Looking to the Heavens

IRAS 10082-5647
Credit: ESA/Hubble & NASA

The pearly wisps surrounding the central star IRAS 10082-5647 in this Hubble image certainly draw the eye towards the heavens. The divine-looking cloud is a reflection nebula, made up of gas and dust glowing softly by the reflected light of nearby stars, in this case a young Herbig Ae/Be star.

The star, like others of this type, is still a relative youngster, only a few million years old. It has not yet reached the so-called main sequence phase, where it will spend around 80% of its life creating energy by burning hydrogen in its core. Until then the star heats itself by gravitational collapse, as the material in the star falls in on itself, becoming ever denser and creating immense pressure which in turn gives off copious amounts of heat.

Stars only spend around 1% of their lives in this pre-main sequence phase. Eventually, gravitational collapse will heat the star’s core enough for hydrogen fusion to begin, propelling the star into the main sequence phase, and adulthood.

The Advanced Camera for Surveys aboard the Hubble Space Telescope captured the whorls and arcs of this nebula, lit up with the light from IRAS 10082-5647. Visible (555 nm) and near-infrared (814 nm) filters were used, coloured blue and red respectively. The field of view is around 1.3 by 1.3 arcminutes.

Subaru's 3-D View of Stephan's Quintet

Figure 1: Composite tricolor images of Stephan's Quintet using Hα filters with a recession velocity of 0 (left image) and a recession velocity of 4,200 miles per second (right image).

Figure 2: A diagram of the member galaxies of Stephan's Quintet. NGC7320 is a closer galaxy and has a recession velocity of 0. The remaining four are a group of more distant galaxies 300 million light years away. The researchers believe that the merging of NGC7318A/B and NGC7319's crashing into them are responsible for the active star formation regions in the Hα emitting region around NGC7318A/B.

Subaru Telescope has added another dimension of information about one of the most studied of all compact galaxy groups—Stephan's Quintet. Located within the borders of the constellation Pegasus, Stephan's Quintet consists of a visual grouping of five galaxies, four of which form an actual compact group of galaxies; one additional galaxy appears in images of the group but is much closer than the others. Refinements in observations of the quintet are revealing more about its members. A comparison of images (the left and right images in Figure 1) compiled by using a suite of specialized filters with Subaru's Prime Focus Camera (Suprime-Cam) have shown different types of star-formation activity between the closer galaxy NGC7320 and the more distant galaxies in Stephan's Quintet. They show the quintet in 3-D.

These new images are the product of Suprime-Cam's ability to capture images of objects in a wide field of view and to use specialized filters to focus observations according to particular research objectives. To learn about the star-forming regions in Stephan's Quintet and their structures, observers used special narrowband filters for Hα emissions, which let in a very specific wavelength of light to indicate distinctive hydrogen emissions during active star formation. They used two Hα filters, each with a different recession velocity, i.e. the speed at which the object is moving away from the observer. They used one Hα filter with a recession velocity of 0, which means that the speed at which the object is moving away from the observer is 0 and that it is not far distant. They used another Hα filter with a greater recession velocity of 4200 miles (6,700 km) per second, an indicator of distant objects. In addition to the red color attributed to the Hα emission, blue and green colors assigned to the images from the blue and red filters captured light so that the composite tricolor images aligned with human color perception in red, green, and blue.

Processing of the filtered images resulted in the two different views of Stephan's Quintet shown in Figure 1. The image on the left shows the galaxies when the observers used the Hα filter with a recession velocity of 0 while the one on the right shows them when they used the Hα filter with a recession velocity of 4,200 miles per second. The left image shows Hα emissions that indicate an active star-forming region in the spiral arms of NGC7320 in the lower left quadrant but not in the other galaxies. The right image contrasts with the left and shows a region of Hα emissions in the upper three galaxies but none from NGC7320. Two (NGC7318A and NGC7318B) of the four galaxies are shedding gas because of a collision while a third (NGC7319) is crashing in, creating shock waves that trigger vigorous star formation. Figure 2 depicts the relationship of the galaxies. Gas stripped from these three galaxies during galactic collisions is ionized by two mechanisms: shock waves and strong ultraviolet light emanating from the newborn stars. This ionized gas emits bright light, which the Hα filter reveals. Thus the researchers believe that NGC7319 as well as NGC7318A/B are driving the star-forming regions in the Hα emitting region around NGC7318A/B.

In addition to star-forming activity, the images indicate the distances of the galaxies. Different recession velocities help observers spot cases where objects located at different distances appear in proximity in the same image. The contrasting images show that NGC7320 is closer than the other galaxies, which show active star formation at a significantly higher recession velocity (4,200 miles per second) than NGC7320 (0). NGC7320 is about 50 million light years away while the other four galaxies are about 300 million light years away. This explains the intriguing arrangement of the galaxies in Stephan's Quintet.

Observation Parameters

Object Name: HCG92 (Stephan's Quintet)
Telescope Used: Subaru Telescope (8.2 m diameter primary mirror), prime focus
Instrument Used: Subaru Prime Focus Camera (Suprime-Cam)
Filters: B (0.45 μm), R (0.65 μm), NA656 (0.656 μm), NA671 (0.671 μm)
Composite Color
Schemes: blue (B), green (R), red (NA656; Figure 1 left) blue (B), green (R), red (NA671; Figure 1 right)
Observation Dates: 2009-05-25 (R), 2009-05-25 (NA671) 2009-05-26 (B), 2009-08-22 (NA656)
Exposure Times: 180s×4 (R), 900s×5 (NA671) 300s×4 (B), 240s×4 (NA656)
Picture Orientation: Up corresponds to North, left corresponds to East. The field of view is 6'44" x 6'44".
Coordinates: RA (J2000.0) 22h36m, Dec (J2000.0) +33o58' (constellation Pegasus)

Astronomers Pin Down Galaxy Collision Rate

Galactic Wrecks Far from Earth: These images from NASA's Hubble Space Telescope's ACS in 2004 and 2005 show four examples of interacting galaxies far away from Earth. The galaxies, beginning at far left, are shown at various stages of the merger process. The top row displays merging galaxies found in different regions of a large survey known as the AEGIS. More detailed views are in the bottom row of images. Credit: NASA, ESA, J. Lotz (STScI), M. Davis (University of California, Berkeley), and A. Koekemoer (STScI). Larger image

A new analysis of Hubble surveys, combined with simulations of galaxy interactions, reveals that the merger rate of galaxies over the last 8 billion to 9 billion years falls between the previous estimates.

The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies. And yet, a huge discrepancy exists over how often galaxies coalesced in the past. Measurements of galaxies in deep-field surveys made by NASA’s Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.

