Subaru Telescope Reveals 3D Structure of Supernovae

A research group led by Dr. Masaomi Tanaka (National Astronomical Observatory of Japan), Dr. Koji Kawabata (Hiroshima University), Dr. Takashi Hattori (National Astronomical Observatory of Japan), and Dr. Keiichi Maeda (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe) used the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope to conduct observations that revealed a clumpy 3D structure of supernovae (Figure 1). This finding supports a clumpy 3D scenario of supernovae explosions rather than the widely accepted bipolar explosion scenario. It advances our understanding of how supernovae explode, a process that has been a persistent mystery.

Figure 1: Schematic drawing of the 3D structure (left) and the image of SN 2009mi (right) captured with FOCAS on the Subaru Telescope. This supernova was discovered in the galaxy IC 2151 (in the direction of the constellation Lepas, about 100 million light years away) by Berto Monard in South Africa. (Credit: NAOJ)

The Mystery of Supernovae

Stars heavier than eight solar masses will end their lives with a brilliant explosion called a "supernova". A supernova ejects elements synthesized within its star that are heavier than hydrogen and helium, the main elements of the primeval Universe. The ejection of these heavier elements into interstellar space has enriched the chemical composition of the Universe.

Despite its important role in the evolution of the Universe, the process of how supernovae explosions occur has been unclear. Based on recent numerical simulations, researchers agree that supernovae would not succeed as one-dimensional, spherical events and that multi-dimensional effects are important for understanding their occurrence. Scientists have proposed two main scenarios to explain how supernovae explosions occur: (1) a bipolar explosion facilitated by rotation, and (2) a clumpy 3D explosion driven by convection. However, scientists have not known which scenario is more plausible, because they have not actually observed the shape of supernovae.


Seeing the "Shape" of Supernovae by Polarization

Although it would seem easy to see the shape of supernovae by simply taking a picture of them, observing them is really a very challenging task. Since most supernovae occur in galaxies millions or hundreds of million light years away, they only look like a point, even though they expand at a speed of 10,000 km/s.

The current research team used a special method of detection to reveal the shape of supernovae; they measured so-called "polarization", which supplies information about the direction of vibrating electromagnetic waves. They performed numerical simulations for emissions from supernovae and found clearly different polarization patterns for clumpy and bipolar explosions. An object shows various angles of polarization in a clumpy explosion while it shows a single angle of polarization in a bipolar explosion (Figure 2).

Figure 2: Schematic drawing of the polarization patterns. If a supernova has a clumpy geometry, the polarization has various angles (left), but if it has a bipolar geometry, the polarization has a single angle (right). (Credit: NAOJ)

Based on hypotheses derived from their simulations, the group used the Subaru Telescope's Faint Object Camera and Spectrograph (FOCAS) to conduct polarimetric observations of nearby supernovae; such observations measure the intensity and direction of polarization. Because the researchers did not know when the supernovae would appear, they could not assign an observing time in advance. Fortunately, Subaru Telescope has a Target of Opportunity (ToO) mode that overcomes this difficulty and enables a dynamic allocation of the observing time. Thanks to this mode, the team succeeded in conducting polarimetric observations of two so-called "stripped-envelope supernovae" (SN 2009mi and SN 2009jf), which do not have hydrogen surrounding them and are the best targets for studying explosion geometry.


Revealing a 3D Structure

The team detected the polarization from the two supernovae, which clearly indicated that the supernovae are not usually round. They also found that each supernova had various angles of polarization, a finding consistent with the scenario of a clumpy 3D explosion (Figure 3).

When the team added the two new supernovae to the ones from previous observations, they had a total of six stripped-envelope supernovae, five of which showed the signature of clumpy 3D geometry. The research showed that the clumpy 3D shape is common in supernovae. Although the bipolar explosion scenario is widely accepted, the findings of this research support the clumpy 3D scenario of supernovae explosions. Convective motion in the explosions could account for this clumpy shape. This result serves as a catalyst to further understand how supernovae explosions occur.

Figure 3: Observed polarization around a calcium absorption line of SN 2009jf as a function of velocity caused by expanding motion (Doppler velocity). It shows that the polarization angle changes with wavelength. (Credit: NAOJ)

Research Group

• Masaomi Tanaka (National Astronomical Observatory of Japan [NAOJ])

• Koji S. Kawabata (Hiroshima University, Japan)

• Takashi Hattori (NAOJ)

• Paolo A. Mazzali (Max Planck Institute for Astrophysics, Germany)

• Kentaro Aoki (NAOJ)

• Masanori Iye (NAOJ)

• Keiichi Maeda (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe, Japan)

• Kenichi Nomoto (University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe, Japan)

• Elena Pian (Scuola Normale Superiore, Italy)

• Toshiyuki Sasaki (NAOJ)

• Masayuki Yamanaka (Kyoto University, Japan)

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Multiple Mergers Generate Ultraluminous Infrared Galaxy

A team of astronomers led by Professor Yoshiaki Taniguchi (Ehime University) has concluded that the ultraluminous infrared galaxy (ULIRG) Arp 220 (Figure 1) developed from a multiple merger among four or more galaxies. Their new imaging data from the Subaru Telescope and optical spectroscopy from the W. M. Keck Observatory revealed two tidal tails that facilitated their analysis of Arp 220's development. Because Arp 220 is an archetypal or representative ULIRG, the team's findings facilitate an understanding of ULIRG development in general.

Figure 1: Optical images of Arp 220

Left: Image from the Hubble Space Telescope’s Advanced Camera for Surveys (ACS). (Credit: Hubble Space Telescope)
Right: Image from the Subaru Prime Focus Camera (Suprime-Cam). Huge, complex tidal remnants surround Arp 220. (Credit: Ehime University / NAOJ)

First discovered from the Infrared Astronomical Satellite's (IRAS) all-sky survey in the mid-1980s, ULIRGs are the most luminous class of galaxies in the relatively near or local Universe. Most of their energy output is in the infrared, suggesting that they contain a large amount of dust, an indication of immense star formation.

