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Small Galaxy Springs 'Dark Matter' Surprises
Posted: Monday, July 28, 2003
Source: CSIRO Australia
Astronomers from the University of Cambridge, UK, have found for the first time the true outer limits of a galaxy. They have also shown that the dark matter in this galaxy is not distributed in the way conventional theory predicts.
The team - Professor Gerry Gilmore, Dr Mark Wilkinson, Dr Jan Kleyna and Dr Wyn Evans - presents its results today at the 25th General Assembly of the International Astronomical Union in Sydney, Australia. The work could provide the key to understanding how larger galaxies were formed, including our own Milky Way galaxy.
The researchers studied rare 'dwarf spheroidal' galaxies. These have few visible stars but contain massive amounts of 'dark matter' - a mysterious kind of matter that does not emit its own light or radiation, and therefore cannot be directly observed by astronomers. However, dark matter can be detected by the gravitational pull it exerts on visible objects such as stars.
Astronomers think that dwarf spheroidal galaxies may be the building blocks from which larger, mainstream galaxies were formed.
Some of the dwarf spheroidals - those in our 'Local Group' of galaxies - are close enough for astronomers to be able to trace the movements of their individual stars.
A galaxy is held together by the combined gravity of its stars and dark matter. By studying the motion of stars in some of the dwarf spheroidal galaxies, the researchers have created a picture of how the mass of each galaxy is distributed.
In one dwarf spheroidal, found in the constellation Ursa Minor, the team found a clump of slow-moving stars near the galaxy's centre. They interpreted this clump as the remains of a group of stars known as a globular cluster.
This group of stars flies in the face of the most popular model for how dark matter is distributed in galaxies. The 'lambda cold dark matter' model, which explains very well the large-scale structures in the Universe, predicts that dark matter rapidly increases in density towards the centre of a galaxy. If dark matter were distributed in this way in the Ursa Minor dwarf spheroidal galaxy, the star cluster would have been dispersed. The cluster's existence shows that the dark matter is in fact distributed differently in this galaxy.
Furthermore, additional research into the Ursa Minor dwarf spheroidal has revealed the true edge of that galaxy - the point at which the dark matter stops. In most galaxies the way the stars move indicates that the dark matter extends far beyond the visible starry regions. In the Ursa Minor dwarf spheroidal, however, the stars in its very outermost parts are not moving quickly. This implies that there is little dark matter in the halo surrounding that galaxy.
Perhaps some of the dark matter has been nibbled off at the edges by the nearest massive galaxy (our own Milky Way), allowing some of the stars to slowly wander away. Or maybe the slow-moving stars could be ones that were 'flung out' from the centre of the galaxy to its edges. Whatever the explanation, the finding represents the first detection of the true outer limits of a galaxy.
"Simulations of galaxy formation generally predict the existence of many more small galaxies around the Milky Way than are actually observed," said Gerry Gilmore, Professor of Experimental Philosophy at the Institute of Astronomy at the University of Cambridge. "However, this prediction is based on assumptions about the masses of the galaxies we observe."
"Our work is aimed at determining how much mass is actually present in the dwarf galaxies around the Milky Way. But until we have a rough idea of where the outer limits of these galaxies lie, we cannot claim to have measured their total mass."
This story has been adapted from a news release issued by CSIRO Australia.
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Posted: Friday, July 25, 2003
Source: European Southern Observatory
Revealing The Beast Within: Deeply Embedded Massive Stellar Clusters Discovered In Milky Way Powerhouse
Peering into a giant molecular cloud in the Milky Way galaxy - known as W49 - astronomers from the European Southern Observatory (ESO) have discovered a whole new population of very massive newborn stars. This research is being presented today at the International Astronomical Union's 25th General Assembly held in Sydney, Australia, by ESO-scientist Joao Alves.
With the help of infrared images obtained during a period of excellent observing conditions with the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory (Chile), the astronomers looked deep into this molecular cloud and discovered four massive stellar clusters, with hot and energetic stars as massive as 120 solar masses. The exceedingly strong radiation from the stars in the largest of these clusters is "powering" a 20 light-year diameter region of mostly ionized hydrogen gas (a "giant HII region").
