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Starbursts in dwarf galaxies last 100 times longer than astronomers thought

May 3, 2009 By admin Leave a Comment

Washington, May 1 (ANI): An analysis of archival images of small, or dwarf, galaxies taken by NASA’s Hubble Space Telescope suggests that starbursts, intense regions of star formation, sweep across the whole galaxy and last 100 times longer than astronomers thought.

The longer duration may affect how dwarf galaxies change over time, and therefore may shed light on galaxy evolution.

“Our analysis shows that starburst activity in a dwarf galaxy happens on a global scale,” explained Kristen McQuinn of the University of Minnesota in Minneapolis and leader of the study.

“There are pockets of intense star formation that propagate throughout the galaxy, like a string of firecrackers going off,” she added.

According to McQuinn, the duration of all the starburst events in a single dwarf galaxy would total 200 million to 400 million years.

These longer timescales are vastly more than the 5 million to 10 million years proposed by astronomers who have studied star formation in dwarf galaxies.

“They were only looking at individual clusters and not the whole galaxy, so they assumed starbursts in galaxies lasted for a short time,” McQuinn said.

Dwarf galaxies are considered by many astronomers to be the building blocks of the large galaxies seen today, so the length of starbursts is important for understanding how galaxies evolve.

“Astronomers are really interested to find out the steps of galaxy evolution,” McQuinn said.

“Exploring these smaller galaxies is important because, according to popular theory, large galaxies are created from the merger of smaller, dwarf galaxies. So understanding these smaller pieces is an important part of filling in that scenario,” she added.

McQuinn’s team analyzed archival Advanced Camera for Surveys data of three dwarf galaxies: NGC 4163, NGC 4068, and IC 4662.

Their distances range from 8 million to 14 million light-years away. The trio is part of a survey of starbursts in 18 nearby dwarf galaxies.

Hubble’s superb resolution allowed McQuinn’s team to pick out individual stars in the galaxies and measure their brightness and color.

Two of the galaxies, NGC 4068 and IC 4662, show active, brilliant starburst regions in the Hubble images.

The most recent starburst in the third galaxy, NGC 4163, occurred 200 million years ago and has faded from view.

The team looked at regions of high and low densities of stars, piecing together a picture of the starbursts.

The galaxies were making a few stars, when something, perhaps an encounter with another galaxy, pushed them into high star-making mode.

According to McQuinn, instead of forming eight stars every thousand years, the galaxies started making 40 stars every thousand years, which is a lot for a small galaxy. (ANI)

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Missing planets proof of destructive power of stars’ tides

May 2, 2009 By admin Leave a Comment

Washington, April 28 (ANI): Astronomers have come across first time evidence of the destructive power of stars’ tides, in the form of missing planets outside our solar system.

According to University of Washington astronomer Rory Barnes, the idea that gravitational forces might pull a planet into its parent star has been predicted by computer models only in the last year or so, and this is the first evidence that such planet destruction has already occurred.

“When we look at the observed properties of extrasolar planets, we can see that this has already happened. Some extrasolar planets have already fallen into their stars,” he said.

Computer models can show where planets should line up in a particular star system, but direct observations show that some systems are missing planets close to the stars where models say they should be.

The research involves planets that are close to their parent stars. Such planets can be detected relatively easily by changes in brightness as their orbits pass in front of the stars.

But, because they are so close to each other, the planet and star begin pulling on each other with increasingly strong gravitational force, misshaping the star’s surface with rising tides from its gaseous surface.

“Tides distort the shape of a star. The bigger the tidal distortion, the more quickly the tide will pull the planet in,” Jackson said.

According to Jackson, the destruction is slow but inevitable.

“The orbits of these tidally evolving planets change very slowly, over timescales of tens of millions of years,” Jackson said.

“Eventually, the planet’s orbit brings it close enough to the star that the star’s gravity begins tearing the planet apart,” he added.

“So, either the planet will be torn apart before it ever reaches the surface of the star, or in the process of being torn apart, its orbit eventually will intersect the star’s atmosphere and the heat from the star will obliterate the planet,” he further added.