Astronomers have proposed different scenarios for the development of ULIRGs. Since ULIRGs' huge infrared luminosities (1012 Lsun), powered mostly by a large number of massive stars, are comparable to the high luminosity of quasars, the brightest class of active galactic nuclei, a 1988 scenario (Note 1) proposed that ULIRGs were an intermediate phase in the evolution of quasars after a merger. Another scenario proposed by Professor Taniguchi and his associate in 1998 (Note 2) hypothesized that multiple mergers among several galaxies explained the observational properties. However, a number of questions remained unanswered: 1) How many galaxies were merged into one? and 2) Which types of galaxies were merged into one? Since then, explanations for the origins of ULIRGs have remained controversial. The current team conducted research to help answer these questions and to propose a plausible, data-based explanation for the origin of ULIRGs.

The team made detailed optical imaging observations of Arp 220 using FOCAS (Faint Object Camera and Spectrograph) on the Subaru Telescope and the LRIS (Low Resolution Imaging Spectometer) on the Keck II Telescope. The new imaging data revealed a spectacular pair of tidal tails extending more than 50,000 light years. Intermediate-mass stars (with masses a few to several times that of the Sun), the remains of intense star formation events called "starbursts", dominate the composition of the tidal tails. The presence of an Hα absorption line (Figure 2) led to the first detection of these features. Dr. Kazuya Matsubayashi said, "I was very surprised when I found these Hα absorption features in the two tidal tails."

Figure 2: Images of Arp 220

Left: Hα image taken with the Faint Object Camera and Spectrograph (FOCAS) mounted on the Subaru Telescope. The dark parts in the figure (Hα absorption) pointed out with arrows are three post-starburst regions. Right: For reference, an R band image from Suprime-Cam. (Credit: Ehime University / NAOJ)

What could explain these surprising features? A merger between two galaxies is necessary to cause a starburst in a merging system. Therefore, two post-starburst galaxies could have produced the two long tidal tails. However, four galaxies are needed to generate the two post-starburst galaxies (Figure 3). The post-starburst tidal tails revealed by the new observations suggest a new scenario for the merging history in Arp 220. The team suggests that the two observed tidal tails in Arp 220 need a merger between two advanced (i.e., post-starburst) merger remnants. In sum, four spiral galaxies are necessary to explain the observed post-starburst tidal tails in Arp 220. They conclude that Arp 220 comes from a multiple merger that includes at least four galaxies, not from a typical merger. The team thinks that this conclusion about Arp 220 can be applied to other galaxy groups.

Figure 3: The proposed scenario of multiple mergers for Arp 220

Each pair of spiral galaxies merges into one, resulting in two merged starburst galaxies. As time goes by (200 million years after merging or longer), these galaxies evolve into post-starburst galaxies, which then merge again, resulting in the current Arp 220 with a pair of post-starburst tidal tails. (Credit: Ehime University / NAOJ)

There are a significant number of compact groups of galaxies in the Universe that could lead to multiple mergers. Professor Taniguchi noted, "Some of such compact groups have already merged into one. They are the ULIRGs observed to date." Some galaxies are associated together in a single gravitationally-bound group, and they will inevitably merge into one galaxy within several billion years.

Although ULIRGs are thought to evolve into quasars and then into giant early-type galaxies, future considerations of the evolution of galaxies will need to take into account the impact of multiple mergers, not just major mergers between two galaxies. Professor Taniguchi applied this principle to the fate of our Milky Way Galaxy: "Very recently, NASA announced that our Milky Way Galaxy will merge with the Andromeda Galaxy (M31) into a giant elliptical galaxy within several billion years. Please don't worry. That would only be a merger between two galaxies, so our Milky Way will not evolve into a ULIRG."


Reference:

These results will be published in The Astrophysical Journal, Volume 753, July 10, 2012.


Notes:

1. Sanders, D. B., et al. 1988, ApJ, 325, 74
2. Taniguchi, Y., & Shioya, Y. 1998, ApJ, 501, L167

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How the Universe Escaped its "Dark Ages"

The following release was received from Swinburne University and is reprinted here in its entirety for the convenience of our readers:

An international team of astronomers have uncovered an important clue about how the Universe emerged from its "dark ages" some 13 billion years ago. By looking at nearby galaxies, they can infer what may have happened to the first galaxies of our Universe.

For some time astronomers have known that following the big bang, a dense hydrogen "fog" settled over the Universe. During this time, the light produced by the first stars could only travel short distances before it was absorbed by the fog. They call this period the "dark ages" of the Universe, but little is known about what was happening at this time.

Figure: The Subaru Telescope captured this image of the Sombrero Galaxy (M104), which is a spiral galaxy located in a small galaxy group. According to Dr. Spitler's research, this galaxy may have emerged from the hydrogen fog of the dark ages early in the Universe's history. (Credit: NAOJ)

"During the dark ages, the hydrogen fog condensed in certain places, which allowed the formation of stars, black holes and the first galaxies," said Swinburne University of Technology astrophysicist Dr Lee Spitler.

"These objects were the first significant sources of ultraviolet radiation, which eventually started to burn off the hydrogen fog much like the Sun burns off a morning fog on Earth. We call this process reionisation, because the hydrogen atoms are ionised by the ultraviolet light.

"But what was happening during the Universe's dark ages is somewhat of a mystery because we can't directly see what was going on through the opaque hydrogen fog.

"Obtaining information about reionisation is quite challenging as it occurred so long ago. Since light takes time to reach us, astronomers can observe what was happening at that time, but it is very difficult and pushes modern telescopes to their limits."

To address this problem, an international research team, led by Dr Spitler, tried a different approach: they looked for signs of reionisation in nearby galaxies, which are easier to observe.

"We used nearby galaxies to understand something that happened long ago, in much the same way fossils are used to understand Earth's history," said Swinburne Professor Duncan Forbes.

"We can see regions around galaxies where reionisation has just finished and use that information to understand important questions about the dark ages: What were the first stars like; how were the first galaxies formed; and were there many supermassive black holes?"

When reionisation occurs in a galaxy and clears out the hydrogen fog, it also disrupts the condensation of the fog into locations of new star formation.

The research team looked for signs of this stalled star formation in ancient star clusters and were able to measure when reionisation passed through the region around a galaxy.

By measuring when reionisation took place around three galaxies, including the Milky Way, the researchers found evidence that the hydrogen fog burned off first in isolated, low-density regions of the Universe. A few hundred million years later, reionisation took place in the dense, crowded regions of the Universe.

