NASA's Hubble Harvests Distant Solar System Objects

This is an artist's concept of a craggy piece of Solar System debris that belongs to a class of bodies called trans-Neptunian objects (TNOs). Astronomers culling the data archives of NASA's Hubble Space Telescope have added 14 new TNOs to the catalog. The newfound TNOs range from 25 to 60 miles (40-100 km) across. Their method promises to turn up hundreds more. In this illustration, the distant Sun is reduced to a bright star at a distance of over 3 billion miles. Credit: NASA, ESA, and G. Bacon (STScI)

Cambridge, MA - Beyond the orbit of Neptune reside countless icy rocks known as trans-Neptunian objects (TNOs). One of the biggest, Pluto, is classified as a dwarf planet. The region also supplies us with comets such as famous Comet Halley. Most TNOs are small and receive little sunlight, making them faint and difficult to spot.

Now, astronomers using clever techniques to cull the data archives of NASA's Hubble Space Telescope have added 14 new TNOs to the catalog. Their method promises to turn up hundreds more.

"Trans-Neptunian objects interest us because they are building blocks left over from the formation of the solar system," explained lead author Cesar Fuentes, formerly with the Harvard-Smithsonian Center for Astrophysics and now at Northern Arizona University.

As TNOs slowly orbit the sun, they move against the starry background, appearing as streaks of light in time exposure photographs. The team developed software to analyze hundreds of Hubble images hunting for such streaks. After promising candidates were flagged, the images were visually examined to confirm or refute each discovery.

Most TNOs are located near the ecliptic -- a line in the sky marking the plane of the solar system (since the solar system formed from a disk of material). Therefore, the team searched within 5 degrees of the ecliptic to increase their chance of success.

They found 14 objects, including one binary (two TNOs orbiting each other like a miniature Pluto-Charon system). All were very faint, with most measuring magnitude 25-27 (more than 100 million times fainter than objects visible to the unaided eye).

By measuring their motion across the sky, astronomers calculated an orbit and distance for each object. Combining the distance and brightness (plus an assumed albedo or reflectivity), they then estimated the size. The newfound TNOs range from 25 to 60 miles (40-100 km) across.

Unlike planets, which tend to have very flat orbits (known as low inclination), some TNOs have orbits significantly tilted from the ecliptic (high inclination). The team examined the size distribution of TNOs with low- versus high-inclination orbits to gain clues about how the population has evolved over the past 4.5 billion years.

Generally, smaller trans-Neptunian objects are the shattered remains of bigger TNOs. Over billions of years, these objects smack together, grinding each other down. The team found that the size distribution of TNOs with low- versus high-inclination orbits is about the same as objects get fainter and smaller. Therefore, both populations (low and high inclination) have similar collisional histories.

This initial study examined only one-third of a square degree of the sky, meaning that there is much more area to survey. Hundreds of additional TNOs may lurk in the Hubble archives at higher ecliptic latitudes. Fuentes and his colleagues intend to continue their search.

"We have proven our ability to detect and characterize TNOs even with data intended for completely different purposes," Fuentes said.

This research has been accepted for publication in The Astrophysical Journal and is available online.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts Hubble science operations. Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Caught in the Act: Fireballs Light up Jupiter

A fleeting bright dot on each of these images of Jupiter marks a small comet or asteroid burning up in the atmosphere. The image on the left was taken on June 3, 2010, by amateur astronomer Anthony Wesley, who was visiting a friend in Broken Hill, Australia, when he obtained the image with a 37-centimeter (14.5-inch) telescope. Full image and caption

Detailed observations made by the Hubble Space Telescope have found an answer to the flash of light seen June 3, 2010, on Jupiter. Image credit: NASA/ESA/GSFC/UCB/SSI/Jupiter Impact Science Team. Full image and caption

This image by amateur astronomer Christopher Go, of Cebu, Philippines, confirms that the glimmer amateur astronomer Anthony Wesley saw in his telescope on June 3, 2010, was an object burning up in the Jupiter atmosphere. Image credit: Christopher Go

Amateur astronomers working with professional astronomers have spotted two fireballs lighting up Jupiter's atmosphere this summer, marking the first time Earth-based telescopes have captured relatively small objects burning up in the atmosphere of the giant planet. The two fireballs – which produced bright freckles on Jupiter that were visible through backyard telescopes – occurred on June 3, 2010, and August 20, 2010, respectively.

A new paper that includes both pros and amateurs, led by Ricardo Hueso of the Universidad del País Vasco, Bilbao, Spain, appears today in the Astrophysical Journal Letters. In the paper, astronomers estimate the object that caused the June 3 fireball was 8 to 13 meters (30 to 40 feet) in diameter. The object is comparable in size to the asteroid 2010 RF12 that flew by Earth on Wednesday, Sept. 8, and slightly larger than the asteroid 2008 TC3, which burned up above Sudan two years ago.

An impact of this kind on Earth would not be expected to cause damage on the ground. The energy released by the June 3 fireball as it collided with Jupiter's atmosphere was five to 10 times less than the 1908 Tunguska event on Earth, which knocked over tens of millions of trees in a remote part of Russia. Analysis is continuing on the Aug. 20 fireball, but scientists said it was comparable to the June 3 object.

"Jupiter is a big gravitational vacuum cleaner," said Glenn Orton, a co-author on the paper and an astronomer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It is clear now that relatively small objects, remnants of the formation of the solar system 4.5 billion years ago, still hit Jupiter frequently. Scientists are trying to figure out just how frequently."

Orton and colleagues said this kind of discovery couldn't have been made without amateur astronomers around the world, whose observations of Jupiter provide a near round-the-clock surveillance that would be impossible to do with the long lines of scientists waiting to use the large telescopes. Amateur astronomers, for example, were the first to see the dark spot that appeared on Jupiter in July 2009 as the result of an impact. Professional astronomers are still analyzing that impact.

