Observations of gamma-ray burst reveal surprising ingredients of early galaxies

ScienceDaily (Nov. 2, 2011) — An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics 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 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. 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, 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. From the GROND observations, the astronomers estimated the minimum rate of star formation, which has to be several times higher than the one in our Galaxy. They could, however, only determine a minimum value because the detected emission could be heavily affected (i.e. absorbed) by the presence of dust in the galaxies. The real rate of star formation, once the (unknown) dust absorption has been taken into account, could easily be 50 times higher than in the Milky Way.

The burst was also studied in great detail using ESO's Very Large Telescope (VLT) just one day after it exploded. These 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. 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. 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.

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The above story is reprinted from materials provided by Max-Planck-Institut für extraterrestrische Physik (MPE).

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Journal Reference:

S. Savaglio et al. Super-solar Metal Abundances in Two Galaxies at z~3.57 revealed by the GRB090323 Afterglow Spectrum. Monthly Notices of the Royal Astronomical Society, 2011

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Distant galaxies reveal the clearing of the cosmic fog; New VLT observations chart timeline of reionization

ScienceDaily (Oct. 13, 2011) — 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.

Spotting the Lyman-alpha line for five very distant galaxies [4] allowed the team to do two key things: first, by observing how far the line had been shifted toward the red end of the spectrum, they were able to determine the galaxies' distances, and hence how soon after the Big Bang they could see them [5]. This let them place them in order, creating a timeline which shows how the galaxies' light evolved over time. Secondly, they were able to see the extent to which the Lyman-alpha emission -- which comes from glowing hydrogen within the galaxies -- was reabsorbed by the neutral hydrogen fog in intergalactic space at different points in time.

"We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample," says lead author Laura Pentericci of INAF Rome Astronomical Observatory. "When the Universe was only 780 million years old this neutral hydrogen was quite abundant, filling from 10 to 50% of the Universe' volume. But only 200 million years later the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionisation must have happened quicker than astronomers previously thought."

As well as probing the rate at which the primordial fog cleared, the team's observations also hint at the likely source of the ultraviolet light which provided the energy necessary for reionisation to occur. There are several competing theories for where this light came from -- two leading candidates are the Universe's first generation of stars [6], and the intense radiation emitted by matter as it falls towards black holes.

"The detailed analysis of the faint light from two of the most distant galaxies we found suggests that the very first generation of stars may have contributed to the energy output observed," says Eros Vanzella of the INAF Trieste Observatory, a member of the research team. "These would have been very young and massive stars, about five thousand times younger and one hundred times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent."

The highly accurate measurements required to confirm or disprove this hypothesis, and show that the stars can produce the required energy, require observations from space, or from ESO's planned European Extremely Large Telescope, which will be the world's largest eye on the sky once completed early next decade.

Studying this early period in cosmic history is technically challenging because accurate observations of extremely distant and faint galaxies are needed, a task which can only be attempted with the most powerful telescopes. For this study, the team used the great light-gathering power of the 8.2-metre VLT to carry out spectroscopic observations, targetting galaxies first identified by the NASA/ESA Hubble Space Telescope and in deep images from the VLT.

Notes

[1] The most distant galaxy that has been reported with a distance measured by spectroscopy is at a redshift of 8.6, placing it 600 million years after the Big Bang (eso1041). There is a candidate galaxy thought to be at a redshift of about 10 (480 million years after the Big Bang) identified by the Hubble Space Telescope, but this is awaiting confirmation. The most distant galaxy in this study is at a redshift of 7.1, placing it 780 million years after the Big Bang. The Universe today is 13.7 billion years old. The new sample of five confirmed galaxies with Lyman-alpha detections (out of 20 candidates) includes half of all galaxies known at z>7.

[2] At the time the first stars and galaxies formed, the Universe was filled with electrically neutral hydrogen gas, which absorbs ultraviolet light. As the ultraviolet radiation from these early galaxies excited the gas, making it electrically charged (ionised), it gradually became transparent to ultraviolet light. This process is technically known as reionisation, as there is thought to have been a brief period within the first 100 000 years after the Big Bang in which the hydrogen was also ionised.

[3] The team measured the effects of the hydrogen fog using spectroscopy, a technique which involves splitting and spreading out the light from the galaxy into its component colours, much like a prism splits sunlight into a rainbow.

[4] The team used the VLT to study the spectra of 20 candidate galaxies at redshifts close to 7. These come from deep imaging studies of three separate fields. Of these 20 targets five were found to have clearly detected Lyman-alpha emission. This is currently the only set of spectroscopically confirmed galaxies around z=7.

