Author: Fraser Cain

Using observations of 3,000 quasars discovered by the Sloan Digital Sky Survey (SDSS), scientists have made the most precise measurement to date of the cosmic clustering of diffuse hydrogen gas. These quasars–100 times more than have been used in such analyses in the past–are at distances of eight to ten billion light years, making them among the most distant objects known.

Filaments of gas between the quasars and the Earth absorb light in the quasar’s spectra, allowing researchers to map the gas distribution and to measure how clumpy the gas is on scales of one million light years. The degree of clumping of this gas, in turn, can answer fundamental questions such as whether neutrinos have mass and what the nature of dark energy is, hypothesized to be driving the accelerated expansion of the universe.

“Scientists have long studied the clustering of galaxies to learn about cosmology,” explained Uros Seljak of Princeton University, one of the SDSS researchers. “However, the physics of galaxy formation and clustering is very complicated. In particular, because most of the mass of the universe is made up of dark matter, an uncertainty arises from our lack of understanding of the relation between the distribution of galaxies (which we see) and the dark matter (which we can’t see but the cosmological models predict).” The gas filaments seen in the quasar spectra are thought to be distributed very much like the dark matter, removing this source of uncertainty.

“We have known for several years that quasar spectra are a unique tool for studying the distribution of dark matter in the early universe, but the quantity and quality of the SDSS data have made that vision a reality,” said David Weinberg of Ohio State University, a member of the SDSS team. “It’s amazing that we can learn so much about the structure of the universe 10 billion years ago.”

Seljak and his collaborators on the SDSS combined the analysis of the quasar spectra with measurements of galaxy clustering, gravitational lensing, and ripples in the Cosmic Microwave Background observed by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). This gives the best determination to date of the clustering of matter in the universe from scales of one million light years to many billions of light years. This comprensive view allows detailed comparison with theoretical models for the history and constituents of the universe.

“This is the most rigorous test to date of the predictions of the cosmological model of inflation; inflation passes with flying colors,” added Seljak.

Inflationary theory states that right after the Big Bang the universe underwent a period of extremely rapid acceleration, during which tiny fluctuations were transformed into astronomical-sized wrinkles in space-time, ultimately observable in the clumping of astronomical objects. The theory of inflation predicts a very specific dependence of the degree of clustering with scale, which the current analysis strongly supports. Other scenarios, such as the cyclic universe theory, make very similar predictions and are also in agreement with the latest results.

Early analyses by the WMAP team and others had hinted at deviations in cosmic clustering from the prediction of inflation. If correct, this would have required a major revision of the current paradigm for origin of structure in the universe.

“The new data and the corresponding analysis substantially improves the observational precision of this test,” said Patrick McDonald of Princeton University and one of the finding’s authors. “The new results are in nearly perfect agreement with inflation.”

“The clustering of matter is a precise and powerful test of cosmological models, and the present analysis is consistent with, and extends our previous studies,” agreed Adrian Pope of The Johns Hopkins University, who led an earlier analysis of the clustering of SDSS galaxies.

The new analysis also provides the best information on the mass of the neutrino. Terrestial experiments–resulting in the 2002 Nobel Prize in Physics–have definitively shown that neutrinos have mass, but these experiments could only measure the difference in mass between the three different types of neutrinos known. The presence of neutrinos would affect the cosmic clustering on million-light-year scales, exactly the scales probed with the quasar spectra.

The new analysis suggests that the lightest neutrino mass has to be less than two times the previously measured mass difference. The new measurements also eliminate the possibility of an additional massive neutrino family suggested by some terrestrial experiments.

“Cosmology, the science of the very large, is able to tell us about properties of fundamental particles, such as neutrinos,” said Lam Hui of The U.S. Department of Energy’s Fermi National Accelerator Laboratory, who has been carrying out an independent analysis of these data, together with Scott Burles of MIT and others.

The new analysis also provides further support for the existence of dark energy, and suggests that dark energy is unchanging in time. This analysis provides the best limits on its time evolution to date.

