Monday, April 30, 2012

M83: A Remarkable Outburst from an Old Black Hole

M83
Credit Left image - Optical: ESO/VLT; Close-up - X-ray: NASA/CXC/Curtin University/R.Soria et al., Optical: NASA/STScI/Middlebury College/F.Winkler et al.



NASA's Chandra X-ray Observatory has discovered an extraordinary outburst by a black hole in the spiral galaxy M83, located about 15 million light years from Earth. Using Chandra, astronomers found a new ultraluminous X-ray source (ULX), objects that give off more X-rays than most "normal" binary systems in which a companion star is in orbit around a neutron star or black hole.

On the left is an optical image of M83 from the Very Large Telescope in Chile, operated by the European Southern Observatory. On the right is a composite image showing X-ray data from Chandra in pink and optical data from the Hubble Space Telescope in blue and yellow. The ULX is located near the bottom of the composite image.

In Chandra observations that spanned several years, the ULX in M83 increased in X-ray brightness by at least 3,000 times. This sudden brightening is one of the largest changes in X-rays ever seen for this type of object, which do not usually show dormant periods.

Optical images reveal a bright blue source at the position of the ULX during the X-ray outburst. Before the outburst the blue source is not seen. These results imply that the companion to the black hole in M83 is a red giant star, more than about 500 million years old, with a mass less than about four times the Sun's. According to theoretical models for the evolution of stars, the black hole should be almost as old as its companion.


Astronomers think that the bright, blue optical emission seen during the X-ray outburst must have been caused by a disk surrounding the black hole that brightened dramatically as it gained more material from the companion star.

Another highly variable ULX with an old, red star as a companion to a black hole was found recently in M31. The new ULXs in M83 and M31 provide direct evidence for a population of black holes that are much older and more volatile than those usually considered to be found in these objects.

The researchers estimate a mass range for the M83 ULX from 40 to 100 times that of the Sun. Lower masses of about 15 times the mass of the Sun are possible, but only if the ULX is producing more X-rays than predicted by standard models of how material falls onto black holes.

Evidence was also found that the black hole in this system may have formed from a star surprisingly rich in "metals", as astronomers call elements heavier than helium. The ULX is located in a region that is known, from previous observations, to be rich with metals.

Large numbers of metals increase the mass-loss rate for massive stars, decreasing their mass before they collapse. This, in turn, decreases the mass of the resulting black hole. Theoretical models suggest that with a high metal content only black holes with masses less than about 15 times that of the Sun should form. Therefore, these results may challenge these models.

This surprisingly rich "recipe" for a black hole is not the only possible explanation. It may also be that the black hole is so old that it formed at a time when heavy elements were much less abundant in M83, before seeding by later generations of supernovas. Another explanation is that the mass of the black hole is only about 15 times that of the Sun.

Fast Facts for M83:

Scale: Left image is 17.6 arcmin on a side (~77,000 light years); Close-up: 0.6 x 1.2 arcmin (~2600 x ~5200 light years)
Coordinates: (J2000) RA 13h 37m 00.80s | Dec -29° 51’ 58.60"
Constellation: Hydra
Observation Dates: 12 pointings between April 29, 2000 and Dec 28, 2011
Observation Time: 219 hours 49 min (9 days 3 hours 49 min)
Obs. IDs: 793, 2064, 12992-12996, 13202, 13241, 13248, 14332, 14342
Instrument: ACIS
Also Known As: NGC 5236
References: Soria, R. et al, 2012, ApJ (in press) arXiv:1203.2335
Kaur, A. et al, 2012, A&A 538, A49 arXiv:1109.1547
Middleton, M.J., et al, 2012, MNRAS, 420, 2969 arXiv:1111.1188
Distance Estimate: About 15 million light years

Saturday, April 28, 2012

‘Ridiculously’ Dim Bevy of Stars Found Beyond Milky Way

The Muñoz 1 globular cluster is seen to the right of the Ursa Minor dwarf galaxy in this image from the Canada-France-Hawaii Telescope MegaCam imager. Credit: Geha & Muñoz

Kamuela, HI – A team of American, Canadian and Chilean astronomers have stumbled onto a remarkably faint cluster of stars orbiting the Milky Way that puts out as much light as only 120 modest Sun-like stars. The tiny cluster, called Muñoz 1, was discovered near a dwarf galaxy in a survey of satellites around the Milky Way using the Canada-France-Hawaii Telescope (CFHT) and confirmed using the Keck II telescope, both of which are on Mauna Kea, Hawaii.

“What’s neat about this is it’s the dimmest globular cluster ever found,” said Ricardo Muñoz, an astronomer at the University of Chile and the discoverer of the cluster. A globular cluster is a spherical group of stars bound to each other by gravity so that they orbit around a galaxy as a unit.

“While I was working on the Ursa Minor dwarf galaxy I noticed there was this tiny little object close by,” Muñoz recalled. He made the discovery while he was a postdoctoral associate at Yale University. Most globular clusters have in the range of 100,000 stars. Muñoz 1 has something like 500 stars. “This is very surprising,” he said.

“It’s ridiculously dim,” agreed Yale astronomer Marla Geha. “There are individual stars that would far outshine this entire globular cluster.” That puts Muñoz 1 head-to-head with the Segue 3 globular cluster (also orbiting the Milky Way) as the dimmest troupe of old stars ever found.

Muñoz 1’s discovery was the result of a survey done with the CFHT MegaCam imager in 2009 and 2010. It was then confirmed by spectroscopic study using the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope. The researchers will be publishing their results soon in The Astrophysical Journal Letters.

The Keck data was critical for the study, said Geha, because it sorted out whether or not Muñoz 1 and the Ursa Minor dwarf galaxy were moving together.

“Nearly every galaxy has an entourage of globular clusters,” said Geha, “so we first thought that Muñoz 1 might be associated with the nearby Ursa Minor dwarf galaxy.” By using spectroscopic data to measure the relative velocities of the cluster and the dwarf galaxy, they discovered quite the opposite was the case.

“The velocities turned out to be wildly different,” said Geha. So the fact that they are near each other is just a coincidence, she said. What has been seen is more like a single snapshot of two cars traveling near each other and apparently together, but they really have different destinations and are traveling at very different speeds. Analysis of the brightness and colors of the stars belonging to Muñoz 1 and Ursa Minor also suggests that the tiny cluster is actually located about 100,000 light years in front of the dwarf galaxy.

As for how Muñoz 1 came to be so dim, a likely scenario is that it has gradually lost stars over the eons, said Geha. It’s also possible it was stripped of stars by passing through the Milky Way. But the direction of the cluster’s movement is not yet known, so it’s not known whether it has passed through the Milky Way.