Anthony Wesley, an amateur astronomer from Murrumbateman, Australia, who was also the first to take a picture of that dark spot on Jupiter in July 2009, was the first to see the tiny flash on June 3. Amateur astronomers had their telescopes trained on Jupiter that day because they were in the middle of "Jupiter season," when the planet is high in the sky and at its largest size, as seen by backyard telescopes.

Wesley was visiting an amateur astronomer friend about 1,000 kilometers (600 miles) away in Broken Hill, and he set a digital video camera to record images from his telescope at about 60 frames per second. He was watching the live video on a computer screen at his friend's house when he saw a two-and-a-half-second-long flash of light near the limb of the planet.

"It was clear to me straight away it had to be an event on Jupiter," he said. "I'm used to seeing other momentary flashes in the camera from cosmic ray impacts, but this was different. Cosmic ray strikes last only for one frame of video, whereas this flash gradually brightened and then faded over 133 frames."

Wesley sent a message out on his e-mail list of amateur and professional astronomers, which included Orton. After receiving Wesley's e-mail, Christopher Go of Cebu, Philippines -- who like Wesley, is an amateur astronomer -- checked his own recordings and confirmed that he had seen a flash, too.

Before Wesley's work, scientists didn't know these small-size impacts could be observed, Hueso explained. "The discovery of optical flashes produced by objects of this size helps scientists understand how many of these objects are out there and the role they played in the formation of our solar system," Hueso said.

For three days afterward, Hueso and colleagues looked for signs of the impact in high-resolution images from larger telescopes: NASA's Hubble Space Telescope, Gemini Observatory telescopes in Hawaii and Chile, the Keck telescope in Hawaii, the NASA Infrared Telescope Facility in Hawaii and the European Southern Observatory's Very Large Telescope in Chile. Scientists analyzed the images for thermal disruptions and chemical signatures seen in previous images of Jupiter impacts. In this case, they saw no signs of debris, which allowed them to limit the size of the impactor.

Based on all these images, and particularly those obtained by Wesley and Go, the astronomers were able to confirm the flash came from some kind of object – probably a small comet or asteroid – that burned up in Jupiter's atmosphere. The impactor likely had a mass of about 500 to 2,000 metric tons (1 million to 4 million pounds), probably about 100,000 times less massive than the object in July 2009.

Calculations also estimated this June 3 impact released about 1 to 4 quadrillion joules (300 million to 1 billion watt-hours) of energy. The second fireball, on Aug. 20, was detected by the amateur Japanese astronomer Masayuki Tachikawa and later confirmed by Aoki Kazuo and Masayuki Ishimaru. It flashed for about 1.5 seconds. The Keck telescope, observing less than a day later, also found no subsequent debris remnants. Scientists are still analyzing this second flash.

Although collisions of this size had never before been detected on Jupiter, some previous models predicted around one collision of this kind a year. Another predicted up to 100 such collisions. Scientists now believe the frequency must be closer to the high end of the scale.

"It is interesting to note that whereas Earth gets smacked by a 10-meter-sized object about every 10 years on average, it looks as though Jupiter gets hit with the same-sized object a few times each month," said Don Yeomans, manager of the Near-Earth Object Program Office at JPL, who was not involved in the paper. "The Jupiter impact rate is still being refined and studies like this one help to do just that."

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

Deadly Tides Mean Early Exit for Hot Jupiters

An artist's depiction of an early stage in the destruction of a hot Jupiter by its star. Credit: NASA/GSFC/Frank Reddy. View Larger

Bad news for planet hunters: most of the "hot Jupiters" that astronomers have been searching for in star clusters were likely destroyed long ago by their stars. In a paper accepted for publication by the Astrophysical Journal, John Debes and Brian Jackson of NASA's Goddard Space Flight Center in Greenbelt, Md., offer this new explanation for why no transiting planets (planets that pass in front of their stars and temporarily block some of the light) have been found yet in star clusters. The researchers also predict that the planet hunting being done by the Kepler mission is more likely to succeed in younger star clusters than older ones.

"Planets are elusive creatures," says Jackson, a NASA Postdoctoral Program fellow at Goddard, "and we found another reason that they're elusive."

When astronomers began to search for planets in star-packed globular clusters about 10 years ago, they hoped to find many new worlds. One survey of the cluster called 47 Tucanae (47 Tuc), for example, was expected to find at least a dozen planets among the roughly 34,000 candidate stars. "They looked at so many stars, people thought for sure they would find some planets," says Debes, a NASA Postdoctoral Program fellow at Goddard. "But they didn't."

More than 450 exoplanets (short for "extrasolar planets," or planets outside our solar system) have been found, but "most of them have been detected around single stars," Debes notes.

"Globular clusters turn out to be rough neighborhoods for planets," explains Jackson, "because there are lots of stars around to beat up on them and not much for them to eat." The high density of stars in these clusters means that planets can be kicked out of their solar systems by nearby stars. In addition, the globular clusters surveyed so far have been rather poor in metals (elements heavier than hydrogen and helium), which are the raw materials for making planets; this is known as low metallicity.

Debes and Jackson propose that hot Jupiters—large planets that are at least 3 to 4 times closer to their host stars than Mercury is to our sun—are quickly destroyed. In these cramped orbits, the gravitational pull of the planet on the star can create a tide—that is, a bulge—on the star. As the planet orbits, the bulge on the star points a little bit behind the planet and essentially pulls against it; this drag reduces the energy of the planet's orbit, and the planet moves a little closer to the star. Then the bulge on the star gets bigger and saps even more energy from the planet's orbit. This continues for billions of years until the planet crashes into the star or is torn apart by the star's gravity, according to Jackson's model of tidal orbital decay.

"The last moments for these planets can be pretty dramatic, as their atmospheres are ripped away by their stars' gravity," says Jackson. "It has even been suggested recently the hot Jupiter called WASP-12B is close enough to its star that it is currently being destroyed."

Debes and Jackson modeled what would have happened in 47 Tuc if the tidal effect were unleashed on hot Jupiters. They recreated the range of masses and sizes of the stars in that cluster and simulated a likely arrangement of planets. Then they let the stars' tides go to work on the close-in planets. The model predicted that so many of these planets would be destroyed, the survey would come up empty-handed. "Our model shows that you don't need to consider metallicity to explain the survey results," says Debes, "though this and other effects will also reduce the number of planets."