This suggests that galaxies in crowded regions of the Universe were more likely to be shrouded in very dense pockets of hydrogen fog. Such dense regions would therefore require larger numbers of light sources and more time to burn off the fog compared to regions with relatively light fog.

"Understanding how reionisation moved through the Universe is very challenging, but of enormous importance in astronomy. Our technique provides a novel way to tackle this problem," Dr Spitler said.

The researchers used the Keck and Subaru telescopes in Hawaii for this work, which has been published in the Monthly Notices of the Royal Astronomical Society. In addition to Dr Spitler and Professor Forbes at Swinburne, the research team included: Dr Aaron Romanowsky and Professor Jean Brodie at the University of California at Santa Cruz and Professor Jürg Diemand and Professor Ben Moore at the University of Zurich, Switzerland.

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Discovery of the Most Distant Galaxy in the Cosmic Dawn

A team of astronomers led by Takatoshi Shibuya (The Graduate University for Advanced Studies, Japan), Dr. Nobunari Kashikawa (National Astronomical Observatory of Japan), Dr. Kazuaki Ota (Kyoto University), and Dr. Masanori Iye (National Astronomical Observatory of Japan) has used the Subaru and Keck Telescopes to discover the most distant galaxy ever found, SXDF-NB1006-2, at a distance of 12.91 billion light years from the Earth. This galaxy is slightly farther away than GN-108036, which Subaru Telescope discovered last year and was the most distant galaxy discovered at the time (Note 1). In addition, the team's research verified that the proportion of neutral hydrogen gas in the 750-million-year-old early Universe was higher than it is today. These findings help us to understand the nature of the early Universe during the "cosmic dawn", when the light of ancient celestial objects and structures appeared from obscurity.

Astronomers think that the our Universe began 13.7 billion years ago at the Big Bang. The exteme temperature and density of this fireball decreased rapidly as its volume increased. Hot cosmic plasma composed mainly of protons and electrons recombined to form neutral hydrogen atoms within 380,000 years after the Big Bang; this was the beginning of the cosmic "dark age." From then on, the gas continued to cool and fluctuated in density. About 200 to 500 million years after the Big Bang, the dense parts of neutral hydrogen clouds contracted under their own gravity, and the first stars and galaxies formed. The radiation from this first generation of stars started to heat and reionize the hydrogen in nearby space, eventually leading to the reionization of the entire Universe. This was the era of "cosmic reionization" (Figure 1) or the "cosmic dawn". The current team focused their research on identifying the exact epoch of the cosmic dawn in an effort to answer major astronomical questions about the history of our Universe.

Figure 1: Cosmic history from the Big Bang to the present

Credit: NAOJ

How did the team design research to explore such an ancient, extremely distant time? Their first steps were to conduct a survey of distant galaxies and measure their number and brightness. Because light from the distant Universe takes time to reach the Earth, identification of more distant galaxies allows astronomers to trace farther back in time and locate the epoch of the cosmic dawn. However, neutral hydrogen in intergalactic space dimmed the light from galaxies before the cosmic dawn and made them more difficult to observe. Because the team needed to search a vast area for objects in the far distant Universe (Note 2), they used the prime focus camera mounted on the Subaru Telescope (Suprime-Cam) for their initial surveys. Suprime-Cam captures images of objects in a wide field of view from the large, 8.2 m primary mirror of the Subaru Telescope and is well-known for discovering faint, far distant galaxies and then measuring the amount of neutral hydrogen in the early Universe (Note 3). The use of Suprime-Cam was even more compelling with the 2008 installation of new detectors with a sensitivity about twice as high as their predecessors, particularly in the red wavelengths (Note 4).

Armed with the most sensitive eyes in the world, the researchers could carry out surveys for extremely distant galaxies (beyond redshift 7, where the majority of energy output from galaxies is detected in red wavelengths). To fine-tune their survey even more, a team led by Dr. Iye constructed a new special filter named NB1006 through which they could selectively identify the light of distant galaxies at a redshift of nearly 7.3.

The team used Suprime-Cam, complete with its new, highly sensitive detectors, attached the NB1006 filter to observe two specifically designated regions of the sky for detailed study: the Subaru Deep Field and the Subaru XMM-Newton Deep Survey Field. After a total of 37 hours in 7 nights of observations in these wide fields, the team carefully processed the images they had obtained. Shibuya measured the color of 58,733 objects in the images and identified four galaxy candidates at a redshift of 7.3. A careful investigation of the brightness variation of the objects allowed the team to narrow down the number of candidates to two.

Then it was necessary for the team to make spectroscopic observations to confirm the nature of these candidates. They observed the two galaxy candidates with two spectrographs, the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope and the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck Telescope, and identified one candidate for which a characteristic emission line of distant galaxies could be detected.

The current team found that the proportion of neutral hydrogen was increasing in the far distant Universe. They concluded that about 80 percent of the hydrogen gas in the ancient Universe, 12.91 billion years ago at a redshift of 7.2, was neutral.

In sum, this careful research plan and procedures, including the appropriate removal of contaminations that could lead to false results, resulted in the successful discovery and confirmation of the most distant galaxy ever discovered: SXDF-NB1006-2 (Figures 2 and 3). In addition, the findings gave the team confidence that they were observing an object during the last phase of the cosmic dawn.

Figure 2: Color composite image of the Subaru XMM-Newton Deep Survey Field. Right panel: The red galaxy at the center of the image is the most distant galaxy, SXDF-NB1006-2. Left panels: Close-ups of the most distant galaxy. Credit: NAOJ)

Figure 3: One- and two- dimensional spectra of SXDF-NB1006-2 obtained with the spectrograph DEIMOS on the Keck Telescope. The red arrow points to a spectral line (the asymmetric Lyman-alpha line) that strongly supports the identification of the the galaxy in the ancient Universe. The grey shaded area covers the wavelength range heavily contaminated by night-sky emission lines of hydroxyl (OH). (Credit: NAOJ)

