hu, Associate Professor in Astronomy & Astrophysics. ?That?s possible under our standard theories of gravity, but it?s not expected.? Whatever dark energy is, Frieman said, ?it?s likely to have profound implications for fundamental physics.?

The DES collaboration consists of researchers at Chicago, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Lawrence Berkeley National Laboratory and the Cerro Tololo Inter-American Observatory, as well as groups from the United Kingdom and Barcelona, Spain. Funding for the $20 million project is likely to come primarily from the U.S. Department of Energy, European funding agencies, the member institutions, and other agencies and sources.

Frieman heads the University?s component of the collaboration. Joining him and Hu in the collaboration are John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics and the College; Scott Dodelson, Professor in Astronomy & Astrophysics and the Physical Sciences Collegiate Division; Stephen Kent, Associate Professor in Astronomy & Astrophysics; Erin Sheldon, Fellow in the Kavli Institute for Cosmological Physics; and Risa Wechsler, Hubble Fellow in the Kavli Institute for Cosmological Physics. Frieman and Dodelson also are members of Fermilab?s Theoretical Astrophysics Group, which Dodelson heads, while Kent heads Fermilab?s Experimental Astrophysics Group.

The DES will entail installing a 520-megapixel camera on the existing four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. ?This would be larger than any existing optical camera in the world,? Frieman said.

A few hundred megapixels may not sound like much, Frieman said, ?but they?re not the same pixels that go into your hand-held. They have much higher sensitivity. They?re high-precision, high-efficiency detectors.? Furthermore, the camera will allow the scientists to survey the sky 10 times faster than they could at any existing U.S. observatory.

?The camera that?s now on the telescope just has too small a field of view. It would take us many decades to do the survey,? Frieman said.

The new camera will enable the DES to employ four techniques in attempting to discriminate between the two broad explanations for dark energy?the cosmological constant or a breakdown of gravity.

?The first method and the one that really drives the survey design is to count clusters of galaxies,? Frieman said. In this effort it will work in tandem with Carlstrom?s South Pole Telescope, which is scheduled to begin making observations in March 2007.

The SPT will help reveal if dark energy has suppressed the formation of galaxy clusters over the history of the universe. A radio telescope, the SPT will detect galaxy clusters by the way they distort the microwave radiation left over from the big bang. If theorists know how distant and how massive the galaxy clusters are, they can predict how many there should be in the presence of dark energy. The DES will make optical measurements to estimate their distance through the colors of the galaxies and their mass by gravitational lensing, the distortion of light by an intervening galaxy cluster. ?That?s a really elegant test,? Hu said.

The third technique employs gravitational lensing on a cosmic scale. Theorists can predict the effect of the dark energy on the large-scale distribution of the dark matter. With its large survey area, the DES can measure the tiny distortion of the images of galaxies induced by fluctuations in the dark matter density.

The fourth method involves the same technique that led to the 1998 discovery of dark energy: measuring the distance to a certain type of exploding star to reconstruct the expansion history of the universe. Astronomers studied these exploding stars expecting to find that the expansion of the universe had slowed as time went on. They discovered instead an accelerated expansion.

?These techniques complement each other very well,? Frieman said. ?They suffer from different sources of error, so if they agree, that gives you confidence in your result.?

For his part, Hu hopes the tests will reveal some discrepancy between predictions and reality. ?To me that would be the most exciting thing.?

Original Source: University of Chicago News Release

Super star clusters are groups of hundreds of thousands of very young stars packed into an unbelievably small volume. They represent the most extreme environments in which stars and planets can form.

Until now, super star clusters were only known to exist very far away, mostly in pairs or groups of interacting galaxies. Now, however, a team of European astronomers [1] have used ESO’s telescopes to uncover such a monster object within our own Galaxy, the Milky Way, almost, but not quite, in our own backyard!

The newly found massive structure is hidden behind a large cloud of dust and gas and this is why it took so long to unveil its true nature. It is known as “Westerlund 1” and is a thousand times closer than any other super star cluster known so far. It is close enough that astronomers may now probe its structure in some detail.