W49 is one of the most energetic regions of star formation in the Milky Way. With the present discovery, the true sources of the enormous energy have now been revealed for the first time, finally bringing to an end some decades of astronomical speculations and hypotheses.
Giant molecular clouds
Stars form predominantly inside Giant Molecular Clouds which populate our Galaxy, the Milky Way. One of the most prominent of these is W49, which has a mass of a million solar masses. It is located some 37,000 light-years away and is the most luminous star-forming region known in our home galaxy: its luminosity is several million times the luminosity of our Sun. A smaller region within this cloud is denoted W49A - this is one of the strongest radio-emitting areas known in the Galaxy.
Massive stars are excessive in all ways. Compared to their smaller and ligther brethren, they form at an Olympic speed and have a frantic and relatively short life. Formation sites of massive stars are quite rare and, accordingly, most are many thousands of light-years away. For that reason alone, it is in general much more difficult to observe details of massive-star formation.
Moreover, as massive stars are generally formed in the main plane of the Galaxy, in the disc where a lot of dust is present, the first stages of such stars are normally hidden behind very thick curtains. In the case of W49A, less than one millionth of the visible light emitted by a star in this region will find its way through the heavy intervening layers of galactic dust and reach the telescopes on Earth.
And finally, because massive stars just formed are still very deeply embedded in their natal clouds, they are anyway not detectable at optical wavelengths. Observations of this early phase of the lives of heavy stars must therefore be done at longer wavelengths (where the dust is more transparent), but even so, such natal dusty clouds still absorb a large proportion of the light emitted by the young stars.
Infrared observations of W49
Because of this observational obstacle, nobody had ever looked deep enough into the central most dense regions of the W49A molecular cloud - and nobody really knew what was in there. That is, until Joao Alves and his colleague, Nicole Homeier decided to obtain "deep" and penetrating observations of this mysterious area with the SofI near-infrared camera on the 3.5-m New Technology Telescope (NTT) at the ESO La Silla Observatory (Chile).
A series of infrared images was secured during a spell of good weather and very good atmospheric conditions (seeing about 0.5 arcsec). They clearly show the presence of a cluster of stars at the centre of a region of ionized hydrogen gas (an "HII-region") measuring 20 light-years across. In addition, three other smaller clusters of stars were detected in the image.
Altogether, the ESO astronomers were able to identify more than one hundred heavy-weight stars inside W49A, with masses greater than 15 to 20 times the mass of our Sun. Among these, about thirty are located within the 20 light-year central region and about ten in each of the three other clusters.
The discovery of these hot and massive stars solves a long-standing problem concerning W49A: the exceptional brightness (in astronomical terminology: "luminosity") of the entire region requires the energetic output from about one hundred massive stars, and nobody had ever seen them. But here they are on the deep and sharp SofI images!
Formation scenarios
The presence of such a large number of very massive stars spread over the entire region suggests that star formation in the various regions of W49A must have happened rather simultaneously from different seeds and not, as some theories propose, by a "domino-type" chain effect where stellar winds of fast particles and the emitted radiation of newly formed massive stars trigger another burst of star formation in the immediate neighbourhood.
The present research results also imply that star formation in W49A began earlier and extends over a larger area than previously thought.
Joao Alves is sure that this news will be received with interest by his colleagues: "W49A has long been known to radio astronomers as one of the most powerful star-forming region in the Galaxy with 30 or so massive baby-stars of the O-type, very deeply embedded in their parental cloud. What we have found is in fact quite amazing: this stellar maternity ward is much bigger than we first thought and it has not stopped forming stars yet. We now have evidence for no less than more than one hundred such stars in this region, way beyond the few dozen known until now".
Nicole Homeier adds: "Above all, we uncovered four massive clusters in there, with stars as massive as 120 times the mass of our Sun - real 'beasts' that bombard their surroundings with incredibly intense stellar winds and strong ultraviolet light. This is not a nice place to live - and imagine, this is all inside our so-called 'quiet Galaxy'!"
