When It Comes to Galaxies, Diversity Is Everywhere

Credit: NASA, ESA, J. Dalcanton and B. Williams
(University of Washington)

ABOUT THIS IMAGE: These images taken with NASA's Hubble Space Telescope are close-up views of four galaxies from a large survey of nearby galaxies.

The galaxies have very different masses and sizes and showcase the diversity of galaxies found in the ANGST study. Although the galaxies are separated by many light-years, they are presented as if they are all at the same distance to show their relative sizes.

The images, taken with Hubble's Advanced Camera for Surveys, reveal rich detail in the stellar populations and in the interstellar dust scattered between the stars. Hubble's sharp views reveal the colors and brightnesses of individual stars, which astronomers used to derive the history of star formation in each galaxy.

In the composite image at the top, NGC 253 is ablaze with the light from thousands of young, blue stars. The spiral galaxy is undergoing intense star formation. The image demonstrates the sharp "eye" of the Advanced Camera, which resolved individual stars. The dark filaments are clouds of dust and gas. NGC 253 is the dominant galaxy in the Sculptor Group of galaxies and it resides about 13 million light-years from Earth.

In the view of the spiral galaxy NGC 300, second from top, young, blue stars are concentrated in spiral arms that sweep diagonally through the image. The yellow blobs are glowing hot gas that has been heated by radiation from the nearest young, blue stars. NGC 300 is a member of the Sculptor Group of galaxies and it is located 7 million light-years away.

The dark clumps of material scattered around the bright nucleus of NGC 3077, the small, dense galaxy at bottom, left, are pieces of wreckage from the galaxy's interactions with its larger neighbors. NGC 3077 is a member of the M81 group of galaxies and it resides 12.5 million light-years from Earth.

The image at bottom, right, shows a swarm of young, blue stars in the diffuse dwarf irregular galaxy NGC 4163. NGC 4163 is a member of a group of dwarf galaxies near our Milky Way and is located roughly 10 million light-years away.

These galaxies are part of a detailed survey called the ACS Nearby Galaxy Survey Treasury program (ANGST). In the census, Hubble observed roughly 14 million stars in 69 galaxies. The survey explored a region called the "Local Volume," and the galaxy distances ranged from 6.5 million light-years to 13 million light-years from Earth. The Local Volume resides beyond the Local Group of galaxies, an even nearer collection of a few dozen galaxies within about 3 million light-years of our Milky Way Galaxy.

There's an old saying in astronomy: "Galaxies are like people. They're only normal until you get to know them." That view is supported by a group of astronomers after using NASA's Hubble Space Telescope to study a large number of galaxies in our cosmic backyard.

The detailed survey, called the ACS Nearby Galaxy Survey Treasury (ANGST) program, observed roughly 14 million stars in 69 galaxies. The survey explored a region called the "Local Volume," and the galaxy distances ranged from 6.5 million light-years to 13 million light-years from Earth. The Local Volume resides beyond the Local Group of galaxies, an even nearer collection of a few dozen galaxies within about 3 million light-years of our Milky Way Galaxy.

A typical galaxy contains billions of stars but looks "smooth" when viewed through a telescope, because the stars are blurred together. In contrast, the galaxies in the new survey are close enough to Earth that the sharp "eyes" of Hubble's Advanced Camera for Surveys and Wide Field Planetary Camera 2 can resolve their brightest individual stars. By measuring the brightness and colors of these stars, scientists can derive the local history of star formation within a galaxy and can tease out subtle features in a galaxy's shape.

"Past Hubble observations of the local neighborhood have provided dramatic insights into the star-formation histories of individual galaxies, but the number of galaxies studied in detail has been rather small," said Julianne Dalcanton of the University of Washington in Seattle and leader of the ANGST survey. The survey's results were submitted to The Astrophysical Journal Supplement Series. Another paper that details the star-formation history in galaxy M81 has been submitted to The Astronomical Journal.

"Instead of picking and choosing particular galaxies to study, our survey will be complete by virtue of looking at 'all' the galaxies in the region. This gives us a multi-color picture of when and where all the stars in the local universe formed."

Many stars in nearby galaxies are the fossil equivalents of the active star formation seen in galaxies in the distant universe. "When we look back in time at distant, young galaxies, we see lots of vigorous star formation. However, we can only guess as to what those galaxies might eventually turn into," Dalcanton explained. "Using the galaxies in the nearby universe as a 'fossil record,' we can compare them with young galaxies far away. This comparison gives us a history of star formation and provides a better understanding of the masses, structures, and environments of the galaxies."

Early results of the ANGST survey show the rich diversity of galaxies. Some galaxies are made up entirely of ancient stars, while others have been forming stars nearly continuously during their whole lives. There are even a few examples of galaxies that have only started forming stars in the recent past.

"With these images, we can see what makes each galaxy unique," said team member Benjamin Williams of the University of Washington. "When we look at the distribution and development of stars in each survey galaxy, we can learn how differences in the galaxies' histories have produced the diversity of galaxy shapes and colors."

The ANGST survey also includes maps of many large galaxies, including M81. "With these maps, we can track when the different parts of the galaxy formed," explained Evan Skillman of the University of Minnesota, describing work by students Dan Weisz of the University of Minnesota and Stephanie Gogarten of the University of Washington.

In a separate paper describing the star-formation history in M81, astronomers confirmed that massive spiral galaxies formed most of their stars in the early universe. Analyzing M81's outer disk, the astronomers found that most of the stars formed more than 7 billion years ago, when the universe was half its present age. M81 and other mammoth galaxies also experienced rapid enrichment of heavy elements, such as carbon, through the deaths of massive stars in supernova explosions. "We were surprised by how quickly the elements formed and how the subsequent star-formation rate for the bulk of the stars in M81 changed after that," said Williams, the paper's lead author.

"This rich survey will add to Hubble's legacy, providing a foundation for future studies," Dalcanton said. "The ANGST sample offers superb targets for future multi-wavelength surveys, which will allow us to combine the star-formation maps with the properties of gas and dust in the galaxies. With this information, we will be able to trace the complete cycle of star formation in detail."

CONTACT
Donna Weaver/Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Julianne Dalcanton
University of Washington, Seattle, Wash.
206-685-2155
jd@astro.washington.edu

Evan Skillman
University of Minnesota, Minneapolis, Minn.
612-624-9523
skillman@astro.umn.edu

The Wild, Hidden Cousin of SN 1987A

ESO PR Photo 32/08

SN 1996cr in Circinus Galaxy

Powerful nearby supernova caught by web of telescopes

Over a decade after it exploded, one of the nearest supernovae in the last 25 years has been identified. This result was made possible by combining data from the vast online archives from many of the world’s premier telescopes.