Perhaps the most intriguing aspect of the discovery is the possibility that Muñoz 1 may be hinting that there are many more such globular clusters in the Galactic halo. After all, the CFHT survey covered only 40 square degrees of sky out of 40,000 square degrees in the entire sky.

“Assuming that we’re not just lucky to have found something very rare, there could be many others out there,” said Geha.

“To truly understand its nature, we will need to measure its mass,” added Munoz. To do that, astronomers would need to measure the velocities of individual stars in the cluster and see how they move with relation to each other. That, in turn, reveals the overall mass of the cluster. A lot of mass would suggest there is a lot of dark matter holding the cluster together, and maybe even qualify the cluster as the smallest, darkest galaxy ever discovered. Right now the Segue 1 dwarf galaxy holds that record. Geha was also involved in measurements with the Keck DEIMOS instrument that confirmed the nature of Segue 1.

“The goal of this survey was to understand the difference between dwarf galaxies and globular clusters,” said Geha. Muñoz 1 suggests there may be plenty of borderline objects out there waiting to be found, which could help sort that matter out.

A pdf of the paper is available at http://www.cfht.hawaii.edu/en/news/Munoz1/munoz12.pdf.

# # #

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system which cancels out much of the interference caused by Earth’s turbulent atmosphere. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

The Canada-France-Hawaii Telescope is a joint facility of National Research Council of Canada, Centre National de la Recherche Scientifique of France, and University of Hawaii.

Friday, April 27, 2012

Cassini Finds Saturn Moon has Planet-Like Qualities

Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during Cassini's flyby on June 11, 2004. Image Credit: NASA/JPL/Space Science Institute. Full image and caption

This panel of images shows the nearly spherical shape of Saturn's moon Phoebe, as derived from imaging obtained from NASA's Cassini spacecraft. Each image represents a 90-degree turn. Image credit: NASA/JPL-Caltech/SSI/Cornell . Full image and caption - enlarge image

PASADENA, Calif. -- Data from NASA's Cassini mission reveal Saturn's moon Phoebe has more planet-like qualities than previously thought.

Scientists had their first close-up look at Phoebe when Cassini began exploring the Saturn system in 2004. Using data from multiple spacecraft instruments and a computer model of the moon's chemistry, geophysics and geology, scientists found Phoebe was a so-called planetesimal, or remnant planetary building block. The findings appear in the April issue of the Journal Icarus.

"Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out," said Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Objects like Phoebe are thought to have condensed very quickly. Hence, they represent building blocks of planets. They give scientists clues about what conditions were like around the time of the birth of planets and their moons."

Cassini images suggest Phoebe originated in the far-off Kuiper Belt, the region of ancient, icy, rocky bodies beyond Neptune's orbit. Data show Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center. Its average density is about the same as Pluto, another object in the Kuiper Belt. Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

Saturn is surrounded by a cloud of irregular moons that circle the planet in orbits tilted from Saturn's orbit around the sun, the so-called equatorial plane. Phoebe is the largest of these irregular moons and also has the distinction of orbiting backward in relation to the other moons. Saturn's large moons appear to have formed from gas and dust orbiting in the planet's equatorial plane. These moons currently orbit Saturn in that same plane.

"By combining Cassini data with modeling techniques previously applied to other solar system bodies, we've been able to go back in time and clarify why it is so different from the rest of the Saturn system," said Jonathan Lunine, a co-author on the study and a Cassini team member at Cornell University, Ithaca, N.Y.

Analyses suggest that Phoebe was born within the first 3 million years of the birth of the solar system, which occurred 4.5 billion years ago. The moon may originally have been porous but appears to have collapsed in on itself as it warmed up. Phoebe developed a density 40 percent higher than the average inner Saturnian moon.

Objects of Phoebe's size have long been thought to form as "potato-shaped" bodies and remained that way over their lifetimes. If such an object formed early enough in the solar system's history, it could have harbored the kinds of radioactive material that would produce substantial heat over a short timescale. This would warm the interior and reshape the moon.

"From the shape seen in Cassini images and modeling the likely cratering history, we were able to see that Phoebe started with a nearly spherical shape, rather than being an irregular shape later smoothed into a sphere by impacts," said co-author Peter Thomas, a Cassini team member at Cornell.

Phoebe likely stayed warm for tens of millions of years before freezing up. The study suggests the heat also would have enabled the moon to host liquid water at one time. This could explain the signature of water-rich material on Phoebe's surface previously detected by Cassini.

The new study also is consistent with the idea that several hundred million years after Phoebe cooled, the moon drifted toward the inner solar system in a solar-system-wide rearrangement. Phoebe was large enough to survive this turbulence.

More than 60 moons are known to orbit Saturn, varying drastically in shape, size, surface age and origin. Scientists using both ground-based observatories and Cassini's cameras continue to search for others.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for the agency's Science Mission Directorate in Washington. The California Institute of Technology in Pasadena manages JPL for NASA.

For more information on the Cassini mission, visit: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

Thursday, April 26, 2012

NASA's WISE Catches Aging Star Erupting With Dust

It's a dust bunny of cosmic proportions. Astronomers used images from NASA's Wide-field Infrared Survey Explorer, or WISE, to locate an aging star shedding loads of dust (orange dot at upper left). Only one other star, called Sakurai's object, has been caught erupting with such large amounts of dust. The process is a natural part of aging for stars like our sun. As they puff up into red giants, they shed dust that is later recycled back into other stars, planets, and in the case of our solar system, living creatures. Image credit: NASA/JPL-Caltech. Full image and caption

PASADENA, Calif. -- Images from NASA's Wide-field Infrared Survey Explorer (WISE) reveal an old star in the throes of a fiery outburst, spraying the cosmos with dust. The findings offer a rare, real-time look at the process by which stars like our sun seed the universe with building blocks for other stars, planets and even life.

The star, catalogued as WISE J180956.27-330500.2, was discovered in images taken during the WISE survey in 2010, the most detailed infrared survey to date of the entire celestial sky. It stood out from other objects because it glowed brightly with infrared light. When compared to images taken more than 20 years ago, astronomers found the star was 100 times brighter.

"We were not searching specifically for this phenomenon, but because WISE scanned the whole sky, we can find such unique objects," said Poshak Gandhi of the Japan Aerospace Exploration Agency (JAXA), lead author of a new paper to be published in the Astrophysical Journal Letters.

Results indicate the star recently exploded with copious amounts of fresh dust, equivalent in mass to our planet Earth. The star is heating the dust and causing it to glow with infrared light.