The study, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Md., analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time. Lotz’s team found that galaxies gained quite a bit of mass through collisions with other galaxies. Large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently. In one of the first measurements of smashups between dwarf and massive galaxies in the distant universe, Lotz’s team found these mergers happened three times more often than encounters between two hefty galaxies.

“Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies,” Lotz explains.

The team’s results are accepted for publication appeared in The Astrophysical Journal.

The problem with previous Hubble estimates is that astronomers used different methods to count the mergers.

“These different techniques probe mergers at different ‘snapshots’ in time along the merger process,” Lotz says. “It is a little bit like trying to count car crashes by taking snapshots. If you look for cars on a collision course, you will only see a few of them. If you count up the number of wrecked cars you see afterwards, you will see many more. Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers than those that searched for galaxies with disturbed shapes, evidence that they’re in smashups.”

To figure out how many encounters happen over time, Lotz needed to understand how long merging galaxies would look like “wrecks” before they settle down and begin to look like normal galaxies again.

That’s why Lotz and her team turned to highly detailed computer simulations to help make sense of the Hubble photographs. The team made simulations of the many possible galaxy collision scenarios and then mapped them to Hubble images of galaxy interactions.

Creating the computer models was a time-consuming process. Lotz’s team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. “Viewing the simulations was akin to watching a slow-motion car crash,” Lotz says.

The simulations followed the galaxies for 2 billion to 3 billion years, beginning at the first encounter and continuing until the union was completed, about a billion years later.

“Our simulations offer a realistic picture of mergers between galaxies,” Lotz says.

In addition to studying the smashups between giant galaxies, the team also analyzed encounters among puny galaxies. Spotting collisions with small galaxies are difficult because the objects are so dim relative to their larger companions.

“Dwarf galaxies are the most common galaxy in the universe,” Lotz says. “They may have contributed to the buildup of large galaxies. In fact, our own Milky Way galaxy had several such mergers with small galaxies in its recent past, which helped to build up the outer regions of its halo. This study provides the first quantitative understanding of how the number of galaxies disturbed by these minor mergers changed with time.”

Lotz compared her simulation images with pictures of thousands of galaxies taken from some of Hubble’s largest surveys, including the All-Wavelength Extended Groth Strip International Survey (AEGIS), the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey (GOODS), as well as mergers identified by the DEEP2 survey with the W.M. Keck Observatory in Hawaii. She and other groups had identified about a thousand merger candidates from these surveys but initially found very different merger rates.

“When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results,” Lotz says.

Her next goal is to analyze galaxies that were interacting around 11 billion years ago, when star formation across the universe peaked, to see if the merger rate rises along with the star formation rate. A link between the two would mean galaxy encounters incite rapid star birth.

In addition to Lotz, the coauthors of the paper include Patrik Jonsson of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass; T. J. Cox of Carnegie Observatories in Pasadena, Calif.; Darren Croton of the Centre for Astrophysics and Supercomputing at Swinburne University of Technology in Hawthorn, Australia; Joel R. Primack of the University of California, Santa Cruz; Rachel S. Somerville of the Space Telescope Science Institute and The Johns Hopkins University in Baltimore, Md.; and Kyle Stewart of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

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. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

CONTACT

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

Jennifer Lotz
Space Telescope Science Institute, Baltimore, Md.
410-338-4467
lotz@stsci.edu

'Pacman' Nebula Gets Some Teeth

In visible light, the star-forming cloud catalogued as NGC 281 in the constellation of Cassiopeia appears to be chomping through the cosmos, earning it the nickname the "Pacman" nebula after the famous Pac-Man video game of the 1980s. Image Credit: NASA/JPL-Caltech/UCLA. Full image and caption

To visible-light telescopes, this star-forming cloud appears to be chomping through the cosmos, earning it the nickname the "Pacman" nebula, like the famous Pac-Man video game that debuted in 1980. When viewed in infrared light by NASA's Wide-field Infrared Survey Explorer, or WISE, the Pacman takes on a new appearance. In place of its typical, triangle-shaped mouth is a new set of lower, sharp-looking teeth. The Pacman is located at the top of the picture, taking a bite in the direction of the upper left corner.

The teeth are actually pillars where new stars may be forming. These structures were formed when radiation and winds from massive stars in a central cluster blew gas and dust away, leaving only the densest of material in the pillars. The red dots sprinkled throughout the picture are thought to be the youngest stars, still forming in cocoons of dust.

The Pacman nebula, also called NGC 281, is located 9,200 light years away in the constellation Cassiopeia.

This image was made from observations by all four infrared detectors aboard WISE. Blue and cyan (blue-green) represent infrared light at wavelengths of 3.4 and 4.6 microns, respectively, which is primarily from stars, the hottest objects pictured. Green and red represent light at 12 and 22 microns, respectively, which is primarily from warm dust (with the green dust being warmer than the red dust).

JPL is managed for NASA by the California Institute of Technology in Pasadena.

Faraway Eris is Pluto's Twin

PR Image eso1142a
Artist’s impression of the dwarf planet Eris

PR Image eso1142b
The occultation of the dwarf planet Eris in November 2010

PR Image eso1142c
Artist’s impression of the dwarf planet Eris and its moon Dysnomia

PR Image eso1142d
Path of the shadow of the dwarf planet Eris during the occultation of November 2010

PR Image eso1142e
Artist’s impression of the dwarf planet Eris

Videos

PR Video eso1142a
ESOcast 38: Faraway Eris is Pluto’s twin

PR Video eso1142b
Path of the shadow of the dwarf planet Eris during the occultation of November 2010

PR Video eso1142c

Artist’s animation showing the dwarf planet Eris and its moon Dysnomia

PR Video eso1142d
Path of the shadow of the dwarf planet Eris during the occultation of November 2010

PR Video eso1142e
Animation of the principle of the occultation

Dwarf planet sized up accurately as it blocks light of faint star

Astronomers have accurately measured the diameter of the faraway dwarf planet Eris for the first time by catching it as it passed in front of a faint star. This event was seen at the end of 2010 by telescopes in Chile, including the Belgian TRAPPIST telescope at ESO’s La Silla Observatory. The observations show that Eris is an almost perfect twin of Pluto in size. Eris appears to have a very reflective surface, suggesting that it is uniformly covered in a thin layer of ice, probably a frozen atmosphere. The results will be published in the 27 October 2011 issue of the journal Nature.

In November 2010, the distant dwarf planet Eris passed in front of a faint background star, an event called an occultation. These occurrences are very rare and difficult to observe as the dwarf planet is very distant and small. The next such event involving Eris will not happen until 2013. Occultations provide the most accurate, and often the only, way to measure the shape and size of a distant Solar System body.