[5] Because the Universe is expanding, the wavelength of light from objects gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, the characteristic red colour this gives to extremely distant objects has become known as 'redshift'. Although it is technically a measure of how the colour of an object's light has been affected, it is also by extension a measure both of the object's distance, and of how long after the Big Bang we see it.

[6] Astronomers classify stars into three categories, known as Population I, Population II and Population III. Population I stars, like our Sun, are rich in heavier elements synthesised in the hearts of older stars and in supernova explosions: as they are made up from the wreckage of previous generations of stars, they only came into existence later in the Universe. Population II stars have fewer heavy elements in them and are predominantly made up of the hydrogen, helium and lithium created during the Big Bang. These are older stars, though there are still many of them in existence in the Universe today. Population III stars have never been directly observed, though they are thought to have existed in the early years of the Universe. As these contained only the material created during the Big Bang, they contained no heavier elements at all. Because of the role of heavier elements in the formation of stars, only very large stars with very short lifespans were able to form at this stage, and so all the Population III stars quickly ended their lives in supernovae in the early years of the Universe. Up to now, no solid evidence of Population III stars has been confirmed even in observations of very distant galaxies.

More information

This research was presented in a paper "Spectroscopic Confirmation of z7 LBGs: Probing the Earliest Galaxies and the Epoch of Reionization," to appear in the Astrophysical Journal.

The team is composed of L.Pentericci (INAF Osservatorio Astronomico di Roma, Rome, Italy [INAF-OAR]), A. Fontana (INAF-OAR), E. Vanzella (INAF Osservatorio Astronomico di Trieste, Trieste, Italy [INAF-OAT]), M. Castellano (INAF-OAR), A. Grazian (INAF-OAR), M. Dijkstra (Max-Planck-Institut fur Astrophysik, Garching, Germany), K. Boutsia (INAF-OAR), S. Cristiani (INAF-OAT), M. Dickinson (National Optical Astronomy Observatory, Tucson, USA), E. Giallongo (INAF-OAR), M. Giavalisco (University of Massachusetts, Amherst, USA), R. Maiolino (INAF-OAR), A. Moorwood (ESO, Garching), P. Santini (INAF-OAR).

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by European Southern Observatory - ESO.

Note: If no author is given, the source is cited instead.

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View the original article here

Distant galaxies reveal the clearing of the cosmic fog; New VLT observations chart timeline of reionization

ScienceDaily (Oct. 13, 2011) — 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.

Spotting the Lyman-alpha line for five very distant galaxies [4] allowed the team to do two key things: first, by observing how far the line had been shifted toward the red end of the spectrum, they were able to determine the galaxies' distances, and hence how soon after the Big Bang they could see them [5]. This let them place them in order, creating a timeline which shows how the galaxies' light evolved over time. Secondly, they were able to see the extent to which the Lyman-alpha emission -- which comes from glowing hydrogen within the galaxies -- was reabsorbed by the neutral hydrogen fog in intergalactic space at different points in time.

"We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample," says lead author Laura Pentericci of INAF Rome Astronomical Observatory. "When the Universe was only 780 million years old this neutral hydrogen was quite abundant, filling from 10 to 50% of the Universe' volume. But only 200 million years later the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionisation must have happened quicker than astronomers previously thought."

As well as probing the rate at which the primordial fog cleared, the team's observations also hint at the likely source of the ultraviolet light which provided the energy necessary for reionisation to occur. There are several competing theories for where this light came from -- two leading candidates are the Universe's first generation of stars [6], and the intense radiation emitted by matter as it falls towards black holes.

"The detailed analysis of the faint light from two of the most distant galaxies we found suggests that the very first generation of stars may have contributed to the energy output observed," says Eros Vanzella of the INAF Trieste Observatory, a member of the research team. "These would have been very young and massive stars, about five thousand times younger and one hundred times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent."

The highly accurate measurements required to confirm or disprove this hypothesis, and show that the stars can produce the required energy, require observations from space, or from ESO's planned European Extremely Large Telescope, which will be the world's largest eye on the sky once completed early next decade.

Studying this early period in cosmic history is technically challenging because accurate observations of extremely distant and faint galaxies are needed, a task which can only be attempted with the most powerful telescopes. For this study, the team used the great light-gathering power of the 8.2-metre VLT to carry out spectroscopic observations, targetting galaxies first identified by the NASA/ESA Hubble Space Telescope and in deep images from the VLT.