The supernova, called SN 1996cr, was first singled out in 2001 by Franz Bauer. Bauer noticed a bright, variable source in the Circinus spiral galaxy, using NASA's Chandra X-ray Observatory. Although the source displayed some exceptional properties Bauer and his Penn State colleagues could not identify its nature confidently at the time.

It was not until years later that Bauer and his team were able to confirm that this object was a supernova. Clues from a spectrum obtained by ESO’s Very Large Telescope led the team to start the real detective work of searching through data from 18 different telescopes, both ground- and space-based, nearly all of which existed. Because this object was found in an interesting nearby galaxy, the public archives of these telescopes contained abundant observations.

The data show that SN 1996cr is among the brightest supernovae ever seen in radio and X-rays. It also bears many striking similarities to the famous supernova SN 1987A, which occurred in a neighbouring galaxy only 160 000 light-years from Earth.

“This supernova appears to be a wild cousin of SN 1987A,” says Bauer. “The two look alike in many ways, except this newer supernova is intrinsically a thousand times brighter in radio and X-rays.”

Visible-light images from the archives of the Anglo-Australian Telescope in Australia show that SN 1996cr exploded sometime between 28 February 1995 and 15 March 1996, but it is the only one of the five nearest supernovae of the last 25 years that was not seen shortly after the explosion.

Other major X-ray observatories in orbit like ROSAT and ASCA did not detect SN 1996cr, but since it was first detected by Chandra in 2001 it has become steadily brighter. Previously, SN 1987A was the only known supernova with an X-ray output that increased over time.

“It’s a bit of a coup to find SN 1996cr like this, and we could never have nailed it without the serendipitous data taken by all of these telescopes. We've truly entered a new era of ‘internet astronomy’,” said Bauer.

The combined data, in conjunction with theoretical work, have led the team to develop a model for the explosion. Before the parent star exploded, it cleared out a large cavity in the surrounding gas, either via a strong wind or from an outburst from the star late in its life. So the blast wave from the explosion itself could expand relatively unimpeded into this cavity. Once the blast wave hit the dense material surrounding SN1996cr, the impact caused the system to glow brightly in X-ray and radio emission. The X-ray and radio emission from SN 1987A is probably fainter because the surrounding material is less compact.

Astronomers think that both SN 1987A and SN 1996cr show evidence for these pre-explosion clear-outs by a star doomed to explode. Having two nearby examples suggests that this type of activity could be relatively common during the death of massive stars.

“Not only does our work suggest that SN 1987A isn’t as unusual as previously thought, but it also teaches us more about the tremendous upheavals that massive stars can undergo over their lifetimes,” said co-author Vikram Dwarkadas of the University of Chicago.

Notes

These results will appear in an upcoming issue of The Astrophysical Journal (F.E. Bauer et al., Supernova 1996cr: SN 1987A's Wild Cousin?, http://arxiv.org/abs/0804.3597) Other co-authors on this paper include Niel Brandt (Penn State, USA), Stefan Immler (NASA Goddard Space Flight Center), Norbert Bartel (York University, Canada), and Michael Bietenholz (York University and Hartebeesthoek Radio Observatory, South Africa). Franz Bauer was formerly at Penn State.

The Circinus galaxy is indeed a rather interesting object, with rings of gas being ejected from the galaxy. It is 13 million light-years away from the Milky Way.


Contact

Franz Bauer
Columbia University, USA Phone: +1 718-637-3071
E-mail: feb (at) astro.columbia.edu

ESO Press Officer: Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

The Hibernating Stellar Magnet

ESO PR Photo 31/08

The Hibernating Stellar Magnet

First Optically Active Magnetar-Candidate Discovered

Astronomers have discovered a most bizarre celestial object that emitted 40 visible-light flashes before disappearing again. It is most likely to be a missing link in the family of neutron stars, the first case of an object with an amazingly powerful magnetic field that showed some brief, strong visible-light activity.

This weird object initially misled its discoverers as it showed up as a gamma-ray burst, suggesting the death of a star in the distant Universe. But soon afterwards, it exhibited some unique behaviour that indicates its origin is much closer to us. After the initial gamma-ray pulse, there was a three-day period of activity during which 40 visible-light flares were observed, followed by a brief near-infrared flaring episode 11 days later, which was recorded by ESO's Very Large Telescope. Then the source became dormant again.

"We are dealing with an object that has been hibernating for decades before entering a brief period of activity", explains Alberto J. Castro-Tirado, lead author of a paper in this week's issue of Nature.

The most likely candidate for this mystery object is a 'magnetar' located in our own Milky Way galaxy, about 15 000 light-years away towards the constellation of Vulpecula, the Fox. Magnetars are young neutron stars with an ultra-strong magnetic field a billion billion times stronger than that of the Earth. “A magnetar would wipe the information from all credit cards on Earth from a distance halfway to the Moon,” says co-author Antonio de Ugarte Postigo. "Magnetars remain quiescent for decades. It is likely that there is a considerable population in the Milky Way, although only about a dozen have been identified."

Some scientists have noted that magnetars should be evolving towards a pleasant retirement as their magnetic fields decay, but no suitable source had been identified up to now as evidence for this evolutionary scheme. The newly discovered object, known as SWIFT J195509+261406 and showing up initially as a gamma-ray burst (GRB 070610), is the first candidate. The magnetar hypothesis for this object is reinforced by another analysis, based on another set of data, appearing in the same issue of Nature.


Notes

Castro-Tirado A. J. et al. 2008, Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars, Nature, 25 September 2008. A paper by Alex Stefanescu et al. in the same issue of Nature confirms the magnetar hypothesis. 

The team is composed of A. J. Castro-Tirado, A. de Ugarte Postigo, J. Gorosabel, M. Jelinek, M. A. Guerrero, F. J. Aceituno, R. Cunniffe, P. Kubanek, S. Vitek (IAA-CSIC, Granada, Spain), T. A. Fatkhullin, V. V. Sokolov, E. Sonbas, S. A. Trushkin, N. N. Bursov, and N. A. Nizhelskij (SAO, Russian Academy of Science), P. Ferrero, D. A. Kann, S. Klose, and S. Schulze (Thuringer Landessternwarte Tautenburg, Germany), D. Sluse (Laboratoire d'Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL) Switzerland), M. Bremer and J.M.Winters (IRAM, Saint Martin d d'Heres, France), D. Nuernberger (ESO, Santiago, Chile), D. Perez-Ramirez (Universidad de Jaen, Spain and University of Leicester, UK), J. French, G. Melady, L. Hanlon, and B. McBreen (University College Dublin, Ireland), K. Leventis and S. B. Markoff (University of Amsterdam, The Netherlands), S. Leon (IRAM, Granada, Spain), A. Kraus (Max-Planck-Institut für Radioastronomie, Bonn, Germany), A. C. Wilson (University of Texas, Austin, USA), R. Hudec (Astronomical Institute of the Czech Academy of Sciences), M. Durant, J.M. Gonzalez-Perez, and T. Shahbaz (IAC, La Laguna, Spain), S. Guziy (Nikolaev State University, Ukraine), S. B. Pandey (Aryabhatta Research Institute of Observational-Sciences, India), L. Pavlenko (Crimean Astrophysical Observatory, Ukraine), C. Sanchez-Fernandez (European Space Astronomy Centre, Madrid, Spain), and L. Sabau-Graziati (INTA, Madrid, Spain). Antonio de Ugarte Postigo is now affiliated with ESO, Chile. 