The candidate star for the occultation was identified by studying pictures from the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory. The observations were carefully planned and carried out by a team of astronomers from a number of (mainly French, Belgian, Spanish and Brazilian) universities using — among others — the TRAPPIST [1] (TRAnsiting Planets and PlanetesImals Small Telescope, eso1023) telescope, also at La Silla.

“Observing occultations by the tiny bodies beyond Neptune in the Solar System requires great precision and very careful planning. This is the best way to measure Eris’s size, short of actually going there,” explains Bruno Sicardy, the lead author.

Observations of the occultation were attempted from 26 locations around the globe on the predicted path of the dwarf planet’s shadow — including several telescopes at amateur observatories, but only two sites were able to observe the event directly, both of them located in Chile. One was at ESO’s La Silla Observatory using the TRAPPIST telescope, and the other was located in San Pedro de Atacama and used two telescopes [2]. All three telescopes recorded a sudden drop in brightness as Eris blocked the light of the distant star.

The combined observations from the two Chilean sites indicate that Eris is close to spherical. These measurements should accurately measure its shape and size as long as they are not distorted by the presence of large mountains. Such features are, however, unlikely on such a large icy body.

Eris was identified as a large object in the outer Solar System in 2005. Its discovery was one of the factors that led to the creation of a new class of objects called dwarf planets and the reclassification of Pluto from planet to dwarf planet in 2006. Eris is currently three times further from the Sun than Pluto.

While earlier observations using other methods suggested that Eris was probably about 25% larger than Pluto with an estimated diameter of 3000 kilometres, the new study proves that the two objects are essentially the same size. Eris’s newly determined diameter stands at 2326 kilometres, with an accuracy of 12 kilometres. This makes its size better known than that of its closer counterpart Pluto, which has a diameter estimated to be between 2300 and 2400 kilometres. Pluto’s diameter is harder to measure because the presence of an atmosphere makes its edge impossible to detect directly by occultations. The motion of Eris’s satellite Dysnomia [3] was used to estimate the mass of Eris. It was found to be 27% heavier than Pluto [4]. Combined with its diameter, this provided Eris’s density, estimated at 2.52 grams per cm3 [5].

“This density means that Eris is probably a large rocky body covered in a relatively thin mantle of ice,” comments Emmanuel Jehin, who contributed to the study [6].

The surface of Eris was found to be extremely reflective, reflecting 96% of the light that falls on it (a visible albedo of 0.96 [7]). This is even brighter than fresh snow on Earth, making Eris one of the most reflective objects in the Solar System, along with Saturn’s icy moon Enceladus. The bright surface of Eris is most likely composed of a nitrogen-rich ice mixed with frozen methane — as indicated by the object's spectrum — coating the dwarf planet’s surface in a thin and very reflective icy layer less than one millimetre thick.

“This layer of ice could result from the dwarf planet’s nitrogen or methane atmosphere condensing as frost onto its surface as it moves away from the Sun in its elongated orbit and into an increasingly cold environment,” Jehin adds. The ice could then turn back to gas as Eris approaches its closest point to the Sun, at a distance of about 5.7 billion kilometres.

The new results also allow the team to make a new measurement for the surface temperature of the dwarf planet. The estimates suggest a temperature for the surface facing the Sun of -238 Celsius at most, and an even lower value for the night side of Eris.

“It is extraordinary how much we can find out about a small and distant object such as Eris by watching it pass in front of a faint star, using relatively small telescopes. Five years after the creation of the new class of dwarf planets, we are finally really getting to know one of its founding members,” concludes Bruno Sicardy.

Notes

[1] TRAPPIST is one of the latest robotic telescopes installed at the La Silla Observatory. With a main mirror just 0.6 metres across, it was inaugurated in June 2010 and is mainly dedicated to the study of exoplanets and comets. The telescope is a project funded by the Belgian Fund for Scientific Research (FRS-FNRS), with the participation of the Swiss National Science Foundation, and is controlled from Liège.

[2] The Caisey Harlingten and ASH2 telescopes.

[3] Eris is the Greek goddess of chaos and strife. Dysnomia is Eris’ daughter and the goddess of lawlessness.

[4] Eris’s mass is 1.66 x 1022 kg, corresponding to 22% of the mass of the Moon.

[5] For comparison, the Moon’s density is 3.3 grams per cm3, and water’s is 1.00 gram per cm3.

[6] The value of the density suggests that Eris is mainly composed of rock (85%), with a small ice content (15%). The latter is likely to be a layer, about 100 kilometre thick, that surrounds the large rocky core. This very thick layer of mostly water ice is not to be confused with the very thin layer of frozen atmosphere on Eris’s surface that makes it so reflective.

[7] The albedo of an object represents the fraction of the light that falls on it that is scattered back into space rather than absorbed. An albedo of 1 corresponds to perfect reflecting white, while 0 is totally absorbing black. For comparison, the Moon’s albedo is only 0.136, similar to that of coal.

More information

This research was presented in a paper to appear in the 27 October 2011 issue of the journal Nature.

