Category: Astronomy

Dark gamma ray burst GRB020819. Image credit: Keck. Click to enlarge.
Virtually everything we know about the Universe comes to us through the agency of light. Unlike matter, light is uniquely suited to travel the vast distances across space to our instruments. Most astronomical phenomena however are persistent and repeatable – we can rely on them to “hang around” for long-term observation or “come back around” on a regular basis. But this isn’t so for gamma ray bursts (GRB’s) – those mysterious cosmological events that supercharge photons (and sub-atomic particles) with absurdly high energy levels.

The first detected celestial GRB occurred during nuclear arms treaty monitoring in 1967. That event required years of analysis before its extraterrestrial origin was confirmed. After this discovery, primitive triangulation methods were put in place using detectors located on various space probes within the Interplanetary Network (IPN). Such methods required a great deal of number crunching and made instant follow-up using Earth-based instruments impossible. Despite the delays involved, hundreds of gamma ray sources were catalogued. Today – even using the Internet – it would still require several days to respond using an IPN-type detection approach.

All this began to change in 1991 when NASA put the Compton Gamma Ray Observatory (CGRO) into space using space shuttle Atlantis as part of its “Great Observatories” program. Within four months of scanning the sky, CGRO made it clear to astronomers that the Universe underwent sporadic and widely distributed gamma ray paroxysms on an almost daily basis – paroxysms caused by cataclysmic events that hurl vast quantities of gamma and other high-energy radiation across the abyss of space-time.

But CGRO had one main limitation – although it could detect gamma rays and alert astronomers quickly, it wasn’t particularly accurate as to where such events happened in space. Because of this large “error circle”, astronomers were unable to locate the visible light “afterglow” of such events. Despite this limitation, CGRO went on to detect hundreds of continuous, periodic, and episodic gamma ray sources – including supernovae, pulsars, black holes, quasars, and even the Earth itself! Meanwhile CGRO also discovered something unsuspected – certain pulsars acted as narrow band transmitters of gamma rays without accompanying visible light – and therein lay astronomer?s first sense of “dark” GRBs.

Today we know that “dark pulsars” are not the only “dark” sources of gamma rays in the Universe. Astronomers have determined that some small portion of episodic (one-time-only) GRBs are also low in visible light, and they – like anyone tickled by the unusual and inexplicable – want to know why. In fact GRB’s are so unique that aficionados may often be heard saying “When you’ve seen one GRB, you’ve seen one GRB”.

The first satellite to simplify optical detection of GRB afterglows was BeppoSAX. Developed by the Italian Space Agency in the mid 1990’s, BeppoSAX launched April 30, 1996 from Cape Canaveral and continued to detect and pinpoint X-ray emission sources until 2002. BeppoSax’s error circle was small enough to enable optical astronomers to rapidly track down many GRB afterglows for detailed study in visible light using earth-based instruments.

BeppoSAX re-entered the Earth’s atmosphere April 29, 2003, but by this time NASA’s replacement (HETE-2 the High Energy Transient Explorer-2) was already several years on station in low-earth orbit. Instrument’s on HETE-2 (its first incarnation HETE failed to separate from the third stage of its Pegasus rocket in 1996) expanded the range of X-ray detection and provided even tighter error circles – just the thing astronomers needed to improve their response time in locating GRB afterglows.

Two years and a few months later (Monday, August 19, 2002) HETE-2 set off the bells and whistles as a strong gamma ray source was detected somewhere near the head of the constellation Pisces the Fishes. That event (designated GRB 020819) caused a series of astronomical observatories to begin capturing radio-frequency, near-infrared, and visible light photons in an effort to determine just where the event occurred and help make sense of the phenomenon driving it.

According to the paper “The Radio Afterglow and Host Galaxy of the Dark GRB 020819” published May 2, 2005 by an international team of investigators (including Pall Jakobsson of the Niels Bohr Institute, Copenhagen Denmark who proofed this article), within 4 hrs of detection the 1 meter Siding Spring Observatory (SSO) telescope in Australia was turned to a region of space less than 1/7th the apparent diameter of the Moon. 13 hours later, a second, slightly larger instrument – the 1.5 meter P60 unit at Mt. Palomar – also joined the chase. Neither instrument – despite capturing light as faint as magnitude 22 – caught anything unusual for that region of space. However a large and extremely photogenic 19.5 magnitude face-on barred spiral galaxy fell nicely within the grasp of their instruments.

Fifteen days later, the 10 meter Keck ESI instrument on Mauna Kea, Hawaii imaged the same region in blue and red light down to magnitude 26.9. At this optical depth, a distinct 24th magnitude “blob” (suspected to be an HII star-formation region) could be seen 3 arc seconds north of the spiral galaxy. A final attempt to detect anything further was made January 1, 2003 – again using the Keck 10 meter. No change was seen in optical light emanating from the region of GRB 020819. All this confirmed that no visible afterglow accompanied the gamma ray outburst detected by HETE-2 some 134 days earlier. The investigating team had their “dark gamma ray burster”. Later would come the task of figuring out just what the heck it was – or at least was not…

Periodically throughout the cycle of optical and near-infrared inspection, the region of the burst was monitored in radio-wave frequencies. Using the VLA (Very Large Array – consisting of 27 Y-configured 25 meter dishes located fifty miles west of Socorro, New Mexico) the team succeeded in capturing a dwindling trail of 8.48 Ghz radiation and identified its locale.

First radio waves from GRB 020819 were collected 1.75 days after the HETE-2 alert. By day 157, rf energy levels flattened to the point where the source could no longer be seen with confidence. However by this time, its location had been pinpointed to the “blob” three arc-seconds north of the core of the previously uncharted spiral galaxy. Unfortunately – due to its faintness – the distance to the blob itself could not be determined spectrographically – however the galaxy was found to lie some 6.2 BLY away and enjoys “high-confidence” in terms of having a relationship with the source.

