Category: Astronomy

Scientists at NASA and the University of Kansas say that a mass extinction on Earth hundreds of millions of years ago could have been triggered by a star explosion called a gamma-ray burst. The scientists do not have direct evidence that such a burst activated the ancient extinction. The strength of their work is their atmospheric modeling — essentially a “what if” scenario.

The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere’s protective ozone layer. Recovery could take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun could kill much of the life on land and near the surface of oceans and lakes, and disrupt the food chain.

Gamma-ray bursts in our Milky Way galaxy are indeed rare, but the scientists estimate that at least one nearby likely hit the Earth in the past billion years. Life on Earth is thought to have appeared at least 3.5 billion years ago. This research, supported by a NASA Astrobiology grant, represents a thorough analysis of the “mass extinction” hypothesis first announced by members of this science team in September 2003.

“A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life,” said Dr. Adrian Melott of the Department of Physics and Astronomy at the University of Kansas. “We don’t know exactly when one came, but we’re rather sure it did come — and left its mark. What’s most surprising is that just a 10-second burst can cause years of devastating ozone damage.”

A scientific paper describing this finding appears in Astrophysical Journal Letters. The lead author is Brian Thomas, a Ph.D. candidate at University of Kansas whom Melott advises.

Gamma-ray bursts are the most powerful explosions known. Most originate in distant galaxies, and a large percentage likely arise from explosions of stars over 15 times more massive than our Sun. A burst creates two oppositely-directed beams of gamma rays that race off into space.

Thomas says that a gamma-ray burst may have caused the Ordovician extinction 450 million years ago, killing 60 percent of all marine invertebrates. Life was largely confined to the sea, although there is evidence of primitive land plants during this period.

In the new work, the team used detailed computer models to calculate the effects of a nearby gamma-ray burst on the atmosphere and the consequences for life.

Thomas, with Dr. Charles Jackman of NASA’s Goddard Space Flight Center in Greenbelt, Md., calculated the effect of a nearby gamma-ray burst on the Earth’s atmosphere. Gamma rays, a high-energy form of light, can break molecular nitrogen (N2) into nitrogen atoms, which react with molecular oxygen (O2) to form nitric oxide (NO). NO will destroy ozone (O3) and produce nitrogen dioxide (NO2). NO2 will then react with atomic oxygen to reform NO. More NO means more ozone destruction. Computer models show that up to half the ozone layer is destroyed within weeks. Five years on, at least 10 percent is still destroyed.

Next Thomas and fellow student Daniel Hogan, an undergraduate, calculated the effect of ultraviolet radiation on life. Deep-sea creatures living several feet below water would be protected. Surface-dwelling plankton and other life near the surface, however, would not survive. Plankton is the foundation of the marine food chain.

Dr. Bruce Lieberman, a paleontologist at the University of Kansas, originated the idea that a gamma-ray burst specifically could have caused the great Ordovician extinction, 200 million years before the dinosaurs. An ice age is thought to have caused this extinction. But a gamma-ray burst could have caused a fast die-out early on and also could have triggered the significant drop in surface temperature on Earth.

“One unknown variable is the rate of local gamma-ray bursts,” said Thomas. “The bursts we detect today originated far away billions of years ago, before the Earth formed. Among the billions of stars in our Galaxy, there’s a good chance that a massive one relatively nearby exploded and sent gamma rays our way.” The Swift mission, launched in November 2004, will help determine recent burst rates. Other team members are Dr. Claude Laird of the University of Kansas, and Drs. Richard Stolarski, John Cannizzo, and Neil Gehrels of NASA Goddard.

Original Source: NASA News Release

Researchers at the University of Birmingham have used the new generation of X-ray space observatories to study fossil galaxies – ancient galaxy groups in which all of the large galaxies have gradually merged to form one central giant galaxy.

The astronomers discovered a remarkable concentration of dark and normal matter in the cores of these isolated star systems, compared with the mass distribution in normal galaxy groups.

Many galaxies, including our Milky Way, reside in groups. Sometimes they experience close encounters with other members of the group. Computer simulations predict that such interactions cause large galaxies to spiral slowly towards the centre of the group, where they can merge to form a single giant galaxy, which progressively swallows all its neighbours.

Since many galaxy groups possess extended halos of hot gas and dark matter, it was predicted ten years ago that a class of systems dubbed fossil groups should exist, in which all the major galaxies have merged to form one central giant galaxy. This would be surrounded by an X-ray-bright cloud of hot gas that extends outward to many galactic radii.