Ron Gilliland, who is at the Space Telescope Science Institute in Baltimore and participated in the 47 Tuc survey, says, "This analysis of tidal interactions of planets and their host stars provides another potentially good explanation—in addition to the strong correlation between metallicity and the presence of planets—of why we failed to detect exoplanets in 47 Tuc."

In general, Debes and Jackson's model predicts that one-third of the hot Jupiters will be destroyed by the time a cluster is a billion years old, which is still juvenile compared to our solar system (about 4-1/2 billion years old). 47 Tuc has recently been estimated to be more than 11 billion years old. At that age, the researchers expect more than 96% of the hot Jupiters to be gone.

The Kepler mission, which is searching for hot Jupiters and smaller, Earth-like planets, gives Debes and Jackson a good chance to test their model. Kepler will survey four open clusters—groups of stars that are not as dense as globular clusters—ranging from less than half a billion to nearly 8 billion years old, and all of the clusters have enough raw materials to form significant numbers of planets, Debes notes. If tidal orbital decay is occurring, Debes and Jackson predict, Kepler could find up to three times more Jupiter-sized planets in the youngest cluster than in the oldest one. (An exact number depends on the brightness of the stars, the planets' distance from the stars, and other conditions.)

"If we do find planets in those clusters with Kepler," says Gilliland, a Kepler co-investigator, "looking at the correlations with age and metallicity will be interesting for shaping our understanding of the formation of planets, as well as their continued existence after they are formed."

If the tidal orbital decay model proves right, Debes adds, planet hunting in clusters may become even harder. "The big, obvious planets may be gone, so we'll have to look for smaller, more distant planets," he explains. "That means we will have to look for a much longer time at large numbers of stars and use instruments that are sensitive enough to detect these fainter planets."

The Kepler mission is managed by NASA's Ames Research Center, Moffett Field, Calif., for the Science Mission Directorate at NASA Headquarters in Washington.

Related links:

20 Years of Hubble: Exoplanets

Elizabeth Zubritsky
NASA's Goddard Space Flight Center

Rosette Nebula: The Heart of a Rose

Credit: X-ray (NASA/CXC/SAO/J. Wang et al),

Optical (DSS & NOAO/AURA/NSF/KPNO 0.9-m/T. Rector et al)

JPEG (1.2 MB) - Tiff (37.1 MB) - PS (14.8 MB) -More Images
Zoom-In (flash)

This composite image shows the Rosette star formation region, located about 5,000 light years from Earth. Data from the Chandra X-ray Observatory are colored red and outlined by a white line (roll your mouse over the image above). The X-rays reveal hundreds of young stars in the central cluster and fainter clusters on either side. Optical data from the Digitized Sky Survey and the Kitt Peak National Observatory (purple, orange, green and blue) show large areas of gas and dust, including giant pillars that remain behind after intense radiation from massive stars has eroded the more diffuse gas.

A recent Chandra study of the cluster on the right side of the image, named NGC 2237, provides the first probe of the low-mass stars in this satellite cluster. Previously only 36 young stars had been discovered in NGC 2237, but the Chandra work has increased this sample to about 160 stars. The presence of several X-ray emitting stars around the pillars and the detection of an outflow -- commonly associated with very young stars -- originating from a dark area of the optical image indicates that star formation is continuing in NGC 2237. By combining these results with earlier studies, the scientists conclude that the central cluster formed first, followed by expansion of the nebula, which triggered the formation of the two neighboring clusters, including NGC 2237.

This work was led by Junfeng Wang of the Harvard-Smithsonian Center for Astrophysics. The co-authors were Eric Feigelson, Leisa Townsley, Pat Broos and Gordon Garmire from Penn State University, Carlos Roman-Zuniga from the German-Spanish Astronomical Center in Spain, and Elizabeth Lada from the University of Florida.

Fast Facts for Rosette Nebula:

Scale: Image is 1 degree across (about 87 light years).
Category: Normal Stars & Star Clusters
Coordinates (J2000) RA 06h 31m 52.00s Dec +04° 55' 57.00"
Constellation: Monoceros
Observation Date: 4 pointings on 5-6 Jan 2001, 1 Jan 2004, 9 Feb 2007
Observation Time: 50 hours (2 days 2 hours)
Obs. ID: 1874-1877, 3750, 8454
Color Code: Optical (Purple, Orange, Green, Blue); X-ray (Red)
Instrument: ACIS
References: Wang et al, 2010 ApJ 716:474-489
Distance Estimate: About 5,000 light years

A Nearby Galactic Exemplar

PR Image eso1037a
Wide Field Imager view of the southern spiral NGC 300

PR Image eso1037b
NGC 300 in the constellation of Sculptor

PR Video eso1037a
Panning across the southern spiral NGC 300

PR Video eso1037b

Zooming into the southern spiral NGC 300

ESO has released a spectacular new image of NGC 300, a spiral galaxy similar to the Milky Way, and located in the nearby Sculptor Group of galaxies. Taken with the Wide Field Imager (WFI) at ESO’s La Silla Observatory in Chile, this 50-hour exposure reveals the structure of the galaxy in exquisite detail. NGC 300 lies about six million light-years away and appears to be about two thirds the size of the full Moon on the sky.

Originally discovered from Australia by the Scottish astronomer James Dunlop early in the nineteenth century, NGC 300 is one of the closest and most prominent spiral galaxies in the southern skies and is bright enough to be seen easily in binoculars. It lies in the inconspicuous constellation of Sculptor, which has few bright stars, but is home to a collection of nearby galaxies that form the Sculptor Group [1]. Other members that have been imaged by ESO telescopes include NGC 55 (eso0914), NGC 253 (eso1025, eso0902) and NGC 7793 (eso0914). Many galaxies have at least some slight peculiarity, but NGC 300 seems to be remarkably normal. This makes it an ideal specimen for astronomers studying the structure and content of spiral galaxies such as our own.