"Observing this period of explosive change while it is actually ongoing is very rare," said co-author Issei Yamamura of JAXA. "These dust eruptions probably occur only once every 10,000 years in the lives of old stars, and they are thought to last less than a few hundred years each time. It's the blink of an eye in cosmological terms."

The aging star is in the "red giant" phase of its life. Our own sun will expand into a red giant in about 5 billion years. When a star begins to run out of fuel, it cools and expands. As the star puffs up, it sheds layers of gas that cool and congeal into tiny dust particles. This is one of the main ways dust is recycled in our universe, making its way from older stars to newborn solar systems. The other way, in which the heaviest of elements are made, is through the deathly explosions, or supernovae, of the most massive stars.

"It's an intriguing glimpse into the cosmic recycling program," said Bill Danchi, WISE program scientist at NASA Headquarters in Washington. "Evolved stars, which this one appears to be, contribute about 50 percent of the particles that make up humans."

Astronomers know of one other star currently pumping out massive amounts of dust. Called Sakurai's Object, this star is farther along in the aging process than the one discovered recently by WISE.

After Poshak and his team discovered the unusual, dusty star with WISE, they went back to look for it in previous infrared all-sky surveys. The object was not seen at all by the Infrared Astronomical Satellite (IRAS), which flew in 1983, but shows up brightly in images taken as part of the Two Micron All-Sky Survey (2MASS) in 1998.

Poshak and his colleagues calculated the star appears to have brightened dramatically since 1983. The WISE data show the dust has continued to evolve over time, with the star now hidden behind a very thick veil. The team plans to follow up with space- and ground-based telescopes to confirm its nature and to better understand how older stars recycle dust back into the cosmos.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages and operates WISE for NASA's Science Mission Directorate in Washington. The spacecraft was put into hibernation mode after it scanned the entire sky twice, completing its main objectives. The principal investigator for WISE, Edward Wright, is at the University of California, Los Angeles. The mission was selected competitively under NASA's Explorers Program managed by the agency's Goddard Space Flight Center in Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory in Logan, Utah. The spacecraft was built by Ball Aerospace & Technologies Corp. in Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology (Caltech) in Pasadena. Caltech manages JPL for NASA.

The IRAS mission was a collaborative effort between NASA (JPL), the Netherlands and the United Kingdom. The 2MASS mission was a joint effort between Caltech, the University of Massachusetts and NASA (JPL). Data are archived at the Infrared Processing and Analysis Center at Caltech.

More information about WISE is online at: http://www.nasa.gov/wise , http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise


Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

J.D. Harrington 202-358-0321
Headquarters, Washington
j.d.harrington@nasa.gov

Dusty Stellar Nurseries from the Dark Side of a Galaxy

Left-hand panel: The red colors in this image show the galaxy M66 as it appears at the sub-mm wavelength of 850 microns, while the white background shows the galaxy as it appears in visible light. Regions of cold dust that appear as dark streaks in the white image glow brightly in the red image. Right-hand panel: The SCUBA-2 image at 850 microns seen on its own. Credit: VLT/ESO, JAC, G. Bendo. Full size image (JPG, 1.5MB)

One of the world’s most powerful cameras, SCUBA-2 is producing its first detailed images of our neighbouring galaxies, revealing previously undetected vast pockets of star formation where the next generation of stars is being created. The light from these stars is usually obscured by dust, but at the sub-millimetre wavelengths that the camera is designed for, these dust lanes actually glow brightly. The images are revealed in the week of the 25th anniversary of the James Clerk Maxwell Telescope (27 April 2012) on which SCUBA-2 is mounted.

"This exquisite image from the galaxy M66 in the constellation Leo is exactly the promising start we were hoping for," said Dr. Stephen Serjeant, the team's co-leader from The Open University. "This is a wonderfully exciting taste of things to come."

When looking up at the Milky Way, an irregular pattern of dark regions obscures the light of the stars. The dark patches are caused by clouds of dust trailing through the spiral arms and blocking out the starlight that would otherwise reveal vast pockets of star formation, or stellar nurseries. These dark lanes are not exclusive to the Milky Way, but can be found in all spiral galaxies.

SCUBA-2, led by STFC’s UK Astronomy Technology Centre in Edinburgh is the most powerful camera ever developed for observing light at sub-milimetre wavelengths, 1000 times longer than we can see with our eyes. This makes it possible to detect stellar nurseries usually obscured by dust that are so remote the light they emit left them within the first billion years after the big bang.

University of Edinburgh astrophysicist Professor James Dunlop said: "These beautiful new images from SCUBA-2 show energy conservation in action, as the same dust which absorbs the blue optical light (obscuring the stars in the optical images) can be seen to re-emit at the much longer wavelengths accessible to SCUBA-2."

This image promises to be the first of many stunning results from the James Clerk Maxwell Telescope Nearby Galaxy Legacy Survey (NGLS). The main aim of the survey is to understand how the broader environment of a galaxy affects its gas and dust content. For example, galaxies in dense clusters can lose their gas and dust through interactions with other galaxies in the cluster or simply by the head wind they feel while moving through the hot gas trapped inside the cluster. The NGLS is an international collaboration led by astronomers from Canada, the Netherlands, and the United Kingdom which is using SCUBA-2 to observe 150 galaxies in the local universe.

The NGLS team has spent much of the last five years studying molecular hydrogen emission using another instrument on the James Clerk Maxwell Telescope. "It is very exciting to now see the first results from the SCUBA-2 side of our programme starting to come in," says Professor Christine Wilson, the Principal Investigator from McMaster University in Canada. "We have a unique sample of galaxies that we are studying and having SCUBA-2 data will let us measure their gas and dust content. Gas and dust usually go hand-in-hand in galaxies, but from time to time, you find a surprise."

The James Clerk Maxwell Telescope
is situated at 14,000 feet atop Mauna Kea in Hawaii

Credit: Nik Szymanek. Full size image (JPG, 1.5MB)


Notes

Sub-millimetre Light

Sub-millimetre wavelengths are much smaller wavelengths than emitted by a typical radio station, but longer wavelengths than light waves or infrared wavelengths.

They are typically measured in microns, also called micrometres. One micron is one millionth of a metre, one 10,000th of a centimetre, or one 25,000th of an inch.

Submillimetre astronomy is most sensitive to very cold gas and dust. For example, a source with a temperature of 10 K (-263°C) emits most of its energy in a broad spectral region centred around 300 microns. Such very cold material is associated with objects in formation, that is, the mysterious earliest evolutionary stages of galaxies, stars and planets. If one wants to understand the origins of these most fundamental of astronomical structures, the submillimetre is the waveband of choice.