Notes

[1] The most distant galaxy that has been reported with a distance measured by spectroscopy is at a redshift of 8.6, placing it 600 million years after the Big Bang (eso1041). There is a candidate galaxy thought to be at a redshift of about 10 (480 million years after the Big Bang) identified by the Hubble Space Telescope, but this is awaiting confirmation. The most distant galaxy in this study is at a redshift of 7.1, placing it 780 million years after the Big Bang. The Universe today is 13.7 billion years old. The new sample of five confirmed galaxies with Lyman-alpha detections (out of 20 candidates) includes half of all galaxies known at z>7.

[2] At the time the first stars and galaxies formed, the Universe was filled with electrically neutral hydrogen gas, which absorbs ultraviolet light. As the ultraviolet radiation from these early galaxies excited the gas, making it electrically charged (ionised), it gradually became transparent to ultraviolet light. This process is technically known as reionisation, as there is thought to have been a brief period within the first 100 000 years after the Big Bang in which the hydrogen was also ionised.

[3] The team measured the effects of the hydrogen fog using spectroscopy, a technique which involves splitting and spreading out the light from the galaxy into its component colours, much like a prism splits sunlight into a rainbow.

[4] The team used the VLT to study the spectra of 20 candidate galaxies at redshifts close to 7. These come from deep imaging studies of three separate fields. Of these 20 targets five were found to have clearly detected Lyman-alpha emission. This is currently the only set of spectroscopically confirmed galaxies around z=7.

[5] Because the Universe is expanding, the wavelength of light from objects gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, the characteristic red colour this gives to extremely distant objects has become known as 'redshift'. Although it is technically a measure of how the colour of an object's light has been affected, it is also by extension a measure both of the object's distance, and of how long after the Big Bang we see it.

[6] Astronomers classify stars into three categories, known as Population I, Population II and Population III. Population I stars, like our Sun, are rich in heavier elements synthesised in the hearts of older stars and in supernova explosions: as they are made up from the wreckage of previous generations of stars, they only came into existence later in the Universe. Population II stars have fewer heavy elements in them and are predominantly made up of the hydrogen, helium and lithium created during the Big Bang. These are older stars, though there are still many of them in existence in the Universe today. Population III stars have never been directly observed, though they are thought to have existed in the early years of the Universe. As these contained only the material created during the Big Bang, they contained no heavier elements at all. Because of the role of heavier elements in the formation of stars, only very large stars with very short lifespans were able to form at this stage, and so all the Population III stars quickly ended their lives in supernovae in the early years of the Universe. Up to now, no solid evidence of Population III stars has been confirmed even in observations of very distant galaxies.

More information

This research was presented in a paper "Spectroscopic Confirmation of z7 LBGs: Probing the Earliest Galaxies and the Epoch of Reionization," to appear in the Astrophysical Journal.

The team is composed of L.Pentericci (INAF Osservatorio Astronomico di Roma, Rome, Italy [INAF-OAR]), A. Fontana (INAF-OAR), E. Vanzella (INAF Osservatorio Astronomico di Trieste, Trieste, Italy [INAF-OAT]), M. Castellano (INAF-OAR), A. Grazian (INAF-OAR), M. Dijkstra (Max-Planck-Institut fur Astrophysik, Garching, Germany), K. Boutsia (INAF-OAR), S. Cristiani (INAF-OAT), M. Dickinson (National Optical Astronomy Observatory, Tucson, USA), E. Giallongo (INAF-OAR), M. Giavalisco (University of Massachusetts, Amherst, USA), R. Maiolino (INAF-OAR), A. Moorwood (ESO, Garching), P. Santini (INAF-OAR).

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How the Milky Way killed off nearby galaxies

ScienceDaily (Oct. 18, 2011) — Two researchers from Observatoire Astronomique de Strasbourg have revealed for the first time the existence of a new signature of the birth of the first stars in our galaxy, the Milky Way. More than 12 billion years ago, the intense ultraviolet light from these stars dispersed the gas of our Galaxy's nearest companions, virtually putting a halt to their ability to form stars and consigning them to a dim future. Now Pierre Ocvirk and Dominique Aubert, members of the Light in the Dark Ages of the Universe (LIDAU) collaboration, have explained why some galaxies were killed off, while stars continued to form in more distant objects.

The two scientists publish their results in the October issue of the letters of the journal Monthly Notices of the Royal Astronomical Society.