This picture from the Wide Field Imager (WFI) at ESO’s La Silla Observatory in Chile was assembled from many individual images taken through a large set of different filters with a total exposure time close to 50 hours. The data was acquired over many observing nights, spanning several years. The main purpose of this extensive observational campaign was to take an unusually thorough census of the stars in the galaxy, counting both the number and varieties of the stars, and marking regions, or even individual stars, that warrant deeper and more focussed investigation. But such a rich data collection will also have many other uses for years to come. By observing the galaxy with filters that isolate the light coming specifically from hydrogen and oxygen, the many star-forming regions along NGC 300’s spiral arms are shown with particular clarity in this image as red and pink clouds. With its huge field of view, 34 x 34 arcminutes, similar to the apparent size of the full Moon in the sky, the WFI is an ideal tool for astronomers to study large objects such as NGC 300.

NGC 300 is also the home of many interesting astronomical phenomena that have been studied with ESO telescopes. ESO astronomers recently discovered the most distant and one of the most massive stellar-mass black holes yet found (eso1004) in this galaxy, as the partner of a hot and luminous Wolf–Rayet star in a binary system. NGC 300 and another galaxy, NGC 55, are slowly spinning around and towards each other, in the early stages of a lengthy merging process (eso0914). The current best estimate of the distance to the NCG 300 was also determined by astronomers using ESO’s Very Large Telescope at the Paranal Observatory (eso0524), among others.

Notes

[1] Although it is normally considered as member of the Sculptor Group, the most recent distance measurements show that NGC 300 lies significantly closer to us than many of the other galaxies in the group and may be only loosely associated with them.

More information

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, 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 VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Contacts

Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey telescopes Press Officer
Garching bei München, Germany
Tel: +49 89 3200 6655

Email: rhook@eso.org

Spirals eat dwarfs: Galactic tendrils shed light on evolution of spiral galaxies

Spiral galaxies grow by swallowing smaller dwarf galaxies. As they are digested, these dwarf galaxies are severely distorted, forming structures such as surreal tendrils and stellar streams that surround their captors. Now, for the first time, a new survey has detected such tell-tale structures in galaxies more distant than our immediate galactic neighbourhood. This opens up the possibility of testing our current views of galaxy evolution in a new way.

Figure 1: Stellar streams around the galaxy M 63: remnants of a satellite galaxy that M 63 has swallowed. The central part is an ordinary positive image; in the outer regions, the negative of the image is shown. In this way, the faint structures that are the target of this survey are more readily discerned. This galaxy's distance from Earth is around 30 million light-years. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of galaxies. Image: D. Martínez-Delgado (MPIA) [Larger version for download]

Figure 2: Examples for traces of spirals swallowing dwarf galaxies, as found with the new survey. For all images, the central part is an ordinary positive image. In the outer regions, the negative of the image is shown. In this way, the faint structures that are the target of this survey are more readily discerned. Wisps, plumes, stellar streams, partially disrupted satellites or stellar cloud indicate that we are witnessing merger processes. Image: D. Martínez-Delgado (MPIA) [Größere Version zum Herunterladen]

Left column from top to bottom:
M 63, NGC 4651, NGC 7531, NGC 5866.
Right column from top to bottom:
NGC 1084, NGC 4651, NGC 3521, NGC 1055.

Figure 3: Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated structures consisting of stars, which are known as tidal streams, as shown in this artist's impression. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of galaxies. Image: Jon Lomberg [Larger version for download]

Figure 4: Closer inspection of this spiral galaxy, NGC 1055 in the constellation Cetus, reveals a weak box-shaped halo of stars: a trace of NGC 1055 having swallowed and assimilated a smaller satellite galaxy. This galaxy's distance from Earth is 60 million light-years. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of galaxies.

Image: K. Crawford (Rancho Del Sol Obs.) in collaboration with D. Martínez-Delgado (MPIA and IAC) et al. [Larger version for download]

Around the Milky Way galaxy and in the vicinity of our immediate cosmic neighborhood, known as the “Local Group” of galaxies, traces of spiral galaxies swallowing dwarf galaxies have been known since 1997. But the Local group with its three spiral galaxies and numerous dwarfs is much too small a sample to see whether theoretical predictions of the frequency of such digestive processes match observations. Now, for the first time, a new survey has managed to detect the tell-tale tendrils of galactic digestion beyond the Local Group. An international group of researchers led by David Martínez-Delgado (Max Planck Institute for Astronomy and Instituto de Astrofísica de Canarias) has completed a pilot survey of spiral galaxies at distances of up to 50 million light-years from Earth, discovering the tell-tale signs of spirals eating dwarfs.

When a spiral galaxy is approached by a much smaller companion, such as a dwarf galaxy, the larger galaxy's uneven gravitational pull severely distorts the smaller star system. Over the course of a few billions of years, tendril-like structures develop that can be detected by sensitive observation. In one typical outcome, the smaller galaxy is transformed into an elongated “tidal stream” consisting of stars that, over the course of additional billions of years, will join the galaxy's regular stellar inventory through a process of complete assimilation. The study shows that major tidal streams with masses between 1 and 5 percent of the galaxy's total mass are quite common in spiral galaxies.

Detailed simulations depicting the evolution of galaxies predict both tidal streams and a number of other distinct features that indicate mergers, such as giant debris clouds or jet-like features emerging from galactic discs. Interestingly, all these various features are indeed seen in the new observations – impressive evidence that current models of galaxy evolution are indeed on the right track.

The ultra-deep images obtained by Delgado and his colleagues open the door to a new round of systematic galactic interaction studies. Next, with a more complete survey that is currently in progress, the researchers intend to subject the current models to more quantitative tests, checking whether current simulations make the correct predictions for the relative frequency of the different morphological features.