The 42 scientists used data taken by eight telescopes worldwide, including the BOOTES-2 robotic telescope at EELM-CSIC, the WATCHER telescope at Boyden Observatory (South Africa), the 0.8-m IAC80 at Teide Observatory (Spain), the Flemish 1.2-m Mercator telescope at Observatorio del Roque de los Muchachos (Spain), the Tautenburg 1.34-m telescope (Germany), the 1.5-m at Observatorio de Sierra Nevada (IAA-CSIC), the 6.0-m BTA in Russia, the 8.2-m VLT at ESO in Chile and the IRAM 30-m Pico Veleta y Plateau de Bure telescopes, together with the SWIFT (NASA) and XMM-Newton (ESA) satellites. 

Neutron stars is the bare, condensed remain of a massive star which started its life with between eight and fifteen times the mass of the Sun, but then expelled its outer layers following a supernova explosion. Such stars are only around 20 kilometres in diameter, yet are more massive than the Sun. Magnetars are neutron stars with magnetic fields hundreds of times more intense than the average neutron star fields. The energy release during one flare in the course of a period of activity can amount to the energy released by the Sun in 10 000 years.

Contacts

Alberto J. Castro-Tirado
IAA-CSIC, Granada, Spain
Phone: +34 958 23 05 91
E-mail: ajct (at) iaa.es

Antonio de Ugarte Postigo
ESO, Chile
Phone: +56 2 463 3062
Mobile: +34 61 60 31 753
E-mail: adeugart (at) eso.org

ESO Press Officer: Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

Spacecraft discovers solar winds weakening

This artist's impression shows the Sun's heliosphere.

Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

Credit: NASA, ESA

Data from the Ulysses spacecraft, a joint NASA-European Space Agency mission, show the Sun has reduced its output of solar wind to the lowest levels since accurate readings became available. The Sun's current state could reduce the natural shielding that envelops our solar system.

"The Sun's million mile-per-hour solar wind inflates a protective bubble, or heliosphere, around the solar system. It influences how things work here on Earth and even out at the boundary of our solar system where it meets the galaxy," says Dave McComas, Ulysses' solar wind instrument principal investigator and senior executive director at the Southwest Research Institute in San Antonio. "Ulysses data indicate the solar wind's global pressure is the lowest we have seen since the beginning of the space age."

The Sun's solar wind plasma is a stream of charged particles ejected from the Sun's upper atmosphere. The solar wind interacts with every planet in our solar system. It also defines the border between our solar system and interstellar space.

This border, called the heliopause, is a bubble-shaped boundary surrounding our solar system where the solar wind's strength is no longer great enough to push back the wind of other stars. The region around the heliopause also acts as a shield for our solar system, warding off a significant portion of the cosmic rays outside the galaxy.

"Galactic cosmic rays carry with them radiation from other parts of our galaxy," says Ed Smith, NASA's Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena, California. "With the solar wind at an all-time low, there is an excellent chance the heliosphere will diminish in size and strength. If that occurs, more galactic cosmic rays will make it into the inner part of our solar system."

Galactic cosmic rays are of great interest to NASA. Cosmic rays are linked to engineering decisions for unmanned interplanetary spacecraft and exposure limits for astronauts traveling beyond low-Earth orbit.

In 2007, Ulysses made its third rapid scan of the solar wind and magnetic field from the Sun's south pole to the north pole. When the results were compared with observations from the previous solar cycle, the strength of the solar wind pressure and the magnetic field embedded in the solar wind was found to have decreased by 20 percent. The field strength near the spacecraft has decreased by 36 percent.

"The Sun cycles between periods of great activity and lesser activity," Smith says. "Right now, we are in a period of minimal activity that has stretched on longer than anyone anticipated."

Ulysses was the first mission to survey the space environment over the Sun's poles. Data from Ulysses have forever changed the way scientists view our star and its effects. The venerable spacecraft has lasted more than 17 years, or almost four times its expected mission lifetime.

The Ulysses solar wind findings were published in a recent edition of Geophysical Research Letters.

The Ulysses spacecraft was carried into Earth orbit aboard space shuttle Discovery on October 6, 1990. From Earth orbit it was propelled toward Jupiter, passing the planet on February 8, 1992. Jupiter's immense gravity bent the spacecraft's flight path downward and away from the plane of the planets' orbits. This placed Ulysses into a final orbit around the Sun that would take it over the north and south poles.

Astronomy.com

Dusty Disk Evidence of Planetary Collision

Credit: Artwork by Lynette Cook

What astronomers had expected to be a run-of-the-mill protoplanetary disk turned out to be evidence of a much more intriguing story. While observing the sun-like star BD 20 307, a team of astronomers noticed a large disk of dust surrounding the star. Usually, this is evidence of planetary formation around younger stars. The 8 planets (and plutoids…) in our own solar system formed out of just such a disk. Disks like this aren't generally found around older stars, though, and when the age of the star was calculated to be several billion years old, the source of the dust appears to come from a rare event: it is the resulting debris of two planets slamming into each other.

Using data from the Chandra X-ray Observatory, and taking the brightness using one of Tennessee State University's automated telescopes in Arizona, the team first discovered BD 20 307 to in fact be part of a close binary pair. Not only that, but the system was much older than previously thought: several billions of years old, rather than a few hundred million. The system is 300 light-years away from Earth in the constellation Ares.

The curiously large amount of dust orbiting BD 20 307 is 1 million times the amount of dust than is found in our own solar system, and orbits at a distance from the star that is similar to the orbits of Earth and Venus around our own Sun. The abundance of dust particles in this orbit – and around such a mature star – led scientists to the conclusion that it was created by the violent collision of two exoplanets.

Benjamin Zuckerman, UCLA professor of physics and astronomy and co-author of a paper on the discovery said, "It's as if Earth and Venus collided with each other. Astronomers have never seen anything like this before. Apparently, major catastrophic collisions can take place in a fully mature planetary system." Zuckerman and his team will report their findings in the December issue of the Astrophysical Journal.

Normally, warm disks of dust surround younger star systems, out of which larger and larger structures can form, eventually yielding planets. To find a disk of dust in around a star that is several billions of years old is odd, because the pressure of stellar radiation pushes out the lighter dust over time, and the larger chunks either form planets and asteroids, or break down in collisions and get blown away.