Westerlund 1 contains hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two-thousand times larger than the Sun (as large as the orbit of Saturn)! Indeed, if the Sun were located at the heart of this remarkable cluster, our sky would be full of hundreds of stars as bright as the full Moon. Westerlund 1 is a most unique natural laboratory for the study of extreme stellar physics, helping astronomers to find out how the most massive stars in our Galaxy live and die.

From their observations, the astronomers conclude that this extreme cluster most probably contains no less than 100,000 times the mass of the Sun, and all of its stars are located within a region less than 6 light-years across. Westerlund 1 thus appears to be the most massive compact young cluster yet identified in the Milky Way Galaxy.

Super Star Clusters
Stars are generally born in small groups, mostly in so-called “open clusters” that typically contain a few hundred stars. From a wide range of observations, astronomers infer that the Sun itself was born in one such cluster, some 4,500 million years ago.

In some active (“starburst”) galaxies, scientists have observed violent episodes of star formation (see, for example, ESO Press Photo 31/04), leading to the development of super star clusters, each containing several million stars.

Such events were obviously common during the Milky Way’s childhood, more than 12,000 million years ago: the many galactic globular clusters – which are nearly as old as our Galaxy (e.g. ESO PR 20/04) – are indeed thought to be the remnants of early super star clusters.

All super star clusters so far observed in starburst galaxies are very distant. It is not possible to distinguish their individual stars, even with the most advanced technology. This dramatically complicates their study and astronomers have therefore long been eager to find such clusters in our neighbourhood in order to probe their structure in much more detail.

Now, a team of European astronomers [1] has finally succeeded in doing so, using several of ESO’s telescopes at the La Silla observatory (Chile).

Westerlund 1
The open cluster Westerlund 1 is located in the Southern constellation Ara (the Altar constellation). It was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970 – 74). This cluster is behind a huge interstellar cloud of gas and dust, which blocks most of its visible light. The dimming factor is more than 100,000 – and this is why it has taken so long to uncover the true nature of this particular cluster.

In 2001, the team of astronomers identified more than a dozen extremely hot and peculiar massive stars in the cluster, so-called “Wolf-Rayet” stars. They have since studied Westerlund 1 extensively with various ESO telescopes.

They used images from the Wide Field Imager (WFI) attached to the 2.2-m ESO/MPG as well as from the SUperb Seeing Imager 2 (SuSI2) camera on the ESO 3.5-m New Technology Telescope (NTT). From these observations, they were able to identify about 200 cluster member stars.

To establish the true nature of these stars, the astronomers then performed spectroscopic observations of about one quarter of them. For this, they used the Boller & Chivens spectrograph on the ESO 1.52-m telescope and the ESO Multi-Mode Instrument (EMMI) on the NTT.

An Exotic Zoo
These observations have revealed a large population of very bright and massive, quite extreme stars. Some would fill the solar system space within the orbit of Saturn (about 2,000 times larger than the Sun!), others are as bright as a million Suns.

Westerlund 1 is obviously a fantastic stellar zoo, with a most exotic population and a true astronomical bonanza. All stars identified are evolved and very massive, spanning the full range of stellar oddities from Wolf-Rayet stars, OB supergiants, Yellow Hypergiants (nearly as bright as a million Suns) and Luminous Blue Variables (similar to the exceptional Eta Carinae object – see ESO PR 31/03).

All stars so far analysed in Westerlund 1 weigh at least 30-40 times more than the Sun. Because such stars have a rather short life – astronomically speaking – Westerlund 1 must be very young. The astronomers determine an age somewhere between 3.5 and 5 million years. So, Westerlund 1 is clearly a “newborn” cluster in our Galaxy!

The Most Massive Cluster
Westerlund 1 is incredibly rich in monster stars – just as one example, it contains as many Yellow Hypergiants as were hitherto known in the entire Milky Way!

“If the Sun were located at the heart of Westerlund 1, the sky would be full of stars, many of them brighter than the full Moon”, comments Ignacio Negueruela of the Universidad de Alicante in Spain and member of the team.