The team is composed of B. Sicardy (LESIA-Observatoire de Paris (OBSPM), CNRS, Université Pierre et Marie Curie (UPMC), Université Paris-Diderot (Paris 7), Institut Universitaire de France (IUF), France) , J. L. Ortiz (Instituto de Astrofísica de Andalucía (CSIC), Spain), M. Assafin (Observatório do Valongo/UFRJ (OV/UFRJ), Brazil), E. Jehin (Institut d'Astrophysique de I'Université de Liège (IAGL), Belgium), A. Maury (San Pedro de Atacama Celestial Explorations, Chile), E. Lellouch (LESIA, CNRS, UPMC, Paris 7), R. Gil Hutton ( Complejo Astronómico El Leoncito (CASLEO) and San Juan National University, Argentina), F. Braga-Ribas (LESIA, CNRS, UPMC, Paris 7, France, and Observatório Nacional/MCT (ON/MCT), Brazil), F. Colas (OBSPM, IMCCE, UPMC, CNRS, France), D. Hestroffer (OBSPM, IMCCE, UPMC, CNRS, France), J. Lecacheux (LESIA-OBSPM, CNRS, UPMC, Paris 7, IUF, France), F. Roques (LESIA-OBSPM, CNRS, UPMC, Paris 7, IUF, France), P. Santos Sanz (LESIA-OBSPM, CNRS, UPMC, Paris 7, IUF, France), T. Widemann (LESIA-OBSPM, CNRS, UPMC, Paris 7, IUF, France), N. Morales (CSIC, Spain), R. Duffard (CSIC, Spain), A. Thirouin (CSIC, Spain), A. J. Castro-Tirado (CSIC, Spain), M. Jelínek (CSIC, Spain), P. Kubánek (CSIC, Spain), A. Sota (CSIC, Spain), R. Sánchez-Ramírez (CSIC, Spain), A. H. Andrei (OV/UFRJ, ON/MCT, Brazil), J. I. B. Camargo (OV/UFRJ, ON/MCT, Brazil), D. N. da Silva Neto (ON/MCT, Centro Universitário Estadual da Zona Oeste (UEZO), Brazil), A. Ramos Gomes Jr (OV/UFRJ, Brazil), R. Vieira Martins (OV/UFRJ, ON/MCT, Brazil, OBSPM, IMCCE, UPMC, CNRS, France), M. Gillon (IAGL, Belgium), J. Manfroid (IAGL, Belgium), G. P. Tozzi (INAF, Osservatorio Astrofisico di Arcetri, Italy), C. Harlingten (Caisey Harlingten Observatory, UK), S. Saravia (San Pedro de Atacama Celestial Explorations, Chile), R. Behrend (Observatoire de Genève, Switzerland), S. Mottola (DLR – German Aerospace Center, Germany), E. García Melendo (Fundació Privada Observatori Esteve Duran, Institut de Ciències de I'Espai (CSIC-IEEC), Spain), V. Peris ( Observatori Astronòmic, Universitat de València (OAUV), Spain), J. Fabregat (OAUV, Spain), J. M. Madiedo ( Universidad de Huelva, Facultad de Ciencias Experimentales, Spain), L. Cuesta (Centro de Astrobiología (CSIC-INTA), Spain), M. T. Eibe (CSIC-INTA, Spain), A. Ullán (CSIC-INTA, Spain), F. Organero ( Observatorio astronómico de La Hita, Spain), S. Pastor (Observatorio de la Murta, Spain), J. A. de los Reyes (Observatorio de la Murta, Spain), S. Pedraz (Calar Alto Observatory, Centro Astronómico Hispano Alemán, Spain), A. Castro (Sociedad Astronómica Malagueña, Centro Cultural José María Gutiérrez Romero, Spain), I. de la Cueva (Astroimagen, Spain), G. Muler (Observatorio Nazaret, Spain), I. A. Steele (Liverpool JMU, UK), M. Cebrián (Instituto de Astrofísica de Canarias (IAC), Spain), P. Montañés-Rodríguez (IAC, Spain), A. Oscoz (IAC, Spain), D. Weaver (Observatório Astronomico Christus, Colégio Christus, Brazil), C. Jacques (Observatório CEAMIG-REA, Brazil), W. J. B. Corradi (Departamento de Física – Instituto de Ciências Exatas – Universidade Federal de Minas Gerais (ICEx–UFMG), Brazil), F. P. Santos (Departamento de Física, ICEx–UFMG, Brazil), W. Reis (Departamento de Física, ICEx–UFMG, Brazil), A. Milone (Instituto Nacional de Pesquisas Espaciais (INPE-MCT), Brazil), M. Emilio ( Universidade Estadual de Ponta Grossa, O.A. – DEGEO, Brazil), L. Gutiérrez (Instituto de Astronomía, Universidad Nacional Autónoma de México (UNAM), México), R. Vázquez (Instituto de Astronomía, UNAM, México) & H. Hernández-Toledo (Instituto de Astronomía, UNAM, México).

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

Links

Photos of La Silla
Article about TRAPPIST in September 2011 issue of the ESO Messenger
TRAPPIST webpage
Video of event observed using the TRAPPIST telescope
Observations of the event observed by the Caisey Harlingten 50cm telescope at San Pedro de Atacama

Contacts

Bruno Sicardy
LESIA-Observatoire de Paris, CNRS, Université Pierre et Marie Curie
Paris, France
Tel: +33 (0)1 45 07 71 15
Cell: +33 (0)6 19 41 26 15
Email: bruno.sicardy@obspm.fr

Emmanuel Jehin
Institut d'Astrophysique de I'Université de Liège,
Liège, Belgium
Tel: +32 (0)4 3669726
Email: ejehin@ulg.ac.be

Richard Hook
ESO, La Silla, Paranal, E-ELT & Survey Telescopes Press Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Portrait of an Imperfect but Beautiful Spiral

NGC 3368 - Messier 96
Credit: ESO/Oleg Maliy

Not all spiral galaxies have to be picture-perfect to be striking. Messier 96, also known as NGC 3368, is a case in point: its core is displaced from the centre, its gas and dust are distributed asymmetrically and its spiral arms are ill-defined. But this portrait, taken with the FORS1 instrument on ESO’s Very Large Telescope, shows that imperfection is beauty in Messier 96. The galaxy's core is compact but glowing, and the dark dust lanes around it move in a delicate swirl towards the nucleus. And the spiral arms, patchy rings of young blue stars, are like necklaces of blue pearls.

Messier 96 lies in the constellation of Leo (The Lion). It is the largest galaxy in the Leo I group of galaxies; including its outermost spiral arms, it spans some 100 000 light-years in diameter — about the size of our Milky Way. Its graceful imperfections likely result from the gravitational pull of other members in the group, or are perhaps due to past galactic encounters.

A multitude of background galaxies peers through the dusty spiral. Perhaps the most striking of these objects is an edge-on galaxy that — because of a chance alignment — appears to interrupt the outermost spiral arm to the upper left of Messier 96's core.

This image was processed by ESO using the observational data found by Oleg Maliy from Ukraine, who participated in ESO's Hidden Treasures 2010 astrophotography competition [1], organised in October–November 2010, for everyone who enjoys making beautiful images of the night sky using astronomical data obtained with professional telescopes. The image was made with data taken at visible and infrared wavelengths through B, V, and I filters.

Notes

[1] ESO’s Hidden Treasures 2010 competition gave amateur astronomers the opportunity to search through ESO’s vast archives of astronomical data, hoping to find a well-hidden gem that needed polishing by the entrants. To find out more about Hidden Treasures, visit http://www.eso.org/public/outreach/hiddentreasures/.

CID 1711 and CID 3083: Close Encounters of the Galactic Kind

CID 1711 and CID 3083
Credit X-ray: NASA/CXC/IPMU/J.Silverman et al;
Optical: NASA/STScI/Caltech/N.Scoville et al.

JPEG (448.5 kb) - Tiff (74.1 MB) -PS (92.6 MB)

Image with Scalebar - More Images

Astronomers have used a large survey to test a prediction that close encounters between galaxies can trigger the rapid growth of supermassive black holes. Key to this work was Chandra's unique ability to pinpoint actively growing black holes through the X-rays they generate.

The researchers looked at 562 pairs of galaxies ranging in distances from about 3 billion to 8 billion light years from Earth. They found that the galaxies in the early stages of an encounter with another were more likely than isolated, or "lonelier" galaxies to have actively growing black holes in their cores.

