VLT Observations of Gamma-ray Burst …

…Reveal Surprising Ingredients of Early Galaxies

Artist’s impression of a gamma-ray burst shining through two young galaxies in the early Universe

An international team of astronomers has used the brief but brilliant light of a distant gamma-ray burst as a probe to study the make-up of very distant galaxies. Surprisingly the new observations, made with ESO’s Very Large Telescope, have revealed two galaxies in the young Universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early Universe will drive the formation of many new stars and may be the trigger for gamma-ray bursts.

Gamma-ray bursts are the brightest explosions in the Universe [1]. They are first spotted by orbiting observatories that detect the initial short burst of gamma rays. After their positions have been pinned down, they are then immediately studied using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. One such burst, called GRB 090323 [2], was first spotted by the NASA Fermi Gamma-ray Space Telescope. Very soon afterwards it was picked up by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile (eso1049) and then studied in great detail using ESO’s Very Large Telescope (VLT) just one day after it exploded.

The VLT observations show that the brilliant light from the gamma-ray burst had passed through its own host galaxy and another galaxy nearby. These galaxies are being seen as they were about 12 billion years ago [3]. Such distant galaxies are very rarely caught in the glare of a gamma-ray burst.

“When we studied the light from this gamma-ray burst we didn’t know what we might find. It was a surprise that the cool gas in these two galaxies in the early Universe proved to have such an unexpected chemical make-up,” explains Sandra Savaglio (Max-Planck Institute for Extraterrestrial Physics, Garching, Germany), lead author of the paper describing the new results. “These galaxies have more heavy elements than have ever been seen in a galaxy so early in the evolution of the Universe. We didn’t expect the Universe to be so mature, so chemically evolved, so early on.”

As light from the gamma-ray burst passed through the galaxies, the gas there acted like a filter, and absorbed some of the light from the gamma-ray burst at certain wavelengths. Without the gamma-ray burst these faint galaxies would be invisible. By carefully analysing the tell-tale fingerprints from different chemical elements the team was able to work out the composition of the cool gas in these very distant galaxies, and in particular how rich they were in heavy elements.

It is expected that galaxies in the young Universe will be found to contain smaller amounts of heavier elements than galaxies at the present day, such as the Milky Way. The heavier elements are produced during the lives and deaths of generations of stars, gradually enriching the gas in the galaxies [4]. Astronomers can use the chemical enrichment in galaxies to indicate how far they are through their lives. But the new observations, surprisingly, revealed that some galaxies were already very rich in heavy elements less than two billion years after the Big Bang. Something unthinkable until recently.

The newly discovered pair of young galaxies must be forming new stars at a tremendous rate, to enrich the cool gas so strongly and quickly. As the two galaxies are close to each other they may be in the process of merging, which would also provoke star formation when the gas clouds collide. The new results also support the idea that gamma-ray bursts may be associated with vigorous massive star formation.

Energetic star formation in galaxies like these might have ceased early on in the history of the Universe. Twelve billion years later, at the present time, the remains of such galaxies would contain a large number of stellar remnants such as black holes and cool dwarf stars, forming a hard to detect population of “dead galaxies”, just faint shadows of how they were in their brilliant youths. Finding such corpses in the present day would be a challenge.

“We were very lucky to observe GRB 090323 when it was still sufficiently bright, so that it was possible to obtain spectacularly detailed observations with the VLT. Gamma-ray bursts only stay bright for a very short time and getting good quality data is very hard. We hope to observe these galaxies again in the future when we have much more sensitive instruments, they would make perfect targets for the E-ELT,” concludes Savaglio.

Notes
[1] Gamma-ray bursts lasting longer than two seconds are referred to as long bursts and those with a shorter duration are known as short bursts. Long bursts, including the one in this study, are associated with supernova explosions of massive young stars in star-forming galaxies. Short bursts are not well understood, but are thought to originate from the merger of two compact objects such as neutron stars.

[2] The name refers to the date on which the burst was discovered, in this case it was spotted on 23 March 2009.

[3] The galaxies were seen at a redshift of 3.57, meaning that they are seen as they were 1.8 billion years after the Big Bang.

[4] The material produced by the Big Bang, 13.7 billion years ago, was almost entirely hydrogen and helium. Most heavier elements, such as oxygen, nitrogen and carbon, were produced later by thermonuclear reactions inside stars and fed back into the reserves of gas within galaxies as these stars die. So, it is expected that the amount of heavier elements in most galaxies gradually increases as the Universe ages.

www.eso.org

Advertisements

Distant Galaxies Reveal The Clearing of the Cosmic Fog

New VLT observations chart timeline of reionisation

Artist’s impression of galaxies at the end of the era of reionisation

Scientists have used ESO’s Very Large Telescope to probe the early Universe at several different times as it was becoming transparent to ultraviolet light. This brief but dramatic phase in cosmic history — known as reionisation — occurred around 13 billion years ago. By carefully studying some of the most distant galaxies ever detected, the team has been able to establish a timeline for reionisation for the first time. They have also demonstrated that this phase must have happened quicker than astronomers previously thought.
An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang [1].