The large quantity of very massive stars implies that Westerlund 1 must contain a huge number of stars. “In our Galaxy, explains Simon Clark of the University College London (UK) and one of the authors of this study, “there are more than 100 solar-like stars for every star weighing 10 times as much as the Sun. The fact that we see hundreds of massive stars in Westerlund 1 means that it probably contains close to half a million stars, but most of these are not bright enough to peer through the obscuring cloud of gas and dust”. This is ten times more than any other known young clusterin the Milky Way.

Westerlund 1 is presumably much more massive than the dense clusters of heavy stars present in the central region of our Galaxy, like the Arches and Quintuplet clusters. Further deep infrared observations will be required to confirm this.

This super star cluster now provides astronomers with a unique perspective towards one of the most extreme environments in the Universe. Westerlund 1 will certainly provide new opportunities in the long-standing quest for more and finer details about how stars, and especially massive ones, do form.

… and the Most Dense
The large number of stars in Westerlund 1 was not the only surprise awaiting Clark and his colleagues. From their observations, the team members also found that all these stars are packed into an amazingly small volume of space, indeed less than 6 light-years across. In fact, this is more or less comparable to the 4 light-year distance to the star nearest to the Sun, Proxima Centauri!

It is incredible: the concentration in Westerlund 1 is so high that the mean separation between stars is quite similar to the extent of the Solar System.

“With so many stars in such a small volume, some of them may collide”, envisages Simon Clark. “This could lead to the formation of an intermediate-mass black hole more massive than 100 solar masses. It may well be that such a monster has already formed at the core of Westerlund 1.”

The huge population of massive stars in Westerlund 1 suggests that it will have a very significant impact on its surroundings. The cluster contains so many massive stars that in a time span of less than 40 million years, it will be the site of more than 1,500 supernovae. A gigantic firework that may drive a fountain of galactic material!

Because Westerlund 1 is at a distance of only about 10,000 light-years, high-resolution cameras such as NAOS/CONICA on ESO’s Very Large Telescope can resolve its individual stars. Such observations are now starting to reveal smaller stars in Westerlund 1, including some that are less massive than the Sun. Astronomers will thus soon be able to study this exotic galactic zoo in great depth.

More information
The research presented in this ESO Press Release will soon appear in the leading research journal Astronomy and Astrophysics (“On the massive stellar population of the Super Star Cluster Westerlund 1” by J.S. Clark and colleagues). The PDF file is available at the A&A web site. A second paper (“Further Wolf-Rayet stars in the starburst cluster Westerlund 1”, by Ignacio Negueruela and Simon Clark) will also soon be published in Astronomy and Astrophysics. It is available as astro-ph/0503303.
A Spanish press release issued by Universidad de Alicante is available on the web site of Ignacio Negueruela.

Note
[1]: The team is composed of Simon Clark (University College London, UK), Ignacio Negueruela (Universidad de Alicante, Spain), Paul Crowther (University of Sheffield, UK), Simon Goodwin (University of Wales, Cardiff, UK), Rens Waters (University of Amsterdam) and Sean Dougherty (Dominion Radio Astrophysical Observatory).

Original Source: ESO News Release

Image credit: Hubble
Spiral galaxy PGC 69457 is located near the boundary of fall constellations Pegasus and Aquarius some 3 degrees south of third magnitude Theta Pegasi – but don’t dig out that 60mm refractor to look for it. The galaxy is actually some 400 million light years away and has an apparent brightness of magnitude 14.5. So next fall may be a good time to hook up with that “astro-nut” friend of yours who is always heading off into the sunset to get well away from city lights sporting a larger, much larger, amateur instrument…

But there are plenty of 14th magnitude galaxies in the sky – what makes PGC 69457 so special?

To begin with most galaxies don’t “block” the view of an even more distant quasar (QSO2237+0305). And should others exist, few have just the right distribution of high-density bodies needed to cause light to “bend” in a way that an otherwise invisible object is visible. With PGC 69457 you get not one – but four – separate 17th magnitude views of the same quasar for the trouble of setting up one 20 inch truss tube dobsonian. Is it worth it? (Can you say “quadruple your observing pleasure”?)