Remarkably, these cutting-edge results were obtained with the telescopes of ambitious amateur astronomers: For their observations, the researchers used telescopes with apertures between 10 and 50 cm, equipped with commercially available CCD cameras. The telescopes are robotic (that is, they can be controlled remotely), and are located at two private observatories in the US and one in Australia. The results attest to the power of systematic work that is possible even with smaller instruments: While larger telescopes have the undeniable edge in detecting very distant, but comparatively bright star systems such as active galaxies, this survey provides some of the deepest insight yet when it comes to detecting ordinary galaxies that are similar to our own cosmic home, the Milky Way.[1]

Contact

Dr. David Martínez-Delgado (Principal investigator of this study)

Max Planck Institute for Astronomy

Phone: +49 6221 528 455

E-Mail: delgado@mpia.de

Dr. Markus Pössel (Public relations)

Max Planck Institute for Astronomy

Phone: +49 6221 528 261

E-Mail: pr@mpia.de

End Note

[1] As a measure of faintness of the observed galaxies, the limit of the current survey was at 29.5 magnitudes per square arcsecond.

Background information

The research described here will be published as a letter in the October issue of the Astronomical Journal as D. Martínez-Delgado et al., “Stellar Tidal Streams in Spiral Galaxies of the Local Volume: A Pilot Survey with Modest Aperture Telescopes”. An electronic preprint is avalailable under http://arxiv.org/abs/1003.4860

The observations were carried out with 50 cm telescopes at Black Bird Observatory (New Mexico, USA) and Ranco del Sol (California, USA), the 37 cm telescope at Moorook (South Australia) and the 16 cm telescope at New Mexico Skies (New Mexico, USA).

The research group consists of David Martínez-Delgado (Max Planck Institute for Astronomy, Heidelberg, Germany, and Instituto de Astrofísica de Canarias, Spain), R. Jay Gabany (Black Bird Observatory), Ken Crawford (Rancho del Sol Observatory), Stefano Zibbeti and Hans-Walter Rix (Max Planck Institute for Astronomy), Steven R. Majewski and David A. McDavid (University of Virginia), Jürgen Fliri (Instituto de Astrofísica de Canarias and Observatoire de Paris, Meudon), Julio A. Carballo-Bello and Ignacio Trujillo (Instituto de Astrofísica de Canarias), Daniella C. Bardalez-Gagliuffi (MIT and Instituto de Astrofísica de Canarias), Jorge Penarriubio (Cambridge University) and Mischa Schirmer (Argelander Institute for Astronomy, Bonn University).

Picture-Perfect Pluto Practice

New Horizons image of Neptune and its largest moon, Triton, taken during the annual checkout in late June 2010. The image, taken when the planet was more than 2 billion miles from New Horizons, is a combination of two 9.967-second exposures.

Neptune’s giant moon Triton is often called Pluto’s “twin” – so what better practice target, then, for New Horizons’ telescopic camera?

New Horizons’ Long Range Reconnaissance Imager (LORRI) snapped several photos of Neptune during the latest annual systems checkout, which ended July 30. Neptune was 23.2 astronomical units (about 2.15 billion miles!) from New Horizons when LORRI took aim at the gas giant planet — and Triton made a cameo appearance in these images.

“That we were able to see Triton so close to Neptune, which is approximately 100 times brighter, shows us that the camera is working exactly as designed,” says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory. “This was a good test for LORRI.”

Weaver points out that the solar phase angle (the spacecraft-planet-Sun angle) was 34 degrees and the solar elongation angle (planet-spacecraft-Sun angle) was 95 degrees. Only New Horizons can observe Neptune at such large solar phase angles, which he says is key to studying the light-scattering properties of Neptune's and Triton’s atmospheres.

“As New Horizons has traveled outward across the solar system, we’ve been using our imagers to make just such special-purpose studies of the giant planets and their moons because this is a small but completely unique contribution that New Horizons can make — because of our position out among the giant planets,” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute.

Triton is slightly larger than Pluto, 1,700 miles (2,700 kilometers) in diameter compared to Pluto’s 1,500 miles (2,400 kilometers). Both objects have atmospheres composed mostly of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures approaching minus-400 degrees Fahrenheit. Triton is widely believed to have been a member of the Kuiper Belt (as Pluto still is) that was captured into orbit around Neptune, probably during a collision early in the solar system’s history.

New Horizons first photographed Triton in 2008, during its second annual checkout, at a smaller phase angle (21.4 degrees) and larger distance (25.08 AU from New Horizons).

Spitzer Finds a Flavorful Mix of Asteroids

This image, taken by NASA's Near Earth Asteroid Rendezvous mission in 2000, shows a close-up view of Eros, an asteroid with an orbit that takes it somewhat close to Earth. NASA's Spitzer Space Telescope observed Eros and dozens of other near-Earth asteroids as part of an ongoing survey to study their sizes and compositions using infrared light. Image credit: NASA/JHUAPL. Full image and caption

New research from NASA's Spitzer Space Telescope reveals that asteroids somewhat near Earth, termed near-Earth objects, are a mixed bunch, with a surprisingly wide array of compositions. Like a piñata filled with everything from chocolates to fruity candies, these asteroids come in assorted colors and compositions. Some are dark and dull; others are shiny and bright. The Spitzer observations of 100 known near-Earth asteroids demonstrate that the objects’ diversity is greater than previously thought.

The findings are helping astronomers better understand near-Earth objects as a whole -- a population whose physical properties are not well known.

"These rocks are teaching us about the places they come from," said David Trilling of Northern Arizona University, Flagstaff, lead author of a new paper on the research appearing in the September issue of Astronomical Journal. "It's like studying pebbles in a streambed to learn about the mountains they tumbled down."

After nearly six years of operation, in May 2009, Spitzer used up the liquid coolant needed to chill its infrared detectors. It is now operating in a so-called "warm" mode (the actual temperature is still quite cold at 30 Kelvin, or minus 406 degrees Fahrenheit). Two of Spitzer's infrared channels, the shortest-wavelength detectors on the observatory, are working perfectly.