The first stars of the Universe appeared about 150 million years after the Big Bang. Back then, the hydrogen and helium gas filling the universe was cold enough for its atoms to be electrically neutral. As the ultraviolet (UV) light of the first stars propagated through this gas, it broke apart the proton-electron pairs that make up hydrogen atoms, returning them to the so-called plasma state they experienced in the first moments of the Universe. This process, known as reionisation, also resulted in significant heating, which had dramatic consequences: the gas became so hot that it escaped the weak gravity of the lowest mass galaxies, thereby depriving them of the material needed to form stars.

It is now widely accepted that this process can explain the small number and large ages of the stars seen in the faintest dwarf galaxy satellites of the Milky Way. It also helps scientists understand why galaxies like the Milky Way have so few satellites around them -- the 'missing satellites' problem. The stripping out of gas from these galaxies makes them sensitive probes of the UV radiation in the reionisation epoch.

The satellite galaxies are also relatively close, from 30000 to 900000 light-years away, which allows us to study them in great detail, something that will be enhanced by the coming generation of larger telescopes. Comparing the population of their stars in each galaxy with its position could give us a unique insight into the structure of the UV radiation emitted from the earliest stars in the Milky Way.

Until now, models for this process assumed that the radiation leading to the removal of gas from galaxy satellites was produced collectively by all the large galaxies nearby, resulting in a uniform background of UV light. The new model put together by the two French researchers proves this assumption wrong.

Ocvirk and Aubert looked at the way the invisible 'dark matter' that makes up about 23% of the Universe structured itself with the stars in our Galaxy and its environs from shortly after the Big Bang to the present day. They used the high resolution numerical simulation Via Lactea II to model the formation of stars in gas trapped in the dark matter haloes that envelop galaxies, and then to describe how this gas reacted to UV radiation.

Pierre Ocvirk comments, "This is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the centre of the Milky Way on its satellite galaxies.

'In contrast to previous models, the radiation field produced is not uniform, but decreases in intensity as one moves away from the centre of the Milky Way.

'The satellite galaxies close to the galactic centre see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. At the same time, the more remote satellite galaxies experience on average a weaker irradiation. Therefore they manage to keep their gas longer, and form more stars. As a consequence they are easier to detect and appear more numerous."

The new model appears to be a close match to observations of our Galaxy and its neighbourhood and suggests that the first stars of our galaxy played a major role in the photo-evaporation of the satellite galaxies' gas, adds Dr Ocvirk. "It is not large nearby galaxies but our own that caused the demise of its tiny neighbours, asphyxiating them through its intense radiation."

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Journal Reference:

Pierre Ocvirk, Dominique Aubert. A signature of the internal reionisation of the Milky Way? Monthly Notices of the Royal Astronomical Society, 2011 [link]

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NASA's Spitzer finds distant galaxies grazed on gas

ScienceDaily (July 1, 2011) — Galaxies once thought of as voracious tigers are more like grazing cows, according to a new study using NASA's Spitzer Space Telescope.

Astronomers have discovered that galaxies in the distant, early universe continuously ingested their star-making fuel over long periods of time. This goes against previous theories that the galaxies devoured their fuel in quick bursts after run-ins with other galaxies.

"Our study shows the merging of massive galaxies was not the dominant method of galaxy growth in the distant universe," said Ranga-Ram Chary of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, Calif. "We're finding this type of galactic cannibalism was rare. Instead, we are seeing evidence for a mechanism of galaxy growth in which a typical galaxy fed itself through a steady stream of gas, making stars at a much faster rate than previously thought."

Chary is the principal investigator of the research, appearing in the Aug. 1 issue of the Astrophysical Journal. According to his findings, these grazing galaxies fed steadily over periods of hundreds of millions of years and created an unusual amount of plump stars, up to 100 times the mass of our sun.

"This is the first time that we have identified galaxies that supersized themselves by grazing," said Hyunjin Shim, also of the Spitzer Science Center and lead author of the paper. "They have many more massive stars than our Milky Way galaxy."

Galaxies like our Milky Way are giant collections of stars, gas and dust. They grow in size by feeding off gas and converting it to new stars. A long-standing question in astronomy is: Where did distant galaxies that formed billions of years ago acquire this stellar fuel? The most favored theory was that galaxies grew by merging with other galaxies, feeding off gas stirred up in the collisions.

Chary and his team addressed this question by using Spitzer to survey more than 70 remote galaxies that existed 1 to 2 billion years after the Big Bang (our universe is approximately 13.7 billion years old). To their surprise, these galaxies were blazing with what is called H alpha, which is radiation from hydrogen gas that has been hit with ultraviolet light from stars. High levels of H alpha indicate stars are forming vigorously. Seventy percent of the surveyed galaxies show strong signs of H alpha. By contrast, only 0.1 percent of galaxies in our local universe possess this signature.