SCUBA-2 Key Facts

  • Size: 3m (height), 2.4m (width), 2.6m (depth)
  • Weight: 4.5 tonnes (about three times the weight of a typical car)
  • Temperature of detectors: 0.1K = -272.9°C = -459.2°F
  • Submillimetre camera with 5120 pixels (4 sub arrays x 1280 pixels) at each wavelength band
  • Provides a unique wide-field submillimetre imaging capability at 450 and 850 microns
  • Hundreds of times faster at mapping large areas of sky than predecessor SCUBA to the same signal-to-noise
  • Uses superconducting transition edge sensors as the light-sensitive elements
  • Addresses a wide-range of scientific issues including how galaxies, stars and planets form
  • Acts as a wide-field "pathfinder" for the new generation of submillimetre interferometers (e.g. SMA and ALMA)
A 2001 survey by the US-based Space Telescope Science Institute revealed that scientific results from SCUBA-2's predecessor, SCUBA had been cited almost as often as those from the Hubble Space Telescope, and much more so than those from any other ground-based facility or satellite project.

The project was funded by the Science and Technology Facilities Council (STFC), the Joint Astronomy Centre (JAC), and the Canada Foundation for Innovation (CFI).

James Clerk Maxwell Telescope

  • The James Clerk Maxwell Telescope (JCMT) is the world's largest single-dish submillimetre-wave telescope.
  • It collects faint submillimetre-wavelength signals with its 15 metre diameter dish.
  • It is situated near the summit of Mauna Kea on the Big Island of Hawaii, at an altitude of approximately 4000 metres (14000 feet) above sea level.
  • It is operated by the Joint Astronomy Centre, on behalf of the UK Science and Technology Facilities Council, the Canadian National Research Council, and the Netherlands Organisation for Scientific Research.

The JCMT webpage can be found at http://www.jach.hawaii.edu/JCMT/

McMaster University

The McMaster Physics and Astronomy webpage can be found at www.physics.mcmaster.ca

The Open University

The Open University Physical Sciences webpage can be found at www8.open.ac.uk/science/physical-science

Leiden Observatory
The Leiden Observatory webpage can be found at www.strw.leidenuniv.nl

Images

Images can be found and downloaded here


Contacts

Lucy Stone
Press Officer
STFC Rutherford Appleton Laboratory
Tel: +44 (0)1235 445627/07920870125

Please note that it is best to contact these individuals by email.

Stephanie Hills
STFC Media Manager
Desk: +44 (0)1235 445398
Email: stephanie.hills@stfc.ac.uk

Dr Holly Thomas
Joint Astronomy Centre
Desk: +1 808-969-6531
Fax: +1 808-961-6516
Email: h.thomas@jach.hawaii.edu

Science Contacts

Please note that it is best to contact these individuals by email.

Prof. Christine Wilson (NGLS PI)
Department of Physics and Astronomy,
McMaster University,
Hamilton, Ontario, L8S 4M1
Canada
Tel: 1 905 525 9140 (ext)27483
email: wilson@physics.mcmaster.ca

Dr Stephen Serjeant
Head of Astronomy,
The Open University,
Milton Keynes, MK7 6AA, UK
Tel: +44 (0)1908 652724
Mob: +44 (0)7946 605913
email: s.serjeant@open.ac.uk

Dr Antonio Chrysostomou
Associate Director, JCMT
Joint Astronomy Centre
Desk: +1 808-969-6512
Email: a.chrysostomou@jach.hawaii.edu

Further Information

University of Edinburgh's Institute for Astronomy
The Open University
McMaster University
Leiden Observatory
STFC

Under 'dark halo' old galaxies have many more stars

Omega Centauri: the tiny red stars (blue is hot red is cold) are just the sort of faint stars that can be imaged in a nearby cluster like this one but cannot be seen in distant galaxies. However, by measuring their combined mass contribution it is possible to discover that old galaxies are dominated by little red stars like these. Click here for a high res image

Some of the oldest galaxies in the Universe have three times more stellar mass, and so many more stars, than all current models of galaxy evolution predict. The finding comes from the Atlas3D international team, led by Michele Cappellari (Oxford), and including ASTRON astronomers Paolo Serra, Raffaella Morganti and Tom Oosterloo, who found a way to remove the 'halo' of dark matter that has clouded previous calculations.

The team's analysis means that all current models, which assumed for decades that the light we observe from a galaxy can be used to infer its stellar mass, will have to be revised. It also suggests that researchers have a new riddle to ponder: exactly how galaxies forming so early in the life of the Universe got to be massive so fast. A report of the research is published in this week's Nature.

'The light we see from galaxies is just the tip of the iceberg, but what we really need to measure are galaxy masses that all models directly predict,' said Dr Michele Cappellari of Oxford University's Department of Physics, who led the work. 'Galaxies can contain huge numbers of small stars, planets or black holes that have lots of mass but give out very little or no light at all. Up until now models assumed that stellar light could be used to infer the stellar masses and any remaining discrepancy with the observed total mass could be hidden behind a 'halo' of dark matter. Our analysis shows that they can't hide any longer: galaxies are diverse and some have many more stars and are even stranger than we'd assumed.'

Up to now the key limitation on what it was possible to say about the stellar mass of galaxies was the difficulty in separating this out from the mass contributed by dark matter. Various attempts from independent groups failed to provide a conclusive answer. he new analysis succeeded thanks to the availability of two-dimensional maps of stellar motions for a large sample of galaxies, combined with sophisticated models. By disentangling stellar mass from dark matter the team was able to show that instead of the relationship between observable light and stellar mass being universal, it varies between different types of galaxies - with some older galaxies having three times the mass suggested by the light they give off.

'The question of how you should turn light from a galaxy into a prediction of its mass has been hotly debated but up until now nobody has been able to kill off the idea that there's a simple and universal way to convert observed light into mass,' said Dr Cappellari. 'We now think we've done that by eliminating the 'fuzziness' in models caused by dark matter. It's exciting because it reveals how much more there is to discover about how galaxies, and the early Universe itself, evolved.'