But the phenomenon behind such a view is even more interesting to professional astronomers. What can we learn from such a unique effect?

The theory is already well established – Albert Einstein predicted it in his “General Theory of Relativity” of 1915. Einstein’s core idea was that an observer undergoing acceleration and one stationary in a gravitational field could not tell the difference between the two on their “weight”. By exploring this idea to its fullest, it became clear that not only matter but light (despite being massless) undergoes the same sort of confusion. Because of this, light approaching a gravitational field at an angle is “accelerated toward” the source of the gravity – but because the velocity of light is constant such acceleration only effects light’s path and wavelength – not its actual speed.

Gravitational lensing itself was first detected during the total solar eclipse of 1919. This was seen as a slight shift in the positions of stars near the Sun’s corona as captured on photographic plates. Because of this observation, we now know that you don’t need a lens to bend light – or even water to refract the image of those Koi swimming in the pond. Light like matter takes the path of least resistance and that means following the gravitational curve of space as well as the optical curve of a lens. The light from QSO2237+0305 is only doing what comes naturally by surfing the contours of “space-time” arcing around dense stars lying along the line of sight from a distant source through a more neighboring galaxy. The really interesting thing about Einstein’s Cross comes down to what it tells us about all the masses involved – those in the galaxy that refracts the light, and the Big One in the heart of the quasar that sources it.

In their paper “Reconstruction of the microlensing light curves of the Einstein Cross” Korean astrophysicist Dong-Wook Lee (et al) of Sejong University in association with Belgian astrophysicist J. Surdez (et al) of the University of Liege, found evidence of an accretion disk surrounding the black hole in Quasar QSO2237+0305. How is such a thing possible at the distances involved?

Lenses in general “collect and focus light” and those “gravitational lenses” (Lee at al posit a minimum of five low-mass but highly condensed bodies) within PGC 69457, do the same. In this way, light from a quasar that would normally travel well away from our instruments “wraps around” the galaxy to come toward us. Because of this we “see” 100,000 times more detail than otherwise possible. But there is a catch: Despite getting 100,000 times more resolution, we still only see light, not detail. And because there are several masses refracting light in the galaxy, we see more than one view of the quasar.

To get useful information from the quasar, you have to collect light over long periods of time (months to years) and use special analytical algorithms to pull the resulting data together. The method used by Lee and associates is called LOHCAM (LOcal Hae CAustic Modeling). (HAE itself is an acronym for High Amplification Events). Using LOHCAM and data available from OGLE (Optical Gravitational Lensing Experiment) and GLIPT (Gravitational Lens International Time Project), the team determined not only that LOHCAM works as hoped but that QSO2237+0305 may include a detectable accretion disk (from which it draws matter to power its light engine). The team has also determined the approximate mass of the quasars black hole, the size of the ultraviolet region radiating from it, and estimated the transverse motion of the black hole as it moves relative to the spiral galaxy.

The central black hole in Quasar QSO2237+0305 is thought to have a combined mass of 1.5 billion Suns – a value rivaling those of the largest central black holes ever discovered. Such a mass number represents 1 percent of the total number of stars in our own Milky Way galaxy. Meanwhile and by comparison, QSO2237+0305’s black hole is roughly 50 times more massive than that in the center of our own galaxy.

Based on “double-peaks” in luminosity from the quasar, Lee et al used LOHCAM to also determine the size of QSO2237+0305’s accretion disk, its orientation, and detected a central obscuration region around the black hole itself. The disk itself is roughly 1/3rd of a light year in diameter and is turned face on towards us.

Impressed? Well let’s also add that the team has determined the minimum number of microlenses and related masses found in the lensing galaxy. Depending on transverse velocity assumed (in LOHCAM modeling), the smallest range from that of a gas giant – such as the planet Jupiter – through that of our own Sun.