The collision between the planets took place within the past few hundred thousand years, though it is possible that it happened even more recently. Such a colossal collision raises the question of how the orbits of the two planets became destabilized, and whether such a collision could happen in our own solar system.

"The stability of planetary orbits in our own solar system has been considered for nearly two decades by astronomer Jacques Laskar in France and, more recently, by Konstantin Batygin and Greg Laughlin in the U.S.A. Their computer models predict planetary motions into the distant future and they find a small probability for collisions of Mercury with Earth or Venus sometime in the next billion years or more. The small probability of this happening may be related to the rarity of very dusty planetary systems like BD+20 307," said paper co-author Gregory Henry, astronomer at Tennessee State University (TSU).

Source: EurekAlert
Universe Today

Scientists Detect "Dark Flow:" Matter From Beyond the Visible Universe

Galaxy clusters like 1E 0657-56 (inset) seem to be drifting toward a 20-degree-wide patch of sky (ellipse) between the constellations of Centaurus and Vela.

Credit: NASA/WMAP/A. Kashlinsky et al.

Just as unseen dark energy is increasing the rate of expansion of the universe, there's something else out there causing an unexpected motion in distant galaxy clusters. Scientists believe the cause is the gravitational attraction of matter that lies beyond the observable universe, and they are calling it "Dark Flow," in the vein of two other cosmological mysteries, dark matter and dark energy. "The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase," said lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "The distribution of matter in the observed universe cannot account for this motion."

"We never expected to find anything like this," he said.

Using NASA's Wilkinson Microwave Anisotropy Probe's (WMAP) three-year view of the microwave background and a catalog of clusters, the astronomers detected hundreds of galaxy clusters that appear to be carried along by a mysterious cosmic flow. The bulk cluster motions are traveling at nearly 2 million miles per hour. The clusters are heading toward a 20-degree patch of sky between the constellations of Centaurus and Vela.

Several astronomers teamed up to identify some 700 X-ray clusters that exhibited a subtle spectral shift. This sample includes objects up to 6 billion light-years — or nearly half of the observable universe — away.

They found this motion is constant out to at least a billion light-years. "Because the dark flow already extends so far, it likely extends across the visible universe," Kashlinsky says.

The finding flies in the face of predictions from standard cosmological models, which describe such motions as decreasing at ever greater distances.

Cosmologists view the microwave background - a flash of light emitted 380,000 years after the big bang - as the universe's ultimate reference frame. Relative to it, all large-scale motion should show no preferred direction.

Big-bang models that include a feature called inflation offer a possible explanation for the flow. Inflation is a brief hyper-expansion early in the universe's history. If inflation did occur, then the universe we can see is only a small portion of the whole cosmos.

WMAP data released in 2006 support the idea that our universe experienced inflation. Kashlinsky and his team suggest that their clusters are responding to the gravitational attraction of matter that was pushed far beyond the observable universe by inflation. "This measurement may give us a way to explore the state of the cosmos before inflation occurred," he says.

The next step is to narrow down uncertainties in the measurements. "We need a more accurate accounting of how the million-degree gas in these galaxy clusters is distributed," says Atrio-Barandela.

"We’re assembling an even larger and deeper catalog of X-ray clusters to better measure the flow," Ebeling adds. The researchers also plan to extend their analysis by using the latest WMAP results, released in March.

The result will appear in the October 20 edition of Astrophysical Journal Letters, which is available electronically this week.

Universe Today

Pinning down the Milky Way's spin

ESO PR Photo 30/08
Cepheids in the Solar Neighbourhood

Pulsating stars enable a new precise determination of the rotation of our Galaxy

New, very precise measurements have shown that the rotation of the Milky Way is simpler than previously thought. A remarkable result from the most successful ESO instrument HARPS, shows that a much debated, apparent 'fall' of neighbourhood Cepheid stars towards our Sun stems from an intrinsic property of the Cepheids themselves.

The result, obtained by a group of astrophysicists led by Nicolas Nardetto, will soon appear in the journal Astronomy & Astrophysics.

Since Henrietta Leavitt's discovery of their unique properties in 1912, the class of bright, pulsating stars known as Cepheids has been used as a distance indicator. Combined with velocity measurements, the properties of Cepheids are also an extremely valuable tool in investigations of how our galaxy, the Milky Way, rotates.

"The motion of Milky Way Cepheids is confusing and has led to disagreement among researchers," says Nardetto. "If the rotation of the Galaxy is taken into account, the Cepheids appear to 'fall' towards the Sun with a mean velocity of about 2 km/s."

A debate has raged for decades as to whether this phenomenon was truly related to the actual motion of the Cepheids and, consequently, to a complicated rotating pattern of our galaxy, or if it was the result of effects within the atmospheres of the Cepheids.

Nardetto and his colleagues observed eight Cepheids with the high precision HARPS spectrograph, attached to the 3.6-m ESO telescope at La Silla, 2400 m up in the mountains of the Chilean Atacama Desert. HARPS, or the High Accuracy Radial Velocity Planetary Searcher, is best known as a very successful planet hunter, but it can also be used to resolve other complicated cases, where its ability to determine radial velocities - the speed with which something is moving towards or away from us - with phenomenally high accuracy is invaluable. "Our observations show that this apparent motion towards us almost certainly stems from an intrinsic property of Cepheids," says Nardetto.

The astronomers found that the deviations in the measured velocity of Cepheids were linked to the chemical elements in the atmospheres of the Cepheids considered. "This result, if generalised to all Cepheids, implies that the rotation of the Milky Way is simpler than previously thought, and is certainly symmetrical about an axis," concludes Nardetto.

Notes

Nardetto, N., Stoekl, A., Bersier, D. & Barnes, T. G., High resolution spectroscopy for Cepheids distance determination. III. A relation between γ-velocities and γ-asymmetries", to appear in Astronomy & Astrophysics.

Nicolas Nardetto was at the Max-Planck-Institute for Radio Astronomy, Bonn, Germany, when doing this research.

Contact

Nicolas Nardetto
Departamento de Astronomia
Universidad de Concepcion, Chile
Phone: + 56 41 220 33 66
E-mail: nnardetto (at) astro-udec.cl

ESO Press Officer:
Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org

ESO Press Officer in Chile:
Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

NASA Satellite Sees Oldest-Ever Gamma-Ray Burst, Long Before Milky Way Existed

GRB 080913 exploded Sept. 13 at a whopping distance of 12.8 billion light-years away in the constellation Eridanus. The box indicates the sky area shown in the Swift image.
Credit: DSS/STScI/AURA

This image merges the view through Swift's UltraViolet and Optical Telescope, which shows bright stars, and its X-ray Telescope, which captures the burst (orange and yellow).
Credit: NASA/Swift/Stefan Immler

Whenever satellites like NASA's Swift sees a gamma-ray burst, it's really a look back in time. Now, scientists have seen one that happened farther back in time than any other seen before.