The new observations have allowed astronomers to establish a timeline for what is known as the age of reionisation [2] for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time.
The new results, which will appear in the Astrophysical Journal, build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the last three years.

“Archaeologists can reconstruct a timeline of the past from the artifacts they find in different layers of soil. Astronomers can go one better: we can look directly into the remote past and observe the faint light from different galaxies at different stages in cosmic evolution,” explains Adriano Fontana, of INAF Rome Astronomical Observatory who led this project. “The differences between the galaxies tell us about the changing conditions in the Universe over this important period, and how quickly these changes were occurring.”

Different chemical elements glow brightly at characteristic colours. These spikes in brightness are known as emission lines. One of the strongest ultraviolet emission lines is the Lyman-alpha line, which comes from hydrogen gas [3]. It is bright and recognisable enough to be seen even in observations of very faint and faraway galaxies….. Continue reading Distant Galaxies Reveal The Clearing of the Cosmic Fog

Feast your Eyes on the Fried Egg Nebula


Astronomers have used ESO’s Very Large Telescope to image a colossal star that belongs to one of the rarest classes of stars in the Universe, the yellow hypergiants. The new picture is the best ever taken of a star in this class and shows for the first time a huge dusty double shell surrounding the central hypergiant. The star and its shells resemble an egg white around a yolky centre, leading the astronomers to nickname the object the Fried Egg Nebula.

The monster star, known to astronomers as IRAS 17163-3907 [1], has a diameter about a thousand times bigger than our Sun. At a distance of about 13 000 light-years from Earth, it is the closest yellow hypergiant found to date and new observations show it shines some 500 000 times more brightly than the Sun [2].

“This object was known to glow brightly in the infrared but, surprisingly, nobody had identified it as a yellow hypergiant before,” said Eric Lagadec (European Southern Observatory), who led the team that produced the new images….. Continue reading Feast your Eyes on the Fried Egg Nebula

The Star That Should Not Exist

A team of European astronomers has used ESO’s Very Large Telescope (VLT) to track down a star in the Milky Way that many thought was impossible. They discovered that this star is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it. This intriguing composition places it in the “forbidden zone” of a widely accepted theory of star formation, meaning that it should never have come into existence in the first place. The results will appear in the 1 September 2011 issue of the journal Nature.

A faint star in the constellation of Leo (The Lion), called SDSS J102915+172927 [1], has been found to have the lowest amount of elements heavier than helium (what astronomers call “metals”) of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old.

“A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed,” [2] said Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Germany and Observatoire de Paris, France), lead author of the paper. “It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”

The team analysed the properties of the star using the X-shooter and UVES instruments on the VLT [3]. This allowed them to measure how abundant the various chemical elements were in the star. They found that the proportion of metals in SDSS J102915+172927 is more than 20 000 times smaller than that of the Sun [4][5].

“The star is faint, and so metal-poor that we could only detect the signature of one element heavier than helium — calcium — in our first observations,” said Piercarlo Bonifacio (Observatoire de Paris, France), who supervised the project. “We had to ask for additional telescope time from ESO’s Director General to study the star’s light in even more detail, and with a long exposure time, to try to find other metals.”

Cosmologists believe that the lightest chemical elements — hydrogen and helium — were created shortly after the Big Bang, together with some lithium [6], while almost all other elements were formed later in stars. Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is.

“The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found,” adds Lorenzo Monaco (ESO, Chile), also involved in the study.

Also very surprising was the lack of lithium in SDSS J102915+172927. Such an old star should have a composition similar to that of the Universe shortly after the Big Bang, with a few more metals in it. But the team found that the proportion of lithium in the star was at least fifty times less than expected in the material produced by the Big Bang.

“It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star.” Bonifacio added.

The researchers also point out that this freakish star is probably not unique. “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case,” concludes Caffau.

Notes
[1] The star is catalogued in the Sloan Digital Sky Survey or SDSS. The numbers refer to the object’s position in the sky.