Previous studies using ultraviolet-light telescopes found about six times less star formation than Spitzer, which sees infrared light. Scientists think this may be due to large amounts of obscuring dust, through which infrared light can sneak. Spitzer opened a new window onto the galaxies by taking very long-exposure infrared images of a patch of sky called the GOODS fields, for Great Observatories Origins Deep Survey.

Further analyses showed that these galaxies furiously formed stars up to 100 times faster than the current star-formation rate of our Milky Way. What's more, the star formation took place over a long period of time, hundreds of millions of years. This tells astronomers that the galaxies did not grow due to mergers, or collisions, which happen on shorter timescales. While such smash-ups are common in the universe -- for example, our Milky Way will merge with the Andromeda galaxy in about 5 billion years -- the new study shows that large mergers were not the main cause of galaxy growth. Instead, the results show that distant, giant galaxies bulked up by feeding off a steady supply of gas that probably streamed in from filaments of dark matter.

Chary said, "If you could visit a planet in one of these galaxies, the sky would be a crazy place, with tons of bright stars, and fairly frequent supernova explosions."

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer Space Telescope mission for the agency's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA.

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

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'Hidden' galaxies of the universe have lower amounts of heavier elements

ScienceDaily (June 14, 2011) — A unique example of some of the lowest surface brightness galaxies in the universe have been found by an international team of astronomers lead by the Niels Bohr Institute. The galaxy has lower amounts of heavier elements than other known galaxies of this type. The discovery means that small low surface brightness galaxies may have more in common with the first galaxies formed shortly after the Big Bang than previously thought.

The results have been published in Monthly Notices of the Royal Astronomical Society Letters.

As the name implies, the galaxies are faint and therefore difficult to find and challenging to observe. The galaxy called ESO 546-G34 is a nearly 20 year old observation that no one had previously taken much notice of. The observation has now been analysed using new methods and it is only now that astronomers have realised how special it is.

"The galaxy gives us an idea of how the galaxies must have looked before star formation really got going," explains Lars Mattsson, an astrophysicist at the Dark Cosmology Centre at the Niels Bohr Institute, University of Copenhagen. The discovery was made in collaboration with astronomers at Uppsala University and the Astronomical Observatory in Kiev.

The evolution of galaxies

A galaxy consists of many millions or billions of stars. Stars are formed when giant gas clouds condense and form a ball of glowing gas -- a star. A star produces energy through the fusion of hydrogen into helium, which fuses into carbon and oxygen and further into heavier and heavier elements. The process of conversion from gases to heavier elements takes anywhere from hundreds of thousands of years to billions of years.

Most of the known galaxies that have only formed small amounts of the heavy elements are young galaxies that are undergoing gigantic outbursts of star formation. This makes them incredibly bright and easier to observe. One type of galaxy with bursts of star formation is called blue compact galaxies, as newly formed stars emit a bluish light.

'Unevolved' dwarf galaxy

The galaxy that has been observed is small and contains only extremely small amounts of the heavier elements. That it consists mostly of the gases hydrogen and helium and is so faint means that it has only just begun to form stars.

"Our analysis shows that while a large, mature galaxy like our own galaxy, the Milky Way, is composed of around 15-20 percent gas, this faint little galaxy is composed of up to 50 percent gas and is very poor in heavier elements. This means that it is very unevolved," explains Lars Mattsson.

The theory is that the very small faint galaxies collide with each other and the greater concentration of gas material and dynamical disturbance boosts star formation and thereby form the larger blue, compact galaxies.

"ESO 546-G34 is a left over dwarf galaxy that doesn't seem to have collided with other galaxies. This gives us unique insight into how the earliest galaxies in the universe may have looked," explains Lars Mattsson.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Copenhagen, via EurekAlert!, a service of AAAS.

Journal Reference:

Lars Mattsson, Leonid S. Pilyugin, Nils Bergvall. ESO 546-G34: the most-metal-poor, low surface brightness galaxy? Monthly Notices of the Royal Astronomical Society: Letters, 2011; DOI: 10.1111/j.1745-3933.2011.01077.x

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'Hidden' galaxies of the universe have lower amounts of heavier elements

ScienceDaily (June 14, 2011) — A unique example of some of the lowest surface brightness galaxies in the universe have been found by an international team of astronomers lead by the Niels Bohr Institute. The galaxy has lower amounts of heavier elements than other known galaxies of this type. The discovery means that small low surface brightness galaxies may have more in common with the first galaxies formed shortly after the Big Bang than previously thought.

