Wednesday, July 11, 2012 1:10:24 AM
Dark matter’s tendrils revealed
Map of galaxy clusters Abell 222 and 223, with a filament of dark matter connecting them. The contours and the blue color represent the distribution of mass, as determined by weak lensing.
Jörg Dietrich, University of Michigan/University Observatory Munich
[ http://arstechnica.com/science/2012/07/dark-matter-is-the-thread-connecting-galaxy-clusters/ ]
Dark-matter filaments, such as the one bridging the galaxy clusters Abell 222 and Abell 223, are predicted to contain more than half of all matter in the Universe.
Jörg Dietrich, University of Michigan/University Observatory Munich
Direct measurement of a dark-matter ‘filament’ confirms its existence in a galaxy supercluster.
Zeeya Merali
04 July 2012
A ‘finger’ of the Universe’s dark-matter skeleton, which ultimately dictates where galaxies form, has been observed for the first time. Researchers have directly detected a slim bridge of dark matter joining two clusters of galaxies, using a technique that could eventually help astrophysicists to understand the structure of the Universe and identify what makes up the mysterious invisible substance known as dark matter.
According to the standard model of cosmology, visible stars and galaxies trace a pattern across the sky known as the cosmic web, which was originally etched out by dark matter — the substance thought to account for almost 80% of the Universe’s matter. Soon after the Big Bang, regions that were slightly denser than others pulled in dark matter, which clumped together and eventually collapsed into flat ‘pancakes’. “Where these pancakes intersect, you get long strands of dark matter, or filaments,” explains Jörg Dietrich, a cosmologist at the University Observatory Munich in Germany. Clusters of galaxies then formed at the nodes of the cosmic web, where these filaments crossed.
The presence of dark matter is usually inferred by the way its strong gravity bends light travelling from distant galaxies that lie behind it — distorting their apparent shapes as seen by telescopes on Earth. But it is difficult to observe this 'gravitational lensing' by dark matter in filaments because they contain relatively little mass.
Dietrich and his colleagues got around this problem by studying a particularly massive filament, 18 megaparsecs long, that bridges the galaxy clusters Abell 222 and Abell 223. Luckily, this dark bridge is oriented so that most of its mass lies along the line of sight to Earth, enhancing the lensing effect, explains Dietrich. The team examined the distortion of more than 40,000 background galaxies, and calculated that the mass in the filament is between 6.5 × 10\13 and 9.8 × 10\13 times the mass of the Sun. Their results are reported in Nature today1.
Mass equation
By examining X-rays from plasma in the filament, observed by the XMM-Newton spacecraft2, the team calculated that no more than 9% of the filament's mass could be made up of hot gas. The team's computer simulations suggest that roughly another 10% of the mass could be due to visible stars and galaxies. The bulk, therefore, must be dark matter, says Dietrich.
Mark Bautz, an astrophysicist at the Massachusetts Institute of Technology in Cambridge, notes that astrophysicists do not know precisely how visible matter follows the paths laid out by dark matter. “What’s exciting is that in this unusual system we can map both dark matter and visible matter together and try to figure out how they connect and evolve along the filament,” he says. Japan’s Astro-H X-ray space telescope, due to launch in 2014, will be able to characterize the ionization state and temperature of the plasma in the filament, which will help to discriminate between different models of how the structure formed.
Refining the technique could also help to pin down the identity of dark matter — whether it is a cold (slow-moving) particle or a warm (fast-moving) one, like a neutrino — because different particles will clump differently along the filament. The Euclid space mission, due to launch in 2019, will provide more lensing data. “This will complement direct dark-matter searches, for example at the Large Hadron Collider,” says Alexandre Refregier, a cosmologist at ETH Zurich, the Swiss Federal Institute of Technology in Zurich.