When we first discovered the large halos of hot gas in which some very compact groups of galaxies are embedded, we realised that just a few billion years of further evolution would leave a single, giant, merged galaxy sitting at the centre of a bright X-ray halo, said Trevor Ponman, the leader of the Birmingham group who made this prediction and then discovered the first fossil group in 1994.

Theories also suggested that fossil groups which fall into even larger clusters of galaxies may account for the giant elliptical galaxies which are often found in the centres of such clusters.

The Birmingham team has observed six likely fossil groups in the past two years, taking advantage of the sharp vision of NASAs Chandra X-Ray Space Observatory and the high sensitivity of ESAs orbiting XMM-Newton X-ray observatory. The six fossil groups are located up to two billion light years from Earth. The teams main objective was to explore the mechanisms by which fossil groups and giant elliptical galaxies are formed.

The key to the study was the distribution of dark matter in the fossil groups. This mysterious matter comprises over 80 per cent of the mass of the Universe, yet its nature is unknown. Dark matter has never been detected directly, but its presence is inferred through its gravitational influence on ordinary matter.

The large elliptical galaxy NGC 6482 was of special interest to the team, since it is the closest known fossil group, and could be studied in great detail. This isolated giant, which shines with the equivalent of 110 billion Suns, is located 100 million light years away in the constellation Hercules. Using Chandras Advanced CCD Imaging Spectrometer, Habib Khosroshahi, Trevor Ponman and Laurence Jones, used observations of the hot gas to trace the distribution of dark matter in NGC 6482. The gas is heated to a temperature of 10 million degrees Celsius, mainly due to shock heating as a result of gravitational collapse.

Speaking today at the RAS National Astronomy Meeting in Birmingham, Habib Khosroshahi described the discovery of a remarkable concentration of dark matter in the core of NGC 6482. Khosroshahi also described two more examples of high mass concentration in more massive and more distant fossil galaxies studied by both the Chandra and the XMM-Newton telescopes, although the case of NGC 6482 is unique, since it is possible to probe the centre of the system with higher accuracy.

According to Khosroshahi, the concentration of mass at the centre of these ancient galaxy groups, which is mostly in the form of dark matter, was found to be typically five times higher than in normal galaxy groups with similar mass and halo size. This central concentration of mass supports the idea that fossil groups such as NGC 6482 are very old structures which collapsed long before the typical groups of galaxies formed. “The explanation for such a centralised dark matter distribution could be that the system formed at very high redshift when the Universe was very young and dense, said Khosroshahi.

The great advantage of fossil groups compared to normal groups is that no major galaxy interaction, which can stir the hot gas, is taking place, he added. Therefore, they provide ideal laboratories to study the properties of visible matter in the form of gas and stars as well as their container, the dark matter.

Original Source: RAS News Release

Distant galaxies undergoing intense bursts of star formation have been shown by NASA’s Chandra X-ray Observatory to be fertile growing grounds for the largest black holes in the Universe. Collisions between galaxies in the early Universe may be the ultimate cause for both the accelerated star formation and black hole growth.

By combining the deepest X-ray image ever obtained with submillimeter and optical observations, an international team of scientists has found evidence that some extremely luminous adolescent galaxies and their central black holes underwent a phenomenal spurt of growth more than 10 billion years ago. This concurrent black hole and galaxy growth spurt is only seen in these galaxies and may have set the stage for the birth of quasars – distant galaxies that contain the largest and most active black holes in the Universe.

“The extreme distances of these galaxies allow us to look back in time, and take a snapshot of how today’s largest galaxies looked when they were producing most of their stars and growing black holes,” said David Alexander of the University of Cambridge, UK, and lead author of a paper in the April 7, 2005 issue of Nature that describes this work.

The galaxies studied by Alexander and his colleagues are known as submillimeter galaxies, so-called because they were originally identified by the James Clerk Maxwell submillimeter telescope (JCMT) on Mauna Kea in Hawaii. The submillimeter observations along with optical data from Keck indicate these galaxies had an unusually large amount of gas. The gas in each galaxy was forming into stars at a rate of about one per day, or 100 times the present rate in the Milky Way galaxy. The Chandra X-ray data show that the supermassive black holes in the galaxies were also growing at the same time.

These galaxies are very faint and it is only with the deepest observations of the Universe that they can be detected at all. “The deeper we look into the Universe with Chandra, the more fascinating things we find” says Niel Brandt of Penn State University in University Park. “Who knows what nature has in store for us as we push the boundaries yet further.”