One of the mission's new "warm" programs is to survey about 700 near-Earth objects, cataloguing their individual traits. By observing in infrared, Spitzer is helping to gather more accurate estimates of asteroids' compositions and sizes than what is possible with visible light alone. Visible-light observations of an asteroid won't differentiate between an asteroid that is big and dark, or small and light. Both rocks would reflect the same amount of visible sunlight. Infrared data provide a read on the object's temperature, which then tells an astronomer more about the actual size and composition. A big, dark rock has a higher temperature than a small, light one because it absorbs more sunlight.

Trilling and his team have analyzed preliminary data on 100 near-Earth asteroids so far. They plan to observe 600 more over the next year. There are roughly 7,000 known near-Earth objects out of a population expected to number in the tens to hundreds of thousands.

"Very little is known about the physical characteristics of the near-Earth population," said Trilling. "Our data will tell us more about the population, and how it changes from one object to the next. This information could be used to help plan possible future space missions to study a near-Earth object."

The data show that some of the smaller objects have surprisingly high albedos (an albedo is a measurement of how much sunlight an object reflects). Since asteroid surfaces become darker with time due to exposure to solar radiation, the presence of lighter, brighter surfaces for some asteroids may indicate that they are relatively young. This is evidence for the continuing evolution of the near-Earth object population.

In addition, the fact that the asteroids observed so far have a greater degree of diversity than expected indicates that they might have different origins. Some might come from the main belt between Mars and Jupiter, and others could come from farther out in the solar system. This diversity also suggests that the materials that went into making the asteroids -- the same materials that make up our planets -- were probably mixed together like a big solar-system soup very early in its history.

The research complements that of NASA's Wide-field Infrared Survey Explorer, or WISE, an all-sky infrared survey mission also up in space now. WISE has already observed more than 430 near-Earth objects -- of these, more than 110 are newly discovered.

In the future, both Spitzer and WISE will tell us even more about the "flavors" of near-Earth objects. This could reveal new clues about how the cosmic objects might have dotted our young planet with water and organics -- ingredients needed to kick-start life.

Other authors of the paper include Cristina Thomas, also from Northern Arizona University; Michael Mueller and Marco Delbo of the Observatoire de la Côte d'Azur, Nice, France; Joseph Hora, Giovanni Fazio, Howard Smith and Tim Spahr of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.; Alan Harris of the DLR Institute of Planetary Research, Berlin, Germany (DLR is Germany's space agency and stands for Deutsches Zentrum für Luft- und Raumfahrt); Bidushi Bhattacharya of the NASA Herschel Science Center at the California Institute of Technology, Pasadena; Steve Chesley and Amy Mainzer of NASA's Jet Propulsion Laboratory, Pasadena, Calif.; Bill Bottke of the Southwest Research Institute, Boulder, Colo.; Josh Emery of the University of Tennessee, Knoxville; Bryan Penprase of the Pomona College, Claremont, Calif.; and John Stansberry of the University of Arizona, Tucson.

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, also in Pasadena. Caltech manages JPL for NASA. For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer .

JPL manages the Wide-field Infrared Survey Explorer for NASA's Science Mission Directorate, Washington. The principal investigator, Edward Wright, is at UCLA. The mission was competitively selected under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA. More information is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu .

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

New Hubble Observations of Supernova 1987A Trace Shock Wave

This image shows the entire region around supernova 1987A. The most prominent feature in the image is a ring with dozens of bright spots. A shock wave of material unleashed by the stellar blast is slamming into regions along the ring's inner regions, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded. Credit: NASA, ESA, K. France (University of Colordo, Boulder), and P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)

Hubble's view of Supernova 1987A over time

An international team of astronomers using the Hubble Space Telescope reports a significant brightening of the emissions from Supernova 1987A. The results, which appear in this week's Science magazine, are consistent with theoretical predictions about how supernovae interact with their immediate galactic environment.

The team observed the supernova remnant in optical, ultraviolet, and near-infrared light. They studied the interaction between the ejecta from the stellar explosion and a glowing 6-trillion-mile-diameter ring of gas encircling the supernova remnant. The gas ring was probably shed some 20,000 years before the supernova exploded. Shock waves resulting from the impact of the ejecta onto the ring have brightened 30 to 40 pearl-like "hot spots" in the ring. These blobs likely will grow and merge together in the coming years to form a continuous, glowing circle.

"We are seeing the effect a supernova can have in the surrounding galaxy, including how the energy deposited by these stellar explosions changes the dynamics and chemistry of the environment," said University of Colorado at Boulder Research Associate Kevin France of the Center for Astrophysics and Space Astronomy. "We can use these new data to understand how supernova processes regulate the evolution of galaxies."

Discovered in 1987, Supernova 1987A is the closest exploding star to Earth to be detected since 1604 and it resides in the nearby Large Magellanic Cloud, a dwarf galaxy adjacent to our own Milky Way Galaxy.

CONTACT

Jim Scott

University of Colorado, Boulder, Colo.

303-492-3114

jim.scott@colorado.edu

Christine Pulliam

Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

617-495-7463

cpulliam@cfa.harvard.edu

Ray Villard

Space Telescope Science Institute, Baltimore, Md.

410-338-4514

villard@stsci.edu

CW Leonis

CW Leonis

Image Credit: ESA / SPIRE / PACS / MESS

The red giant carbon star CW Leonis is too bright to be seen well by the SPIRE and PACS cameras, but it is releasing material in a violent stellar wind. Some of that material is seen as a “bow shock” to the left of the star in this image, glowing as it piles up against the interstellar medium. Observations with the PACS and SPIRE spectrometers have shown that water vapour is being formed deep down near the surface of the star; a place where it was previously thought to be impossible.

When astronomers discovered an unexpected cloud of water vapour around the old star CW Leonis in 2001, using the Submillimeter Wave Astronomy Satellite (SWAS), they immediately began searching for the source. Water is known to be present around several types of stars, but CW Leonis is a “carbon star” and therefore thought not to produce water. Initially they suspected the star’s heat must be evaporating comets or even dwarf planets to produce the water.