Gamma-ray bursts or "GRBs" are the most powerful and brightest explosions of energy in our universe. They last only a few milliseconds to several minutes and they outshine all other sources of gamma rays combined. Astronomers now think that most GRBs, those lasting 2 seconds or longer, are associated with the explosive deaths of massive stars. These stars collapse and explode when they run out of nuclear fuel.

Now, NASA's Swift satellite has found the most distant gamma-ray burst ever detected! The blast was named "GRB 080913." The GRB number is actually the date YYMMDD of the burst, with letters used for the first, second, etc. burst of the day. This burst came from an exploding star 12.8 billion light-years away. A light-year is the distance light travels, at a speed 186,000 miles per second, in one year.

"This is the most amazing burst Swift has seen," says the mission’s lead scientist Neil Gehrels at Goddard Space Flight Center in Greenbelt, Md. "It's coming to us from near the edge of the visible universe."

Because light moves at a set speed, looking farther into the universe means looking back in time. GRB 080913's "lookback time" reveals that the burst occurred less than 825 million years after the universe began. Scientists think that the universe is 13.7 billion years old. That means this gamma-ray burst happened 12.8 billion years ago! That's long before our galaxy, called the Milky Way, even existed. Scientists believe the galaxy formed about 10 billion years ago.

The star that created this gamma-ray burst died when the universe was less than one-seventh its present age. "This burst accompanies the death of a star from one of the universe’s early generations," says Patricia Schady of the Mullard Space Science Laboratory at University College London, who is organizing Swift observations of the event.

The star's gamma-rays were registered on NASA's Swift satellite at 1:47 a.m. EDT on Sept. 13. The spacecraft established the burst's location in the constellation Eridanus. The previous record holder was a burst from about 12.1 billion years ago.

Goddard Space Flight Center

Water Hit with Young Star's Best Shot

A jet of gas firing out of a very young star can be seen ramming into a wall of material in this infrared image from NASA's Spitzer Space Telescope.

Credit: NASA/JPL-Caltech/A. Tappe (Harvard-Smithsonian CfA)

Water is being blasted to pieces by a young star's laser-like jets, according to new observations from NASA's Spitzer Space Telescope.

The discovery provides a better understanding of how water -- an essential ingredient for life as we know it -- is processed in emerging solar systems.

"This is a truly unique observation that will provide important information about the chemistry occurring in planet-forming regions, and may give us insights into the chemical reactions that made water and even life possible in our own solar system," said Achim Tappe, of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

A young star forms out of a thick, rotating cloud of gas and dust. Like the two ends of a spinning top, powerful jets of gas emerge from the top and bottom of the dusty cloud. As the cloud shrinks more and more under its own gravity, its star eventually ignites and the remaining dust and gas flatten into a pancake-like disk, from which planets will later form. By the time the star ignites and stops accumulating material from its cloud, the jets will have died out.

Tappe and his colleagues used Spitzer's infrared eyes to cut through the dust surrounding a nascent star, called HH 211-mm, and get a better look at its jets. These particular jets are exceptionally young at 1,000 years old, and they are some of the most collimated, or focused, known. An instrument on Spitzer called a spectrometer analyzed light from one of the jets, revealing information about its molecules.

To the astronomers' surprise, Spitzer picked up the signature of rapidly spinning fragments of water molecules, called hydroxyl, or OH. In fact, the hydroxyl molecules have absorbed so much energy (through a process called excitation) that they are rotating around with energies equivalent to 28,000 Kelvin (27,700 degrees Celsius). This far exceeds normal expectations for gas streaming out of a stellar jet. Water, which is abbreviated H2O, is made up of two oxygen atoms and one hydrogen; hydroxyl, or OH, contains one oxygen and one hydrogen atom.

The results reveal that the jet is ramming its head into a wall of material, vaporizing ice right off the dust grains it normally coats. The jet is hitting the material so fast and hard that a shock wave is also being produced.

"The shock from colliding atoms and molecules generates ultraviolet radiation, which will break up water molecules, leaving extremely hot hydroxyl molecules," said Tappe.

Tappe said this same process of ice being vaporized off dust occurs in our own solar system, when the sun vaporizes ice in approaching comets. In addition, the water that now coats our world is thought to have come from icy comets that vaporized as they rained down on a young Earth.

Tappe is the lead author of a paper on this topic, which was published in a recent issue of the Astrophysical Journal. Co-authors on the paper include Charlie Lada, and August Muench, also of the Harvard-Smithsonian Center for Astrophysics; and J. H. Black, of the Chalmers University of Technology, in Onsala, Sweden.

Spitzer Space Telescope

Galaxy Silhouettes

Overlapping Galaxies 2MASX J00482185-2507365
Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA

NASA's Hubble Space Telescope has captured a rare alignment between two spiral galaxies. The outer rim of a small, foreground galaxy is silhouetted in front of a larger background galaxy. Skeletal tentacles of dust can be seen extending beyond the small galaxy's disk of starlight.

Such outer dark dusty structures, which appear to be devoid of stars, like barren branches, are rarely so visible in a galaxy because there is usually nothing behind them to illuminate them. Astronomers have never seen dust this far beyond the visible edge of a galaxy. They do not know if these dusty structures are common features in galaxies.

Understanding a galaxy's color and how dust affects and dims that color are crucial to measuring a galaxy's true brightness. By knowing the true brightness, astronomers can calculate the galaxy's distance from Earth.

Astronomers calculated that the background galaxy is 780 million light-years away. They have not as yet calculated the distance between the two galaxies, although they think the two are relatively close, but not close enough to interact. The background galaxy is about the size of the Milky Way Galaxy and is about 10 times larger than the foreground galaxy.

Most of the stars speckled across this image belong to the nearby spiral galaxy NGC 253, which is out of view to the right. Astronomers used Hubble's Advanced Camera for Surveys to snap images of NGC 253 when they spied the two galaxies in the background. From ground-based telescopes, the two galaxies look like a single blob. But the Advanced Camera's sharp "eye" distinguished the blob as two galaxies, cataloged as 2MASX J00482185-2507365. The images were taken on Sept. 19, 2006.

The results have been submitted for publication in The Astronomical Journal.

For additional information, contact:

Donna Weaver/Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Benne Holwerda
Space Telescope Science Institute, Baltimore, Md.
/University of Cape Town, South Africa
holwerda@stsci.edu

Object Name: 2MASX J00482185-2507365

Acknowledgment: B. Holwerda (Space Telescope Science Institute)
and J. Dalcanton (University of Washington)

First Picture of Likely Planet around Sun-like Star

Photo Credit: Gemini Observatory

Gemini adaptive optics image of 1RSX J160929.1-210524 and its likely ~8 Jupiter-mass companion (within red circle). This image is a composite of J-, H- and K-band near-infrared images. All images obtained with the Gemini Altair adaptive optics system and the Near-Infrared Imager (NIRI) on the Gemini North telescope.