[2] Widely accepted star formation theories state that stars with a mass as low as SDSS J102915+172927 (about 0.8 solar masses or less) could only have formed after supernova explosions enriched the interstellar medium above a critical value. This is because the heavier elements act as “cooling agents”, helping to radiate away the heat of gas clouds in this medium, which can then collapse to form stars. Without these metals, the pressure due to heating would be too strong, and the gravity of the cloud would be too weak to overcome it and make the cloud collapse. One theory in particular identifies carbon and oxygen as the main cooling agents, and in SDSS J102915+172927 the amount of carbon is lower than the minimum deemed necessary for this cooling to be effective.

[3] X-shooter and UVES are VLT spectrographs — instruments used to separate the light from celestial objects into its component colours and allow detailed analysis of the chemical composition. X-shooter can capture a very wide range of wavelengths in the spectrum of an object in one shot (from the ultraviolet to the near-infrared). UVES is the Ultraviolet and Visual Echelle Spectrograph, a high-resolution optical instrument.

[4] The star HE 1327-2326, discovered in 2005, has the lowest known iron abundance, but it is rich in carbon. The star now analysed has the lowest proportion of metals when all chemical elements heavier than helium are considered.

[5] ESO telescopes have been deeply involved in many of the discoveries of the most metal-poor stars. Some of the earlier results were reported in eso0228 and eso0723 and the new discovery shows that observations with ESO telescopes have let astronomers make a further step closer to finding the first generation of stars.

[6] Primordial nucleosynthesis refers to the production of chemical elements with more than one proton a few moments after the Big Bang. This production happened in a very short time, allowing only hydrogen, helium and lithium to form, but no heavier elements. The Big Bang theory predicts, and observations confirm, that the primordial matter was composed of about 75% (by mass) of hydrogen, 25% of helium, and trace amounts of lithium.
http://www.eso.org/public/news/eso1132/

Mysterious Blob in the Ancient Universe Explained

Giant Space Blob Glows from Within
VLT finds primordial cloud of hydrogen to be centrally powered

Observations from ESO’s Very Large Telescope have shed light on the power source of a rare vast cloud of glowing gas in the early Universe. The observations show for the first time that this giant “Lyman-alpha blob” — one of the largest single objects known — must be powered by galaxies embedded within it. The results appear in the 18 August issue of the journal Nature.

A team of astronomers has used ESO’s Very Large Telescope (VLT) to study an unusual object called a Lyman-alpha blob [1]. These huge and very luminous rare structures are normally seen in regions of the early Universe where matter is concentrated. The team found that the light coming from one of these blobs is polarised [2]. In everyday life, for example, polarised light is used to create 3D effects in cinemas [3]. This is the first time that polarisation has ever been found in a Lyman-alpha blob, and this observation helps to unlock the mystery of how the blobs shine.

We have shown for the first time that the glow of this enigmatic object is scattered light from brilliant galaxies hidden within, rather than the gas throughout the cloud itself shining.” explains Matthew Hayes (University of Toulouse, France), lead author of the paper.

Lyman-alpha blobs are some of the biggest objects in the Universe: gigantic clouds of hydrogen gas that can reach diameters of a few hundred thousand light-years (a few times larger than the size of the Milky Way), and which are as powerful as the brightest galaxies. They are typically found at large distances, so we see them as they were when the Universe was only a few billion years old. They are therefore important in our understanding of how galaxies formed and evolved when the Universe was younger. But the power source for their extreme luminosity, and the precise nature of the blobs, has remained unclear.

The team studied one of the first and brightest of these blobs to be found. Known as LAB-1, it was discovered in 2000, and it is so far away that its light has taken about 11.5 billion years to reach us (redshift 3.1). With a diameter of about 300 000 light-years it is also one of the largest known, and has several primordial galaxies inside it, including an active galaxy [4].

There are several competing theories to explain Lyman-alpha blobs. One idea is that they shine when cool gas is pulled in by the blob’s powerful gravity, and heats up. Another is that they are shining because of brilliant objects inside them: galaxies undergoing vigorous star formation, or containing voracious black holes engulfing matter. The new observations show that it is embedded galaxies, and not gas being pulled in, that power LAB-1.

The team tested the two theories by measuring whether the light from the blob was polarised. By studying how light is polarised astronomers can find out about the physical processes that produced the light, or what has happened to it between its origin and its arrival at Earth. If it is reflected or scattered it becomes polarised and this subtle effect can be detected by a very sensitive instrument. To measure polarisation of the light from a Lyman-alpha blob is, however, a very challenging observation, because of their great distance.

These observations couldn’t have been done without the VLT and its FORS instrument. We clearly needed two things: a telescope with at least an eight-metre mirror to collect enough light, and a camera capable of measuring the polarisation of light. Not many observatories in the world offer this combination.” adds Claudia Scarlata (University of Minnesota, USA), co-author of the paper.