CW Leonis is a red giant star which is only a few times the mass of the Sun but has expanded to hundreds of times its size. Nuclear fusion reactions deep inside the star are converting helium into carbon, much of which has ended up in the outer layers of the star’s atmosphere. It is this abundance of Carbon in the atmospheres of these types of stars that has previously led scientists to believe that water could not exist; with so much carbon all of the oxygen should be locked up in carbon monoxide (CO).

Now, the SPIRE and PACS spectrometers ave shown that the water vapour is being formed much closer to the star itself than preivously thought, at temperatures approaching 1000oC. These results, published in Nature, suggests that the water is being created by a previously unsuspected chemical process where ultraviolet radiation from interstellar space is breaking up the carbon monoxide and releasing oxygen atoms that can then react with hydrogen to form water molecules.

Ultraviolet starlight would normally be blocked by the material flowing from the star as its outer layers billow out in a stellar wind. It was already known that the stellar wind is “clumpy” but the Herschel results have shown that some regions around the star must have no wind. These empty regions allow the ultraviolet light to reach the deepest layers of the star’s atmosphere and initiate the chemical reactions that produce the water.

About this image: CW Leonis as seen by SPIRE and PACS, with the emission lines due to water also shown. Image credit: ESA / SPIRE / PACS / MESS Consortia

The water is detected by the fact that it preferentially emits light with specific wavelengths, with the relative brightnesses of the "emission lines" indicating the conditions in which the water is forming.

The wavelength range of the SPIRE and PACS spectrometers, between them covering 60--670 micrometers (frequencies of 0.45--5 THz), allows astronomers to detect elements and molecules previously impossible. The wide range now also makes it possible to determine the temperature od the environment in which the water is forming, which wasn't possible before.

The Superwind Galaxy NGC 4666

PR Image eso1036a
The superwind galaxy NGC 4666

PR Image eso1036b
Wide-field view of the field around NGC 4666

PR Video eso1036a
Zooming in on NGC 4666

PR Video eso1036b
Panning across the field of NGC 4666

The galaxy NGC 4666 takes pride of place at the centre of this new image, made in visible light with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. NGC 4666 is a remarkable galaxy with very vigorous star formation and an unusual “superwind” of out-flowing gas. It had previously been observed in X-rays by the ESA XMM-Newton space telescope, and the image presented here was taken to allow further study of other objects detected in the earlier X-ray observations.

The prominent galaxy NGC 4666 in the centre of the picture is a starburst galaxy, about 80 million light-years from Earth, in which particularly intense star formation is taking place. The starburst is thought to be caused by gravitational interactions between NGC 4666 and its neighbouring galaxies, including NGC 4668, visible to the lower left. These interactions often spark vigorous star-formation in the galaxies involved.

A combination of supernova explosions and strong winds from massive stars in the starburst region drives a vast flow of gas from the galaxy into space — a so-called “superwind”. The superwind is huge in scale, coming from the bright central region of the galaxy and extending for tens of thousands of light-years. As the superwind gas is very hot it emits radiation mostly as X-rays and in the radio part of the spectrum and cannot be seen in visible light images such as the one presented here.

This image was made as part of a follow-up to observations made with the ESA XMM-Newton space telescope in X-rays. NGC 4666 was the target of the original XMM-Newton observations, but thanks to the telescope’s wide field-of-view many other X-ray sources were also seen in the background. One such serendipitous detection is a faint galaxy cluster seen close to the bottom edge of the image, right of centre. This cluster is much further away from us than NGC 4666, at a distance of about three billion light-years.

In order to fully understand the nature of astronomical objects, researchers must study them at several wavelengths. This is because light of different wavelengths can tell us about different physical processes taking place. In this case the Wide Field Imager (WFI) [1] observations were made in visible light to further investigate these serendipitously detected X-ray objects — a good example of how astronomers using different telescopes work together to explore the Universe.

Notes

[1] The WFI is a joint project between the European Southern Observatory (ESO), the Max-Planck-Institut für Astronomie (MPIA) in Heidelberg (Germany) and the Osservatorio Astronomico di Capodimonte (OAC) in Naples (Italy).

More information

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, 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 VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links
Science paper: A&A 449, 837
Image of the galaxy in X-rays

Contacts

Richard Hook
ESO La Silla/Paranal & E-ELT Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org

Jörg Dietrich
Physics Department, University of Michigan
USA
Tel: +1 734 615 4256
Email: jorgd@umich.edu

Spectrum of Young Extrasolar Planet Yields Surprising Results

Keck II image of the young extrasolar planet HR 8799 b, seen as the point source in center of image. The bright light from the parent star HR 8799 is seen in background in yellow/red and has been removed in an annular region centered on the planet. Credit: Brendan Bowler and Michael Liu, IfA/Hawaii

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California and NASA. Credit: Pablo McLoud/WMKO

Kamuela, HI - Astronomers at the University of Hawaii have measured the temperature of a young gas-giant planet around another star using the W. M. Keck Observatory, and the results are puzzling. They have found that its atmosphere is unlike that of any previously studied extrasolar planet.

By obtaining a spectrum of its emitted light, the astronomers determined the temperature of the planet. As a result, they found that current theoretical models of gas-giant planets did a poor job of explaining all the data. The team suspects that the reason is dust in the planet’s atmosphere. Models with normal amounts of dust do not resemble this planet, but models with exceptionally thick dust clouds do a much better job. It therefore appears that young gas-giant planets are extremely cloudy.

“We are at a point where not only can we directly image planets around other stars, but we can begin to study the properties of their atmospheres in detail. Direct spectroscopy of exoplanets is the future of this field,” said Mr. Brendan Bowler, a graduate student at the University of Hawaii and the lead author of the study.

The planet, known as HR 8799 b, is one of three gas-giant planets orbiting the star HR 8799, located 130 light-years away from Earth in the constellation Pegasus. (For reference, the distance to the nearest nighttime star from Earth is about four light-years.) HR 8799 b is the lowest-mass planet around the star, about seven times the mass of Jupiter. This multiplanet system was discovered by direct imaging in 2008, and now, only a year and a half later, astronomers have obtained a spectrum of one of its planets. The spectrum of a planet contains much more information than a single image: it can reveal the temperature, chemical composition, and cloud properties of the planet.