Astronomers have unveiled what is likely the first picture of a planet around a normal star similar to the Sun.

Three University of Toronto scientists used the Gemini North telescope on Mauna Kea in Hawai‘i to take images of the young star 1RXS J160929.1-210524 (which lies about 500 light-years from Earth) and a candidate companion of that star. They also obtained spectra to confirm the nature of the companion, which has a mass about eight times that of Jupiter, and lies roughly 330 times the Earth-Sun distance away from its star. (For comparison, the most distant planet in our solar system, Neptune, orbits the Sun at only about 30 times the Earth-Sun distance.) The parent star is similar in mass to the Sun, but is much younger.

“This is the first time we have directly seen a planetary mass object in a likely orbit around a star like our Sun,” said David Lafrenière, lead author of a paper submitted to the Astrophysical Journal Letters and also posted online. “If we confirm that this object is indeed gravitationally tied to the star, it will be a major step forward.”

Until now, the only planet-like bodies that have been directly imaged outside of the solar system are either free-floating in space (i.e. not found around a star), or orbit brown dwarfs, which are dim and make it easier to detect planetary-mass companions.

The existence of a planetary-mass companion so far from its parent star comes as a surprise, and poses a challenge to theoretical models of star and planet formation. "This discovery is yet another reminder of the truly remarkable diversity of worlds out there, and it's a strong hint that nature may have more than one mechanism for producing planetary mass companions to normal stars,” said Ray Jayawardhana, team member and author of a forthcoming book on extrasolar planets entitled Worlds Beyond.

The team’s Gemini observations took advantage of adaptive optics technology to dramatically reduce distortions caused by turbulence in Earth’s atmosphere. The near-infrared images and spectra of the suspected planetary object indicate that it is too cool to be a star or even a more massive brown dwarf, and that it is young. Taken together, such findings confirm that it is a very young, very low-mass object at roughly the same distance from Earth as the star.

Even though the likelihood of a chance alignment between such an object and a similarly young star is rather small, it will take up to two years to verify that the star and its likely planet are moving through space together. “Of course it would be premature to say that the object is definitely orbiting this star, but the evidence is extremely compelling. This will be a very intensely studied object for the next few years!” said Lafrenière.

Near-infrared spectra of 1RSX J160929.1-210524 and its candidate companion. The primary's spectrum (row A) is as expected for a temperature of about 4000 K (spectral type K7). The candidate companion's spectrum (black curves repeated in rows B--F) is compared with the spectra of two young brown dwarfs (red curves on rows B--C; spectral types M9 and L1) and two older, cooler brown dwarfs (red curves on rows D--E; L3 and L6. The `triangular' shape of the left part of the companion's spectrum is in much better agreement with the two young brown dwarfs, indicating the candidate companion has low gravity; in turn, this implies it has not yet fully contracted and thus is still young. The companion spectrum and those of all comparison objects have been normalised to be the same on the right-hand side. The fact that, compared to the young brown dwarfs, the candidate companion is slightly fainter in the left-hand part *indicates that it it is cooler*, more like the field L3 brown dwarf. The comparison with models (row F) confirms that the companion has low gravity, and thus is young.

Team member Marten van Kerkwijk described the group’s search method. “We targeted young stars so that any planetary mass object they hosted would not have had time to cool, and thus would still be relatively bright,” he said. “This is one reason we were able to see it at all.”

The Jupiter-sized body has an estimated temperature of about 1800 Kelvin (about 1500ºC), much hotter than our own Jupiter, which has a temperature of about 160 Kelvin (-110ºC), and its likely host is a young star of type K7 with an estimated mass of about 85% that of the Sun. For more characteristics of the star and planet see this table from the paper.

The work that led to this discovery is part of a survey of more than 85 stars in the Upper Scorpius association, a group of young stars formed about 5 million years ago. It uses the Gemini telescope's high-resolution adaptive optics capabilities to determine the different types of companions that can form around young stars: stars, brown dwarfs, or planetary mass objects. “This discovery certainly has us looking forward to what other surprises nature has in stock for us,” said Van Kerkwijk.

The preprint of the paper is available at http://arxiv.org/abs/0809.1424

Gemini Observatory

The Double Firing Burst

ESO PR Photo 28/08
A Gamma-Ray Burst with Two Jets

Brightest Gamma-Ray

Burst Provides Wealth of Information on How Stars Explode

Astronomers from around the world combined data from ground- and space-based telescopes to paint a detailed portrait of the brightest explosion ever see. The observations reveal that the jets of the gamma-ray burst called GRB 080319B were aimed almost directly at the Earth.

GRB 080319B was so intense that, despite happening halfway across the Universe, it could have been seen briefly with the unaided eye (ESO 08/08). In a paper to appear in the 11 September issue of Nature, Judith Racusin of Penn State University, Pennsylvania (USA), and a team of 92 co-authors report observations across the electromagnetic spectrum that began 30 minutes before the explosion and followed it for months afterwards.

"We conclude that the burst's extraordinary brightness arose from a jet that shot material almost directly towards Earth at almost the speed of light - the difference is only 1 part in 20 000," says Guido Chincarini, a member of the team.

Gamma-ray bursts are the Universe's most luminous explosions. Most occur when massive stars run out of fuel. As a star collapses, it creates a black hole or neutron star that, through processes not fully understood, drives powerful gas jets outward. As the jets shoot into space, they strike gas previously shed by the star and heat it, thereby generating bright afterglows.

The team believes the jet directed toward Earth contained an ultra-fast component just 0.4 degrees across (this is slightly smaller than the apparent size of the Full Moon). This jet is contained within another slightly less energetic jet about 20 times wider.

The broad component is more typical of other bursts. "Perhaps every gamma-ray burst has a narrow jet, but astronomers miss it most of the time," says team member Stefano Covino. "We happened to view this monster down the barrel of the very narrow and energetic jet, and the chance for this nearly head-on alignment to occur is only about once a decade," added his colleague Cristiano Guidorzi.

GRB 080319B was detected by the NASA/STFC/ASI Swift satellite towards the constellation of Boötes, the "Herdsman". A host of ground-based telescopes reacted promptly to study this new object in the sky, including ESO's Very Large Telescope, which was the first to provide the distance of the object, 7.5 billion light-years. The visible light from the burst was detected by a handful of wide-field cameras worldwide that are mounted on telescopes constantly monitoring a large fraction of the sky. One of these was the TORTORA camera mounted on the 0.6-m REM telescope at ESO's La Silla Observatory (ESO 26/07).

TORTORA's rapid imaging provides the most detailed look yet at the visible light associated with the initial blast of a gamma-ray burst. "We've been waiting a long time for this one," says TORTORA senior scientist Grigory Beskin of Russia's Special Astrophysical Observatory. The data collected simultaneously by TORTORA and the Swift satellite allowed astronomers to explain the properties of this burst.