Although finding just one galaxy at a critical epoch is exciting by itself, it is not a sufficient sample to characterize the entire epoch. Precise measurement of the number of galaxies during the cosmic dawn requires surveys of even wider fields. The scheduled 2012 installation of Subaru's new instrument, Hyper Suprime-Cam (HSC) will allow researchers to observe a field of view seven times greater than that of Suprime-Cam and opens the door to a huge galaxy sample beyond redshift 7. Observations with HSC are steps in the direction of uncovering the dark periods of the Universe and understanding the physical properties and formation of the first stars and galaxies. Shibuya summarized the team's future intent and hopes: "By conducting an extremely wide HSC survey for distant galaxies beyond redshift seven, we will find the mechanisms of the cosmic reionization in a variety of ways, not just by investigating their number and brightness." Dr. Iye, the leader of the Thirty Meter Telescope (TMT) project at the National Astronomical Observatory of Japan (NAOJ), added, "We have been pushing the limits of 8-10 m class telescopes to detect distant galaxies. The 30 m mirror of the TMT will be able to gather up to ten times more light than current large telescopes and detect faint light from galaxies up to a redshift of 14. The day is not so far off when the mysteries of the dark ages of the Universe and the physical properties of the first galaxies will be revealed."

Table 1: Distance ranking of galaxies that were confirmed by precise calculations from spectra in spectroscopic observations. (Credit: NAOJ)

Notes:

1. GN-108036 is the galaxy at redshift 7.213 discovered in an observation described in "Discovery of a Vigorous Star-Forming Galaxy at the Cosmic Dawn"

2. The galaxy populations that emit Lyman Alpha emission lines are those that enable investigations of the proportion of neutral hydrogen in the early Universe.

3. For more information on the discovery of objects in the ancient Universe, go to descriptions in
"Cosmic Archeology Uncovers the Universe's Dark Ages"

4. A more detailed explanation of the new CCDs is in a press release labeled "Suprime-Cam Upgraded with Ultra-Sensitive CCDs"

Reference:

These results will be published in the June 20, 2012, edition of the Astrophysical Journal. This research was supported by The Japan Society for the Promotion of Science through Grant-in-Aid for Scientific Research 23340050 and 19104004. The authors of the paper are:
Takatoshi Shibuya, The Graduate University for Advanced Studies, Japan
Nobunari Kashikawa, National Astronomical Observatory of Japan
Kazuaki Ota, Kyoto University
Masanori Iye, National Astronomical Observatory of Japan
Masami Ouchi, University of Tokyo
Hisanori Furusawa, National Astronomical Observatory of Japan
Kazuhiro Shimasaku, University of Tokyo
Takashi Hattori, Subaru Telescope

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Subaru Telescope Pioneers the Use of Adaptive Optics for Optical Observations

Figure 1: Kyoto3DII at the Nasmyth focus of the Subaru Telescope. The size of the instrument, including the frame, is 2 m high X 2 m wide X 1 m deep. The black box on the left side of the image is part of AO 188. (Credit: NAOJ)

Unlike space telescopes, ground-based telescopes must deal with observational distortions from atmospheric turbulence that degrades the spatial resolution of images. Adaptive optics systems (Note 2) correct for the distortion of light in real time and facilitate the production of high-resolution images. However, the AO systems of large, ground-based telescopes have only been used with infrared instruments. The turbulence of Earth's atmosphere distorts optical light more rapidly and significantly than infrared light. Therefore, the technical challenge of an AO system operating in optical wavelengths is to make faster and finer corrections of light distortion to obtain higher resolution images. Given the huge light-gathering capacity of the Subaru Telescope's 8.2 m primary mirror and the high performance of its AO 188 system in the infrared, the research team hypothesized that this system could also yield high-resolution images at optical wavelengths.

After using numerical simulations to confirm their hypothesis, they developed the connection between AO 188 and Kyoto3DII, an optical instrument that can operate in four modes. Because Kyoto3DII has to be positioned properly at each focus, the team designed and made a new frame mount for observations with the instrument at Nasmyth focus. The team also had to make a beam-splitter specialized for use with optical instruments. Making such a change is difficult, because the span of optical wavelengths is so short relative to infrared ones, but the team accomplished this task. On April 3, 2012 they carried out a test observation with the Kyoto3DII coupled with AO 188 and, for the first time, succeeded in performing full-scale, AO assisted scientific observations at optical wavelengths. Figure 2 shows the difference between the images obtained through this observation and those captured without AO. The team's images display the stars more clearly and at a higher spatial resolution (Note 3). The magnified images in Figure 2 show that that what looks like a very faint star when observed without AO appears as double stars when observed with AO (magnified Figure 2).

Figure 2: Images of the globular cluster M3, a region 50 arcseconds X 35 arcseconds at the observed wavelength of 660 nm and an exposure time of 10 seconds. Upper left panel: Image without using AO. Upper right panel: AO image. Lower panels are magnified images of parts of the upper panels. (Credit: NAOJ)

Kyoto3DII can operate in multiple modes, performing not only standard imaging and slit spectroscopy but also integral field spectroscopy, which has a square field of view and is a powerful tool for investigating the detailed structures of extended and multiple objects. The successful connection of Kyoto3DII with AO 188 enables the research team to carry out integral field spectroscopic observations with high resolution at optical wavelengths. Further analysis of the data will allow the astronomers to estimate the ionized state and gas motion of NGC 4151.

Figure 3: Images of NGC 4151, which has an active galactic nucleus at the center, taken by using Kyoto3DII in the integral field spectroscopy mode with an exposure time of 120 seconds. Left four panels: Images without using AO. Right four panels: Images with use of AO. Within each of the four panels, the high-resolution images of continuum emissions from stars and the active galactic nucleus (upper left), emission lines from hydrogen (upper right), sulfur (lower left), and argon (lower right) were obtained simultaneously. Continuum emissions refer thermally produced light. (Credit: NAOJ)

The team expressed their enthusiasm for the scientific promise of their newly developed basis for AO-assisted optical observations: "Using the combination of Kyoto3DII and AO 188, we hope to reveal the detailed structures of nearby galaxies and the formation processes of distant galaxies."

Note:

1. Please see details of the technical specifications and applications of Kyoto3DII at:

http://cosmos.phys.sci.ehime-u.ac.jp/~kazuya/p-3DII/index.html

2. Please see the previous press release about adaptive optics and AO 188:

http://www.naoj.org/Pressrelease/2006/11/20/index.html

3. The spatial resolution of images obtained with AO improved from 0.5 arcseconds to 0.2 arcseconds. The radius at which 50% of the energy was encompassed improved with AO from 0.60 arcseconds to 0.50 arcseconds.