The X-ray observations also showed that the black holes are surrounded by a dense shroud of gas and dust. This is probably the material that will be consumed by the growing black holes.

Hubble Space Telescope observations indicate that most of the submillimeter galaxies are actually two galaxies that are colliding and merging. Recent sophisticated computer simulations performed by Tiziana Di Matteo of Carnegie Mellon University in Pittsburgh, Penn., and her collaborators have shown that such mergers drive gas toward the central regions of galaxies, triggering a burst of star formation and providing fuel for the growth of a central black hole.

“It is exciting that these recent observations are in good agreement with our simulation,” says Di Matteo, “We seem to be converging on a consistent picture of galaxy formation with both observations and theory.” In particular, this work will help scientists to understand the observed link in the present epoch between the total mass of stars in the central bulges of large galaxies and the size of their central, supermassive black holes.

The James Clerk Maxwell Telescope (JCMT) is operated on behalf of the United Kingdom, Canada & Netherlands by the Joint Astronomy Centre. With its 15-meter (50-foot) diameter dish the JCMT detects light with “submillimeter” wavelengths, between infrared light and radio waves on the wavelength scale. The W. M. Keck Observatory is operated by the California Association for Research in Astronomy.

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate, Washington. Northrop Grumman of Redondo Beach, Calif., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at: http://chandra.harvard.edu and http://chandra.nasa.gov

Original Source: Chandra News Release

When the distant planetoid Sedna was discovered on the outer edges of our solar system, it posed a puzzle to scientists. Sedna appeared to be spinning very slowly compared to most solar system objects, completing one rotation every 20 days. Astronomers hypothesized that this world possessed an unseen moon whose gravity was slowing Sedna’s spin. Yet Hubble Space Telescope images showed no sign of a moon large enough to affect Sedna.

New measurements by Scott Gaudi, Krzysztof (Kris) Stanek and colleagues at the Harvard-Smithsonian Center for Astrophysics (CfA) have cleared up this mystery by showing that a moon wasn’t needed after all. Sedna is rotating much more rapidly than originally believed, spinning once on its axis every 10 hours. This shorter rotation period is typical of planetoids in our solar system, requiring no external influences to explain.

“We’ve solved the case of Sedna’s missing moon. The moon didn’t vanish because it was never there to begin with,” said Gaudi.

Sedna is an odd world whose extreme orbit takes it more than 45 billion miles from the Sun, or more than 500 astronomical units (where one astronomical unit is the average Earth-Sun distance of 93 million miles). Sedna never approaches the Sun any closer than 80 astronomical units, and takes 10,000 years to complete one orbit. In comparison, Pluto’s 248-year-long oval orbit takes it between 30 and 50 astronomical units from the Sun.

“Up until now, Sedna appeared strange in every way it had been studied. Every property of Sedna that we’d been able to measure was atypical,” said Gaudi. “We’ve shown that Sedna’s rotation period, at least, is entirely normal.”

Sedna appears unusual in other ways besides its orbit. First and foremost, it is one of the largest known “minor planets,” with an estimated size of 1,000 miles compared to Pluto’s 1,400 miles. Sedna also displays an unusually red color that is still unexplained.

Initial measurements indicated that Sedna’s rotation period was also extreme – extremely long compared to other solar system residents. By measuring small brightness fluctuations, scientists estimated that Sedna rotated once every 20-40 days. Such slow rotation likely would require the presence of a nearby large moon whose gravity could apply the brakes and slow Sedna’s spin. As a result of this interpretation, artist’s concepts released when Sedna’s discovery was announced showed a companion moon. One month later, images taken by NASA’s Hubble Space Telescope demonstrated that no large moon existed.

In true detective fashion, Gaudi and his colleagues re-investigated the matter by observing Sedna using the new MegaCam instrument on the 6.5-meter-diameter MMT Telescope at Mount Hopkins, Ariz. They measured Sedna’s brightness looking for telltale, periodic brightening and dimming that would show how fast Sedna rotates.

As noted by Matthew Holman, one of the members of the CfA team, “The variation in Sedna’s brightness is quite small and could have been easily overlooked.”

Their data fits a computer model in which Sedna rotates once every 10 hours or so. The team’s measurements definitively rule out a rotation period shorter than 5 hours or longer than 10 days.

While these data solve one mystery of Sedna, other mysteries remain. Chief among them is the question of how Sedna arrived in its highly elliptical, eons-long orbit.