As a result of such investigations astronomers are now learning more and more about a class of cataclysmic events that results in massive fluxes of high and low energy photons while almost completely skipping intermediate frequencies – such as ultraviolet, visible, and near-infrared of light. Is there anything that could account for this?

Based on learning from GRB 020819, the team explored three fireball-shock models of how dark GRBs might occur. Of the three (an even expansion of high energy gases into a homogenous medium, even expansion into a stratified medium, and a collimated jet penetrating either type medium), the best fit against GRB 020819 behaviors was that of an even expansion of high energy gases into a homogenous medium of other gases (a model first proposed by the astrophysicist R. Sari et al in 1998). The virtue of this isotropic-expansion model being (in the words of the investigating team) that “only a modest amount of extinction must be invoked” to account for the absence of visible light.

In addition to narrowing the range of possible scenarios associated with dark GRBs, the team concluded that “GRB 020819, a relatively nearby burst, is only one of two of the 14 GRB’s localized to within (2 arc minutes using) HETE-2 that does not have a reported OA. This lends support to the recent proposition that the dark burst fraction is far lower than previously suggested, perhaps as small as 10%.”

Written by Jeff Barbour

Artist illustration of a superflare on a young star. Image credit: NASA. Click to enlarge.
New results from NASA’s Chandra X-ray Observatory imply that X-ray super-flares torched the young Solar System. Such flares likely affected the planet-forming disk around the early Sun, and may have enhanced the survival chances of Earth.

By focusing on the Orion Nebula almost continuously for 13 days, a team of scientists used Chandra to obtain the deepest X-ray observation ever taken of this or any star cluster. The Orion Nebula is the nearest rich stellar nursery, located just 1,500 light years away.

These data provide an unparalleled view of 1400 young stars, 30 of which are prototypes of the early Sun. The scientists discovered that these young suns erupt in enormous flares that dwarf – in energy, size, and frequency — anything seen from the Sun today.

“We don’t have a time machine to see how the young Sun behaved, but the next best thing is to observe Sun-like stars in Orion,” said Scott Wolk of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “We are getting a unique look at stars between one and 10 million years old – a time when planets form.”

A key result is that the more violent stars produce flares that are a hundred times as energetic as the more docile ones. This difference may specifically affect the fate of planets that are relatively small and rocky, like the Earth.

“Big X-ray flares could lead to planetary systems like ours where Earth is a safe distance from the Sun,” said Eric Feigelson of Penn State University in University Park, and principal investigator for the international Chandra Orion Ultradeep Project. “Stars with smaller flares, on the other hand, might end up with Earth-like planets plummeting into the star.”

According to recent theoretical work, X-ray flares can create turbulence when they strike planet-forming disks, and this affects the position of rocky planets as they form. Specifically, this turbulence can help prevent planets from rapidly migrating towards the young star.

“Although these flares may be creating havoc in the disks, they ultimately could do more good than harm,” said Feigelson. “These flares may be acting like a planetary protection program.”

About half of the young suns in Orion show evidence for disks, likely sites for current planet formation, including four lying at the center of proplyds (proto-planetary disks) imaged by Hubble Space Telescope. X-ray flares bombard these planet-forming disks, likely giving them an electric charge. This charge, combined with motion of the disk and the effects of magnetic fields should create turbulence in the disk.

The numerous results from the Chandra Orion Ultradeep Project will appear in a dedicated issue of The Astrophysical Journal Supplement in October, 2005. The team contains 37 scientists from institutions across the world including the US, Italy, France, Germany, Taiwan, Japan and the Netherlands.

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

Two of the four H.E.S.S. telescopes in Namibia. Image credit: HESS. Click to enlarge.
Our planet is exposed to almost four dozen octaves of electro-magnetic radiation from the Universe around us. Of those, half-a-dozen octaves can be detected from the Earth’s surface. During the 1990’s several extraordinary new octaves were added with the advent of high-sensitivity CCD imagers and modern computing systems. Today we can track super-high energy gamma rays back to their sources in space ? even while safely ensconced in the Earth?s protective mantle of air.

Well before the turn of the third millennium, it was realized that high-energy photons penetrating the air causes a secondary form of light known as Cherenkov Radiation (CR). CR was first observed by Pierre and Marie Curie when investigating radioactivity at the turn of the 20th century. It wasn’t until the mid-1930s that the hauntingly beautiful “blue-white” glow given off by glass in the presence of radioactivity was studied in detail.

CR was first fully investigated by the Russian experimentalist P. A. Cherenkov in 1936. Cherenkov found that whenever high-energy photons (or particles) pass through a transparent gas, liquid, or glass at velocities greater than the speed of light for that substance, a shower of secondary light is created. In terms of the Earth’s atmosphere, such showers typically occur as gamma rays approach within 10 km’s of sea level and the resulting luminosity projects a light cone (or “light pool”) roughly 250ms in diameter.

Enter the Max Plank Institute of Physics (MPIK) of Heidelberg, Germany in the early 1990s.

In 1992 MPIK tested the first in a series of prototypes intended to develop a full scale IACT (Imaging Atmospheric Cherenkov Telescopes) system. That instrument (CT1) proved that CR showers could be detected using CCDs. It also showed that computers could accurately log a CR shower’s time and position in the sky. A later instrument (CT2) increased CR sensitivity and resolution by adding aperture. Meanwhile improvements were made to associated imaging, data processing, and sky sensing components. By combining four CT2-class instruments together, the first full IACT system was developed in 1995 (CT3). Because of this progress, MPIK’s own website could later say that “Ground-based Imaging Atmospheric Cherenkov Telescopes have become the most efficient experimental technique for the observation of cosmic gamma rays in the TeV energy range.”