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Subaru Telescope Discovers the Most Distant Protocluster of Galaxies

Using the Subaru Telescope, a team of astronomers led by Jun Toshikawa (The Graduate University for Advanced Studies, Japan), Dr. Nobunari Kashikawa (National Astronomical Observatory of Japan), and Dr. Kazuaki Ota (Kyoto University) has discovered the most distant protocluster of galaxies ever found--one that existed less than one billion years after the Big Bang. Since protoclusters are ancestors of today's massive clusters of galaxies, this discovery of a protocluster in the early Universe advances our understanding of how large-scale structures form and how galaxies evolve.

The nearby or "local" universe, an area that extends about 380 million light-years away from Earth, contains many galaxy clusters, i.e., gravitationally bound groups of about 100 to more than 1000 galaxies. These clusters are connected with each other and make up a huge network of galaxies called the "large-scale structure" of the Universe. Such configurations raise fundamental questions: When and how did these structures form in the history of the Universe?

Astronomers think that the Universe started out as an almost homogeneous mass that spread uniformly. Small fluctuations in the initial mass distribution increased by gravity over the 13.7 billion years of the Universe's age and produced the recent array of clusters. Because clusters contain a larger number of old and massive galaxies than those found in isolated galaxies, astronomers speculate that developing clusters may significantly affect the evolution of their member galaxies. Therefore, understanding the details of cluster formation (Note 1) is an essential step in addressing key issues of structure formation and galaxy evolution. A necessary part of this process is an investigation of all stages of cluster formation from beginning to end, which is why the current team gave particular emphasis to studying the birth of clusters.

The team focused on this phase of cluster formation by searching very distant galaxies that existed in the early Universe. Such observations present challenges for a couple of reasons. First, the light from more distant galaxies is faint and difficult to detect. Second, protoclusters in the early Universe are rare. The use of the Subaru Telescope allowed the team to overcome these difficulties. The telescope not only has an 8.2 m primary mirror with large light-gathering power but also offers the advantage of the Subaru Prime Focus Camera (Suprime-Cam) with a wide-field imaging capability. These features are particularly beneficial for discovering faint and rare objects in the distant Universe.

The team chose to observe the Subaru Deep Field, a 0.25 square-degree-wide field in the northern sky near the constellation Coma Barenices. The Subaru Deep Field is one of the most suitable regions for finding protoclusters in the early Universe; the area is not only deep and wide but has been intensively observed with the Subaru Telescope, which has detected very faint galaxies. When the team searched for distant galaxies in the Subaru Deep Field and investigated their distribution, they found a region with a surface number density five times greater than the average (Fig. 1).

Figure 1: Left panel: The distribution of galaxies 12.7 billion years ago. White circles represent the galaxies, and their size represents the luminosity of the galaxies. The color contour represents the density; the redder the color, the higher the density. The reddest region appears in the lower part of the figure. (Credit: NAOJ). Right panel: An enlargement from the map that shows the area around the cluster. (Credit: NAOJ)

The astronomers then used Subaru's Faint Object Camera and Spectrograph (FOCAS) to conduct a spectroscopic observation, which confirmed that most of the galaxies located in the highly dense region lay in a narrow area in the line-of-sight. This concentration of galaxies could not be explained by chance. On the basis of their observations with the Subaru Telescope, the team confirmed the existence of a protocluster 12.72 billion years ago (Fig. 2)--the most distant protocluster found with its distance established by spectroscopic observations (Note 2). The astronomers were able to directly observe this cluster of galaxies at an early stage in galaxy evolution, when structures were beginning to form in the early Universe. This discovery will be an important step on the way to understanding structure formation and galaxy evolution.

Figure 2: A close-up of the central region of the protocluster. Objects circled in red are galaxies 12.7 billion light-years away. (Credit: NAOJ) .

Although the team also investigated the properties of the galaxies in the protocluster (Note 3), they did not find a significant difference between the protocluster galaxies and other galaxies in the field. The astronomers speculate that the characteristic features of cluster galaxies in the nearby Universe occurred in later stages of cluster development, not during their birth (Note 4). Close examination of the internal structure of the protocluster showed that it could consist of subgroups of galaxies, merging together to form a more massive cluster (Note 5).

The team will continue their research with the Subaru Telescope's forthcoming Hyper-Suprime Camera (HSC), which has an imaging capability with a field of view seven times wider than Suprime-Cam. The astronomers expect to use HSC to reveal how many protoclusters existed in the early Universe and to provide a better picture of protoclusters in general. Toshikawa summarized the team's intent: "By continually working to find such distant protoclusters, we can understand cluster formation more clearly."

Reference:

These results were published in the May 1, 2012, edition of the Astrophysical Journal. This research was supported by The Japan Society for the Promotion of Science through Grant-in-Aid for Scientific Research 23340050. The authors of the paper are:

Jun Toshikawa, The Graduate University for Advanced Studies, Japan
Nobunari Kashikawa, National Astronomical Observatory of Japan
Kazuaki Ota, Kyoto University
Tomoki Morokuma, University of Tokyo
Takatoshi Shibuya, The Graduate University for Advanced Studies, Japan
Masao Hayashi, National Astronomical Observatory of Japan
Tohru Nagao, associate professor, Kyoto University
Linhua Jiang, University of Arizona
Matthew A. Malkan, University of California
Eiichi Egami, University of Arizona
Kazuhiro Shimasaku, University of Tokyo
Kentaro Motohara, University of Tokyo
Yoshifumi Ishizaki, The Graduate University for Advanced Studies, Japan


Notes:

1. Protoclusters more than 12.7 billion light-years away also closely relate to issues of cosmic reionization.

2. Prior to this research, Ouchi et al. used the Subaru Telescope and discovered the most distant protocluster ever found in 2005. In 2012 Trenti et al. found a protocluster candidate beyond 12.7 billion light-years ago, but spectroscopic observations have not confirmed their distances.

3. This research investigated luminosities and star formation rates.

4. Investigation of other properties such as mass, age and color is necessary to conclude whether the properties of protoclusters are different from those of field galaxies.