Nature | doi:10.1038/nature.2012.10951 [ http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11224.html ( http://dx.doi.org/10.1038/nature11224 )]
*
Related stories and links
From nature.com
Galaxy clusters caught in motion
29 June 2012
http://www.nature.com/doifinder/10.1038/nature.2012.10920
Gran Sasso: Chamber of physics
23 May 2012
http://www.nature.com/doifinder/10.1038/485435a
Survey finds no hint of dark matter near Solar System
19 April 2012
http://www.nature.com/doifinder/10.1038/nature.2012.10494
Physicists hunt for dark forces
03 April 2012
http://www.nature.com/doifinder/10.1038/484013a
From elsewhere
Jörg Dietrich
http://www.usm.lmu.de/people/dietrich/index.html
Mark Bautz
http://spie.org/app/profiles/viewer.aspx?profile=ZILSSO
Alexandre Refregier
http://www.astro.ethz.ch/refregier/
*
© 2012 Nature Publishing Group, a division of Macmillan Publishers Limited
http://www.nature.com/news/dark-matter-s-tendrils-revealed-1.10951 [with comments]
===
Hubble Unmasks Ghost Galaxies
Astronomers used the NASA/ESA Hubble Space Telescope to unmask the dim, star-starved dwarf galaxy Leo IV. This Hubble image demonstrates why astronomers had a tough time spotting this small-fry galaxy: it is practically invisible. The image shows how the handful of stars from the sparse galaxy are virtually indistinguishable from the background.
Residing 500 000 light-years from Earth, Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around our Milky Way galaxy. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the Universe’s mass.
Credit: NASA, ESA, and T. Brown (STScI)
Astronomers used the NASA/ESA Hubble Space Telescope to unmask the dim, star-starved dwarf galaxy Leo IV. These Hubble images demonstrate why astronomers had a tough time spotting this small-fry galaxy.
The image at left shows part of the galaxy, outlined by the white rectangular box. The box measures 83 light-years wide by 163 light-years long. The few stars in Leo IV are lost amid neighboring stars and distant galaxies.
A close-up view of the background galaxies within the box is shown in the middle image.
The image at right shows only the stars in Leo IV. The galaxy, which contains several thousand stars, is composed of Sun-like stars, fainter, red dwarf stars, and some red giant stars brighter than the Sun. Astronomers discovered Leo IV in Sloan Digital Sky Survey images by spotting a region where a clump of stars was huddled closer together than stars in areas around it.
Residing 500 000 light-years from Earth, Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around our Milky Way galaxy. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the Universe’s mass.
Credit: NASA, ESA, and T. Brown (STScI)
These images reveal the small and faint star-starved dwarf galaxy, Leo IV, a close neighbour of our Milky Way galaxy.
Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around the Milky Way. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the universe’s mass.
The wide image, taken by the Sloan Digital Sky Survey, is a view of Leo IV and the surrounding neighbourhood. The galaxy resides 500 000 light-years from Earth. The dotted line marks the galaxy’s boundaries, measuring about 1100 light-years wide. The small white box outlines the Hubble Space Telescope’s view.
Leo IV has so few stars, roughly several thousand, that astronomers had difficulty identifying it as a galaxy. Astronomers discovered Leo IV in Sloan Digital Sky Survey images by spotting a region where a clump of stars was huddled closer together than stars in areas around it. The dwarf galaxy is composed of Sun-like stars, fainter, red dwarf stars, and some red giant stars brighter than the Sun.
Hubble’s close-up view is shown in the inset at right, measuring 483 light-years wide. Astronomers used Hubble to measure the ages of the stars in Leo IV and two other ultra-faint dwarf galaxies.
The measurements revealed that the stars in all three galaxies are more than 13 billion years old, almost as old as the 13.7-billion-year-old universe. Because the stars in these galaxies are so ancient and share the same age, astronomers suggest that a global event, such as reionisation, shut down star formation in them.
Reionisation is a transitional phase in the early universe when the first stars burned off a fog of cold hydrogen.
The Hubble image is a composite of exposures taken in January 2012 by the Advanced Camera for Surveys.
Credit: NASA, ESA, and T. Brown (STScI)
These illustrations, taken from computer simulations, show a swarm of dark matter clumps around our Milky Way galaxy. Some of the dark-matter concentrations are massive enough to spark star formation. Dark matter is an invisible substance that accounts for most of the universe’s mass.