This research is part of the Atlas3D project and is part-funded by the Science and Technology Facilities Council, the UK sponsors of astronomy and of the William Herschel Telescope (WHT) which was used by the team. More information about the project and its team can be found here: http://www-astro.physics.ox.ac.uk/atlas3d/

For more information contact Prof. Dr Tom Oosterloo +31 (0)521 595 100 or Dr Michele Cappellari +44 (0)1865 273647

Wednesday, April 25, 2012

Dawn Reveals Secrets of Giant Asteroid Vesta

These composite images from the framing camera aboard NASA’s Dawn spacecraft show three views of a terrain with ridges and grooves near Aquilia crater in the southern hemisphere of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption - View animation

These composite images from the framing camera aboard NASA’s Dawn spacecraft show three views of the comparatively fresh crater named Vibidia on the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

These images of Tarpeia crater, near the south pole of the giant asteroid Vesta, were obtained by the visible and infrared mapping spectrometer on NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCLA/INAF. Full image and caption

This colorized image from NASA’s Dawn mission shows temperature variations at Tarpeia crater, near the south pole of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCLA/INAF. Full image and caption

This set of images from NASA's Dawn mission shows topography of the southern hemisphere of the giant asteroid Vesta and a map of Vesta’s gravity variations that have been adjusted to account for Vesta’s shape. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

PASADENA, Calif. – Findings from NASA's Dawn spacecraft reveal new details about the giant asteroid Vesta, including its varied surface composition, sharp temperature changes and clues to its internal structure. The findings were presented today at the European Geosciences Union meeting in Vienna, Austria, and will help scientists better understand the early solar system and processes that dominated its formation.

Images from Dawn's framing camera and visible and infrared mapping spectrometer, taken 420 miles (680 kilometers) and 130 miles (210 kilometers) above the surface of the asteroid, show a variety of surface mineral and rock patterns. Coded false-color images help scientists better understand Vesta's composition and enable them to identify material that was once molten below the asteroid's surface.

Researchers also see breccias, which are rocks fused during impacts from space debris. Many of the materials seen by Dawn are composed of iron- and magnesium-rich minerals, which often are found in Earth's volcanic rocks. Images also reveal smooth pond-like deposits, which might have formed as fine dust created during impacts settled into low regions.

"Dawn now enables us to study the variety of rock mixtures making up Vesta's surface in great detail," said Harald Hiesinger, a Dawn participating scientist at Münster University in Germany. "The images suggest an amazing variety of processes that paint Vesta's surface."

At the Tarpeia crater near the south pole of the asteroid, Dawn imagery revealed bands of minerals that appear as brilliant layers on the crater's steep slopes. The exposed layering allows scientists to see farther back into the geological history of the giant asteroid.

The layers closer to the asteroid's surface bear evidence of contamination from space rocks bombarding Vesta. Layers below preserve more of their original characteristics. Frequent landslides on the slopes of the craters also have revealed other hidden mineral patterns.

"These results from Dawn suggest Vesta's 'skin' is constantly renewing," said Maria Cristina De Sanctis, lead of the visible and infrared mapping spectrometer team based at Italy's National Institute for Astrophysics in Rome.

Dawn has given scientists a near 3-D view into Vesta's internal structure. By making ultra-sensitive measurements of the asteroid's gravitational tug on the spacecraft, Dawn can detect unusual densities within its outer layers. Data now show an anomalous area near Vesta's south pole, suggesting denser material from a lower layer of Vesta has been exposed by the impact that created a feature called the Rheasilvia basin. The lighter, younger layers coating other parts of Vesta's surface have been blasted away in the basin.

Dawn obtained the highest-resolution surface temperature maps of any asteroid visited by a spacecraft. Data reveal temperatures can vary from as warm as minus 10 degrees Fahrenheit (minus 23 degrees Celsius) in the sunniest spots to as cold as minus 150 degrees Fahrenheit (minus 100 degrees Celsius) in the shadows. This is the lowest temperature measurable by Dawn's visible and infrared mapping spectrometer. These findings show the surface responds quickly to illumination with no mitigating effect of an atmosphere.

"After more than nine months at Vesta, Dawn's suite of instruments has enabled us to peel back the layers of mystery that have surrounded this giant asteroid since humankind first saw it as just a bright spot in the night sky," said Carol Raymond, Dawn deputy principal investigator at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We are closing in on the giant asteroid's secrets."

Launched in 2007, Dawn began its exploration of the approximately 330-mile-wide (530-kilometers) asteroid in mid-2011. The spacecraft's next assignment will be to study the dwarf planet Ceres in 2015. These two icons of the asteroid belt have been witness to much of our solar system's history.

Dawn's mission is managed by JPL for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Ala. UCLA is responsible for overall Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are international partners on the mission team. The California Institute of Technology in Pasadena manages JPL for NASA.

To view the new images and for more information about Dawn, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov .


Jia-Rui Cook
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0850
jccook@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

Do the Milky Way’s companions spell trouble for dark matter?

The galaxy pair UGC 9618 / VV 340, two spiral galaxies at the beginning of a collision. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

The Milky Way, the galaxy we live in, consists of around three hundred thousand million stars as well as large amounts of gas and dust arranged with arms in a flat disk that wind out from a central bar. The diameter of the main part of the Milky Way is about 100,000 light years, meaning that a beam of light takes 100,000 years to travel across it. A number of smaller satellite galaxies and spherical clusters of stars (so-called globular clusters) orbit at various distances from the main Galaxy.

Conventional models for the origin and evolution of the universe (cosmology) are based on the presence of ‘dark matter’, invisible material thought to make up about 23% of the content of the cosmos that has never been detected directly. In this model, the Milky Way is predicted to have far more satellite galaxies than are actually seen.

In their effort to understand exactly what surrounds our Galaxy, the scientists used a range of sources from twentieth century photographic plates to images from the robotic telescope of the Sloan Deep Sky Survey. Using all these data they assembled a picture that includes bright ‘classical’ satellite galaxies, more recently detected fainter satellites and the younger globular clusters.

“Once we had completed our analysis, a new picture of our cosmic neighbourhood emerged”, says Pawlowski. The astronomers found that all the different objects are distributed in a plane at right angles to the galactic disk. The newly-discovered structure is huge, extending from as close as 33,000 light years to as far away as one million light years from the centre of the Galaxy.

Team member Pavel Kroupa, professor for astronomy at the University of Bonn, adds “We were baffled by how well the distributions of the different types of objects agreed with each other”. As the different companions move around the Milky Way, they lose material, stars and sometimes gas, which forms long streams along their paths. The new results show that this lost material is aligned with the plane of galaxies and clusters too. “This illustrates that the objects are not only situated within this plane right now, but that they move within it”, says Pawlowski. “The structure is stable.”

The various dark matter models struggle to explain this arrangement. “In the standard theories, the satellite galaxies would have formed as individual objects before being captured by the Milky Way”, explains Kroupa. “As they would have come from many directions, it is next to impossible for them to end up distributed in such a thin plane structure.”

Postdoctoral researcher and team member Jan Pflamm-Altenburg suggests an alternative explanation. “The satellite galaxies and clusters must have formed together in one major event, a collision of two galaxies.” Such collisions are relatively common and lead to large chunks of galaxies being torn out due to gravitational and tidal forces acting on the stars, gas and dust they contain, forming tails that are the birthplaces of new objects like star clusters and dwarf galaxies.