5. The interesting structure in Fig.1 is elongated toward the upper left of the protocluster. A large-scale structure may begin to form in this early Universe. Only the Subaru Telescope could find this large feature.

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Subaru-Led Team Discovers a Rare Stellar Disk of Quartz Dust

A research team of Japanese astronomers led by Dr. Hideaki Fujiwara (Subaru Telescope) has discovered a main-sequence star that is surrounded by a rare disk of quartz dust. Collisions of planetesimals, building blocks for planets, may have produced the dusty quartz ring during planet formation around the star. Based on observations with the AKARI and Spitzer infrared space telescopes, this recently discovered, intriguing feature of a stellar system may open new doors for research on the mineralogical nature of extrasolar planetary systems.

Since the 1995 discovery of the first extrasolar planet, 51 Pegasi b, about 700 planets orbiting around stars other than our Sun have been discovered so far. Understanding the origins and evolution of extrasolar planets has become one of the hottest and the most fundamental themes in modern astronomy. The team of scientists led by Dr. Fujiwara conducted research exploring this new frontier and concentrated their efforts on finding debris disks that could indicate planet formation.

According to a widely accepted recent scenario of planet formation, rocky planets like the Earth begin as an aggregation of cosmic dust and then continued their development as an accumulation of planetesimals, rocky planetary building blocks, around young stars. A substantial amount of dust would be generated from the collisions of planetesimals around main-sequence stars in the later phase of the planet formation process. Light absorbed from the central star would heat the grains of debris dust and re-emit its energy in infrared wavelengths. In light of this information, the team decided to focus their initial research on finding stars that have disks around them. They used the brand-new infrared map of the entire sky based on observations with AKARI, the Japanese infrared satellite to conduct their search.

The team discovered that the sun-like star HD 15407A, located in the Constellation Perseus 180 light years away from Earth, emits very bright infrared light relative to its visible light (Figure 1). Since HD 15407A is a main-sequence star, active collisions of planetesimals around it could produce a large amount of dust and emit infrared light.

Figure 1: (Left) Infrared image of HD 15407A obtained by the observations of AKARI (Field of View: 0.15 degree x 0.15 degree). (Right) Infrared spectrum of HD 15407A obtained by the observations from Spitzer. Two bumps seen around 9 and 21 microns indicate the presence of micron-sized quartz dust. (Credit: Univ. of Tokyo and NAOJ.)

The team conducted follow-up observations of the star using Spitzer, the infrared space telescope operated by Caltech and NASA. Spitzer's high sensitivity and sharp spatial resolution in the infrared allowed the researchers to identify the amount and mineralogical nature of the debris dust. Their examination of the Spitzer spectrum of HD 15407A revealed that at least 100 trillion tons of tiny particles of quartz dust orbit the star. They also found that the dust is located about 1 AU (astronomical unit) from the central star in the so-called "terrestrial planet region." This is the first study that not only reveals the presence of a quartz disk of dust around a sun-like main-sequence star but also precisely determines its amount and spatial distribution.

Figure 2: Illustration of the HD 15407A System Based on Observations from AKARI and Spitzer, showing the dusty quartz disk orbiting the star. (Credit: Univ. of Tokyo and NAOJ.)

Quartz dust seems to be a rather rare kind of dust in the Universe, and it is not yet clear how the quartz dust observed around HD 15407A formed. Since the Earth's crust contains a large amount of quartz-like minerals, this suggests that the quartz dust around HD 15407A might come from the surface layers of large rocky bodies colliding with other planetesimals orbiting the star. Team leader Fujiwara commented, "We have used two infrared space telescopes to discover a precious star with a rare disk of quartz dust that may contribute to revealing the secrets of planetary materials. We would like to examine this star more thoroughly by combining theory with laboratory studies to further our understanding of the planet formation process."

This research was published in the April 20, 2012 issue of The Astrophysical Journal Letters and was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (23103002) from MEXT, Japan.

Reference:

Fujiwara et al. 2012, "Silica-Rich Bright Debris Disk around HD 15407A", The Astrophysical Journal Letters, 749, L29 (2012).

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Mapping Galaxy Formation in Dual Mode

A team of astronomers led by David Sobral (Leiden Observatory and Royal Observatory of Edinburgh) has explored the synergies between the Subaru Telescope and the United Kingdom Infra-Red Telescope (UKIRT) to locate numerous distant galaxies in the ancient universe and investigate their star formation activity. By combining narrow-band filter (Note 1) observations from both the Subaru Telescope and the UKIRT, the team has been able to obtain clean panoramic maps of parts of the distant universe about 9 billion years ago. This dual mode of surveying faint galaxies provides a powerful technique for selecting and studying star-forming galaxies during their formation and evolution.

Astronomers rely on detailed observations of astronomical objects outside of our own Milky Way Galaxy to understand how galaxies developed into what they are today. By comparing the properties of galaxies at different ages of the Universe, scientists can investigate their formation and evolution. However, current samples of the distant Universe lack the size and volume to answer such questions as: When was the peak of galaxy formation activity? Which physical processes propelled such activity? The current research team has developed and applied a technique for overcoming some common problems: a) missing many galaxies by looking at only one emission line and b) contamination of findings by less accurate measurements of galaxy distance and properties.

The dual-line technique takes advantage of a unique combination of the capabilities of the 8.2 m Subaru Telescope and the 3.8 m UKIRT to view very distant galaxies over wide areas. The combined Subaru-UKIRT survey uses two filters-- a narrow-band filter on the Subaru Telescope to look for oxygen emission lines (Note 2) and another narrow-band filter on UKIRT to look for hydrogen emission lines-- and yields a panoramic view of the distant universe about 9 billion years ago that one survey alone could not provide. Team leader Sobral explains the technique: “We are looking at the same process(es), i.e., star formation activity, through two different perspectives, which greatly expands our view of what is going on in these distant galaxies. It’s similar to listening to stereo music with earphones. If we listen with just one earphone, we won’t hear part of the vocals/instruments. Only when we use two earphones can we fully appreciate the complete range of sounds.”

Figure 1: An example of how the dual narrow-band technique works. UKIRT images (top) are obtained with a narrow- (left) and a broad-band filter (middle), and, after subtraction, a galaxy with an emission line (right) is identified. Subaru images (bottom) are obtained with different narrow- and broad-band filters, and the same procedure is followed. Galaxies with emission lines in both data sets are automatically confirmed to be galaxies forming stars at a distance of about 9 billion light years away.