In the first panel, thousands of clumps of dark matter coexist with our Milky Way galaxy, shown in the center.
The green blobs in the second panel are those dark-matter chunks massive enough to obtain gas from the intergalactic medium and trigger ongoing star formation, eventually creating dwarf galaxies.
In the third panel, the red blobs are ultra-faint dwarf galaxies that stopped forming stars long ago. New Hubble Space Telescope observations of three of the puny galaxies reveal that star-making in these faint galaxies shut down more than 13 billion years ago.
The synchronised shutdown is evidence that a global event, such as reionisation, swept through the early universe. Reionisation is a transitional phase in the early universe when the first stars burned off a fog of cold hydrogen.
Popular theory predicts that most of the Milky Way’s satellites contain few, if any, stars and are instead dominated by dark matter. More than a dozen small-fry galaxies have been found so far, all by the Sloan Digital Sky Survey, which scanned just a quarter of the sky.
Credit: J. Tumlinson (STScI)
heic1211 - Science Release
10 July 2012
Astronomers have used the NASA/ESA Hubble Space Telescope to study some of the smallest and faintest galaxies in our cosmic neighbourhood. These galaxies are fossils of the early Universe: they have barely changed for 13 billion years. The discovery could help explain the so-called “missing satellite” problem, where only a handful of satellite galaxies have been found around the Milky Way, against the thousands that are predicted by theories.
Astronomers have puzzled over why some extremely faint dwarf galaxies spotted in our Milky Way galaxy’s backyard contain so few stars. The galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the Universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey. But an international team of astronomers needed the NASA/ESA Hubble Space Telescope to help solve the mystery of why these galaxies are starved of stars, and why so few of them have been found.
Hubble views of three of these small galaxies, the Hercules, Leo IV and Ursa Major dwarf galaxies, reveal that they all started forming stars more than 13 billion years ago — and then abruptly stopped — all in the first billion years after the Universe was born in the Big Bang. In fact, the extreme age of their stars is similar to Messier 92, the oldest known globular cluster [1] in the Milky Way.
“These galaxies are all ancient and they’re all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies,” said Tom Brown of the Space Telescope Science Institute in Baltimore, USA, the study’s leader. “The most likely explanation is a process called reionisation.”
The relic galaxies are evidence for a transitional phase in the early Universe that shut down star-making factories in tiny galaxies. This phase seems to coincide with the time when the first stars burned off a fog of cold hydrogen, a process called reionisation. In this period, which began in the first billion years after the Big Bang, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionising the Universe’s cool hydrogen gas. [2]
The same radiation that sparked universal reionisation also appears to have squelched star-making activities in dwarf galaxies, such as those in Brown’s study. The small irregular galaxies were born about 100 million years before reionisation began and had just started to churn out stars at that time. Roughly 2000 light-years wide, these galaxies are the lightweight cousins of the more luminous and higher-mass star-making dwarf galaxies near our Milky Way. Unlike their higher-mass relatives, the puny galaxies were not massive enough to shield themselves from the harsh ultraviolet light. What little gas they had was stripped away as the flood of ultraviolet light rushed through them. Their gas supply depleted, the galaxies could not make new stars.
The discovery could help explain the so-called “missing satellite problem,” where only a few dozen dwarf galaxies have been observed around the Milky Way while the computer simulations predict that thousands should exist. One possible explanation for the low number discovered to date is that there has been very little, or even no star formation in the smallest of these dwarf galaxies, leaving them virtually invisible.
The Sloan survey recently uncovered more than a dozen of these galaxies in our cosmic neighbourhood. These have very few stars — only a few hundred or thousand — but a great deal of dark matter, the underlying scaffolding upon which galaxies are built. Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas and stars, while in these so-called ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100. Astronomers think the rest of the sky should contain dozens more of these ultra-faint dwarf galaxies with few stars, and the evidence for squelched star formation in the smallest of these dwarfs suggests that there may be still thousands more with essentially no stars at all.