Jackson hopes new observations will provide new lines of evidence to investigate how a star’s tides can destroy planets.

“For example, the rotation rates of stars tend to drop, so older stars tend to spin more slowly than younger stars,” he said.

“However, if a star has recently consumed a planet, the addition of the planet’s orbital angular momentum will cause the star to rapidly increase its spin rate. So, we would like to look for stars that are spinning too fast for their age,” he added. (ANI)

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Asteroids age quickly because of a ‘sun tan’

April 23, 2009 By admin Leave a Comment

Munich, April 23 (ANI): A new study has revealed that asteroid surfaces age and redden much faster than previously thought – in less than a million years, all thanks to solar winds.

“Asteroids seem to get a ‘sun tan’ very quickly,” said lead author Pierre Vernazza. “But not, as for people, from an overdose of the Sun’s ultraviolet radiation, but from the effects of its powerful wind,” he added.

It has long been known that asteroid surfaces alter in appearance with time.

The observed asteroids are much redder than the interior of meteorites found on Earth, but the actual processes of this “space weathering” and the timescales involved were controversial.

Thanks to observations of different families of asteroids using ESO’s New Technology Telescope at La Silla and the Very Large Telescope at Paranal, as well as telescopes in Spain and Hawaii, Vernazza’s team have now solved the puzzle.

When two asteroids collide, they create a family of fragments with “fresh” surfaces.

The astronomers found that these newly exposed surfaces are quickly altered and change color in less than a million years – a very short time compared to the age of the Solar System.

“The charged, fast moving particles in the solar wind damage the asteroid’s surface at an amazing rate,” said Vernazza.

Unlike human skin, which is damaged and aged by repeated overexposure to sunlight, it is, perhaps rather surprisingly, the first moments of exposure (on the timescale considered) – the first million years – that causes most of the aging in asteroids.

By studying different families of asteroids, the team has also shown that an asteroid’s surface composition is an important factor in how red its surface can become.

After the first million years, the surface “tans” much more slowly. At that stage, the color depends more on composition than on age.

Moreover, the observations reveal that collisions cannot be the main mechanism behind the high proportion of “fresh” surfaces seen among near-Earth asteroids.

Instead, these “fresh-looking” surfaces may be the results of planetary encounters, where the tug of a planet has “shaken” the asteroid, exposing unaltered material.

Thanks to these results, astronomers will now be able to understand better how the surface of an asteroid, which often is the only thing we can observe, reflects its history. (ANI)

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Butterflies use wings to send both ‘sexy’ and ‘repulsive’ signals

April 2, 2009 By admin Leave a Comment

Washington, April 2 (ANI): The eyespots of some butterflies serve to both attract mates and ward off predators, according to new research by Yale University biologists.

The researchers say that butterflies seem able to both attract mates and ward off predators by using different sides of their wings.

“You want to be noticeable and desirable for mates, but other onlookers, including predators, are paying attention to those signals as well,” says Jeffrey Oliver, a postdoctoral associate in Yale’s Department of Ecology and Evolutionary Biology.

Oliver joined forces with Yale biologist Antonia Monteiro to study whether the eyespots on the upperside of butterflies’ wings – specifically, those of bush brown butterflies – serve a different purpose than the ones on the underside.

The researchers used different evolutionary models for their study.

They found that the eyespots on the upperside of the butterflies’ wings appeared to evolve much more quickly than those on the underside, meaning they appear and disappear frequently through the course of evolution.

According to them, the finding is consistent with the theory that these are used to attract mates, as signals used for sexual selection tend to evolve faster than others.

Oliver claims that his group’s study is the first to employ evolutionary history models to show that a species can use the same signal on different areas of its body to communicate different messages.

He says that butterflies can flash hidden eyespot on their forewings to confuse predators and give themselves time to escape.

While the researchers have yet to find out how the upperside eyespots communicate with potential mates, it is thought that they might help butterflies identify each other and thus would help keep different species from cross-mating.