Notes

Racusin, J. L. et al. 2008, Broadband observations of the naked-eye gamma-ray burst GRB 080319B, Nature, 11 September 2008.

The TORTORA camera is mounted on the Italian REM telescope at ESO's La Silla Observatory in Chile. It was built and is being operated by a collaboration between Italy's Bologna State University and Brera Observatory and Russia's Special Astrophysical Observatory and Institute of Precise Instrumentation. The REM team is composed of G. Chincarini, E. Molinari, F.M. Zerbi, L.A. Antonelli, S. Covino, P. Conconi, L. Nicastro, E. Palazzi, M. Stefanon, V. Testa, G. Tosti, F. Vitali, A. Monfardini, F. D'Alessio, P. D'Avanzo, D. Fugazza, G. Malaspina, S.D. Vergani, S. Campana, P. Goldoni, D. Guetta, N. Masetti, E.J.A. Meurs, L. Norci, E. Pian, A. Fernandez-Soto, L. Stella, G. Tagliaferri, G. Ihle, L. Gonzalez, A. Pizarro, P. Sinclair, and J. Valenzuela.

The TORTORA team comprises G. Beskin, S. Karpov, S. Bondar, A Guarnieri (TORTORA Italian PI), C. Bartolini, G. Greco, A. Piccioni, D. Nanni, F. Terra, and E. Molinari.

Swift is managed by NASA's Goddard Space Flight Center. It was built and is being operated in collaboration with Penn State, the Los Alamos National Laboratory, and General Dynamics in the U.S.; Brera Observatory and the Italian Space Agency in Italy; the University of Leicester and Mullard Space Science Laboratory in the United Kingdom; plus partners in Germany and Japan.

Contacts:

Guido Chincarini, Stefano Covino, Cristiano Guidorzi

University of Miliano Bicocca and INAF-Brera, Italy

Phone: +39 039 597 10 74

Cell phone: +39 340 280 36 12

E-mail: guido.chincarini (at) brera.inaf.it

ESO
European Organisation for Astronomical Research
in the Southern Hemisphere

Mind the Gap

ESO PR Photo 27a/08
Planet-forming Disc

VLT instrument hints at the presence of planets in young gas discs

Astronomers have been able to study planet-forming discs around young Sun-like stars in unsurpassed detail, clearly revealing the motion and distribution of the gas in the inner parts of the disc. This result, which possibly implies the presence of giant planets, was made possible by the combination of a very clever method enabled by ESO's Very Large Telescope.

Planets could be home to other forms of life, so the study of exoplanets ranks very high in contemporary astronomy. More than 300 planets are already known to orbit stars other than the Sun, and these new worlds show an amazing diversity in their characteristics. But astronomers don't just look at systems where planets have already formed - they can also get great insights by studying the discs around young stars where planets may currently be forming. "This is like going 4.6 billion years back in time to watch how the planets of our own Solar System formed," says Klaus Pontoppidan from Caltech, who led the research.

Pontoppidan and colleagues have analysed three young analogues of our Sun that are each surrounded by a disc of gas and dust from which planets could form. These three discs are just a few million years old and were known to have gaps or holes in them, indicating regions where the dust has been cleared and the possible presence of young planets.

The new results not only confirm that gas is present in the gaps in the dust, but also enable astronomers to measure how the gas is distributed in the disc and how the disc is oriented. In regions where the dust appears to have been cleared out, molecular gas is still highly abundant. This can either mean that the dust has clumped together to form planetary embryos, or that a planet has already formed and is in the process of clearing the gas in the disc.

For one of the stars, SR 21, a likely explanation is the presence of a massive giant planet orbiting at less than 3.5 times the distance between the Earth and the Sun, while for the second star, HD 135344B, a possible planet could be orbiting at 10 to 20 times the Earth-Sun distance. The observations of the third star, TW Hydrae, may also require the presence of one or two planets.

"Our observations with the CRIRES instrument on ESO's Very Large Telescope clearly reveal that the discs around these three young, Sun-like stars are all very different and will most likely result in very different planetary systems," concludes Pontoppidan. "Nature certainly does not like to repeat herself" [1].

"These kinds of observations complement the future work of the ALMA observatory, which will be imaging these discs in great detail and on a larger scale," adds Ewine van Dishoeck, from Leiden Observatory, who works with Pontoppidan.

To study the gaps in dust discs that are the size of the Solar System around stars that are located up to 400 light-years away is a daunting challenge that requires a clever solution and the best possible instruments [2].

"Traditional imaging cannot hope to see details on the scale of planetary distances for objects located so far away," explains van Dishoeck. "Interferometry can do better but won't allow us to follow the motion of the gas."

Astronomers used a technique known as 'spectro-astrometric imaging' to give them a window into the inner regions of the discs where Earth-like planets may be forming. They were able not only to measure distances as small as one-tenth the Earth-Sun distance, but to measure the velocity of the gas at the same time [3].

"The particular configuration of the instrument and the use of adaptive optics allows astronomers to carry out observations with this technique in a very user-friendly way: as a consequence, spectro-astrometric imaging with CRIRES can now be routinely performed," says team member Alain Smette, from ESO [4].

Notes for editors

Pontoppidan, K. M. et. al. 2008, Spectro-Astrometric Imaging of Molecular Gas Within Protoplanetary Disk Gaps, Astrophysical Journal, 684, 1323, 10 September 2008. Team members are Klaus M. Pontoppidan, Geoffrey A. Blake, and Michael J. Ireland (California Institute of Technology, Pasadena, USA), Ewine F. van Dishoeck (Leiden Observatory, The Netherlands, and Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany - MPE), Alain Smette (ESO, Chile), and Joanna Brown (MPE).

[1] The discs are about an hundred astronomical units (AU - the mean distance between the Earth and the Sun, or 149.6 million kilometres) across, but the stars are more than 200 light-years away (one light-year is 200 000 AU). To resolve structures on 1 AU scales in these systems corresponds to reading the license plate on a car at a distance of 2000 km - roughly the distance from Stockholm to Lisbon.

[2] CRIRES, the near-infrared spectrograph attached to ESO's Very Large Telescope, is fed from the telescope through an adaptive optics module which corrects for the blurring effect of the atmosphere and so makes it possible to have a very narrow slit with a high spectral dispersion: the slit width is 0.2 arcsecond and the spectral resolution is 100 000. Using spectro-astrometry, an ultimate spatial resolution of better than 1 milli-arcsecond is achieved.