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The full text of this Press Release, with two photos (ESO PR Photos 21a-b/03) and all related links, is available at:
http://www.eso.org/outreach/press-rel/pr-2003/pr-20-03.html
More information
The research described in this press release is presented in a research article in the professional research journal Astrophysical Journal ("Uncovering the Beast: Discovery of Embedded Massive Stellar Clusters in W49A" by Joao Alves and Nicole Homeier, Volume 589, pp. L45-L49). It is also one of the topics addressed by Joao Alves during his talk given at the General Assembly of the International Astronomical Union in Sydney on Tuesday, July 22, 2003.
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Cosmic dust culprit unmasked Posted: Thursday, July 17, 2003
Tim Radford
Thursday July 17, 2003
The Guardian
British astronomers have solved one of the darker riddles of the universe - the culprit behind the gargantuan clouds of cosmic dust.
There are around 100 billion galaxies each containing an estimated 100 billion stars in the visible universe. Puzzlingly, almost half of all the visible light from distant stars is blocked by shadowy clouds of dust.
A team led by Loretta Dunne of Cardiff University reports today in Nature that they focused on fine grains of carbon and rock floating in Cassiopeia A, 11,000 light years from Earth, and measured 1,000 times more dust than had been previously detected.
Cassiopeia A is a remnant of a supernova, a star that exploded, showering stellar shrapnel into the cosmos. The conclusion is that cosmic dust is made and then spread by supernovae.
Stars burn hydrogen in enormous thermonuclear reactions, and in the course of doing so forge more complicated atoms, starting with helium. Star factories made all the higher elements in the periodic table, and must also have played a hand in making more complicated molecules found in space, such as water, diamond, amino acids, silicates, formaldehyde and alcohol.
"Effectively, we live on a very large collection of cosmic dust grains," Dr Dunne said. "The question of the origin of cosmic dust is in fact that of the origin of our planets and others."
Cassiopeia A was once a star 30 times the mass of the sun. The dust and debris from the explosion is still travelling outwards at speeds of 10,000km a second. "This is over 1,000 times what has been seen before," said Steve Eales of Cardiff. "Cassiopeia A must have been extremely efficient at creating dust from the elements available."
http://www.guardian.co.uk
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Out of Africa Posted: Wednesday, July 9, 2003
How long ago did our ancestors begin to migrate from Africa? Evidence from a massive volcanic explosion 74,000 years ago in South-east Asia is giving researchers clues about these first colonists, says Stephen Oppenheimer
09 July 2003, Independent.co.uk
Much has been made of the evidence from the so-called Adam and Eve genes, which support the notion that all modern humans alive today have descended from ancestors living in Africa within the past 200,000 years. A recent find of skeletons in Ethiopia, dated to 160,000 years ago, confirms the final transitions between pre-modern and anatomically modern humans in Africa. But the fine details and dates of early human explorers do not just come from advances in the study of genes and bones. Traces of a great natural disaster may allow us to pinpoint just when humans first left Africa.
The scene of the disaster is Lake Toba which today is a popular tourist spot in Sumatra. Toba is the largest lake in South-east Asia - 100km long and 31km wide - and, at 450m deep, it is one of the deepest in the world. Tourists may be unaware that Toba is also the world's largest active volcanic crater. About 74,000 years ago the volcanic eruption of Toba caused the biggest explosion of the past two million years. This "mega-bang", dwarfing the historic eruption of Krakatoa, caused a six-year "nuclear winter" and released ash in a huge plume that spread to the north-west, covering the Indian sub-continent in a blanket of ash between one and three metres deep. The Toba eruption is a valuable date mark, since the ash covered such a wide area, and can still be chemically identified today.
New genetic evidence, using genes only passed down through our mothers, suggests that the ancestors of all non-Africans left Africa, as a single group, via Aden as much as 80,000 years ago. If the date is correct, their descendants could have reached South-east Asia well before the Toba explosion, most likely by beachcombing along the coast of the Indian Ocean. Stone tools have been found covered by volcanic ash from Toba in Kota Tampan in the Malay Peninsula. Were these the tools of modern humans, therefore backing up the genetic evidence for modern humans reaching Sumatra before the Toba explosion? Or were they the tools of an earlier human species? Recent re-dating of the volcanic ash layer at Kota Tampan, which is several metres thick, has forced an extraordinary reappraisal of the significance of the site.