“No evidence of dark energy changing in time has emerged so far, and the possibility that the universe will be torn apart by a big rip in the future is substantially reduced by these new results,” said Alexey Makarov of Princeton University, who also took part in this research.

Original Source: SDSS News Release

A major breakthrough in pinpointing some of the most primordial and violently star forming galaxies in the Universe has been made by a joint collaboration of UK and US astronomers using the Spitzer Space Telescope to resolve primordial galaxies initially detected by the James Clerk Maxwell telescope [JCMT]. UK astronomers from the University of Kent, The Royal Observatory Edinburgh and the University of Oxford teamed up with American cosmologists to finally identify these elusive galaxies. The work will be published in the Astrophysical Journal Supplement Spitzer Special Issue in September 2004.

Back in 1995, the UK’s SCUBA camera (Sub-millimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope in Hawaii, which detects light with wavelengths just under a millimetre, began finding fuzzy traces of very distant, primordial galaxies. Some of these are either too distant or too dusty to be seen even by the Hubble Space Telescope. But SCUBA’s images on their own, and those of other similar cameras, are not fine enough: within the fuzzy SCUBA detections are sometimes many galaxies. So astronomers have spent enormous effort following up these SCUBA galaxies on other telescopes, particularly radio telescopes, to answer the question: which one is the primordial galaxy, and which ones are in the foreground? But even with the most sensitive radio telescope images ever made, only around half the SCUBA galaxies can be pinpointed unambiguously. Even worse, the radio telescopes miss all of the most distant and most primordial of SCUBA’s galaxies.

UK and US astronomers teamed up to combine Spitzer’s sharp images with SCUBA’s ability to find primordial galaxies. The team were stunned to find all the SCUBA galaxies in Spitzers field of view detected in only ten minutes with Spitzer. These breakthrough observations, described as a watershed by the team, finally give astronomers a way of unambiguously pinpointing even the most distant of SCUBA’s galaxies. This could only be done by combining SCUBA with the Spitzer Space Telescope: SCUBA shows there is a primordial, violent starburst somewhere in the vicinity, which is then pinpointed by Spitzer.

At the same time, Spitzer solved another mystery about SCUBA galaxies. When Galileo first trained a telescope at the Milky Way, he was astonished to find the fuzzy light resolved into many individual stars. This is, in essence, what the team of astronomers have done with the diffuse extragalactic background light seen from all directions at a wavelength of about half a millimetre. By comparing the distinct Spitzer galaxies with the SCUBA data, the team discovered that they had identified the sources of this cosmic background for the first time. This background is caused by an important population of galaxies: most of the stars in the early Universe are created in these galaxies, and star formation is where everything comes from – including the material that makes planets like our own. Finding where this star formation happens tells us, in a sense, where we came from. Identifying most of these galaxies is a second coup for the joint UK/US team.

Dr. Stephen Serjeant (University of Kent, UK) said, Our Spitzer Space Telescope images picked our galaxies out astonishingly quickly, in only ten minutes, when the community has been pouring effort into detecting them. This really is pioneering work and a great triumph for the Spitzer Space Telescope and the UKs SCUBA camera. To cap it all, at the same time weve found the galaxies that dominate the star formation in the early Universe. The Earth and everything on it is made from the dust created in stars like those people, trees, beef burgers, the lot.

Dr. Rob Ivison (Royal Observatory Edinburgh, UK) said, In 10 minutes, the Spitzer Space Telescope has managed to pinpoint the galaxies we have been chasing for 7 years. We can finally begin to sort the babies and teenagers of the galaxy world from the adults and senior citizens.

Dr. Herv Dole (University of Arizona USA and IAS, Orsay, France) said, These Spitzer observations were designed as the first joint survey using the MIPS and IRAC instruments on Spitzer, to assess the instrument sensitivities. As a matter of fact, it’s a great technological, operational and scientific success, overwhelming our wildest expectations. This demonstrates the amazing capabilities of Spitzer for studying galaxy evolution at high redshifts; no doubt that deeper and larger ongoing surveys will give even more exciting results!