[3] The core of the spectro-astrometry imaging technique relies on the ability of CRIRES to be positioned very precisely on the sky, while retaining the ability to spread the light into a spectrum so that wavelength differences of 1 part in 100 000 can be detected. More precisely, the astronomers measure the centroid in the spatial direction of a spectrally resolved emission line: effectively, astronomers take a sharp emission line - a clear fingerprint of a molecule in the gas - and use data from several slit positions to locate the sources of particular emission lines, and hence to map the distribution of the gas with much greater precision than can be achieved by straightforward imaging. The astronomers have obtained spectra of the discs centred at wavelengths of 4.715 microns at 6 different position angles.

[4] Alain Smette is the CRIRES Instrument Scientist.

Contacts

Klaus Pontoppidan
California Institute of Technology, Pasadena, USA
Phone: +1 626 395 4900
Mobile: +1 626 679 5793
E-mail: pontoppi (at) gps.caltech.edu

Ewine van Dishoeck
Leiden University, The Netherlands
Phone: +31 71 527 58 14
E-mail: ewine (at) strw.leidenuniv.nl

ESO
European Organisation for Astronomical Research
in the Southern Hemisphere

M110 - Satellite of the Andromeda Galaxy

Credit & Copyright: Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum Astronomia), Hawaiian Starlight

Explanation: Our Milky Way Galaxy is not alone. It is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical and dwarf spheroidal galaxies. Pictured on the lower right is one of the dwarf ellipticals: NGC 205. Like M32, NGC 205 is a companion to the large M31, and can sometimes be seen to the south of M31's center in photographs. The image shows NGC 205 to be unusual for an elliptical galaxy in that it contains at least two dust clouds (at 9 and 2 o'clock - they are visible but hard to spot) and signs of recent star formation. This galaxy is sometimes known as M110, although it was actually not part of Messier's original catalog.

Astronomy Picture of the Day

Perseus A - A Monster Galaxy at the Heart of Perseus Cluster

Credit: X-ray: NASA/CXC/IoA/A.Fabian et al.; Radio: NRAO/VLA/G. Taylor; Optical: NASA/ESA/Hubble Heritage (STScI/AURA) & Univ. of Cambridge/IoA/A. Fabian

The active galaxy NGC 1275 is also a well-known radio source (Perseus A) and a strong emitter of X-rays due to the presence of a black hole in the center of the galaxy. The behemoth also lies at the center of the cluster of galaxies known as the Perseus Cluster. By combining multi-wavelength images into a single composite, the dynamics of the galaxy are more easily visible. Detail and structure from x-ray, optical and radio wavelengths combine for an aesthetically pleasing, but nonetheless violent depiction of events going on at the heart of the galaxy.

Chandra data from the Advanced CCD Imaging Spectrometer (ACIS) covers X-ray energies from 0.3-7keV. Hubble data from the Advanced Camera for Surveys covers optical wavelengths in the red, green and blue. Radio data from NRAO's Very Large Array at 328 MHz was also used. In the composite image, the X-ray data contribute to the soft violet shells around the outside of the center. The pinkish lobes toward the center of the galaxy are from radio frequencies. The radio emission, tracing jets from the black hole, fills the X-ray cavities. Dust lanes, star-forming regions, hydrogen filaments, foreground stars, and background galaxies are contributions from the Hubble optical data.

Chandra

Cassini Images Ring Arcs Among Saturn's Moons

Credit: NASA/JPL/Space Science Institute)

In this image, most of the visible material in the arc lies ahead of Anthe in its orbit. However, over time the moon drifts slowly back and forth with respect to the arc. PASADENA, Calif. -- NASA's Cassini spacecraft has detected a faint, partial ring orbiting with one small moon of Saturn, and has confirmed the presence of another partial ring orbiting with a second moon. This is further evidence that most of the planet's small, inner moons orbit within partial or complete rings.

Recent Cassini images show material, called ring arcs, extending ahead of and behind the small moons Anthe and Methone in their orbits. The new findings indicate that the gravitational influence of nearby moons on ring particles might be the deciding factor in whether an arc or complete ring is formed.

Both Anthe and Methone orbit Saturn in locations, called resonances, where the gravity of the nearby larger moon Mimas disturbs their orbits. Gravitational resonances are also responsible for many of the structures in Saturn's magnificent rings. Mimas provides a regular gravitational tug on each moon, which causes the moons to skip forward and backward within an arc-shaped region along their orbital paths, according to Nick Cooper, a Cassini imaging team associate from Queen Mary, University of London. "When we realized that the Anthe and Methone ring arcs were very similar in appearance to the region in which the moons swing back and forth in their orbits due to their resonance with Mimas, we knew we had a possible cause-and-effect relationship," Cooper said.

Scientists believe the faint ring arcs from Anthe and Methone likely consist of material knocked off these small moons by micrometeoroid impacts. This material does not spread all the way around Saturn to form a complete ring, because of the gravitational resonance with Mimas. That interaction confines the material to a narrow region along the orbits of the moons.

Credit: NASA/JPL/Space Science Institute)

Arrows indicate the positions of Anthe, at top left, and Methone, at bottom right. Micrometeoroid impacts on the moons are the likely source of the arc material.

This is the first detection of an arc of material near Anthe. The Methone arc was previously detected by Cassini's Magnetospheric Imaging Instrument, and the new images confirm its presence. Previous Cassini images show faint rings connected with other small moons either embedded within or near the outskirts of Saturn's main ring system, such as Pan, Janus, Epimetheus and Pallene. Cassini had also previously observed an arc in the G ring, one of Saturn's faint, major rings.

"This is probably the same mechanism responsible for producing the arc in the G ring," said Matthew Hedman, a Cassini imaging team associate at Cornell University in Ithaca, N.Y. Hedman and his Cassini imaging team colleagues previously determined that the G-ring arc is maintained by a gravitational resonance with Mimas, much like the new, small moon arcs. "Indeed, the Anthe arc may be similar to the debris we see in the G-ring arc, where the largest particles are clearly visible. One might even speculate that if Anthe were shattered, its debris might form a structure much like the G ring," Hedman said.

Additional analysis by scientists indicates that, while the gravitational influence of Mimas keeps the Anthe, Methone and G-ring arcs in place, the material that orbits with the moons Pallene, Janus and Epimetheus is not subject to such powerful resonant forces and is free to spread out around the planet, forming complete rings without arcs.

The intricate relationships between these ring arcs and the moons are just one of many such mechanisms that exist in the Saturn system. Cassini Imaging Team Member and Professor Carl Murray, also from Queen Mary, University of London, said, "There are many examples in the Saturn system of moons creating structures in the rings and disturbing the orbits of other moons. Understanding these interactions and learning about their origins can help us to make sense of what we are seeing in the Cassini images."

Images of Anthe and Methone with their ring arcs are available at: http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov and http://ciclops.org.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

Contacts:
Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
carolina.martinez@jpl.nasa.gov

Preston Dyches 720-974-5859
Space Science Institute, Boulder, Colo.
media@ciclops.org

Julia Maddock +44 (0)1793 442 094
Science and Technology Facilities Council
julia.maddock@stfc.ac.uk