IACT systems monitor for CR showers using two or more widely spaced light-collecting mirror assemblies pointed at the same part of the sky. Because CR originates in the Earth’s atmosphere – not well-off in the Universe itself – each mirror sees a shower from a different perspective. The resulting “stereoscopic vision” works like eye and brain to precisely determine the path a gamma ray takes after entering the atmosphere. Based on that data – along with laws governing the way photons move – computers calculate the location of gamma ray source in space. Each ray effectively acts like a luminous finger pointing back toward a distant cosmic source.

By 1998 the first purely astronomical IACT (HEGRA – High Energy Gamma Ray Astronomy) was put into service by MPIK on La Palma in the Canary Islands. HEGRA confirmed dozens of high energy gamma ray sources – many hurling photons of more than 1 terra-electron-volts of energy (the amount of force stored in a single electron accelerated by a trillion volts of electricity). Among them were the Crab Nebula pulsar in Taurus and the giant elliptical galaxy M87 – regent of the Coma-Virgo galaxy cluster.

Today even more advanced IACT systems collect CR. One of the most sophisticated instruments (H.E.S.S – High Energy Stereoscopic System) was developed by MPIK along with a consortium of European scientific and educational organizations. Currently HESS consists of four separate 12m diameter IACTS gathering faint CR light in the dark skies above the 1.8km high Khomas Highlands of Namibia, Africa.

Named after Nobel Prize winning physicist Victor Hess (who discovered cosmic rays in 1912), HESS uses an array of four IACT mirror systems. Each spherical IACT mirror consists of 382, 60cm diameter individually-adjustable sub-mirrors reflecting CR light into a large electronic “camera”. Light focused on the camera is detected by a honeycomb of 960 “smartpixel” photo-multiplier tubes (PMTs). The four IACTs are placed in a square and spaced by 120 meters to give an optimally stereoscopic view of the sky within the 250m light pool caused by a CR event.

Each HESS IACT is ten times more sensitive than its corresponding HEGRA unit – and has to be, for the total amount of CR light in the sky is 10 stellar magnitudes fainter than starlight. HESS IACTS can resolve CR showers caused by photons as “weak” as .1 TeV while discriminating between high-energy particles and photons. Using a pair of IACTS, gamma ray sources can be isolated to less than 5 arc-minutes of angular resolution – roughly 1/6th the apparent size of the full moon. To simplify detection, HESS IACTS can scan 5 degrees of the sky at a time.

One of the fundamental questions before astrophysicists is to determine just how nature manages to pack so much punch into those mass-less, charge-less photons. Currently no terra-electron-volt particle accelerators are on line – and such devices only work with charged particles – not photons. It may fall to IACTS like HESS to lead the way.

In a paper entitled “Observation of the giant radio galaxy M87 at TeV energies using H.E.S.S”, M Beilicke of the Institute for Expermental Physics, Hamburg Germany and associates have used HESS to determine that the giant elliptical galaxy M87 is a strong and possibly periodically variable source of high-energy gamma ray photons.

According to the paper, “M87 is of particular interest for observations of TeV energies. The large jet angle makes it different from the so far observed TeV emitting AGN of the blazar type.” Using HESS, the team determined that high-energy photons originate from a point source centered in the midst of M87 – precisely where it’s AGN is thought to be. Unlike blazars however, M87’s relativistic jets do not point at the Earth.

Meanwhile the team may have also discovered that gamma ray output from M87’s AGN is variable “on time scales of years.” According to M. Bielecke et al, “Such a result would be very important since various models for the TeV gamma-ray production in M87 could be ruled out.” The team goes on to say that “Mechanisms correlated with cosmic rays, large scale jet structures, and exotic dark matter particle annihilation could not explain variability in the TeV gamma ray emission on these time-scales.”

As in many areas of contemporary astronomical investigation, observing M87 across a wide-range of the em band may be essential to understanding how those tiny mass-free wave-particles of light can carry so much ?weight?. There is no doubt that capturing the ‘blue-white” glow of Cherenkov radiation put off by our Earth’s very own atmosphere will play a critical role in making this possible.

Written by Jeff Barbour

Near perfect “Einstein Ring” gravitational lens. Image credit: ESO/VLT. Click to enlarge.
This is Einstein’s Year. One-hundred years ago a little known Swiss patent clerk in the very early years of a scientific career was confronted with a series of paradoxes related to time and space, energy and matter. Gifted with a profound intuition and a powerful imagination, Albert A. Einstein rose out of obscurity to present an entirely new way of looking at natural phenomenon. Einstein showed us all that time had very little to do with clocks, energy has less to do with quantity and more to do with quality, space was not just ?a big square box to put stuff in”, matter and energy were two sides of the same cosmic coin, and gravity had a profound effect on everything – light, matter, time, and space.

Today we use all these principles ? enunciated a century ago – to probe the most distant things in the Universe. Because of Einstein’s investigation of the photoelectric effect, we now understand why light is not continuous but curiously riddled with dark and bright lines telling us when that light was emitted, what emitted it. and the kinds of things touching it in its travels. Because of Einstein’s insight into the conversion of mass and energy, we now understand how distant suns illuminate the cosmos, and how powerful magnetic fields whip particles up to stupendous speeds later to come crashing down on the Earth’s atmosphere. And because gravity is now understood to influence everything, we have learned how distant objects can capture and focus light from even more distant objects.

Although we have yet to find an absolutely perfect instance of gravitational lensing in the Universe, today we are much closer to that ideal. In a paper entitled “Discovery of a high red-shift Einstein Ring” published April 27, 2005, Remi Cabanac of Canada-France-Hawaii Telescope, in Hawaii and associates “report the discovery of a partial Einstein ring … produced by a massive (and seemingly isolated) elliptical galaxy.” Previous to this find, the most complete Einstein ring discovered was documented in 1996 by S.J. Warren of the Imperial College in London. That ring – also one of the few visible in optical light – is slightly less than a half-circle in circumference (170 degrees).