These two composite images show a sample of the pairs of galaxies that are undergoing close encounters in the survey. In these images, the data from NASA's Chandra X-ray Observatory are shown in purple and Hubble Space Telescope data are in gold. In both images, the point-like X-ray source near the center is generated by gas that has been heated to millions of degrees as it falls toward a supermassive black hole located in the middle of its host galaxy. The other faint X-ray emission may be caused by hot gas associated with the pair of galaxies.

The authors of the study estimate that nearly one-fifth of all moderately active black holes are found in galaxies undergoing the early stages of an interaction. This leaves open the question of what events are responsible for fueling the remaining 80% of growing black holes. Some of these may involve the late stages of mergers between two galaxies. Less violent events such as gas falling in from the halo of the galaxy, or the disruption of small satellite galaxies are also likely to play an important role.

The survey used in this research is called the Cosmic Evolution Survey (COSMOS), which covers two square degrees on the sky with observations from several major space-based observatories including Chandra and Hubble. Accurate distance information about the galaxies was also derived from optical observations with the European Southern Observatory's Very Large Telescope. The researchers compared a sample of 562 galaxies in pairs with 2726 solo galaxies to come to their conclusions.

A paper describing this work has been accepted for publication in The Astrophysical Journal. The study was led by John Silverman from the Institute for the Physics and Mathematics of the Universe (IPMU) at the University of Tokyo in Japan. There are 54 co-authors from various institutions around the world.

Fast Facts for CID 1711:

Scale: Image is 15 arcsec across (363,000 light years)
Category: Quasars & Active Galaxies
Coordinates: (J2000) RA 09h 59m 34.658s | Dec +01° 56' 51.24"
Constellation: Sextans
Observation Dates: 4 pointings between 4/6 and 4/13 2007
Observation Time: 55 hours 33 min (2 days 7 hours 33 min)
Obs. IDs: 8019, 8020, 8025-8026
Color Code: X-ray (Purple); Optical (Yellow)
Instrument: ACIS
References: Silverman, J. et al 2011 ApJ (in press); arXiv:1109.1292
Distance Estimate: 6.7 billion light years (z=0.77)
Release Date: October 25, 2011

Fast Facts for CID 3083:

Scale: Image is 15 arcsec across (249,000 light years)
Category: Quasars & Active Galaxies
Coordinates: (J2000) RA 09h 59m 21.53s | Dec +01° 55' 45.18"
Constellation: Sextans
Observation Dates: 4 pointings between 4/6 and 4/13 2007
Observation Time: 55 hours 33 min (2 days 7 hours 33 min)
Obs. IDs: 8019, 8020, 8025-8026
Color Code: X-ray (Purple); Optical (Yellow)
Instrument: ACIS
References: Silverman, J. et al 2011 ApJ (in press); arXiv:1109.1292
Distance Estimate: 4.0 billion light years (z=0.371)

NASA Telescopes Help Solve Ancient Supernova Mystery

This image combines data from four different space telescopes to create a multi-wavelength view of all that remains of the oldest documented example of a supernova, called RCW 86. Full image and caption

Infrared images from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) are combined in this image of RCW 86, the dusty remains of the oldest documented example of an exploding star, or supernova. It shows light from both the remnant itself and unrelated background light from our Milky Way galaxy.

Full image and caption - enlarge image

PASADENA, Calif. -- A mystery that began nearly 2,000 years ago, when Chinese astronomers witnessed what would turn out to be an exploding star in the sky, has been solved. New infrared observations from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer, or WISE, reveal how the first supernova ever recorded occurred and how its shattered remains ultimately spread out to great distances.

The findings show that the stellar explosion took place in a hollowed-out cavity, allowing material expelled by the star to travel much faster and farther than it would have otherwise.

"This supernova remnant got really big, really fast," said Brian J. Williams, an astronomer at North Carolina State University in Raleigh. Williams is lead author of a new study detailing the findings online in the Astrophysical Journal. "It's two to three times bigger than we would expect for a supernova that was witnessed exploding nearly 2,000 years ago. Now, we've been able to finally pinpoint the cause."

A new image of the supernova, known as RCW 86, is online at http://go.nasa.gov/pnv6Oy .

In 185 A.D., Chinese astronomers noted a "guest star" that mysteriously appeared in the sky and stayed for about 8 months. By the 1960s, scientists had determined that the mysterious object was the first documented supernova. Later, they pinpointed RCW 86 as a supernova remnant located about 8,000 light-years away. But a puzzle persisted. The star's spherical remains are larger than expected. If they could be seen in the sky today in infrared light, they'd take up more space than our full moon.

The solution arrived through new infrared observations made with Spitzer and WISE, and previous data from NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton Observatory.

The findings reveal that the event is a "Type Ia" supernova, created by the relatively peaceful death of a star like our sun, which then shrank into a dense star called a white dwarf. The white dwarf is thought to have later blown up in a supernova after siphoning matter, or fuel, from a nearby star.

"A white dwarf is like a smoking cinder from a burnt-out fire," Williams said. "If you pour gasoline on it, it will explode."

The observations also show for the first time that a white dwarf can create a cavity around it before blowing up in a Type Ia event. A cavity would explain why the remains of RCW 86 are so big. When the explosion occurred, the ejected material would have traveled unimpeded by gas and dust and spread out quickly.

Spitzer and WISE allowed the team to measure the temperature of the dust making up the RCW 86 remnant at about minus 325 degrees Fahrenheit, or minus 200 degrees Celsius. They then calculated how much gas must be present within the remnant to heat the dust to those temperatures. The results point to a low-density environment for much of the life of the remnant, essentially a cavity.

Scientists initially suspected that RCW 86 was the result of a core-collapse supernova, the most powerful type of stellar blast. They had seen hints of a cavity around the remnant, and, at that time, such cavities were only associated with core-collapse supernovae. In those events, massive stars blow material away from them before they blow up, carving out holes around them.

But other evidence argued against a core-collapse supernova. X-ray data from Chandra and XMM-Newton indicated that the object consisted of high amounts of iron, a telltale sign of a Type Ia blast. Together with the infrared observations, a picture of a Type Ia explosion into a cavity emerged.

"Modern astronomers unveiled one secret of a two-millennia-old cosmic mystery only to reveal another," said Bill Danchi, Spitzer and WISE program scientist at NASA Headquarters in Washington. "Now, with multiple observatories extending our senses in space, we can fully appreciate the remarkable physics behind this star's death throes, yet still be as in awe of the cosmos as the ancient astronomers."

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

JPL manages, and operated, WISE for NASA's Science Mission Directorate. The spacecraft was put into hibernation mode after it scanned the entire sky twice, completing its main objectives. Edward Wright is the principal investigator and is at UCLA. The mission was selected competitively under NASA's Explorers Program managed by the agency's Goddard Space Flight Center in Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory in Logan. The spacecraft was built by Ball Aerospace & Technologies Corp. in Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. More information is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu and http://www.jpl.nasa.gov/wise .