The results have been published in Monthly Notices of the Royal Astronomical Society Letters.

As the name implies, the galaxies are faint and therefore difficult to find and challenging to observe. The galaxy called ESO 546-G34 is a nearly 20 year old observation that no one had previously taken much notice of. The observation has now been analysed using new methods and it is only now that astronomers have realised how special it is.

"The galaxy gives us an idea of how the galaxies must have looked before star formation really got going," explains Lars Mattsson, an astrophysicist at the Dark Cosmology Centre at the Niels Bohr Institute, University of Copenhagen. The discovery was made in collaboration with astronomers at Uppsala University and the Astronomical Observatory in Kiev.

The evolution of galaxies

A galaxy consists of many millions or billions of stars. Stars are formed when giant gas clouds condense and form a ball of glowing gas -- a star. A star produces energy through the fusion of hydrogen into helium, which fuses into carbon and oxygen and further into heavier and heavier elements. The process of conversion from gases to heavier elements takes anywhere from hundreds of thousands of years to billions of years.

Most of the known galaxies that have only formed small amounts of the heavy elements are young galaxies that are undergoing gigantic outbursts of star formation. This makes them incredibly bright and easier to observe. One type of galaxy with bursts of star formation is called blue compact galaxies, as newly formed stars emit a bluish light.

'Unevolved' dwarf galaxy

The galaxy that has been observed is small and contains only extremely small amounts of the heavier elements. That it consists mostly of the gases hydrogen and helium and is so faint means that it has only just begun to form stars.

"Our analysis shows that while a large, mature galaxy like our own galaxy, the Milky Way, is composed of around 15-20 percent gas, this faint little galaxy is composed of up to 50 percent gas and is very poor in heavier elements. This means that it is very unevolved," explains Lars Mattsson.

The theory is that the very small faint galaxies collide with each other and the greater concentration of gas material and dynamical disturbance boosts star formation and thereby form the larger blue, compact galaxies.

"ESO 546-G34 is a left over dwarf galaxy that doesn't seem to have collided with other galaxies. This gives us unique insight into how the earliest galaxies in the universe may have looked," explains Lars Mattsson.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Copenhagen, via EurekAlert!, a service of AAAS.

Journal Reference:

Lars Mattsson, Leonid S. Pilyugin, Nils Bergvall. ESO 546-G34: the most-metal-poor, low surface brightness galaxy? Monthly Notices of the Royal Astronomical Society: Letters, 2011; DOI: 10.1111/j.1745-3933.2011.01077.x

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Just four percent of galaxies have neighbors like the Milky Way

ScienceDaily (May 23, 2011) — How unique is the Milky Way?

To find out, a group of researchers led by Stanford University astrophysicist Risa Wechsler compared the Milky Way to similar galaxies and found that just four percent are like the galaxy Earth calls home.

"We are interested in how the Milky Way fits into the broader context of the universe," said Wechsler. "This research helps us understand whether our galaxy is typical or not, and may provide clues to its formation history."

The research team compared the Milky Way to similar galaxies in terms of luminosity--a measure of how much light is emitted--and distance to other bright galaxies. They found galaxies that have two satellites that are as bright and close by as the Milky Way's two closest satellites, the Large and Small Magellanic Clouds, are rare.

Published in the May 20 issue of the Astrophysical Journal, the findings are based on analyses of data collected from the Sloan Digital Sky Survey (SDSS). The work is the first of three papers that study the properties of the Milky Way's two most massive satellites.

Supported in part by the National Science Foundation (NSF), the SDSS is the most extensive survey of the optical sky performed to date.

In more than eight years of operations, SDSS has obtained images covering more than a quarter of the sky, and created 3-dimensional maps containing more than 930,000 galaxies and 120,000 quasars. For this analysis, Wechsler's group studied more than 20,000 galaxies with properties similar to the Milky Way and investigated the galaxies surrounding these Milky Way "twins," to create a "census" of galaxies similar to the Milky Way in the universe.

The work represents one of the most extensive studies of this kind ever performed.

Scientists can also compare the SDSS data to galaxies simulated by a computer model. Since they are currently unable to see all the way back to the Big Bang, this is one way researchers are trying to understand how the universe as we see it today began.