Remi Cabanac explained that he “discovered the system while observing at the European Southern Observatory Very Large Telescope in Chile with a spectro imager called FORS1.” Remi says he was fullfilling his responsibilties as a service astronomer, “observing for Helmut Jerjen (co-author of the paper) doing deep imaging of nearby dwarf galaxies in the outskirts of a well-known nearby galaxy cluster in Fornax.” Remi continued to say that his “eye got attracted by the very unusual bright arc in the northwest of the field, I knew it was something pretty amazing because lensing arcs are usually very dim, and I was observing in red band whereas arcs are usually blueish.”

To confirm his suspicions of a new discovery Remi “went to the astronomical database but nothing existed under the coordinates.” Later Remi consulted with “Chris Lidman (another co-author and lens expert) and showed him the image. He couldn’t believe it was a lens at first because it was so bright and conspicuous, Chris thought it could be an artefact on the image.” With Chris’ support, Remi “applied for spectroscopic follow-up and realized that it was both a true gravitational lense and a very significant discovery, because the background source was highly amplified and very far away.”

According to the paper, the ring inscribes a “C-shaped” circle of 270 degrees in near-complete circumference with an apparent radius of slightly more than 1 3/4 arc seconds – roughly the size of a star’s “virtual” image seen at high power through a small amateur telescope. The lens galaxy is a giant elliptical similar to M87 in the Virgo-Coma cluster. The lens lies some 7 billion light years distant in the direction of the constellation Fornax (visible from warmer temperate northern hemisphere and southern hemisphere skies). The source galaxy bears a red shift of 3.77 – suggesting a recessionary distance of roughly 11 BLYs. Source and lens galaxy have received the designation FOR J0332-3557 3h32m59s, -35d57m51s and lie proximate to the Fornax galaxy cluster – but well beyond it in terms of real space.

What makes this particular discovery so interesting astronomically is the fact that the lens galaxy is very massive, is in a period of star-birth quiescence, lies at such a great distance from the Earth, and may be isolated from other cluster galaxies in its own spatial locale. Meanwhile the source galaxy is significantly brighter (by one absolute stellar magnitude) than other Lyman break galaxies (galaxies that red-shift the Lyman Break at 912 angstroms into the visible part of the spectrum), is poor in emission line spectra, and recently had completed a cycle of rapid star birth (“starburst”). All these factors combined mean that FOR J0332 could provide a wealth of data concerning galaxy formation before the current inflationary epoch of the Universe.

According to the science team, “One of the key issues in galaxy formation within the current LCDM (Lambda Cold Dark Matter) framework of structure formation is the mass assembly histories of galactic halos.” Current thinking is that galaxies accumulate halo mass – that huge spherical bulge of low luminosity matter surrounding galactic cores – before star formation really kicks in. One way to investigate this idea is to determine how mass-to-light ratios change over time as galaxies evolve. But to do that you need to sample the masses and luminosities of as many galaxies as possible, of a variety of types, over the broadest possible range of space and time.

The discovery of FOR J0332 – and the three other partial Einstein ring objects – helps astronomers by adding examples of galaxies normally undetectable at great distances. From the paper, “Various deep surveys have uncovered different galaxy populations, but the selection criteria produced biased samples: UV-selected and narrow-band selected samples are sensitive to actively star-forming galaxies and biased against quiescent, evolved systems while sub-millimeter and near-infrared surveys select dusty starburst galaxies and very red galaxies respectively.”

What conclusions can we draw based on this discovery?

Remi underscores the significance of this find by saying “The source amplified by the lens is the galaxy with the brightest apparent luminosity ever discovered at such a distance. It will give us unique information on the physical conditions prevailing in the interstellar medium when the universe was only 12% of its present age. The shape of the source is also very important because it gives the amount of mass within the lens at a redshift of z=1. Only a handful of Einstein rings have been discovered at such high redshift. It will give an important measurement at how elliptical galaxy mass evolved through time.”

Written by Jeff Barbour

Applying cutting edge computer science to a wealth of new astronomical data, researchers from the Sloan Digital Sky Survey (SDSS) reported today the first robust detection of cosmic magnification on large scales, a prediction of Einstein’s General Theory of Relativity applied to the distribution of galaxies, dark matter, and distant quasars.

These findings, accepted for publication in The Astrophysical Journal, detail the subtle distortions that light undergoes as it travels from distant quasars through the web of dark matter and galaxies before reaching observers here on Earth.

The SDSS discovery ends a two decade-old disagreement between earlier magnification measurements and other cosmological tests of the relationship between galaxies, dark matter and the overall geometry of the universe.

“The distortion of the shapes of background galaxies due to gravitational lensing was first observed over a decade ago, but no one had been able to reliably detect the magnification part of the lensing signal”, explained lead researcher Ryan Scranton of the University of Pittsburgh.

As light makes its 10 billion year journey from a distant quasar, it is deflected and focused by the gravitational pull of dark matter and galaxies, an effect known as gravitational lensing. The SDSS researchers definitively measured the slight brightening, or “magnification” of quasars and connect the effect to the density of galaxies and dark matter along the path of the quasar light. The SDSS team has detected this magnification in the brightness of 200,000 quasars.

While gravitational lensing is a fundamental prediction of Einstein’s General Relativity, the SDSS collaboration’s discovery adds a new dimension.

“Observing the magnification effect is an important confirmation of a basic prediction of Einstein’s theory,” explained SDSS collaborator Bob Nichol at the University of Portsmouth (UK). “It also gives us a crucial consistency check on the standard model developed to explain the interplay of galaxies, galaxy clusters and dark matter.”

Astronomers have been trying to measure this aspect of gravitational lensing for two decades. However, the magnification signal is a very small effect — as small as a few percent increases in the light coming from each quasar. Detecting such a small change required a very large sample of quasars with precise measurements of their brightness.

“While many groups have reported detections of cosmic magnification in the past, their data sets were not large enough or precise enough to allow a definitive measurement, and the results were difficult to reconcile with standard cosmology,” added Brice Menard, a researcher at the Institute for Advanced Study in Princeton, NJ.