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

Trent J. Perrotto 202-358-0321
NASA Headquarters, Washington
trent.j.perrotto@nasa.gov

New Dwarf Galaxy Discovered near the Andromeda Galaxy

Figure 1: Central region of the GMOS-N observations of And XXIX against the less dense field of stars. The image is 4.3 arcminutes on a side, corresponding to 900 parsecs (or 3000 lightyears square) at the distance of And XXIX.

Figure 2: The solid points show the stars of dwarf galaxy And XXIX observed with GMOS on a color-magnitude diagram; open diamonds were measured from SDSS observations. The near-vertical lines show the predicted location of the red giant branch stars, for two different assumptions about metal abundance. The horizontal line marks the magnitude of the tip of the red giant branch, which implies a distance to And XXIX of about 730 kiloparsecs (nearly 2.4 million light years).

New observations with the Gemini Multi-Object Spectrograph on Gemini North confirm a new dwarf companion to the Andromeda Galaxy.

The Local Group of galaxies includes many smaller dwarf galaxies in addition to the large Milky Way and Andromeda spiral galaxies. These dwarfs tend to contain a high proportion of dark matter, and theories of galaxy formation and dark matter make specific predictions about their presence and quantity. The Local Group contains many fewer dwarf galaxies than “standard” models of galaxy formation in a dark matter-dominated universe predict and at least two outstanding questions remain: 1) whether astronomers have missed some of the dwarfs (which, even though these in the Local Group are relatively close, are hard to see because they are intrinsically faint), or 2) whether dark matter theories need to be revised.

Now, adding new evidence to the debate, Eric Bell (University of Michigan) and collaborators report the discovery of another dwarf spheroidal companion to the Andromeda Galaxy, called Andromeda XXIX (And XXIX). Fainter than some individual stars in the Milky Way (e.g., eta Carina), this dwarf is particularly interesting owing to its distance from the Andromeda Galaxy. It is over 200 kiloparsecs (650 thousand light years) from Andromeda, where the distribution and properties of dwarf companions to Andromeda are very poorly understood. The team first identified the candidate in the Sloan Digital Sky Survey, where it appeared as a slightly more populated region of the sky at the outskirts of the Andromeda Galaxy.

In this new work Bell et al. followed up with deeper observations using GMOS on the Gemini North telescope - allowing them to confirm the dwarf’s existence and measure its properties. The deep GMOS imaging data (Figure 1) show the clear signature of red giant branch stars with no sign of other, more luminous, bluer stars (Figure 2). These data led to its confirmed identification as a dwarf spheroidal galaxy. The team also measured other physical properties of And XXIX, finding that it is typical of other dwarf in the Local Group in terms of size and ellipticity, given its luminosity.

Complete results are accepted for publication in The Astrophysical Journal Letters.

Hubble Sizes up a Dwarf Galaxy

Phoenix Dwarf Galaxy
Credit: ESA/Hubble & NASA
Fullsize Original 1,2 mb

The NASA/ESA Hubble Space Telescope has taken this image of the Phoenix Dwarf Galaxy, which is located 1.4 million light-years away from Earth. It is located in the constellation of Phoenix in the southern sky. The object, a dwarf irregular galaxy, features younger stars in its inner regions and older ones at its outskirts.

Dwarf galaxies are small galaxies composed of a few billion stars, compared to fully-fledged galaxies which can contain hundreds of billions of stars. In the Local Group, there are a number of such dwarf galaxies orbiting the larger galaxies such as the Milky Way or the Andromeda Galaxy. They are thought to have been created by tidal forces in the early stages of the creation of these galaxies, or as a result of collisions between galaxies, forming from ejected streams of material and dark matter from the parent galaxies. The Milky Way galaxy features at least 14 satellite dwarf galaxies orbiting it.

Because of their shape, dwarf irregulars have often been mistaken for globular clusters: they do not feature a bulge or spiral arms like larger galaxies. However, their importance in terms of cosmology is in stark contrast to their unspectacular shapes, as their chemical makeup and high levels of gas are believed to be similar to those of the earliest galaxies that populated the Universe. They are thought to be contemporary versions of some of the remote galaxies observed in deep field galaxy surveys, and can thus help to understand the early stages of galaxy and star formation in the young Universe.

The galaxy was discovered in 1976 by Hans-Emil Schuster and Richard Martin West. Hans-Emil Schuster was acting director of ESO’s La Silla Observatory in Chile and was involved in the selection and testing of the sites for the observatories of both La Silla and Paranal. His great contribution to astronomy and to ESO was recognised by the Chilean government last week when he was awarded the medal of the Order of Bernardo O’Higgins.

Herschel detects abundant water in planet-forming disc

This image shows an artist's impression of the icy protoplanetary disc around the young star TW Hydrae (upper panel) and the spectrum of the disc as obtained using the HIFI spectrometer on ESA's Herschel Space Observatory (lower panel).

By analysing the spectrum, astronomers have detected the emission from cold water vapour in the planet-forming disc. The vapour arises when highly energetic radiation from the central star interacts with icy grains in the disc. The detection thus hints at a copious and otherwise undetectable supply of water ice hidden in the disc's deeper and colder layers.

The graph in the lower panel shows the spectral signature of water vapour in the disc. Water molecules come in two "spin" forms, called ortho and para, in which the two spins of the hydrogen nuclei have different orientations. By comparing the relative amounts of ortho and para water, astronomers can determine the temperatures under which the water formed. Lower ratios indicate cooler temperatures, though in practice the analysis is much more complicated. The ratio of ortho to para water observed in TW Hydrae's protoplanetary disc is low enough to point to the presence of cold water vapour.Credits: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory) HI-RES JPEG (Size: 1196 kb)

This artist's impression illustrates an icy protoplanetary disc around the young star TW Hydrae, located about 175 light-years away in the Hydra, or Sea Serpent, constellation.

Astronomers using the HIFI spectrometer on ESA's Herschel Space Observatory detected copious amounts of cold water vapour, illustrated in blue, emanating from the star's planet-forming disc of dust and gas. The water vapour, corresponding to temperatures lower than 100 K, is distributed across the entire extent of the disc and is likely confined to a thin layer at an intermediate depth in the disc. The vapour arises when highly energetic radiation from the central star interacts with icy grains in the disc, the very same grains that should ultimately coalesce into icy planetesimals, such as comets. The detection thus hints at a copious and otherwise undetectable supply of water ice hidden in the disc's deeper and colder layers.