The team found 190 distant galaxies seen simultaneously through their hydrogen and oxygen lines and has been able to derive how much star formation was occurring in the universe 9 billion years ago and compare that with other studies. The results reveal that the overall population of star-forming galaxies has been continuously decreasing their star formation activity for the last 11 billion years.

Figure 2: Star formation history of the Universe. This shows that star formation activity in the Universe as a whole has been declining over the last 11 billion years. The dual line technique can provide a reliable estimate of star formation activity 9 billion years ago. It allows this estimate by using the two emission lines for the first time; each emission line is shown separately and reveals good agreement about star formation during the same era.

The findings from this research also contribute greater details to our general understanding of how galaxies form and evolve. For the first time, they allow a comparison of dust extinction (i.e., the amount of light absorbed by dust) affecting typical star-forming galaxies today and those that existed 9 billion years ago. Contrary to past assumptions, dust extinction has similar effects on both distant young galaxies, which are much more active, and local ones. This result is very important for accurate measurement of star formation rates at early epochs in the Universe.

Notes:

1. Astronomical filters are used to focus observations on a specific range of wavelengths. Narrow-band filters only transmit a narrow band of lines in the spectrum while broad-band filters transmit light from a wider range of wavelengths.

2. Emission lines refer to bright, narrow lines in a spectrum that derive from light emitted from the hot gas of stars of galaxies at certain wavelengths.


Reference:

• The paper describing this research, "Star formation at z=1.47 from HiZELS: An Ha+[O II] double-blind study", was published in the 2012 Monthly Notice of the Royal Astronomical Society (MNRAS), 420, 1926. The authors of the paper are:

• D. Sobral, Leiden Observatory, Leiden University, the Netherlands & Institute for Astronomy, Royal Observatory of Edinburgh, UK

• P.N. Best, Institute for Astronomy, Royal Observatory of Edinburgh, UK

• Y. Matsuda, Institute for Computational Cosmology, Durham University, UK

• I. Smail, Institute for Computational Cosmology, Durham University, UK

• J.E.Geach, Institute for Computational Cosmology, Durham University, UK and McGill University, Canada

• M. Cirasuolo, Institute for Astronomy, Royal Observatory of Edinburgh, UK and Astronomy Technology Centre, Royal Observatory of Edinburgh, UK

More about → Mapping Galaxy Formation in Dual Mode

Surprising Discovery of a Rare "Emerald-Cut" Galaxy

An international team of astronomers—from Australia, Germany, Switzerland, and Finland—has discovered a rare, rectangular-shaped galaxy (LEDA 074886) that has a striking resemblance to an emerald-cut diamond. While using the Subaru Prime Focus Camera (Suprime-Cam) to look for globular clusters of stars swarming around NGC 1407, a bright, giant galaxy in the Constellation Eridanus and 700 million light years from Earth, the researchers discovered an unusually shaped dwarf galaxy toward the edge of their image. Professor Alister Graham (Swinburne University of Technology, Australia), lead author of the paper describing the research, said, "It's one of those things that just makes you smile because it shouldn't exist, or rather, you don't expect it to exist." Its discovery allows astronomers to obtain useful information for modeling other galaxies.

Figure: False-color image of LEDA 074886 taken with Subaru Telescope's Suprime-Cam. The central contrast has been adjusted to reveal the inner disk/bar-like component. Dr. Lee Spitler (Swinburne University of Technology, Australia) took this image.

Most galaxies in the universe around us exist in one of three forms: ellipsoidal, disk-like (usually in the shape of a flattened circular disk hosting a spiral pattern of stars), or irregular. Dwarf galaxies, probably the most common galaxies in the Universe, are small and have low intrinsic brightness (i.e., luminosity). One of the reasons that LEDA 074886 was hard to find is its dwarf-like status; it has 50 times less stars than our own Milky Way Galaxy, and its distance from Earth is equivalent to that spanned by 700 Milky Way galaxies placed end-to-end. The combined advantages of Subaru's large 8.2m primary mirror and its camera at prime focus gave the researchers such a wide field of view that they could observe objects beyond their intended targets and make the surprising discovery of the emerald-shaped dwarf galaxy. Additional information gleaned from the use of green, red, and infrared filters along with the good image quality seeing in the observation enabled the researchers to see and measure a stellar disk embedded within the rectangular-shaped galaxy. The blue color of the inner disk suggested a younger average age for this stellar population.

The astronomers suspect that the emerald-cut galaxy may resemble an inflated disk seen side-on, like a short cylinder. Research co-author Professor Duncan Forbes (Swinburne University of Technology, Australia) explained, "One possibility is that the galaxy may have formed out of the collision of two spiral galaxies. While the pre-existing stars from the initial galaxies were strewn to large orbits creating the emerald-cut shape, the gas sank to the mid-plane where it condensed to form new stars and the disk that we have observed."

Despite its apparent uniqueness, partly due to its chance orientation, the team has gathered useful information for modeling other galaxies. While the outer rectangular shape is somewhat like galaxy simulations that don't involve the production of new stars, the disk-like structure is comparable with simulations involving star formation. "This highlights the importance of combining lessons learned from both types of past simulations, for better understanding of galaxy evolution," says Professor Graham. When our own disk-shaped Milky Way Galaxy collides with the disk-shaped Andromeda Galaxy in about three billion years from now, we may become inhabitants of a rectangular looking galaxy.