“By measuring the star formation histories of the observed dwarfs, Hubble has supported the theoretical explanation for the paucity of such objects, according to which star formation in the smaller clumps would be shut down by reionisation,” said Jason Tumlinson of the Space Telescope Science Institute, a member of the research team.
The results are published in the 1 July issue of The Astrophysical Journal Letters.
Notes
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The international team of astronomers in this study consists of T. M. Brown (STScI), J. Tumlinson (STScI), M. Geha (Yale), E. N. Kirby (Cal Tech), D. A. VandenBerg (University of Victoria), R. R. Munoz (University of Chile), J. S. Kalirai (STScI), J. D. Simon (Carnegie Institution), R. J. Avila (STScI), P. Guhathakurta (UCO/Lick), A. Renzini (Osservatorio Astronomico di Padova) and, H. C. Ferguson (STScI)
[1] Globular clusters are tightly-bound spherical collections of up to a few hundred thousand stars. They are known to contain some of the most ancient stars in the Universe. In addition, globular clusters are known to have formed in single events, so all the stars in them have the same age.
[2] The period of reionisation is also the limit for how far telescopes can see: the process is what rendered the Universe’s hydrogen gas transparent to ultraviolet light.
Links
Science paper
http://www.spacetelescope.org/static/archives/releases/science_papers/heic1211.pdf
Images of Hubble
http://www.spacetelescope.org/images/archive/category/spacecraft/
Contacts
Tom Brown
Space Telescope Science Institute
Baltimore, USA
Tel: +1-410-338-4902
Email: tbrown@stsci.edu
Alvio Renzini
Osservatorio Astronomico di Padova
Padova, Italy
Tel: +39-049-829-3503
Cell: +39-347-502-2828
Email: alvio.renzini@oapd.inaf.it
Oli Usher
Hubble/ESA
Garching, Germany
Tel: +49-89-3200-6855
Email: ousher@eso.org
http://www.spacetelescope.org/news/heic1211/
===
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Map of galaxy clusters Abell 222 and 223, with a filament of dark matter connecting them. The contours and the blue color represent the distribution of mass, as determined by weak lensing.
Jörg Dietrich, University of Michigan/University Observatory Munich
[ http://arstechnica.com/science/2012/07/dark-matter-is-the-thread-connecting-galaxy-clusters/ ]
Dark-matter filaments, such as the one bridging the galaxy clusters Abell 222 and Abell 223, are predicted to contain more than half of all matter in the Universe.
Jörg Dietrich, University of Michigan/University Observatory Munich
Direct measurement of a dark-matter ‘filament’ confirms its existence in a galaxy supercluster.
Zeeya Merali
04 July 2012
A ‘finger’ of the Universe’s dark-matter skeleton, which ultimately dictates where galaxies form, has been observed for the first time. Researchers have directly detected a slim bridge of dark matter joining two clusters of galaxies, using a technique that could eventually help astrophysicists to understand the structure of the Universe and identify what makes up the mysterious invisible substance known as dark matter.
According to the standard model of cosmology, visible stars and galaxies trace a pattern across the sky known as the cosmic web, which was originally etched out by dark matter — the substance thought to account for almost 80% of the Universe’s matter. Soon after the Big Bang, regions that were slightly denser than others pulled in dark matter, which clumped together and eventually collapsed into flat ‘pancakes’. “Where these pancakes intersect, you get long strands of dark matter, or filaments,” explains Jörg Dietrich, a cosmologist at the University Observatory Munich in Germany. Clusters of galaxies then formed at the nodes of the cosmic web, where these filaments crossed.
The presence of dark matter is usually inferred by the way its strong gravity bends light travelling from distant galaxies that lie behind it — distorting their apparent shapes as seen by telescopes on Earth. But it is difficult to observe this 'gravitational lensing' by dark matter in filaments because they contain relatively little mass.
Dietrich and his colleagues got around this problem by studying a particularly massive filament, 18 megaparsecs long, that bridges the galaxy clusters Abell 222 and Abell 223. Luckily, this dark bridge is oriented so that most of its mass lies along the line of sight to Earth, enhancing the lensing effect, explains Dietrich. The team examined the distortion of more than 40,000 background galaxies, and calculated that the mass in the filament is between 6.5 × 10\13 and 9.8 × 10\13 times the mass of the Sun. Their results are reported in Nature today1.