The breakthrough came earlier this year using a precisely calibrated sample of 13 million galaxies and 200,000 quasars from the SDSS catalog. The fully digital data available from the SDSS solved many of the technical problems that plagued earlier attempts to measure the magnification. However, the key to the new measurement was the development of a new way to find quasars in the SDSS data.

“We took cutting edge ideas from the world of computer science and statistics and applied them to our data,” explained Gordon Richards of Princeton University.

Richards explained that by using new statistical techniques, SDSS scientists were able to extract a sample of quasars 10 times larger than conventional methods, allowing for the extraordinary precision required to find the magnification signal. “Our clear detection of the lensing signal couldn’t have been done without these techniques,” Richards concluded.

Recent observations of the large-scale distribution of galaxies, the Cosmic Microwave Background and distant supernovae have led astronomers to develop a ‘standard model’ of cosmology. In this model, visible galaxies represent only a small fraction of all the mass of the universe, the remainder being made of dark matter.

But to reconcile previous measurements of the cosmic magnification signal with this model required making implausible assumptions about how galaxies are distributed relative to the dominant dark matter. This led some to conclude that the basic cosmological picture was incorrect or at least inconsistent. However, the more precise SDSS results indicate that previous data sets were likely not up to the challenge of the measurement.

“With the quality data from the SDSS and our much better method of selecting quasars, we have put this problem to rest,” Scranton said. “Our measurement is in agreement with the rest of what the universe is telling us and the nagging disagreement is resolved.”

“Now that we’ve demonstrated that we can make a reliable measurement of cosmic magnification, the next step will be to use it as a tool to study the interaction between galaxies, dark matter, and light in much greater detail,” said Andrew Connolly of the University of Pittsburgh.

Original Source: SDSS News Release

The Spitzer Space Telescope (SST) is the fourth and final instrument in NASA’s Great Observatories series. The SST followed the Hubble Space Telescope (HST), Chandra X-Ray, and Compton Gamma Ray Observatories into space on August 25th, 2003. Placed in Earth-trailing heliocentric (solar) orbit, and working under a 2.5 plus year charter within NASA’s Origins Program, the SST revealed first public light in May of 2004 – giving the world a spectacular infrared view of the face-on grand spiral galaxy M51 in Canes Venatici.

Lord Rosse first described M51 as a “spiral nebula” in 1845. It wasn’t until Edwin Hubble resolved faint variable stars within another “M” – M31 – that M51 and other “spiral nebulae” achieved a rank equal with our own Milky Way – Galaxy!

But to name a thing is not to explain it. One of the toughest things to explain about anything is “How did it get to be what it is?”

Well before the release of SST’s image of M51, astronomers had already been given a “heads-up” on a rare instance of a class of distant objects in the heavens – an expansive region of gas and dust glowing faintly yet unattended by stellar light – just the kind of study that could revolutionize the way astronomers understand galaxy formation. NASA’s Origins Program had made a major hit and now the problem was to advance the runner to home using other sources of data…

In a paper entitled “Discovery of a Large ~ 200kpc Gaseous Nebula at z=~2.7 with the Spitzer Space Telescope” (published March 29, 2005), astrophysicist Arjun Dey of the National Optical Astronomy Observatory (NOAO) and colleagues from other organizations (including the SST operations center at the Jet Propulsion Laboratory) pulled together data from across the lower half of the em spectrum – radio to visible light – to paint a picture of early galaxy cluster formation associated with this excited (and exciting) region of dust and gas located some 11.3 BLY’s away in time and space.

In the words of the team, “We report the discovery of a very large spatially extended nebula associated with a luminous mid-infrared source.” To you and me that means they discovered “a long ago, and far away womb of early galactic birth”.

The object (SST24 J1434110+331733) was originally mapped using the SST’s MIPS and IRAC detectors during a mid-infrared survey of spring?s constellation Bootes in late January 2004. After data reduction by JPL personnel, it became clear that SST24 could offer some extremely significant insights into that mysterious era of galactic unfolding when young galaxies are ensconced in the stuff of star formation. But to penetrate this stuff would require expanding the picture of the region using light from across the em spectrum.

In part the need to have other looks at SST24 was driven by the limited aperture of SST’s 0.84 meter mirror and those long wavelengths associated with infrared light. At best, the SST revealed the central third of the nebulosity. (Instruments aboard the SST are limited to 6 arc seconds detail resolution.) Three onboard detectors (the Infrared Array Camera -IRAC, Infrared Spectrograph – IRS, and Multiband Imaging Photometer for Spitzer – MIPS) image and analyze infrared light in the mid to far-infrared wavelengths (3.6-160 micrometers).

Although light observed using the three SST instruments mostly originates from “warm” objects (gases and dust), light from near-optical sources can also be seen after expansionary redshift over vast distances. Interestingly, one particular bright line in that same “near-optical light” was first flagged for astronomical use by astrophysicist Lyman Spitzer – namesake of the SST itself – one of the leading 20th century proponents of infrared astronomy.

Joined with data from other instruments, Dey and his team put together a compelling case for an active galactic nuclei (AGN) within SST24. If verified such an AGN would demonstrate that black holes play an important role in early galaxy evolution. Such an example may very well revolutionize our understanding of galaxy formation by making AGN’s more the cause – rather than the effect – of galaxy group formation…

Visual data used by the team associated with SST24 was collected using the 4m and 2.1m telescopes of the NOAO in Kitt Peak, Arizona. These instruments improved SST resolution by a factor of almost eight times. Other data available in optical light extended the picture of SST24’s energy output. During May and June of 2004, spectrographic information on SST24 (along with foreground and background objects) was gathered in finely-tuned and precisely oriented 1 arc second strips through the 10 meter Keck I instrument on Mauna Kea, Hawaii.