Pawlowski adds, “We think that the Milky Way collided with another galaxy in the distant past. The other galaxy lost part of its material, material that then formed our Galaxy’s satellite galaxies and the younger globular clusters and the bulge at the galactic centre. The companions we see today are the debris of this 11 billion year old collision.”

Kroupa concludes by highlighting the wider significance of the new work. “Our model appears to rule out the presence of dark matter in the universe, threatening a central pillar of current cosmological theory. We see this as the beginning of a paradigm shift, one that will ultimately lead us to a new understanding of the universe we inhabit.”


Images, movie and captions

Images of two galaxies that are set to collide, a galaxy being torn apart in a collision and a movie showing the distribution of satellite galaxies and globular clusters around the Milky Way can be downloaded from http://www.astro.uni-bonn.de/~mpawlow/pr2012.html (The animation is also available on YouTube at http://youtu.be/nUwxv-WGfHM)


Science contacts

Dipl.-Phys. Marcel S. Pawlowski
Argelander-Institut fuer Astronomie der Universitaet Bonn
Tel: +49 228 73 3649
Email: mpawlow@astro.uni-bonn.de

Dr. Jan Pflamm-Altenburg
Argelander-Institut fuer Astronomie der Universitaet Bonn
Tel: +49 228 73 3461
Email: jpflamm@astro.uni-bonn.de

Professor Dr Pavel Kroupa
Argelander-Institut fuer Astronomie der Universitaet Bonn
Tel: 49 228 73 6140
Email: pavel@astro.uni-bonn.de


Media contact

Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
Email: rm@ras.org.uk


Further information

The work appears in "The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way", M. S. Pawlowski, J. Pflamm-Altenburg, P. Kroupa, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper can be downloaded from http://arxiv.org/abs/1204.5176


Notes for editors

The Royal Astronomical Society

The Royal Astronomical Society (RAS: www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

A Cluster Within a Cluster

The star cluster NGC 6604 and its surroundings

PR Image eso1218b
The star cluster NGC 6604 in the constellation of Serpens

PR Image eso1218c
Wide-field view of the sky around the cluster NGC 6604

Videos

Zooming in on the star cluster NGC 6604

Panning across the region of the star cluster NGC 6604

The star cluster NGC 6604 is shown in this new image taken by the Wide Field Imager attached to the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. It is often overlooked in favour of its more prominent neighbour, the Eagle Nebula (also known as Messier 16), that lies a mere wingspan away. But the framing of this picture, which places the star cluster in a landscape of surrounding gas and dust clouds, shows what a beautiful object NGC 6604 is in its own right.

NGC 6604 is the bright grouping towards to the upper left of the image. It is a young star cluster that is the densest part of a more widely scattered association containing about one hundred brilliant blue-white stars [1]. The picture also shows the cluster’s associated nebula — a cloud of glowing hydrogen gas that is called Sh2-54 [2] — as well as dust clouds.

NGC 6604 lies about 5500 light-years away in the constellation of Serpens (The Serpent) and is located about two degrees north of the Eagle Nebula in the night sky (eso0926). The bright stars are easily seen in a small telescope and were first catalogued by William Herschel in 1784. However, the faint gas cloud escaped attention until the 1950s when it was catalogued by Stewart Sharpless on photographs from the National Geographic–Palomar Sky Atlas.

The cluster’s hot young stars are helping a new generation of stars to form in NGC 6604, by collecting star-making material into a compact region with their strong stellar winds and radiation. This second generation of stars will quickly replace the older generation, as although the brightest young stars are massive, they consume their fuel copiously and live short lives.

Aside from aesthetics, NGC 6604 has other reasons to draw the gaze of astronomers, as it has a strange column of hot ionised gas emanating from it. Similar columns of hot gas, which channel outflowing material from young star clusters, have been found elsewhere in the Milky Way and other spiral galaxies, but the example in NGC 6604 is relatively nearby, allowing astronomers to study it in detail.

This particular column (often referred to by astronomers as a “chimney”) is perpendicular to the galactic plane and stretches an incredible 650 light-years in length. Astronomers think that the hot stars within NGC 6604 are responsible for producing the chimney, but more research is needed to fully understand these unusual structures.

Notes

[1] This stellar association is called Serpens OB. The first part of the name refers to the constellation in which it lies and the letters OB refer to the spectral type of the stars. O and B are the two hottest stellar classifications and most stars of these types are very brilliant blue-white stars, and relatively young.

[2] The name Sh2-54 means that the object is the 54th in the second Sharpless catalogue of HII regions, published in 1959.
More information

The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

Photos of the MPG/ESO 2.2-metre Telescope
Other photos taken with the MPG/ESO 2.2-metre Telescope
Photos of La Silla

Contacts

Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 15 37 35 91
Email: rhook@eso.org

Tuesday, April 24, 2012

NASA's Spitzer Finds Galaxy With Split Personality

The infrared vision of NASA's Spitzer Space Telescope has revealed that the Sombrero galaxy -- named after its appearance in visible light to a wide-brimmed hat -- is in fact two galaxies in one. Image credit: NASA/JPL-Caltech. Full image and caption

New observations from NASA's Spitzer Space Telescope reveal the Sombrero galaxy is not simply a regular flat disk galaxy of stars as previously believed, but a more round elliptical galaxy with a flat disk tucked inside. Full image and caption-enlarge image

PASADENA, Calif. -- While some galaxies are rotund and others are slender disks like our spiral Milky Way, new observations from NASA's Spitzer Space Telescope show that the Sombrero galaxy is both. The galaxy, which is a round elliptical galaxy with a thin disk embedded inside, is one of the first known to exhibit characteristics of the two different types. The findings will lead to a better understanding of galaxy evolution, a topic still poorly understood.

"The Sombrero is more complex than previously thought," said Dimitri Gadotti of the European Southern Observatory in Chile and lead author of a new paper on the findings appearing in the Monthly Notices of the Royal Astronomical Society. "The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other."

The Sombrero galaxy, also known as NGC 4594, is located 28 million light-years away in the constellation Virgo. From our viewpoint on Earth, we can see the thin edge of its flat disk and a central bulge of stars, making it resemble a wide-brimmed hat. Astronomers do not know whether the Sombrero's disk is shaped like a ring or a spiral, but agree it belongs to the disk class.

"Spitzer is helping to unravel secrets behind an object that has been imaged thousands of times," said Sean Carey of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena. "It is intriguing Spitzer can read the fossil record of events that occurred billions of years ago within this beautiful and archetypal galaxy."