Mass equation
By examining X-rays from plasma in the filament, observed by the XMM-Newton spacecraft2, the team calculated that no more than 9% of the filament's mass could be made up of hot gas. The team's computer simulations suggest that roughly another 10% of the mass could be due to visible stars and galaxies. The bulk, therefore, must be dark matter, says Dietrich.
Mark Bautz, an astrophysicist at the Massachusetts Institute of Technology in Cambridge, notes that astrophysicists do not know precisely how visible matter follows the paths laid out by dark matter. “What’s exciting is that in this unusual system we can map both dark matter and visible matter together and try to figure out how they connect and evolve along the filament,” he says. Japan’s Astro-H X-ray space telescope, due to launch in 2014, will be able to characterize the ionization state and temperature of the plasma in the filament, which will help to discriminate between different models of how the structure formed.
Refining the technique could also help to pin down the identity of dark matter — whether it is a cold (slow-moving) particle or a warm (fast-moving) one, like a neutrino — because different particles will clump differently along the filament. The Euclid space mission, due to launch in 2019, will provide more lensing data. “This will complement direct dark-matter searches, for example at the Large Hadron Collider,” says Alexandre Refregier, a cosmologist at ETH Zurich, the Swiss Federal Institute of Technology in Zurich.
Nature | doi:10.1038/nature.2012.10951 [ http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11224.html ( http://dx.doi.org/10.1038/nature11224 )]
*
Related stories and links
From nature.com
Galaxy clusters caught in motion
29 June 2012
http://www.nature.com/doifinder/10.1038/nature.2012.10920
Gran Sasso: Chamber of physics
23 May 2012
http://www.nature.com/doifinder/10.1038/485435a
Survey finds no hint of dark matter near Solar System
19 April 2012
http://www.nature.com/doifinder/10.1038/nature.2012.10494
Physicists hunt for dark forces
03 April 2012
http://www.nature.com/doifinder/10.1038/484013a
From elsewhere
Jörg Dietrich
http://www.usm.lmu.de/people/dietrich/index.html
Mark Bautz
http://spie.org/app/profiles/viewer.aspx?profile=ZILSSO
Alexandre Refregier
http://www.astro.ethz.ch/refregier/
*
© 2012 Nature Publishing Group, a division of Macmillan Publishers Limited
http://www.nature.com/news/dark-matter-s-tendrils-revealed-1.10951 [with comments]
===
Hubble Unmasks Ghost Galaxies
Astronomers used the NASA/ESA Hubble Space Telescope to unmask the dim, star-starved dwarf galaxy Leo IV. This Hubble image demonstrates why astronomers had a tough time spotting this small-fry galaxy: it is practically invisible. The image shows how the handful of stars from the sparse galaxy are virtually indistinguishable from the background.
Residing 500 000 light-years from Earth, Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around our Milky Way galaxy. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the Universe’s mass.
Credit: NASA, ESA, and T. Brown (STScI)
Astronomers used the NASA/ESA Hubble Space Telescope to unmask the dim, star-starved dwarf galaxy Leo IV. These Hubble images demonstrate why astronomers had a tough time spotting this small-fry galaxy.
The image at left shows part of the galaxy, outlined by the white rectangular box. The box measures 83 light-years wide by 163 light-years long. The few stars in Leo IV are lost amid neighboring stars and distant galaxies.
A close-up view of the background galaxies within the box is shown in the middle image.
The image at right shows only the stars in Leo IV. The galaxy, which contains several thousand stars, is composed of Sun-like stars, fainter, red dwarf stars, and some red giant stars brighter than the Sun. Astronomers discovered Leo IV in Sloan Digital Sky Survey images by spotting a region where a clump of stars was huddled closer together than stars in areas around it.
Residing 500 000 light-years from Earth, Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around our Milky Way galaxy. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the Universe’s mass.