From the paper’s abstract, “The bright mid-infrared source was first detected in observations made using the Spitzer Space Telescope. Existing broad-band imaging data from the NOAO Deep Wide-Field Survey revealed the mid-infrared source to be associated with a diffuse, spatially-extended, optical counterpart… Spectroscopy and further imaging … reveals the optical source is almost purely line-emitting nebula with little if any, detectable diffuse continuum emission.”

Typically, mature galaxies display a full spectrum of light generated by blackbody radiation from stellar photospheres. Such broadband spectra are usually reinforced by narrow, bright emission lines associated with atomic excitation. But SST24’s spectrum is dominated by a single narrow band of radiation. That band – though redshifted some 3.7 times due to 11.3 BLY’s of recession – associates with the “Lyman Alpha” frequency emitted by hydrogen gas. Usually such Lyman-alpha clouds irradiate by stimulation from distant background quasars. But in the case of SST24, another mechanism may be involved – a black hole source within the nebula itself.

In piecing together SST24’s structure, the science team determined that its AGN is offset from the center of the cloud by nearly one-tenth the cloud’s full extent. Although it is unclear what impact this offset has on galaxy formation, the fact of it must be incorporated into how we model galaxy group formation in the future.

Spectrographic shifts in Lyman alpha light also indicate that the central 100 KLY region of SST24 slowly revolves and contains the mass equivalent of some 6 trillion suns – some 5x that of our own Milky Way and Whirlpool (M51) galaxies combined. SST24 includes a region of space easily encompassing the entire Milky Way and all twelve satellite galaxies.

But SST24 is not totally devoid of star formation. The team reports that “a young star forming galaxy lies near the northern end of the nebula.” That galaxy is reddened by dust, has the same redshift as the Lyman-alpha radiation, plus broad-band radiation associated with star formation. This galaxy gives no indication of having an AGN. Because of this we may soon learn that AGN?s may not play a role essential to the formation of all galaxies.

Although radio-frequency examination of SST24 is difficult (due to resolution issues at long wavelengths), the team points out that its mid-infrared to radio-wave density ratio, “shows remarkable similarity to starburst galaxies…” For this reason parts of SST24 mat be passing through an era of rapid stellar evolution that could quickly lead to the revelation of a full-blown galaxy rich with luminous breeder stars…

SST24 is not the only Lyman-alpha cloud ever detected, but those few discovered are thought extraordinary by the science team: “The rarity of these >100kpc lyman-alpha clouds, their association with powerful AGN and galaxy overdensities, and their energetics all suggest that these regions are the formation sites of the most massive galaxies. If so, understanding the physical conditions and energetics of these systems can provide important insights into the massive galaxy formation process.”

Written by Jeff Barbour

Thanks to data from ESA?s XMM-Newton spacecraft, European astronomers have observed for the first time rotating ?hot spots? on the surfaces of three nearby neutron stars.

This result provides a breakthrough in understanding the ?thermal geography? of neutron stars, and provides the first measurement of very small-sized features on objects hundreds to thousands light-years away. The spots vary in size from that of a football field to that of a golf course.

Neutron stars are extremely dense and fast-rotating stars mainly composed of neutrons. They are extremely hot when they are born, being remnants of supernovae explosions. Their surface temperature is thought to gradually cool down with time, decreasing to less than one million degrees after 100 000 years.

However, astrophysicists had proposed the existence of physical mechanisms by which the electromagnetic energy emitted by neutron stars could be funnelled back into their surface in certain regions. Such regions, or ?hot spots?, would then be reheated and reach temperatures much higher than the rest of the cooling surface. Such peculiar ?thermal geography? of neutron stars, although speculated, could never be observed directly before.

Using XMM-Newton data, a team of European astronomers have observed rotating hot spots on three isolated neutron stars that are well-known X-ray and gamma-ray emitters. The three observed neutron stars are ?PSR B0656-14?, ?PSR B1055-52?, and ?Geminga?, respectively at about 800, 2000 and 500 light-years away from us.

As for normal stars, the temperature of a neutron star is measured through its colour that indicates the energy the star emits. The astronomers have divided the neutron star surfaces into ten wedges and have measured the temperature of each wedge. By doing so, they could observe rise and fall of emission from the star?s surface, as the hot spots disappear and appear again while the star rotates. It is also the first time that surface details ranging in size from less than 100 metres to about one kilometre are identified on the surface of objects hundreds to thousands light-years away.

The team think that the hot spots are most probably linked to the polar regions of the neutron stars. This is where the star?s magnetic field funnels charged particles back towards the surface, in a way somehow similar to the ?Northern lights?, or aurorae, seen at the poles of planets which have magnetic fields, such as Earth, Jupiter and Saturn.

?This result is a first, and a key to understand the internal structure, the dominant role of the magnetic field treading the star interior and its magnetosphere, and the complex phenomenology of neutron stars,? says Patrizia Caraveo, of the Istituto Nazionale di Astrofisica (IASF), Milan, Italy.

?It has been possible only thanks to the new capabilities provided by the ESA XMM-Newton observatory. We look forward to applying our method to many more magnetically isolated neutron stars,? concludes Caraveo.

However, there is still a puzzle for the astronomers. If the three ?musketeers? are predicted to have polar caps of comparable dimensions, why then are the hot spots observed in the three cases so different in size, ranging from 60 metres to one kilometre? What mechanisms rule the difference? Or does this mean some of the current predictions on neutron stars magnetic fields need to be revised?

The result, by Andrea De Luca, Patrizia Caraveo, Sandro Mereghetti, Matteo Negroni (IASF) and Giovanni Bignami of CESR, Toulouse and University of Pavia, is published in the 20 April 05 issue of the Astrophysical Journal (http://www.journals.uchicago.edu/ApJ, vol. 623:1051-1069).