By observing their target for about 15 hours with the Very Large Telescope, the team found that the light from the Lyman-alpha blob LAB-1 was polarised in a ring around the central region, and that there was no polarisation in the centre. This effect is almost impossible to produce if light simply comes from the gas falling into the blob under gravity, but it is just what is expected if the light originally comes from galaxies embedded in the central region, before being scattered by the gas.

The astronomers now plan to look at more of these objects to see if the results obtained for LAB-1 are true of other blobs.

Notes

[1] The name comes from the fact that these blobs emit a characteristic wavelength of light, known as “Lyman-alpha” radiation, that is produced when electrons in hydrogen atoms drop from the second-lowest to the lowest energy level.

[2] When light waves are polarised, their component electric and magnetic fields have a specific orientation. In unpolarised light the orientation of the fields is random and has no preferred direction.

[3] The 3D effect is created by making sure the left and right eyes are seeing slightly different images. The trick used in some 3D cinemas involves polarised light: separate images made with differently polarised light are sent to our left and right eyes by polarising filters in the glasses.

[4] Active galaxies are galaxies whose bright cores are believed to be powered by a vast black hole. Their luminosity comes from material being heated as it is pulled in by the black hole.
http://www.eso.org/public/news/eso1130/

A Spiral in Leo

This new picture from ESO’s Very Large Telescope shows NGC 3521, a spiral galaxy located about 35 million light years away in the constellation of Leo (The Lion). Spanning about 50 000 light-years, this spectacular object has a bright and compact nucleus, surrounded by richly detailed spiral structure.

The most distinctive features of the bright galaxy NGC 3521 are its long spiral arms that are dotted with star-forming regions and interspersed with veins of dust. The arms are rather irregular and patchy, making NGC 3521 a typical example of a flocculent spiral galaxy. These galaxies have “fluffy” spiral arms that contrast with the sweeping arms of grand-design spirals such as the famous Whirlpool galaxy or M 51, discovered by Charles Messier.

NGC 3521 is bright and relatively close-by, and can easily be seen with a small telescope such as the one used by Messier to catalogue a series of hazy and comet-like objects in the 1700s. Strangely, the French astronomer seems to have missed this flocculent spiral even though he identified several other galaxies of similar brightness in the constellation of Leo.

It was only in the year that Messier published the final version of his catalogue, 1784, that another famous astronomer, William Herschel, discovered NGC 3521 early on in his more detailed surveys of the northern skies.  Through his larger, 47-cm aperture, telescope, Herschel saw a “bright center surrounded by nebulosity,” according to his observation notes.

In this new VLT picture, colourful, yet ill defined, spiral arms replace Herschel’s “nebulosity”. Older stars dominate the reddish area in the centre while young, hot blue stars permeate the arms further away from the core.

Oleg Maliy, who participated ESO’s Hidden Treasures 2010 competition [1], selected the data from the FORS1 instrument on ESO’s VLT at the Paranal Observatory in Chile that were used to create this dramatic image. Exposures taken through three different filters that passed blue light (coloured blue), yellow/green light (coloured green), and near-infrared light (coloured red) have been combined to make this picture. The total exposure times were 300 seconds per filter. Oleg’s image of NGC 3521 was a highly ranked entry in the competition, which attracted almost 100 entries.

Notes

[1] ESO’s Hidden Treasures 2010 competition gave amateur astronomers the opportunity to search through ESO’s vast archives of astronomical data, hoping to find a well-hidden gem that needed polishing by the entrants. To find out more about Hidden Treasures, visit http://www.eso.org/public/outreach/hiddentreasures/.

Dark Sky and White Desert…

…Snow pays a rare visit to ESO’s Paranal Observatory

The night sky above Cerro Paranal, the home of ESO’s Very Large Telescope (VLT), is dark and dotted with the bright stars of the Milky Way, and more distant galaxies. But it is very rare to see the ground contrasting with the sky as markedly as in this photograph, which shows a gentle layer of white snow dotted with darker spots of the desert terrain beneath.
The picture was taken last week, shortly before sunrise, by ESO Photo Ambassador Yuri Beletsky, who works as an astronomer at the La Silla Paranal Observatory. He captured not only the beautiful snowy landscape of the Atacama and the mountaintop domes of the VLT, but also an incredible night sky. To the left of the VLT is a satellite trail, and to the right is the trail of a meteor.
Cerro Paranal is a 2600-metre-high mountain located in the Chilean Atacama Desert. It is a very dry place with humidity often dropping below 10 percent and rainfall of less than 10 millimetres per year. Snow, however, does occasionally fall in the desert, providing fleeting but magnificent views such as this one.
http://www.eso.org/public/images/potw1132a/