Credit: NASA, ESA, and T. Brown (STScI)
These images reveal the small and faint star-starved dwarf galaxy, Leo IV, a close neighbour of our Milky Way galaxy.
Leo IV is one of more than a dozen ultra-faint dwarf galaxies found lurking around the Milky Way. These galaxies are dominated by dark matter, an invisible substance that makes up the bulk of the universe’s mass.
The wide image, taken by the Sloan Digital Sky Survey, is a view of Leo IV and the surrounding neighbourhood. The galaxy resides 500 000 light-years from Earth. The dotted line marks the galaxy’s boundaries, measuring about 1100 light-years wide. The small white box outlines the Hubble Space Telescope’s view.
Leo IV has so few stars, roughly several thousand, that astronomers had difficulty identifying it as a galaxy. Astronomers discovered Leo IV in Sloan Digital Sky Survey images by spotting a region where a clump of stars was huddled closer together than stars in areas around it. The dwarf galaxy is composed of Sun-like stars, fainter, red dwarf stars, and some red giant stars brighter than the Sun.
Hubble’s close-up view is shown in the inset at right, measuring 483 light-years wide. Astronomers used Hubble to measure the ages of the stars in Leo IV and two other ultra-faint dwarf galaxies.
The measurements revealed that the stars in all three galaxies are more than 13 billion years old, almost as old as the 13.7-billion-year-old universe. Because the stars in these galaxies are so ancient and share the same age, astronomers suggest that a global event, such as reionisation, shut down star formation in them.
Reionisation is a transitional phase in the early universe when the first stars burned off a fog of cold hydrogen.
The Hubble image is a composite of exposures taken in January 2012 by the Advanced Camera for Surveys.
Credit: NASA, ESA, and T. Brown (STScI)
These illustrations, taken from computer simulations, show a swarm of dark matter clumps around our Milky Way galaxy. Some of the dark-matter concentrations are massive enough to spark star formation. Dark matter is an invisible substance that accounts for most of the universe’s mass.
In the first panel, thousands of clumps of dark matter coexist with our Milky Way galaxy, shown in the center.
The green blobs in the second panel are those dark-matter chunks massive enough to obtain gas from the intergalactic medium and trigger ongoing star formation, eventually creating dwarf galaxies.
In the third panel, the red blobs are ultra-faint dwarf galaxies that stopped forming stars long ago. New Hubble Space Telescope observations of three of the puny galaxies reveal that star-making in these faint galaxies shut down more than 13 billion years ago.
The synchronised shutdown is evidence that a global event, such as reionisation, swept through the early universe. Reionisation is a transitional phase in the early universe when the first stars burned off a fog of cold hydrogen.
Popular theory predicts that most of the Milky Way’s satellites contain few, if any, stars and are instead dominated by dark matter. More than a dozen small-fry galaxies have been found so far, all by the Sloan Digital Sky Survey, which scanned just a quarter of the sky.
Credit: J. Tumlinson (STScI)
heic1211 - Science Release
10 July 2012
Astronomers have used the NASA/ESA Hubble Space Telescope to study some of the smallest and faintest galaxies in our cosmic neighbourhood. These galaxies are fossils of the early Universe: they have barely changed for 13 billion years. The discovery could help explain the so-called “missing satellite” problem, where only a handful of satellite galaxies have been found around the Milky Way, against the thousands that are predicted by theories.
Astronomers have puzzled over why some extremely faint dwarf galaxies spotted in our Milky Way galaxy’s backyard contain so few stars. The galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the Universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey. But an international team of astronomers needed the NASA/ESA Hubble Space Telescope to help solve the mystery of why these galaxies are starved of stars, and why so few of them have been found.
Hubble views of three of these small galaxies, the Hercules, Leo IV and Ursa Major dwarf galaxies, reveal that they all started forming stars more than 13 billion years ago — and then abruptly stopped — all in the first billion years after the Universe was born in the Big Bang. In fact, the extreme age of their stars is similar to Messier 92, the oldest known globular cluster [1] in the Milky Way.