Original Source: ESA News Release

The nebula N214 [1] is a large region of gas and dust located in a remote part of our neighbouring galaxy, the Large Magellanic Cloud. N214 is a quite remarkable site where massive stars are forming. In particular, its main component, N214C (also named NGC 2103 or DEM 293), is of special interest since it hosts a very rare massive star, known as Sk-71 51 [2] and belonging to a peculiar class with only a dozen known members in the whole sky. N214C thus provides an excellent opportunity for studying the formation site of such stars.

Using ESO’s 3.5-m New Technology telescope (NTT) located at La Silla (Chile) and the SuSI2 and EMMI instruments, astronomers from France and the USA [3] studied in great depth this unusual region by taking the highest resolution images so far as well as a series of spectra of the most prominent objects present.

N214C is a complex of ionised hot gas, a so-called H II region [4], spreading over 170 by 125 light-years (see ESO PR Photo 12b/05). At the centre of the nebula lies Sk-71 51, the region’s brightest and hottest star. At a distance of ~12 light-years north of Sk-71 51 runs a long arc of highly compressed gas created by the strong stellar wind of the star. There are a dozen less bright stars scattered across the nebula and mainly around Sk-71 51. Moreover, several fine, filamentary structures and fine pillars are visible.

The green colour in the composite image, which covers the bulk of the N214C region, comes from doubly ionised oxygen atoms [5] and indicates that the nebula must be extremely hot over a very large extent.

The Star Sk-71 51 decomposed
The central and brightest object in ESO PR Photo 12b/05 is not a single star but a small, compact cluster of stars. In order to study this very tight cluster in great detail, the astronomers used sophisticated image-sharpening software to produce high-resolution images on which precise brightness and positional measurements could then be performed (see ESO PR Photo 12c/05). This so-called “deconvolution” technique makes it possible to visualize this complex system much better, leading to the conclusion that the tight core of the Sk-71 51 cluster, covering a ~ 4 arc seconds area, is made up of at least 6 components.

From additional spectra taken with EMMI (ESO Multi-Mode Instrument), the brightest component is found to belong to the rare class of very massive stars of spectral type O2 V((f*)). The astronomers derive a mass of ~80 solar masses for this object but it might well be that this is a multiple system, in which case, each component would be less massive.

Stellar populations
From the unique images obtained and reproduced as ESO PR Photo 12b/05, the astronomers could study in great depth the properties of the 2341 stars lying towards the N214C region. This was done by putting them in a so-called colour-magnitude diagram, where the abscissa is the colour (representative of the temperature of the object) and the ordinate the magnitude (related to the intrinsic brightness). Plotting the temperature of stars against their intrinsic brightness reveals a typical distribution that reflects their different evolutionary stages.

Two main stellar populations show up in this particular diagram (ESO PR Photo 12d/05): a main sequence, that is, stars that like the Sun are still centrally burning their hydrogen, and an evolved population. The main sequence is made up of stars with initial masses from roughly 2-4 to about 80 solar masses. The stars that follow the red line on ESO PR Photo 12d/05 are main sequence stars still very young, with an estimated age of about 1 million years only. The evolved population is mainly composed of much older and lower mass stars, having an age of 1,000 million years.

From their work, the astronomers classified several massive O and B stars, which are associated with the H II region and therefore contribute to its ionisation.

A Blob of Ionised Gas
A remarkable feature of N214C is the presence of a globular blob of hot and ionised gas at ~ 60 arc seconds (~ 50 light-years in projection) north of Sk-71 51. It appears as a sphere about four light-years across, split into two lobes by a dust lane which runs along an almost north-south direction (ESO PR Photo 12d/05). The blob seems to be placed on a ridge of ionised gas that follows the structure of the blob, implying a possible interaction.

The H II blob coincides with a strong infrared source, 05423-7120, which was detected with the IRAS satellite. The observations indicate the presence of a massive heat source, 200,000 times more luminous than the Sun. This is more probably due to an O7 V star of about 40 solar masses embedded in an infrared cluster. Alternatively, it might well be that the heating arises from a very massive star of about 100 solar masses still in the process of being formed.

“It is possible that the blob resulted from massive star formation following the collapse of a thin shell of neutral matter accumulated through the effect of strong irradiation and heating of the star Sk-71 51”, says Mohammad Heydari-Malayeri from the Observatoire de Paris (France) and member of the team.”Such a “sequential star formation” has probably occurred also toward the southern ridge of N214C”.

Newcomer to the Family
The compact H II region discovered in N214C may be a newcomer to the family of HEBs (“High Excitation Blobs”) in the Magellanic Clouds, the first member of which was detected in LMC N159 at ESO. In contrast to the typical H II regions of the Magellanic Clouds, which are extended structures spanning more than 150 light years and are powered by a large number of hot stars, HEBs are dense, small regions usually “only” 4 to 9 light-years wide. Moreover, they often form adjacent to or apparently inside the typical giant H II regions, and rarely in isolation.

“The formation mechanisms of these objects are not yet fully understood but it seems however sure that they represent the youngest massive stars of their OB associations”, explains Frederic Meynadier, another member of the team from the Observatoire de Paris. “So far only a half-dozen of them have been detected and studied using the ESO telescopes as well as the Hubble Space Telescope. But the stars responsible for the excitation of the tightest or youngest members of the family still remain to be detected.”

More information
The research made on N214C has been presented in a paper accepted for publication by the leading professional journal, Astronomy and Astrophysics (“The LMC H II Region N214C and its peculiar nebular blob”, by F. Meynadier, M. Heydari-Malayeri and Nolan R. Walborn). The full text is freely accessible as a PDF file from the A&A web site.

Notes
[1]: The letter “N” (for “Nebula”) in the designation of these objects indicates that they were included in the “Catalogue of H-alpha emission stars and nebulae in the Magellanic Clouds” compiled and published in 1956 by American astronomer-astronaut Karl Henize (1926 – 1993).

[2]: The name Sk-71 51, is the abbreviation of Sanduleak -71 51. The American astronomer Nicholas Sanduleak, while working at the Cerro Tololo Observatory, published in 1970 an important list of objects (stars and nebulae showing emission-lines in their spectra) in the Magellanic Clouds. The “-71” in the star’s name is the declination of the object, while the “51” is the entry number in the catalogue.