The technique the team used to determine the planet’s temperature relies on the chemistry of the planet’s atmosphere. Specifically, the presence or absence of gaseous methane can be used as a thermometer. The team found that HR 8799 b shows little or no methane in its atmosphere. Based on their spectrum and previously obtained images of the planet, and by comparing the observations to theoretical models of low-temperature atmospheres, they estimate the coolest possible temperature for the planet is about 1200 Kelvin (about 1,700 degrees Fahrenheit).

The models, however, did a poor job of reproducing all the data. Current theoretical models predict HR 8799 b should be about 400 Kelvin cooler than they measured, based on the age of the planet and the amount of energy it is currently emitting. The team suspects the discrepancy arises because the planet is much more dusty and cloudy than expected by current models.

“Direct studies of extrasolar planets are just in their infancy. But even at this early stage, we are learning they are a different beast than objects we have known about previously,” said University of Hawaii astronomy professor Michael Liu, coauthor of the study.

The planets around HR 8799 are incredibly faint, about 100,000 times dimmer than their parent star. To obtain the spectrum of HR 8799 b, the team relied on the adaptive optics system of the Keck II Telescope to make an ultra-sharp image of the star for many hours. Then they used the Keck facility instrument called OSIRIS, a special kind of spectrograph, to precisely separate the spectrum of the planet from the light of its parent star.

“Adaptive optics systems on Keck and other large ground-based telescopes make sharper images than even the Hubble Space Telescope. With adaptive optics, we are learning an incredible amount about objects that are smaller than the lowest-mass stars and larger than the most massive gas-giant planets in our solar system,” said Mr. Trent Dupuy, a University of Hawaii graduate student and co-author on the study. Dr. Michael Cushing of the Jet Propulsion Laboratory was also a member of the team announcing these results.

Although over 500 planets have been discovered around other stars, only six planets have been directly imaged. Three of these are around HR 8799 and were discovered in 2008 by Christian Marois of Canada’s National Research Council and collaborators. When it was announced, the discovery represented one of the first direct image of light emitted from extrasolar planets.

A paper describing the study will be published in the Astrophysical Journal later this year. A copy is available here http://arxiv.org/abs/1008.4582.

Abell 1758: Cluster Collisions Switch on Radio Halos

Abell 1758

Credit X-ray (NASA/CXC/SAO/M.Markevitch);

Radio (TIFR/GMRTSAO/INAF/R.Cassano, S.Giacintucci);

Optical (DSS)

JPEG (671.4 kb) - Tiff (37.1 MB) - PS (14.8 MB)

More Images - Zoom-In (flash)

This is a composite image of the northern part of the galaxy cluster Abell 1758, located about 3.2 billion light years from Earth, showing the effects of a collision between two smaller galaxy clusters. Chandra X-ray data (blue) reveals hot gas in the cluster and data from the Giant Metrewave Radio Telescope (GMRT) in India (pink) shows huge "halos" generated by ultra-relativistic particles and magnetic fields over vast scales. Optical data from the Digitized Sky Survey are colored gold.

A study of this galaxy cluster and 31 others with Chandra and the GMRT shows that huge radio halos are generated during collisions between galaxy clusters. This result implies that galaxy clusters with radio halos are still forming, while clusters without this radio emission are not still accumulating large amounts of material. The result also implies that relativistic electrons are likely accelerated by turbulence generated by mergers between clusters

Galaxy clusters are the largest structures in the Universe that are bound together by gravity. They form when smaller clusters or groups of galaxies collide and merge. Collisions between galaxy clusters, such as this one in Abell 1758 and its more famous cousin the Bullet Cluster, are the most energetic events in the Universe since the Big Bang. Their growth rate over the last 7 billion years has been slowed by the effects of dark energy, as shown by previous studies with Chandra.

Fast Facts for Abell 1758:

Scale: Image is about 13 arcmin across (about 12 million light years)
Category: Groups & Clusters of Galaxies, Cosmology/Deep Fields/X-ray Background
Coordinates: (J2000) RA 13h 32m 43.20s | Dec +50° 32' 25.70"
Constellation: Canes Venatici
Observation Date: Aug 28, 2001
Observation Time: 16 hours 40 min
Obs. ID: 2213
Color Code: X-ray (Blue); Radio (Pink); Optical (Gold)
Instrument: ACIS
Distance Estimate: 3.2 Billion Light Years (z=0.28)

Watch Titan Occult a Binary Star System

It is Astrophoto Friday! This week's image shows a unique event that was captured by the 200-inch Hale Telescope armed with adaptive optics.

What might look like Pac Man swallowing a dot is actually Saturn's moon Titan occulting (passing in front of) a binary star system (named NV0435215+200905). The two stars are separated in the sky by just 1.5 arc seconds (One arc second is 1/3600 of a degree).

Because fantastic resolving power of the Hale using adaptive optics you can see that the light of the star nearest to Titan is being refracted by Titan's dense atmosphere. Such events are rare, but valuable. The starlight as it is seen passing through Titan's atmosphere is essentially a probe providing clues as to the density, temperature and wind patterns of this distant world. The team of astronomers (Antonin Bouchez, Michael E. Brown, Mitchell Troy, Rick S. Burruss, Richard G. Dekany and Robert A. West) that observed this event December 20, 2001 was fortunate that both of the stars were seen to pass behind Titan. This provided two passes through Titan's atmosphere - effectively doubling what could be learned from the event.

Be sure to check out the movie of the event. As you watch it will look like Titan is still and the stars are moving behind it. As they pass behind Titan be sure to look for the refracted light of each star on either side of Titan's atmosphere. It is an impressive sight!

The result? Jet stream winds were discovered in Titan's atmosphere.

For those so inclined you can read a pdf of one of the scientific publication that came out of these observations.