Oliver has revealed that his team next plans to use longer evolutionary timescales to study where and how eyespots evolved, as well as whether they developed all at once, or independently over time.

The study appears in the journal Proceedings of the Royal Society B: Biological Sciences. (ANI)

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Black holes that can regulate the rate at which they grow

March 26, 2009 By admin Leave a Comment

Washington, March 26 (ANI): New results from NASA’s Chandra X-ray Observatory have suggested that a special class of black holes have a mechanism for regulating the rate at which they grow, and can shut off the high-speed jets they produce.

Some stellar-mass black holes launch powerful jets of particles and radiation, like seen in quasars, and are called “micro-quasars”.

The new study looks at a famous micro-quasar in our own Galaxy, and regions close to its event horizon, or point of no return.

This system, GRS 1915+105 (GRS 1915 for short), contains a black hole about 14 times the mass of the Sun that is feeding off material from a nearby companion star.

As the material swirls toward the black hole, an accretion disk forms.

This system shows remarkably unpredictable and complicated variability ranging from timescales of seconds to months, including 14 different patterns of variation.

These variations are caused by a poorly understood connection between the disk and the radio jet seen in GRS 1915.

Chandra, with its spectrograph, has observed GRS 1915 eleven times since its launch in 1999.

These studies reveal that the jet in GRS 1915 may be periodically choked off when a hot wind, seen in X-rays, is driven off the accretion disk around the black hole.

The wind is believed to shut down the jet by depriving it of matter that would have otherwise fueled it. Conversely, once the wind dies down, the jet can re-emerge.

“We think the jet and wind around this black hole are in a sort of tug of war,” said Joseph Neilsen, Harvard graduate student and lead author of the research paper. “Sometimes one is winning and then, for reasons we don’t entirely understand, the other one gets the upper hand,” he added.

The latest Chandra results also show that the wind and the jet carry about the same amount of matter away from the black hole.

This is evidence that the black hole is somehow regulating its accretion rate, which may be related to the toggling between mass expulsion via either a jet or a wind from the accretion disk.

Self-regulation is a common topic when discussing supermassive black holes, but this is the first clear evidence for it in stellar-mass black holes.

According to Julia Lee, assistant professor in the Astronomy department at the Harvard-Smithsonian Center for Astrophysics, “It is exciting that we may be on the track of explaining two mysteries at the same time: how black hole jets can be shut down and also how black holes regulate their growth.” By Sarda Lahangir (ANI)

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Cause of glacial earthquakes in Greenland attributed to major ice calving events

January 4, 2009 By Rajiv Gupta Leave a Comment

Washington, Jan 4 (ANI): Scientists have clarified that glacial earthquakes in Greenland are caused by major ice calving events, not glacier lurching.

Satellite observations during the past decade have shown dramatic changes in flow speed on year-to-year timescales at Greenland”s outlet glaciers.

Seismic events traced back to glaciers during the same time period have been interpreted to have resulted from calving events at the glacier terminus or surging events lubricated by subglacial meltwater.

To learn more, M. Nettles and G. Ekstrom from Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, US, conducted geodetic studies at Helheim Glacier, one of Greenland”s largest outlet glaciers, during summer 2007.

They observed several large and sudden increases in flow speed along the length of the glacier. These accelerations coincided with glacial earthquakes and major iceberg calving events.

No offset in the position of the glacier surface was observed during these events.

Instead, modest tsunamis associated with the glacial earthquakes implicate glacier calving as the generator of seismic events, putting to rest the idea that lurching glaciers are responsible for glacial earthquakes at outlet glaciers like Helheim, and demonstrating a link between ice loss and glacier acceleration. (ANI)

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Starbursts in dwarf galaxies last 100 times longer than astronomers thought

Missing planets proof of destructive power of stars’ tides

Asteroids age quickly because of a ‘sun tan’

Black holes that can regulate the rate at which they grow

Cause of glacial earthquakes in Greenland attributed to major ice calving events

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