“These galaxies are all ancient and they’re all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies,” said Tom Brown of the Space Telescope Science Institute in Baltimore, USA, the study’s leader. “The most likely explanation is a process called reionisation.”
The relic galaxies are evidence for a transitional phase in the early Universe that shut down star-making factories in tiny galaxies. This phase seems to coincide with the time when the first stars burned off a fog of cold hydrogen, a process called reionisation. In this period, which began in the first billion years after the Big Bang, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionising the Universe’s cool hydrogen gas. [2]
The same radiation that sparked universal reionisation also appears to have squelched star-making activities in dwarf galaxies, such as those in Brown’s study. The small irregular galaxies were born about 100 million years before reionisation began and had just started to churn out stars at that time. Roughly 2000 light-years wide, these galaxies are the lightweight cousins of the more luminous and higher-mass star-making dwarf galaxies near our Milky Way. Unlike their higher-mass relatives, the puny galaxies were not massive enough to shield themselves from the harsh ultraviolet light. What little gas they had was stripped away as the flood of ultraviolet light rushed through them. Their gas supply depleted, the galaxies could not make new stars.
The discovery could help explain the so-called “missing satellite problem,” where only a few dozen dwarf galaxies have been observed around the Milky Way while the computer simulations predict that thousands should exist. One possible explanation for the low number discovered to date is that there has been very little, or even no star formation in the smallest of these dwarf galaxies, leaving them virtually invisible.
The Sloan survey recently uncovered more than a dozen of these galaxies in our cosmic neighbourhood. These have very few stars — only a few hundred or thousand — but a great deal of dark matter, the underlying scaffolding upon which galaxies are built. Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas and stars, while in these so-called ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100. Astronomers think the rest of the sky should contain dozens more of these ultra-faint dwarf galaxies with few stars, and the evidence for squelched star formation in the smallest of these dwarfs suggests that there may be still thousands more with essentially no stars at all.
“By measuring the star formation histories of the observed dwarfs, Hubble has supported the theoretical explanation for the paucity of such objects, according to which star formation in the smaller clumps would be shut down by reionisation,” said Jason Tumlinson of the Space Telescope Science Institute, a member of the research team.
The results are published in the 1 July issue of The Astrophysical Journal Letters.
Notes
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The international team of astronomers in this study consists of T. M. Brown (STScI), J. Tumlinson (STScI), M. Geha (Yale), E. N. Kirby (Cal Tech), D. A. VandenBerg (University of Victoria), R. R. Munoz (University of Chile), J. S. Kalirai (STScI), J. D. Simon (Carnegie Institution), R. J. Avila (STScI), P. Guhathakurta (UCO/Lick), A. Renzini (Osservatorio Astronomico di Padova) and, H. C. Ferguson (STScI)
[1] Globular clusters are tightly-bound spherical collections of up to a few hundred thousand stars. They are known to contain some of the most ancient stars in the Universe. In addition, globular clusters are known to have formed in single events, so all the stars in them have the same age.
[2] The period of reionisation is also the limit for how far telescopes can see: the process is what rendered the Universe’s hydrogen gas transparent to ultraviolet light.
Links
Science paper
http://www.spacetelescope.org/static/archives/releases/science_papers/heic1211.pdf
Images of Hubble
http://www.spacetelescope.org/images/archive/category/spacecraft/
Contacts
Tom Brown
Space Telescope Science Institute
Baltimore, USA
Tel: +1-410-338-4902
Email: tbrown@stsci.edu
Alvio Renzini
Osservatorio Astronomico di Padova
Padova, Italy
Tel: +39-049-829-3503
Cell: +39-347-502-2828
Email: alvio.renzini@oapd.inaf.it
Oli Usher
Hubble/ESA
Garching, Germany
Tel: +49-89-3200-6855
Email: ousher@eso.org
http://www.spacetelescope.org/news/heic1211/
===
(linked in):
http://investorshub.advfn.com/boards/read_msg.aspx?message_id=63486312 and preceding and following
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Greensburg, KS - 5/4/07
"Eternal vigilance is the price of Liberty."
from John Philpot Curran, Speech
upon the Right of Election, 1790
F6
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