In our own Solar System, comets are thought to have carried water to Earth, creating our oceans. A similar process might be taking place around TW Hydrae, where comets could, over the next several millions of years, transport water to young worlds. The Herschel results demonstrate that vast reservoirs of water are available around stars for creating these hypothetical water worlds. Credits: ESA/NASA/JPL-Caltech. HI-RES JPEG (Size: 1934 kb)

ESA’s Herschel space observatory has found evidence of water vapour emanating from ice on dust grains in the disc around a young star, revealing a hidden ice reservoir the size of thousands of oceans.

TW Hydrae, a star between 5-10 million years old, and only 176 light-years away, is in the final stage of formation, and is surrounded by a disc of dust and gas that may condense to form a complete set of planets.

It is believed that a large proportion of Earth’s water may have come from ice-laden comets that bombarded our world during and after its formation. Recent studies of comet 103P/Hartley 2 with Herschel shed new light on how water may have come to Earth, with its findings of the first Earth-like water in a comet. Until now, however, almost nothing was known about reservoirs in planet-forming discs around other stars.

This new detection is the first of its kind and has been made possible by Herschel’s HIFI instrument.

The tell-tale water vapour signature, believed to be produced when the ice coated dust grains are warmed by interstellar UV radiation, has been detected throughout the disc around TW Hydrae, and, though weaker than expected, it hints at a substantial reservoir of ice. This could be a rich source of water for any planets that form around this young star.

"The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System's evolution, where over millions of years, similar dust grains then coalesced to form comets," says Michiel Hogerheijde of Leiden University in the Netherlands, who led the study.

"These comets we believe became a contributing source of water for the planets."

The scientists ran detailed simulations, combining the new data with previous ground-based observations and some from NASA’s Spitzer telescope. From this they calculated the size of the ice reservoirs in the planet-forming regions.

Their results show that the total amount of water in the disc around TW Hydrae would fill several thousand Earth oceans.

"We already have approved time on Herschel to study more planet-forming regions around three other stars," says Dr Hogerheijde.

"We believe that will show similar results in terms of the water detections, but as our next observations will be of objects up to three times further in distance away, we'll need many more hours of observation time."

This research breaks new ground in understanding water’s role in planet-forming discs and gives scientists a new testing ground for looking at how water came to our own planet.

"With Herschel we can follow the trail of water through all the steps of star and planet formation," comments Göran Pilbratt, Herschel Project Scientist at ESA.

"Here we are studying the 'raw material' for planet formation, which is fundamental to an understanding of how planetary systems such as our own Solar System once formed."

Notes to editors

HIFI is the Heterodyne Instrument for the Far-Infrared spectrometer on the Herschel Space Observatory. It was designed to observe water in a wide variety of objects, and aims to study not only planet-forming discs and star formation, but also galactic evolution. Its capability for highly detailed chemical identification of individual atoms and molecules makes it the instrument of choice for studying chemistry throughout space, particularly around embryonic and dying stars.

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Email: markus.bauer@esa.int
Tel: +31 71 565 6799
Mob: +31 61 594 3 954

Michiel Hogerheijde
Leiden Observatory
Tel: +31 71 527 5590
Email: michiel@strw.leidenuniv.nl

Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int

Record-Breaking Photo Reveals a Planet-sized Object as Cool as the Earth

These two infrared images were taken by the Spitzer Space Telescope in 2004 and 2009. They show a faint object moving through space together with a white dwarf. The brown dwarf, named WD 0806-661 B, is the coldest companion object to be directly imaged outside our solar system. Credit: Kevin Luhman, Penn State University, October 2011

An artist's impression of the coldest imaged companion, named WD 0806-661 B, (right foreground) orbiting at a large distance from a white dwarf --the collapsed-core remnant of a dying star. Credit: NASA Goddard Space Flight Center/Francis Reddy

A second artist's impression of the coldest imaged companion, named WD 0806-661 B, (left foreground) orbiting at a large distance from a white dwarf -- the collapsed-core remnant of a dying star. Credit: Janella Williams

The photo of a nearby star and its orbiting companion -- whose temperature is like a hot summer day in Arizona -- will be presented by Penn State Associate Professor of Astronomy and Astrophysics Kevin Luhman during the Signposts of Planets conference at NASA's Goddard Space Flight Center on 20 October 2011. A paper describing the discovery will be published in the Astrophysical Journal.

"This planet-like companion is the coldest object ever directly photographed outside our solar system," said Luhman, who led the discovery team. "Its mass is about the same as many of the known extra-solar planets -- about six to nine times the mass of Jupiter -- but in other ways it is more like a star. Essentially, what we have found is a very small star with an atmospheric temperature about cool as the Earth's."

Luhman classifies this object as a "brown dwarf," an object that formed just like a star out of a massive cloud of dust and gas. But the mass that a brown dwarf accumulates is not enough to ignite thermonuclear reactions in its core, resulting in a failed star that is very cool. In the case of the new brown dwarf, the scientists have gauged the temperature of its surface to be between 80 and 160 degrees Fahrenheit -- possibly as cool as a human.

Ever since brown dwarfs first were discovered in 1995, astronomers have been trying to find new record holders for the coldest brown dwarfs because these objects are valuable as laboratories for studying the atmospheres of planets with Earth-like temperatures outside our solar system.

Astronomers have named the brown dwarf "WD 0806-661 B" because it is the orbiting companion of an object named "WD 0806-661" -- the "white dwarf" core of a star that was like the Sun until its outer layers were expelled into space during the final phase of its evolution. "The distance of this white dwarf from the Sun is 63 light years, which is very near our solar system compared with most stars in our galaxy," Luhman said.

"The distance of this white dwarf from its brown-dwarf companion is 2500 astronomical units (AU) -- about 2500 times the distance between the Earth and the Sun, so its orbit is very large as compared with the orbits of planets, which form within a disk of dust swirling close around a newborn star," said Adam Burgasser at the University of California, San Diego, a member of the discovery team. Because it has such a large orbit, the astronomers say this companion most probably was born in the same manner as binary stars, which are known to be separated as far apart as this pair, while remaining gravitationally bound to each other.

Luhman and his colleagues presented this new candidate for the coldest known brown dwarf in a paper published in spring 2011, and they now have confirmed its record-setting cool temperature in a new paper that will be published in the Astrophysical Journal.

To make their discovery, Luhman and his colleagues searched through infrared images of over six hundred stars near our solar system. They compared images of nearby stars taken a few years apart, searching for any faint points of light that showed the same motion across the sky as the targeted star. "Objects with cool temperatures like the Earth are brightest at infrared wavelengths," Luhman said. "We used NASA's Spitzer Space Telescope because it is the most sensitive infrared telescope available."

Luhman and his team discovered the brown dwarf WD 0806-661 B moving in tandem with the white dwarf WD 0806-661 in two Spitzer images taken in 2004 and 2009. The images, which together show the movement of the objects, are available here. "This animation is a fun illustration of our technique because it resembles the method used to discovery Pluto in our own solar system," Luhman said.