Spitzer captures a different view of the galaxy than visible-light telescopes. In visible views, the galaxy appears to be immersed in a glowing halo, which scientists had thought was relatively light and small. With Spitzer's infrared vision, a different view emerges. Spitzer sees old stars through the dust and reveals the halo has the right size and mass to be a giant elliptical galaxy.

While it is tempting to think the giant elliptical swallowed a spiral disk, astronomers say this is highly unlikely because that process would have destroyed the disk structure. Instead, one scenario they propose is that a giant elliptical galaxy was inundated with gas more than nine billion years ago. Early in the history of our universe, networks of gas clouds were common, and they sometimes fed growing galaxies, causing them to bulk up. The gas would have been pulled into the galaxy by gravity, falling into orbit around the center and spinning out into a flat disk. Stars would have formed from the gas in the disk.

"This poses all sorts of questions," said Rubén Sánchez-Janssen from the European Southern Observatory, co-author of the study. "How did such a large disk take shape and survive inside such a massive elliptical? How unusual is such a formation process?"

Researchers say the answers could help them piece together how other galaxies evolve. Another galaxy, called Centaurus A, appears also to be an elliptical galaxy with a disk inside it. But its disk does not contain many stars. Astronomers speculate that Centaurus A could be at an earlier stage of evolution than the Sombrero and might eventually look similar.

The findings also answer a mystery about the number of globular clusters in the Sombrero galaxy. Globular clusters are spherical nuggets of old stars. Ellipticals typically have a few thousand, while spirals contain a few hundred. The Sombrero has almost 2,000, a number that makes sense now but had puzzled astronomers when they thought it was only a disk galaxy.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

J.D. Harrington 202-358-0321
Headquarters, Washington
j.d.harrington@nasa.gov

Monday, April 23, 2012

Cassini Sees Objects Blazing Trails in Saturn Ring

This set of six images obtained by NASA's Cassini spacecraft shows trails that were dragged out from Saturn's F ring by objects about a half mile (1 kilometer) in diameter. NASA/JPL-Caltech/SSI/QMUL. Full image and caption

The constant change in Saturn's wavy, wiggly F ring is on display in this set of images obtained by NASA's Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI/QMUL

Images from NASA's Cassini spacecraft have revealed half-mile-sized (kilometer-sized) objects punching through parts of Saturn's F ring, leaving glittering trails behind them. These trails in the rings, which scientists are calling "mini-jets," fill in a missing link in our story of the curious behavior of the F ring. Download video

PASADENA, Calif. – Scientists working with images from NASA's Cassini spacecraft have discovered strange half-mile-sized (kilometer-sized) objects punching through parts of Saturn's F ring, leaving glittering trails behind them. These trails in the rings, which scientists are calling "mini-jets," fill in a missing link in our story of the curious behavior of the F ring. The results will be presented tomorrow at the European Geosciences Union meeting in Vienna, Austria.

"I think the F ring is Saturn's weirdest ring, and these latest Cassini results go to show how the F ring is even more dynamic than we ever thought," said Carl Murray, a Cassini imaging team member based at Queen Mary University of London, England. "These findings show us that the F ring region is like a bustling zoo of objects from a half mile [kilometer] in size to moons like Prometheus a hundred miles [kilometers] in size, creating a spectacular show."

Scientists have known that relatively large objects like Prometheus (as long as 92 miles, or 148 kilometers, across) can create channels, ripples and snowballs in the F ring. But scientists didn't know what happened to these snowballs after they were created, Murray said. Some were surely broken up by collisions or tidal forces in their orbit around Saturn, but now scientists have evidence that some of the smaller ones survive, and their differing orbits mean they go on to strike through the F ring on their own.

These small objects appear to collide with the F ring at gentle speeds – something on the order of about 4 mph (2 meters per second). The collisions drag glittering ice particles out of the F ring with them, leaving a trail typically 20 to 110 miles (40 to 180 kilometers) long. Murray's group happened to see a tiny trail in an image from Jan. 30, 2009 and tracked it over eight hours. The long footage confirmed the small object originated in the F ring, so they went back through the Cassini image catalog to see if the phenomenon was frequent.

"The F ring has a circumference of 550,000 miles [881,000 kilometers], and these mini-jets are so tiny they took quite a bit of time and serendipity to find," said Nick Attree, a Cassini imaging associate at Queen Mary. "We combed through 20,000 images and were delighted to find 500 examples of these rogues during just the seven years Cassini has been at Saturn."

In some cases, the objects traveled in packs, creating mini-jets that looked quite exotic, like the barb of a harpoon. Other new images show grand views of the entire F ring, showing the swirls and eddies that ripple around the ring from all the different kinds of objects moving through and around it.

"Beyond just showing us the strange beauty of the F ring, Cassini's studies of this ring help us understand the activity that occurs when solar systems evolve out of dusty disks that are similar to, but obviously much grander than, the disk we see around Saturn," said Linda Spilker, Cassini project scientist based at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We can't wait to see what else Cassini will show us in Saturn's rings."

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini .

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the mission for NASA's Science Mission Directorate, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colo. JPL is a division of the California Institute of Technology, Pasadena.


Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@jpl.nasa.gov

Hubble Images Searchlight Beams from a Preplanetary Nebula

Egg Nebula
Credit: ESA/Hubble & NASA

The NASA/ESA Hubble Space Telescope has been at the cutting edge of research into what happens to stars like our Sun at the ends of their lives (see for example Hubblecast 51). One stage that stars pass through as they run out of nuclear fuel is the preplanetary, or protoplanetary nebula. This Hubble image of the Egg Nebula shows one of the best views to date of this brief but dramatic phase in a star’s life.

The preplanetary nebula phase is a short period in the cycle of stellar evolution — over a few thousand years, the hot remains of the star in the centre of the nebula heat it up, excite the gas, and make it glow as a planetary nebula. The short lifespan of preplanetary nebulae means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The Egg Nebula, the first to be discovered, was first spotted less than 40 years ago, and many aspects of this class of object remain shrouded in mystery.

At the centre of this image, and hidden in a thick cloud of dust, is the nebula’s central star. While we can’t see the star directly, four searchlight beams of light coming from it shine out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the centre of the nebula.

The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years.

The distance to the Egg Nebula is only known very approximately, the best guess placing it at around 3000 light-years from Earth. This in turn means that astronomers do not have any accurate figures for the size of the nebula (it may be larger and further away, or smaller but nearer).

This image is produced from exposures in visible and infrared light from Hubble’s Wide Field Camera 3.