[3]: The team of astronomers consists of Frederic Meynadier and Mohammad Heydari-Malayeri (LERMA, Paris Observatory, France), and Nolan R. Walborn (Space Telescope Science Institute, USA).

[4]: A gas is said to be ionised when its atoms have lost one or more electrons – in this case by the action of energetic ultraviolet radiation emitted by very hot and luminous stars close by. The heated gas shines mostly in the light of ionized hydrogen (H) atoms, leading to an emission nebula. Such nebulae are referred to as “H II regions”. The well-known Orion Nebula is an outstanding example of that type of nebula, cf. ESO PR Photos 03a-c/01 and ESO PR Photo 20/04.

[5]: The hotter the central object of an emission nebula, the hotter and more excited will be the surrounding nebula. The word “excitation” refers to the degree of ionization of the nebular gas. The more energetic the impinging particles and radiation, the more electrons will be lost and higher is the degree of excitation. In N214C, the central cluster of stars is so hot that the oxygen atoms are twice ionized, i.e. they have lost two electrons.

Original Source: ESO News Release

Detailed new images of the starbirth nursery in the Omega Nebula (M17) have revealed a multi component structure in the envelope of dust and gas surrounding a very young star. The stellar newborn, called M17-SO1, has a flaring torus of gas and dust, and thin conical shells of material above and below the torus. Shigeyuki Sako from University of Tokyo and a team of astronomers from the National Astronomical Observatory of Japan, the Japan Aeorospace Exploration Agency, Ibaraki University, the Purple Mountain Observatory of the Chinese Academy of Sciences, and Chiba University obtained these images and analyzed them in infrared wavelengths in order to understand the mechanics of protoplanetary disk formation around young stars. Their work is described in a detailed article in the April 21, 2005 edition of Nature.

The research team wanted to find a young star located in front of a bright background nebula and use near-infrared observations to image the surrounding envelope in silhouette, in a way comparable to how dentists use X-rays to take images of teeth. Using the Infrared Camera and Spectrograph with Adaptive Optics on the Subaru telescope, the astronomers looked for candidates in and around the Omega Nebula, which lies about 5,000 light-years away in the constellation Sagittarius. They found a large butterfly-shaped near-infrared silhouette of an envelope about 150 times the size of our solar system surrounding a very young star. They made follow-up observations of the region using the Cooled Mid-Infrared Camera and Spectrograph on the Subaru telescope and the Nobeyama Millimeter Array at the Nobeyama Radio Observatory. By combining the results from the near-infrared, mid-infrared, and millimeter wave radio observations, the researchers determined that the M17-SO1 is a protostar about 2.5 to 8 times the mass of the Sun, and that the butterfly-like silhouette reveals an edge-on view of the envelope.

The near-infrared observations reveal the structure of the surrounding envelope with unprecedented levels of detail. In particular, observations using the 2.166 emission line of hydrogen (called the Brackett gamma (Br ?) line) show that the envelope has multiple components instead of one simple structure. Around the equator of the protostar, the torus of dust and gas increases in thickness farther way from the star. Thin cone-shaped shells of material extend away from both poles of the star.

The discovery of the multi-component structure puts new constraints on how an envelope feeds material to a protostellar disk forming within its boundaries. “It’s quite likely that our own solar system looked like M17-SO1 when it was beginning to form,” said Sako. “We hope to confirm the relevance of our discovery for understanding the mechanism of protoplanetary disk formation by using the Subaru telescope to take infrared images with high resolution and high sensitivity of many more young stars.?

Original Source: NOAJ News Release

Image credit: William K. Hartmann/PSI
The oxygen and magnesium content of some of the oldest objects in the universe are giving clues to the lifetime of the solar nebula, the mass of dust and gas that eventually led to the formation of our solar system.
Specimen from the Allende Meteorite

By looking at the content of chondrules and calcium aluminum-rich inclusions (CAIs), both components of the primitive meteorite Allende, Lab physicist Ian Hutcheon, with colleagues from the University of Hawaii at Manoa, the Tokyo Institute of Technology and the Smithsonian Institution, found that the age difference between the two fragments points directly to the lifetime of the solar nebula.

CAIs were formed in an oxygen-rich environment and date to 4.567 billion years old, while chondrules were formed in an oxygen setting much like that on Earth and date to 4.565 billion, or less, years old.

?Over this span of about two million years, the oxygen in the solar nebula changed substantially in its isotopic makeup,? Hutcheon said. ?This is telling us that oxygen was evolving fairly rapidly.?

The research appears in the April 21 edition of the journal Nature.

One of the signatures of CAIs is an enrichment of the isotope Oxygen 16 (O-16). An isotope is a variation of an element that is heavier or lighter than the standard form of the element because each atom has more or fewer neutrons in its nucleus. The CAIs in this study are enriched with an amount of O-16 4 percent more than that found on Earth. And, while 4 percent may not sound like much, this O-16 enrichment is an indelible signature of the oldest solar system objects, like CAIs. CAIs and chondrules are tens of millions of years older than more modern objects in the solar system, such as planets, which formed about 4.5 billion years ago.

?By the time chondrules formed, the O-16 content changed to resemble what we have on Earth today,? Hutcheon said.

In the past, the estimated lifetime of the solar nebula ranged from less than a million years to ten million years. However, through analysis of the mineral composition and oxygen and magnesium isotope content of CAIs and chondrules, the team was able to refine that lifespan to roughly two million years.

?In the past the age difference between CAIs and chondrules was not well-defined,? Hutcheon said. ?Refining the lifetime of the solar nebula is quite significant in terms of understanding how our solar system formed.?

Founded in 1952, Lawrence Livermore National Laboratory has a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy’s National Nuclear Security Administration.

Original Source: LLNL News Release