In order to learn more about possible conditions in the early universe, the group performed computer simulations to recreate the universe from specific sets of starting conditions. Then they compared their simulations to the SDSS data set. In this way, the group was able to test different theories of galaxy formation to determine whether or not each would result in a universe that matches what we see today. The results of their simulation matched the result found in the SDSS data set: just four percent of the simulated galaxies had two satellites like the Magellanic Clouds.

"This is an excellent example of data-enabled science," said Nigel Sharp, of NSF's Division of Astronomical Sciences. "Comparing the 'fake' and 'real' Universes is how we discriminate between successful and unsuccessful theories. This work interconnects three of the four legs of science: theory, observation and simulation, for a powerful scientific result."

Their results also lend support to a leading theory of galaxy formation called the Cold Dark Matter (CDM) theory. This theory provides what many consider to be the simplest explanation for the arrangement of galaxies throughout the universe following the Big Bang. It assumes that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation (dark) and whose constituent particles move slowly (cold). Dark matter, an invisible and exotic material of unknown composition, is believed to influence the distribution of galaxies in space and the overall expansion of the universe. The rareness of this aspect of the Milky Way may provide clues to its formation history.

"Because the presence of two galaxies like the Magellanic Clouds is unusual, we can use them to learn more about our own galaxy," said Wechsler. Using their simulation, the team identified a sample of simulated galaxies that had satellites matching the Milky Way's in terms of their locations and speeds.

"The combination of large surveys of the sky like the SDSS and large samples of simulated galaxies provides a new opportunity to learn about the place of our galaxy in the Universe," said Wechsler. "Future surveys will allow us to extend this study to even dimmer satellite galaxies, to build a full picture of the formation of our galaxy."

The theoretical and numerical work that produced the simulations used as a comparison for the SDSS data were supported by an award funded under the American Recovery and Reinvestment Act of 2009.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by National Science Foundation.

Journal Reference:

Lulu Liu, Brian F. Gerke, Risa H. Wechsler, Peter S. Behroozi, Michael T. Busha. How Common Are the Magellanic Clouds? The Astrophysical Journal, 2011; 733 (1): 62 DOI: 10.1088/0004-637X/733/1/62

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'Dead' galaxies aren't so dead after all

ScienceDaily (June 1, 2011) — University of Michigan astronomers examined old galaxies and were surprised to discover that they are still making new stars. The results provide insights into how galaxies evolve with time. U-M research fellow Alyson Ford and astronomy professor Joel Bregman presented their findings May 31 at a meeting of the Canadian Astronomical Society in London, Ontario.

Using the Wide Field Camera 3 on the Hubble Space Telescope, they saw individual young stars and star clusters in four galaxies that are about 40 million light years away. One light year is about 5.9 trillion miles.

"Scientists thought these were dead galaxies that had finished making stars a long time ago," Ford said. "But we've shown that they are still alive and are forming stars at a fairly low level."

Galaxies generally come in two types: spiral galaxies, like our own Milky Way, and elliptical galaxies. The stars in spiral galaxies lie in a disk that also contains cold, dense gas, from which new stars are regularly formed at a rate of about one sun per year.

Stars in elliptical galaxies, on the other hand, are nearly all billions of years old. These galaxies contain stars that orbit every which way, like bees around a beehive. Ellipticals have little, if any, cold gas, and no star formation was known.

"Astronomers previously studied star formation by looking at all of the light from an elliptical galaxy at once, because we usually can't see individual stars," Ford said. "Our trick is to make sensitive ultraviolet images with the Hubble Space Telescope, which allows us to see individual stars."

The technique enabled the astronomers to observe star formation, even if it is as little as one sun every 100,000 years.

Ford and Bregman are working to understand the stellar birth rate and likelihood of stars forming in groups within ellipticals. In the Milky Way, stars usually form in associations containing from tens to 100,000 stars. In elliptical galaxies, conditions are different because there is no disk of cold material to form stars.

"We were confused by some of the colors of objects in our images until we realized that they must be star clusters, so most of the star formation happens in associations," Ford said.

The team's breakthrough came when they observed Messier 105, a normal elliptical galaxy that is 34 million light years away, in the constellation Leo. Though there had been no previous indication of star formation in Messier 105, Ford and Bregman saw a few bright, very blue stars, resembling a single star 10 to 20 times the mass of the sun.

They also saw objects that aren't blue enough to be single stars, but instead are clusters of many stars. When accounting for these clusters, stars are forming in Messier 105 at an average rate of one sun every 10,000 years, Ford and Bregman concluded. "This is not just a burst of star formation but a continuous process," Ford said.

These findings raise new mysteries, such as the origin of the gas that forms the stars.