“Theorists are working hard to try to figure out where Sedna came from,” said Gaudi.

Astronomers will continue to study this strange world for some time to come.

“This is a completely unique object in our solar system, so anything we can learn about it will be helpful in understanding its origin,” said Stanek.

This research has been submitted to The Astrophysical Journal Letters for publication and is posted online at http://arxiv.org/abs/astro-ph/0503673.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

UK and US astronomers have used the Spitzer Space Telescope and the Hubble Space Telescope to detect light coming from the first stars to form in some of the most distant galaxies yet seen. Speaking on Wednesday 6 April at the RAS National Astronomy Meeting in Birmingham, Dr. Andrew Bunker (University of Exeter) will discuss new evidence that the formation of the first galaxies may have got underway earlier than previously thought.

This observational work using infrared images from Spitzer Space Telescope is essential, since theoretical predictions for the history of star formation in the early Universe are highly uncertain. The team, led by Bunker and graduate student Laurence Eyles (University of Exeter), used Hubble Space Telescope data to identify remote galaxies that were suitable for further study. They then analysed archived images taken at infrared wavelengths with NASAs Spitzer Space Telescope.

These images, obtained as part of the Great Observatory Origins Deep Survey (GOODS) project and the Hubble Ultra Deep Field (UDF), covered a part of the southern sky known as the constellation of Fornax (the Oven). We used the images from the Hubble Ultra Deep Field to identify objects likely to be galaxies 95 per cent of the way across the observable Universe, explained Bunker. These images are our most sensitive picture of the Universe so far, and they enabled us to discover the faintest objects yet. Intervening gas clouds absorbed the light they emitted at visible wavelengths long before it reached Earth, but their infrared light can still be detected – and it is their infrared colours which led the researchers to believe that they lie at such immense distances.

Confirmation of their extreme remoteness was provided by the 10-metre Keck telescopes in Hawaii, the largest optical telescopes in the world. We proved these galaxies are indeed among the most distant known by using the Keck telescopes to take a spectrum, said Dr. Elizabeth Stanway (University of Wisconsin- Madison).

The Keck spectra showed that the galaxies have redshifts of about 6, which means they are so far away that light from them has taken about 13 billion years to reach us. Telescopes show them as they were when the Universe was less than a billion years old – eight billion years before the Earth and Sun formed.

The next step was to learn more about the stars within these most distant galaxies by studying new infrared images of this region of space taken by Spitzer. The Hubble images tell us about the new-born stars, but the new infrared images taken with the Spitzer Space Telescope give us extra information about the light that comes from older stars within these distant galaxies, said Laurence Eyles, who studied the Spitzer images of these objects as part of his research for a doctorate at Exeter.

This is very important, because it tells us that some of these galaxies are already 300 million years old when the Universe is very young. It could be that these were some of the first galaxies to be born, said Michelle Doherty (Institute of Astronomy, Cambridge). Using the Spitzer images, the team was able to weigh the stars in these galaxies by studying the starlight. It seems that in a couple of cases these early galaxies are nearly as massive as galaxies we see around us today, which is a bit surprising when the theory is that galaxies start small and grow by colliding and merging with other galaxies, said Dr. Mark Lacy (Spitzer Science Center).

The real puzzle is that these galaxies seem to be already quite old when the Universe was only about 5 per cent of its current age, commented Professor Richard Ellis of Caltech. This means star formation must have started very early in the history of the Universe – earlier than previously believed. The light from these first stars to ignite could have ended the Dark Ages of the Universe when the galaxies first turned on. It is also likely to have caused the gas between the galaxies to be blasted by starlight – the reionisation which has been detected in the cosmic microwave background by the WMAP satellite.

The results from WMAP and the Hubble Ultra Deep Field complement the new work done by Bunkers team with the Spitzer data. Taken together, they suggest that the Dark Ages ended sometime between 200 and 500 million years after the Big Bang, when the first stars were born.

A paper on these results has been submitted for publication in the Monthly Notices of the Royal Astronomical Society.

Original Source: RAS News Release

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

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

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

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

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

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

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

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

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

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

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

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

Original Source: Penn State News Release

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Original Source: RAS News Release

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

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

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

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

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

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

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

Original Source: ESA News Release

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

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

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

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

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

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

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

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

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

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

Written by Jeff Barbour

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

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

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

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

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

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

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

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

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

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

Original Source: PPARC News Release