In a related new discovery involving a different cool brown dwarf, Penn State Postdoctoral Scholar John Bochanski and his colleagues have made the most detailed measurement yet of ammonia in the atmosphere of an object outside our solar system. "These new data are much higher quality that previously achieved, making it possible to study, in much more detail than ever before, the atmospheres of the coldest brown dwarfs, which most closely resemble the atmospheres that are possible around planets," Bochanski said.

"Brown dwarfs that are far from their companion stars are much easier to study than are planets, which typically are difficult to observe because they get lost in the glare of the stars they orbit," Burgasser said. "Brown dwarfs with Earth-like temperatures allow us to refine theories about the atmospheres of objects outside our solar system that have comparatively cool atmospheres like that of our own planet."

This research was sponsored by grants from the National Science Foundation and the NASA Astrophysics Theory Program.

***

CONTACTS

• Kevin Luhman at Penn State: kluhman@astro.psu.edu, (+1) 814-863-4957

• Mark Kuchner, organizer of the Signposts of Planets conference: (+1) 301-286-5165, marc.j.kuchner@nasa.gov

• Barbara Kennedy (Penn State PIO): 814-863-4682, science@psu.edu

Researchers Explain the Formation of Scheila's Unusual Triple Dust Tails

A research team of planetary scientists and astronomers, primarily from Seoul National University, the National Astronomical Observatory of Japan (NAOJ), the Institute of Space and Astronautical Science (ISAS), and Kobe University, has explained the formation of peculiar triple dust tails from the asteroid Scheila (asteroid #596). The researchers concluded that another asteroid about 20-50 meters in size impacted Scheila from behind on December 3, 2010 and accounted for its unusual brightness and form.

On December 11.4, 2010, Steve Larson of the Catalina Sky Survey noticed an odd brightness from Scheila, an asteroid on the outer region of the main belt of asteroids that orbit in an area between Mars and Jupiter. Three streams of dust appeared to trail from the asteroid. Data from NASA's Swift Satellite and the Hubble Space Telescope suggested that a smaller asteroid's impact was the likely trigger for the appearance of comet-like tails from Scheila. However, questions remained about the date when the dust emission occurred and how the triple dust tails formed. The current research team sought answers to these queries.

Soon after reports of Scheila's unusual brightness, the current research team used the Subaru Prime Focus Camera (Suprime-Cam) on the Subaru Telescope (8.2 m), the Ishigakijima Astronomical Observatory Murikabushi Telescope (1.05 m), and the University of Hawaii 2.2 m Telescope to make optical observations of these mysterious dust trails over a three-month period. The top of Figure 1 shows images of the development of the dust trails taken by the Murikabushi Telescope on the 12th and 19th of December 2010. Although asteroids generally look like points when observed from Earth, Scheila looked like a comet. As the three streaks of dust streamed from the asteroid, their surface brightness decreased. Eventually the dust clouds became undetectable, and then a faint linear structure appeared. The bottom of Figure 1 shows the image obtained by Subaru Telescope on March 2, 2011. Based on these images of the linear structure, the scientists determined a dust emission date of December 3.5+/-1, 2010. Steve Larson of the Catalina Sky Survey noticed that Scheila had a slightly diffuse appearance on December 3.4, 2010. Therefore, it is likely that the collision of the asteroids occurred within the short time between December 2 12:00 UT and December 3 10:00 UT.

To explain the formation of Scheila's triple dust tails, the research team conducted a computer simulation of Scheila's dust emission on December 3th. Their simulation was based on information gained through impact experiments in a laboratory at ISAS, a hypervelocity impact facility and division of the Japan Aerospace Exploration Agency (JAXA). Figure 2 shows the ejecta produced by an oblique impact, which was not a head-on collision. Two prominent features characterize oblique impacts and the shock waves generated by them. One feature, a downrange plume, occurs in a direction downrange from the impact site and results from the fragmentation or sometimes evaporation of the object that impacted another. A second feature occurs during the physical destruction of the impacted object; a shock wave spreads from the impact site, scoops out materials (conical impact ejecta), and forms an impact crater. The axis of the cone of ejecta is roughly perpendicular to the surface at the impact site. The team reasoned that these two processes caused the ejection of Scheila's dust particles and that sunlight pushed them away from the asteroid. After performing a tremendous number of computer simulations under different conditions, they could only duplicate their observed images when an object struck Scheila’s surface from behind (Figures 3 and 4).

Taking all of the evidence into account—their observations and simulations --the research team concluded that there is only one way to explain the mysterious brightness and triple trails of dust from Scheila. A smaller asteroid obliquely impacted Scheila from behind.

Notes:

The following papers will appear in the Astrophysical Journal:

Ishiguro et al. 2011, Astrophysical Journal Letters 740, L11, "Observational Evidences for Impact on the Main-Belt Asteroid (596) Scheila"
Ishiguro et al. 2011, Astrophysical Journal Letters, 741, L24, "Interpretation of (596) Scheila's Triple Dust Tails"

This research was supported by a Basic Research Grant from Seoul National University, by a fundamental research grant (type I) from the National Research Foundation of Korea and by a Grant-in-Aid for Scientific Research on Priority Areas from MEXT, Japan. NAOJ supported the use of the UH 2.2 m Telescope.

Figure 1: Top: Optical images of Scheila at three different epochs with different telescopes. Images of the triple dust tails were taken on the 12th and 19th of December 2010 using the Murikabushi Telescope.
Bottom: Suprime-Cam on the Subaru Telescope captured this image of the linear structure on the 2nd of March 2011.

Figure 2: Sequence of events after an oblique impact: (a) Object impacts another and generates a shock wave; (b) and (c) increased development of two prominent features, i.e., a downrange plume and conical impact ejecta. The downrange plume results from the fragmentation or sometimes the evaporation of the impacting object while the conical impact ejecta come from the physical destruction of the impacted object when the shock wave spreads from the site of impact and scoops up materials from it.

Figure 3: Image showing the result of a vast number of computer simulations to reproduce the shape of Scheila's triple dust tails observed on December 12, 2010. The researchers reasoned that the downrange plume and the conical impact ejecta produced the dust particles, which sunlight pushed away from the asteroid. The image on the right is the best-fit match for the observed image on the left. The downrange plume explains the appearance of the prominent northern feature (1) while the conical impact ejecta explain the remaining two structures (2)(3).

Figure 4: Orbits of the impacting asteroid and Scheila on December 3, 2010, assuming an impact angle of 45° relative to the surface normal vector.
Top: Face-on view, projected on the ecliptic plane.
Bottom: Edge-on view, projected on the X-Z plane.