Universe Today

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