"We're at the beginning of a new line of research, which is very exciting, but at times confusing," Bregman said. "We hope to follow up this discovery with new observations that will really give us insight into the process of star formation in these 'dead' galaxies."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Michigan.

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Why biggest stellar explosions often happen in tiniest galaxies: Ultraviolet probe sheds light on mystery

ScienceDaily (Apr. 21, 2011) — Astronomers using NASA's Galaxy Evolution Explorer may be closer to knowing why some of the most massive stellar explosions ever observed occur in the tiniest of galaxies.

"It's like finding a sumo wrestler in a little 'Smart Car,'" said Don Neill, a member of NASA's Galaxy Evolution Explorer team at the California Institute of Technology in Pasadena, and lead author of a new study published in the Astrophysical Journal.

"The most powerful explosions of massive stars are happening in extremely low-mass galaxies. New data are revealing that the stars that start out massive in these little galaxies stay massive until they explode, while in larger galaxies they are whittled away as they age, and are less massive when they explode," said Neill.

Over the past few years, astronomers using data from the Palomar Transient Factory, a sky survey based at the ground-based Palomar Observatory near San Diego, have discovered a surprising number of exceptionally bright stellar explosions in so-called dwarf galaxies up to 1,000 times smaller than our Milky Way galaxy. Stellar explosions, called supernovae, occur when massive stars -- some up to 100 times the mass of our sun -- end their lives.

The Palomar observations may explain a mystery first pointed out by Neil deGrasse Tyson and John Scalo when they were at the University of Austin Texas (Tyson is now the director of the Hayden Planetarium in New York, N.Y.). They noted that supernovae were occurring where there seemed to be no galaxies at all, and they even proposed that dwarf galaxies were the culprits, as the Palomar data now indicate.

Now, astronomers are using ultraviolet data from the Galaxy Evolution Explorer to further examine the dwarf galaxies. Newly formed stars tend to radiate copious amounts of ultraviolet light, so the Galaxy Evolution Explorer, which has scanned much of the sky in ultraviolet light, is the ideal tool for measuring the rate of star birth in galaxies.

The results show that the little galaxies are low in mass, as suspected, and have low rates of star formation. In other words, the petite galaxies are not producing that many huge stars.

"Even in these little galaxies where the explosions are happening, the big guys are rare," said co-author Michael Rich of UCLA, who is a member of the mission team.

In addition, the new study helps explain why massive stars in little galaxies undergo even more powerful explosions than stars of a similar heft in larger galaxies like our Milky Way. The reason is that low-mass galaxies tend to have fewer heavy atoms, such as carbon and oxygen, than their larger counterparts. These small galaxies are younger, and thus their stars have had less time to enrich the environment with heavy atoms.

According to Neill and his collaborators, the lack of heavy atoms in the atmosphere around a massive star causes it to shed less material as it ages. In essence, the massive stars in little galaxies are fatter in their old age than the massive stars in larger galaxies. And the fatter the star, the bigger the blast that will occur when it finally goes supernova. This, according to the astronomers, may explain why super supernovae are occurring in the not-so-super galaxies.

"These stars are like heavyweight champions, breaking all the records," said Neill.

Added Rich, "These dwarf galaxies are especially interesting to astronomers, because they are quite similar to the kinds of galaxies that may have been present in our young universe, shortly after the Big Bang. The Galaxy Evolution Explorer has given us a powerful tool for learning what galaxies were like when the universe was just a child."

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory in Pasadena manages the mission and built the science instrument. Caltech manages JPL for NASA. The mission was developed under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. Researchers sponsored by Yonsei University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France collaborated on this mission.

Graphics and additional information about the Galaxy Evolution Explorer are online at http://www.nasa.gov/galex/ and http://www.galex.caltech.edu .

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by NASA/Jet Propulsion Laboratory.

Journal Reference:

James D. Neill, Mark Sullivan, Avishay Gal-Yam, Robert Quimby, Eran Ofek, Ted K. Wyder, D. Andrew Howell, Peter Nugent, Mark Seibert, D. Christopher Martin, Roderik Overzier, Tom A. Barlow, Karl Foster, Peter G. Friedman, Patrick Morrissey, Susan G. Neff, David Schiminovich, Luciana Bianchi, José Donas, Timothy M. Heckman, Young-Wook Lee, Barry F. Madore, Bruno Milliard, R. Michael Rich, Alex S. Szalay. The Extreme Hosts of Extreme Supernovae. The Astrophysical Journal, 2011; 727 (1): 15 DOI: 10.1088/0004-637X/727/1/15

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