The Kota Tampan site with its evidence of a Palaeolithic human culture is located in the Lenggong Valley, two-thirds of the way from Africa to Australia. This culture was first identified by the find of large pebble-tools, fashioned on one side only. In the absence of skeletal remains at Kota Tampan, the tools were initially thought by archaeologists in the 1960s to be the work of an earlier human species. On the face of it, these were not sophisticated tools.
No one has done more research into Kota Tampan and the Lenggong Valley culture than archaeologist Professor Zuraina Majid, of the University of Science in Penang, Malaysia. Her extensive work at a number of sites in the Lenggong Valley suggests that the local pebble-tool culture may have persisted continuously right up until only 7,000 years ago. If so, it implies that the oval pebble-tools really were made by modern humans.
Professor Majid's trump card is the much-publicised finding by her team of "Perak Man" in the Lenggong Valley in 1990. Surrounded by the same class of pebble-tools, this complete skeleton of a modern human was dated to about 10,000 years ago. This clear recent association of the pebble-tool culture with modern humans undermines the argument that the Kota Tampan pebble-tools were too crude to be the work of modern humans.
Only with recent re-dating of the thick volcanic ash layer at Kota Tampan has the penny finally dropped. When it was first dated several decades ago, the result came out at 31,000 years old. More recently, several geologists, including the one who did the original dating, have agreed on the evidence that the ash overlying the tools belonged to the 74,000-year-old Toba eruption and carried the same geochemical fingerprint as Toba ash found throughout India.
The dating is critical. If the Kota Tampan pebble-tools were made by modern humans, they would be the oldest precisely dated evidence for modern humans outside Africa. It therefore looks as though the ancestors of the Australians left Africa and arrived in Malaysia on their beachcombing trail well before the great Toba explosion. Perhaps as important as the precision of the dating, this connection between stone tools and ashes in Malaysia puts the first Indian and Pakistani colonists in the direct path of the greatest natural calamity to ever befall any humans.
It is difficult to see how the first colonies in India could have survived. So, we could predict a broad human extinction zone in India placed between East and West Asia. Population recovery from the Indian extinction would have added fresh shoots to the Asian genetic tree with a discontinuity between East and West. This prediction is confirmed by a deep East-West genetic division, still clearly seen in Asia's genetic record. Although descended from the same root lines of the single exodus, Indian maternal branch genetic lines are completely different from those of the Far East and mostly different from those in the West.
Another prediction we can make from the scenario of Asian colonisation before Toba, is that genetic lines both inside Africa and in Asia should show the same tight genetic bottleneck - low diversity followed by dramatic expansion - dated to around 71,000 years ago - and probably the effects of worldwide extinctions resulting from the prolonged volcanic winter. This is indeed the case, providing further evidence that our ancestors had left Africa before the great Toba disaster.
Last but not least, there are also local genetic links near the Lenggong Valley with those first explorers out of Africa. With collaborators in England, Italy and Malaysia, I recently performed a genetic survey of the various aboriginal groups living in this part of the Malay Peninsula. Our results, which have just been completed, combined with dental studies, are consistent with the view that these groups have remained isolated in the jungles of the interior ever since the out-of-Africa trek and may have been descended from some of those first pebble-tool makers in the Peninsula.
One might ask how there could be any surviving links if their ancestors had been buried in volcanic ash. Luckily, Kota Tampan was just at the eastern edge of the great ash fall and there would have been survivors in the rest of the Malay Peninsula re-occupying the Lenggong Valley - bearing the torch of humanity into the modern age.
The archaeological, geographical and genetic evidence from this small region of Malaysia all help to pinpoint our ancient relatives as a part of a single small group who left Africa 80,000 years ago and then went on to populate the rest of the planet. So, from a diversity of early human species who used to occupy the globe, we have reduced dramatically to just one species of modern humans alive today.
'Out of Eden: The Peopling of the World', by Stephen Oppenheimer (Constable, £18.99) is published on 24 July
http://news.independent.co.uk/world/science_medical/story.jsp?story=423003
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