So how does this “hole” thing work?

The OGLE and GLIPT projects monitored changes in the intensity of visual light streaming to us from each of the four 17th magnitude views of the quasar. Since most quasars are unresolvable,due to their great distances in space, by telescope. Fluctuations in luminosity are seen only as a single point of data based on the brightness of the entire quasar. However, QSO2237+0305 presents four images of the quasar and each image highlights luminosity originating from a different perspective of the quasar. By telescopically monitoring all four images simultaneously, slight variations in image intensity can be detected and recorded in terms of magnitude, date, and time. Over several months to years, a considerable number of such “high amplification events” can occur. Patterns emerging out of their occurrence (from one 17th magnitude view to the next) can then be analyzed to show motion and intensity. Out of this a super high resolution view of normally unseen structure within the quasar is possible.

Could you and your friend with that 20 inch dob-newtonian do this?

Sure – but not without some very expensive equipment and a good handle on some complex mathematical imaging algorithms. A nice place to start however might simply be to ogle the galaxy and hang with the cross for awhile…

Written by Jeff Barbour

Why is the universe expanding at an accelerating rate, spreading its contents over ever greater dimensions of space? An original solution to this puzzle, certainly the most fascinating question in modern cosmology, was put forward by four theoretical physicists, Edward W. Kolb of the U.S. Department of Energy’s Fermi National Accelerator Laboratory, Chicago (USA): Sabino Matarrese of the University of Padova; Alessio Notari from the University of Montreal (Canada); and Antonio Riotto of INFN (Istituto Nazionale di Fisica Nucleare) of Padova (Italy). Their study was submitted yesterday to the journal Physical Review Letters.

Over the last hundred years, the expansion of the universe has been a subject of passionate discussion, engaging the most brilliant minds of the century. Like his contemporaries, Albert Einstein initially thought that the universe was static: that it neither expanded nor shrank. When his own Theory of General Relativity clearly showed that the universe should expand or contract, Einstein chose to introduce a new ingredient into his theory. His “cosmological constant” represented a mass density of empty space that drove the universe to expand at an ever-increasing rate.

When in 1929 Edwin Hubble proved that the universe is in fact expanding, Einstein repudiated his cosmological constant, calling it “the greatest blunder of my life.” Then, almost a century later, physicists resurrected the cosmological constant in a variant called dark energy. In 1998, observations of very distant supernovae demonstrated that the universe is expanding at an accelerating rate. This accelerating expansion seemed to be explicable only by the presence of a new component of the universe, a “dark energy,” representing some 70 percent of the total mass of the universe. Of the rest, about 25 percent appears to be in the form of another mysterious component, dark matter; while only about 5 percent comprises ordinary matter, those quarks, protons, neutrons and electrons that we and the galaxies are made of.

“The hypothesis of dark energy is extremely fascinating,” explains Padova’s Antonio Riotto, “but on the other hand it represents a serious problem. No theoretical model, not even the most modern, such as supersymmetry or string theory, is able to explain the presence of this mysterious dark energy in the amount that our observations require. If dark energy were the size that theories predict, the universe would have expanded with such a fantastic velocity that it would have prevented the existence of everything we know in our cosmos.”

The requisite amount of dark energy is so difficult to reconcile with the known laws of nature that physicists have proposed all manner of exotic explanations, including new forces, new dimensions of spacetime, and new ultralight elementary particles. However, the new report proposes no new ingredient for the universe, only a realization that the present acceleration of the universe is a consequence of the standard cosmological model for the early universe: inflation.

“Our solution to the paradox posed by the accelerating universe,” Riotto says, “relies on the so-called inflationary theory, born in 1981. According to this theory, within a tiny fraction of a second after the Big Bang, the universe experienced an incredibly rapid expansion. This explains why our universe seems to be very homogeneous. Recently, the Boomerang and WMAP experiments, which measured the small fluctuations in the background radiation originating with the Big Bang, confirmed inflationary theory.