Source: ESA/Hubble - Space Telescope


Friday, April 20, 2012

Hinode and SOHO Paint An Asymmetrical Picture Of the Sun

In 2008 in the northern hemisphere of the sun (left) Hinode observed large patches of negative polarity, shown in orange. In 2011, the same area showed much smaller patches and a more even distribution of negative and positive (blue) regions. Credit: JAXA/Hinode. View larger - View 2008 only - View 2011 only

Approximately every 11 years the magnetic field on the sun reverses completely – the north magnetic pole switches to south, and vice versa. It’s as if a bar magnet slowly lost its magnetic field and regained it in the opposite direction, so the positive side becomes the negative side. But, of course, the sun is not a simple bar magnet and the causes of the switch, not to mention the complex tracery of moving magnetic fields throughout the eleven-year cycle, are not easy to map out.

Mapping such fields, however, is a crucial part of understanding how – and, in turn, when – the sun will exercise its next flip. This flip coincides with the greatest solar activity seen on the sun in any given cycle, known as "solar maximum."

While the cycle unfolds with seeming regularity every 11 years, in two upcoming papers scientists highlight just how asymmetrical this process actually is. Currently the polarity at the north of the sun appears to have decreased close to zero – that is, it seems to be well into its polar flip from magnetic north to south -- but the polarity at the south is only just beginning to decrease.

"Right now, there's an imbalance between the north and the south poles," says Jonathan Cirtain, a space scientist at NASA's Marshall Space Flight Center in Huntsville, Ala., who is also NASA's project scientist for a Japanese solar mission called Hinode. "The north is already in transition, well ahead of the south pole, and we don't understand why."

One of the two papers relies on Hinode data that shows direct observations of this polar switch. The other paper makes use of a new technique observing microwave radiation from the sun's polar atmosphere to infer the magnetic activity on the surface. The asymmetry described in the papers belies models of the sun that assume that the sun's north and south polarities switch at the same time. In addition, both papers agree that the switch is imminent at the north pole, well in advance of general predictions that solar maximum for this cycle will occur in 2013. Lastly, the direct Hinode results also suggest a need to re-examine certain other solar models as well.

Measuring the magnetic activity near the poles isn't easy because all of our solar telescopes view the sun approximately at its equator, offering only an oblique view of the poles, when they require a top-down view for accurate magnetic measurements. Hinode can observe this activity annually with its high resolution Solar Optical Telescope that can map magnetic fields when observing them from near the equator. The microwave radiation technique described in the second paper makes use of the discovery in 2003 that as the sun moves toward solar maximum, giant eruptions on the sun, called prominence eruptions – which during solar minimum, are concentrated at lower solar latitudes -- begin to travel toward higher latitudes near the poles. In addition, the polar brightness in the microwave wavelengths declines to very low values.

"These prominence eruptions are associated with increased solar activity such as coronal mass ejections or CMEs, so CMEs originating from higher latitudes also point to an oncoming solar maximum," says Nat Gopalswamy. Gopalswamy is a solar scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. who is the first author on the microwave observations paper, which was accepted by The Astrophysical Journal on April 11, 2012. "When we start to see prominence eruptions above 60 degrees latitude on the sun, then we know that we are reaching solar maximum."

These images from Hinode show magnetism in the southern hemisphere in 2009 (left) and 2011 (right). The large blue patches show regions of positive polarity, which remain present even in 2011. Credit: JAXA/Hinode. View larger - View 2009 only - View 2011 only

To look at the prominence eruptions toward the poles, Gopalswamy and his team used observations from Japan's Nobeyama Solar Radio Observatory telescopes and the joint ESA/NASA mission the Solar Heliospheric Observatory (SOHO). They watched the sun in the microwave wavelengths – which are used to observe the area of the sun's atmosphere just above the surface, known as the chromosphere. Gopalswamy created precise techniques to use such microwave radiation to measure the intensity of magnetic activity on the sun's surface at the poles. By mapping the brightness of the microwave radiation throughout the chromosphere, the scientists showed that the intensity at the north pole has already dropped to the threshold that was reached in the last solar maximum cycle, suggesting the onset of solar max there. This is backed by the fact that prominence eruptions are also occurring at high latitudes in the north. Eruption activity in the south half of the sun, however, is only just beginning to increase – the first CME occurred there in early March 2012.

The Hinode data also shows this discrepancy between the north and the south. The Hinode results are reported by a Japanese team, led by Daikou Shiota a solar scientist at RIKEN Institute of Physics and Chemical Research, and were recently submitted to The Astrophysical Journal for publication. Shiota and his team used Hinode to observe the magnetic map of the poles every month since September of 2008. Early maps showed large, strong concentrations of magnetic fields that are almost all magnetically negative in polarity. Recent maps, however, show a different picture. Not only are the patches of magnetism smaller and weaker, but now there is a great deal of positive polarity visible as well. What once pointed to a strongly negative north pole, is now a weakly magnetized, mixed pole that will become neutral – which occurs at solar maximum -- within the month according to the team's predictions.

"This is the first direct observation of this field reversal," says Cirtain. "And it is extremely important to understanding how the sun's magnetism generates the solar cycle."

Ted Tarbell is the principal investigator for Hinode's Solar Optical Telescope at Lockheed Martin in Palo Alto, Calif., and he points out that the direct measurements showed the progress of the pole reversal, and highlights the earlier portion of the cycle in 2008. Typical models of the magnetic flip, suggest that as active regions rotate around the equator, their higher, trailing edge – which is almost always the opposite polarity from the pole in their hemisphere – drift upward, eventually dominating the status quo and turning positive to negative or negative to positive. The Hinode data show that this transition at the north began before such drifting had a chance to occur.

"This is one of the most interesting things in this Hinode paper to me," says Tarbell. "How did the polar reversal start so early, even though the onset of the solar cycle, that is, increased activity at lower latitudes, hadn't begun yet?"

Tarbell thinks these observations mean that this model, too, may need to be re-examined.

Such adjustments to models are of course expected whenever new and better data is collected. Indeed, David Hathaway, who is a solar scientist at NASA's Marshall, and who is a co-author on the microwave observations paper with Gopalswamy, points out that the idea that asymmetries exist in the sun is not completely new. Other work has recently emphasized symptoms of this asymmetry, measuring, for example, more sunspots in the northern hemisphere than in the south at the moment. "But most of the well-developed models don't incorporate the asymmetry in them," Hathaway says. "More complicated models that incorporate asymmetries do exist, but they have other ways in which they fail to match observations."

Continued study on these differences, using the best observatories as well as new techniques for analysis will help expand and improve our understanding of the sun, its 11-year cycle, and the great eruptions that occur on its surface.

Scientists will also keep their eye on the current cycle – numbered Solar Cycle 24 – because a polar switch at the north that is sooner than was expected also implies this may be a fairly small cycle in terms of the number of sunspots and amount of solar activity.

Related Links:
Hinode site
SOHO site

Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, MD