Dr. Steve Willner (Harvard-Smithsonian Center for Astrophysics, USA) said, We expected to detect one or a few of these galaxies, but I was stunned that we detected all of the ones we looked at. The new data finally tell us what these galaxies are all about. We’ve known all along that they had to be far away and rapidly turning all their gas into stars, but now we know their true distances and ages.

Original Source: PPARC News Release

Astronomers using ESA?s X-ray observatory XMM-Newton have detected a small, bright ?hot spot? on the surface of the neutron star called Geminga, 500 light-years away. The hot spot is the size of a football field and is caused by the same mechanism producing Geminga?s X-ray tails. This discovery identifies the missing link between the X-ray and gamma-ray emission from Geminga.

Neutron stars are the smallest kind of stars known. They are the super-dense remnants of massive stars that died in cataclysmic explosions called supernovae. They have been thrown through space like cannonballs and set spinning at a furious rate, with magnetic fields hundreds of billions of times stronger than Earth?s.

In the case of Geminga, this cannonball contains one and a half times the mass of the Sun, squeezed into a sphere just 20 kilometres across and spinning four times every second.

A cloud bustling with electrically charged particles surrounds Geminga. These particles are shepherded by its magnetic and electric fields. ESA?s XMM-Newton observatory had already discovered that some of these particles are ejected into space, forming tails that stream behind the neutron star as it hurtles along.

Scientists did not know whether Geminga?s tails are formed by electrons or by their twin particles with an opposite electrical charge, called positrons. Nevertheless, they expected that, if for instance electrons are kicked into space, then the positrons should be funnelled down towards the neutron star itself, like in an ?own goal?. Where these particles strike the surface of the star, they ought to create a hot spot, a region considerably hotter than the surroundings.

An international team of astronomers, lead by Patrizia Caraveo, IASF-CNR, Italy, has now reported the detection of such a hot spot on Geminga using ESA?s XMM-Newton observatory.

With a temperature of about two million degrees, this hot spot is considerably hotter than the one half million degrees of the surrounding surface. According to this new work, Geminga?s hot spot is just 60 metres in radius.

“This hot spot is the size of a football field,” said Caraveo, “and is the smallest object ever detected outside of our Solar System.” Details of this size can presently be measured only on the Moon and Mars and, even then, only from a spacecraft in orbit around them.

The presence of a hot spot was suspected in the late 1990s but only now can we see it ?live?, emitting X-rays as Geminga rotates, thanks to the superior sensitivity of ESA?s X-ray observatory, XMM-Newton.

The team used the European Photon Imaging Cameras (EPIC) to conduct a study of Geminga, lasting about 28 consecutive hours and recording the arrival time and energy of every X-ray photon that Geminga emitted within XMM-Newton?s grasp.

“In total, this amounted to 76 850 X-ray counts ? twice as many as have been collected by all previous observations of Geminga, since the time of the Roman Empire,” said Caraveo.

Knowing the rotation rate of Geminga and the time of each photon?s arrival meant that astronomers could identify which photons were coming from each region of the neutron star as it rotates.

When they compared photons coming from different regions of the star, they found that the ?colour? of the X-rays, which corresponds to their energy, changed as Geminga rotated. In particular, they could clearly see a distinct colour change when the hot spot came into view and then disappeared behind the star.

This research closes the gap between the X-ray and gamma-ray emission from neutron stars. XMM-Newton has shown that they both can originate through the same physical mechanism, namely the acceleration of charged particles in the magnetosphere of these degenerate stars.

“XMM-Newton?s Geminga observation has been particularly fruitful,” said Norbert Schartel, ESA?s Project Scientist for XMM-Newton. “Last year, it yielded the discovery of the source tails and now it has found its rotating hot spot.”

Caraveo is already applying this new technique to other pulsating neutron stars observed by XMM-Newton looking for hot spots. This research represents an important new tool for understanding the physics of neutron stars.

Original Source: ESA News Release