It is widely believed that during the inflationary expansion early in the history of the universe, very tiny ripples in spacetime were generated, as predicted by Einstein’s theory of General Relativity. These ripples were stretched by the expansion of the universe and extend today far beyond our cosmic horizon, that is over a region much bigger than the observable universe, a distance of about 15 billion light years. In their current paper, the authors propose that it is the evolution of these cosmic ripples that increases the observed expansion of the universe and accounts for its acceleration.

“We realized that you simply need to add this new key ingredient, the ripples of spacetime generated during the epoch of inflation, to Einstein’s General Relativity to explain why the universe is accelerating today,” Riotto says. “It seems that the solution to the puzzle of acceleration involves the universe beyond our cosmic horizon. No mysterious dark energy is required.”

Fermilab’s Kolb called the authors’ proposal the most conservative explanation for the accelerating universe. “It requires only a proper accounting of the physical effects of the ripples beyond our cosmic horizon,” he said.

Data from upcoming experiments will allow cosmologists to test the proposal. “Whether Einstein was right when he first introduced the cosmological constant, or whether he was right when he later refuted the idea will soon be tested by a new round of precision cosmological observations,” Kolb said. “New data will soon allow us to distinguish between our explanation for the accelerated expansion of the universe and the dark energy solution.”

INFN (Istituto Nazionale di Fisica Nucleare), Italy’s national nuclear physics institute, supports, coordinates and carries out scientific research in subnuclear, nuclear and astroparticle physics and is involved in developing relevant technologies.

Fermilab, in Batavia, Illinois, USA, is operated by Universities Research Association, Inc. for the Department of Energy’s Office of Science, which funds advanced research in particle physics and cosmology.

Original Source: Istituto Nazionale di Fisica Nucleare

Astrophysicists in recent years have found evidence for a force they call dark energy in observations from the farthest reaches of the universe, billions of light years away.

Now an international team of researchers has used data from powerful computer models, supported by observations from the Hubble Space Telescope, to find evidence of dark energy right in our own cosmic neighborhood.

The data paint a picture of the universe as a virtual sea of dark energy, with billions of galaxies as islands emerging from the sea, said Fabio Governato, a University of Washington research associate professor of astronomy and a researcher with Italy’s National Institute for Astrophysics.

In 1929 astronomer Edwin Hubble demonstrated that galaxies are moving away from each other, which supported the theory that the universe has been expanding since the big bang. In 1999 cosmologists reported evidence that an unusual force, called dark energy, was actually causing the expansion of the universe to accelerate.

However, the expansion is slower than it would be otherwise because of the tug of gravity among galaxies. As the battle between the attraction of gravity and the repellent force of dark energy plays out, cosmologists are left to ponder whether the expansion will continue forever or if the universe will collapse in a “big crunch.”

In 1997, Governato designed a computer model to simulate evolution of the universe from the big bang until the present. His research group found the model could not duplicate the smooth expansion that had been observed among galaxies around the Milky Way, the galaxy in which Earth resides. In fact, the model produced deviations from a purely radial expansion that were three to seven times higher than astronomers had actually observed, Governato said.

“The observed motion was small, and we could not duplicate it without the presence of dark energy,” he said. “When we added the dark energy, we got a perfect match.”

Governato is one of three authors of a paper describing the work, scheduled for publication in the Monthly Notices of the Royal Astronomical Society, an astronomy journal in the United Kingdom. Co-authors are Andrea Maccio of the University of Zurich in Switzerland and Cathy Horellou of Chalmers University of Technology in Sweden. The work was supported by grants from the National Science Foundation and Vetenskapsr?det, the Swedish Research Council.

The authors, part of an international research collaboration called the N-Body Shop that originated at the UW, ran simulations of universe expansion on powerful supercomputers in Italy and Alaska. Their findings provide supporting evidence for a sea of dark energy surrounding galaxies.

“We studied the properties of galaxies close to the Milky Way instead of looking billions of light years away,” Governato said. “It’s like traveling from Seattle to Portland, Ore., rather than from Seattle to New York, to measure the Earth’s curvature.”

Original Source: University of Washington News Release