Reference:

The paper presenting the results of this research, "Leda 074886: A Remarkable Rectangular-Looking Galaxy", will appear in the Astrophysical Journal. It is tentatively scheduled for the May 1, 2012, Issue 750-1. Authors of the paper are:

Graham, A.W., Swinburne University of Technology, Australia
Spitler, L.R., Swinburne University of Technology, Australia
Forbes, D.A., Swinburne University of Technology, Australia
Lisker, T., Heidelberg University, Germany
Moore, B. University of Zurich, Switzerland
Janz, J., University of Oulu, Finland

More about → Surprising Discovery of a Rare "Emerald-Cut" Galaxy

Subaru Telescope Captures Images of the "Stealth Merger" of Dwarf Galaxies

Figure 1: Suprime-Cam captured this image of the nearby dwarf galaxy NGC 4449 (lower left) and its companion (upper right), a dwarf galaxy that has been gravitationally pulled apart into a stellar stream. It shows individual stars composing the galaxies. R. Jay GaBany (Blackbird Observatory) produced the composite: blue hues in the center of the larger galaxy are brought about by a burst of recent star formation, while the red in its periphery and in its satellite indicates the presence of older, red-giant stars. (Hi-Res image)

An international team of scientists led by David Martinez-Delgado (Max Planck Institute for Astronomy, Germany) has conducted research that reveals a "stealth merger" of dwarf galaxies, where an in-falling satellite galaxy is nearly undetectable by conventional means yet has a substantial influence on its host galaxy. Aaron Romanowsky (University of California Observatories in Santa Cruz) along with graduate student Jacob Arnold (UCSC) used the Subaru Telescope to obtain high-resolution images of individual stars in a dense stream of stars in the outer regions of a nearby dwarf galaxy (NGC 4449); these outlying stars are the remains of an even smaller companion galaxy in the process of merging with its host (Figure 1). NGC 4449, the host galaxy, is the smallest primary galaxy in which a stellar stream from an ongoing merger has been identified and studied in detail. Romanowsky commented, "I don't think I'd ever seen a picture of a galaxy merger where you can see the individual stars. It's really an impressive image."

Martinez-Delgado organized a campaign to follow up on an initial report of the stellar stream in NGC 4449, first detected by Russian astronomers as a mysterious, faint smudge in digitized photographic plates from the Digitized Sky Survey project. Had the objects been slightly fainter, more diffuse, or farther from the host galaxy, the stellar stream could easily have been missed. NGC 4449 is located 12.5 million light years from Earth and is a member of a group of galaxies in the constellation Canes Venatici. Martinez-Delgado recruited astrophotographer R. Jay GaBany (recipient of the 2010 AAS Chambliss Amateur Achievement Award) to obtain deep, wide-field images with the half-meter telescope at Black Bird Observatory; these images confirmed the presence of a faint substructure in the galaxy's halo. Romanowsky and Arnold then took advantage of the wide field of view and light collecting power of Subaru Telescope's 8.2 meter mirror and its prime focus camera (Suprime-Cam) to capture high resolution images of the faint objects in the halo. These final observations at Subaru in 2011 clearly showed the stealth merger of two dwarf galaxies.

Modern cosmological theory posits that large galaxies were built up from smaller ones through an orderly succession of mergers. Although astronomers have observed many mergers involving massive galaxies, it has been difficult to find mergers of two dwarf galaxies. Theory suggests that similar processes of merging should occur on a smaller scale, with small galaxies eating even smaller ones; this is how galaxies grow. Romanowsky commented on the significance of the Subaru image: "Now we have this beautiful image of a dwarf galaxy consuming a smaller dwarf. You can see a smaller galaxy coming in and getting shredded, eventually leaving its stars scattered through the halo of the host galaxy. "

The new observations support the idea that the stellar halos around many dwarf galaxies are the remnants of smaller satellites that were shredded in past merger events. The ongoing merger in NGC 4449 may also be responsible for the intense burst of star formation seen in the galaxy. "This galaxy is famous for its starburst activity, and it seems we've found the reason for that. The gravitational interaction with the incoming galaxy has probably disturbed the gas in the main galaxy and caused it to start forming stars," Romanowsky said.


Notes:

This research was supported by the National Science Foundation (NSF, USA), National Aeronautics and Space Administration (NASA, USA) and the University of California at Santa Cruz-University Affiliated Research Center (UCSC-UARC) Aligned Research Program.


The paper describing the research, “Dwarfs gobbling dwarfs: A stellar tidal stream around NGC 4449 and hierarchical galaxy formation on small scales”, will be published in Astrophysical Journal Letters. Coauthors of this international study include:

D. Martinez-Delgado (Max Planck Institute for Astronomy, Germany)
A. Romanowsky (University of California Observatories at Santa Cruz, USA)
J. Arnold (University of California at Santa Cruz, USA)
R. J. GaBany (astrophotographer, amateur astronomer, USA)
J. Brodie (UC Santa Cruz, USA)
F. Annibali (Astronomical Observatory of Bologna, Italy)
J. Fliri (Observatory of Paris, France)
S. Zibetti (University of Copenhagen, Denmark)
R. van der Marel (Space Telescope Science Institute, USA)
A. Aloisi (Space Telescope Science Institute, USA)
H.-W. Rix (Max Planck Institute for Astronomy, Germany)
A. Macciò (Max Planck Institute for Astronomy, Germany)
T. Chonis (University of Texas at Austin, USA)
J. Carballo-Bello (Canary Astrophysics Institute, Spain)
J. Gallego-Laborda (Fosca Nit Observatory Spain)
M. Merrifield (University of Nottingham, UK)

More about → Subaru Telescope Captures Images of the "Stealth Merger" of Dwarf Galaxies

Subaru's Sharp Eye Confirms Signs of Unseen Planets in the Dust Ring of HR 4796 A

The SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope/HiCIAO) project, a five-year international collaboration launched in 2009 and led by Motohide Tamura of NAOJ (National Astronomical Observatory of Japan) has yielded another impressive image that contributes to our understanding of the link between disks and planet formation. Researchers used Subaru's planet-finder camera, HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics), to take a crisp high-contrast image of the dust ring around HR 4796 A, a young (8-10 million years old) nearby star, only 240 light years away from Earth. The ring consists of dust grains in a wide orbit, roughly twice the size of Pluto's orbit, around the central star. The resolution of the image of the inner edge of the ring is so precise that an offset between its center and the star's position can be measured. Although data from the Hubble Space Telescope led another research group to suspect such an offset, the Subaru data not only confirm its presence but also reveal it to be larger than previously assumed.

Figure 1: Near-infrared (1.6 micron) image of the debris ring around the star HR 4796 A. An astronomical unit (AU) is a unit of length that corresponds to the average distance between the Earth and Sun, almost 92 million miles (over 149 million km).

The ring consists of dust grains in a wide orbit (roughly twice the size of Pluto's orbit) around the central star. Its edge is so precisely revealed that the researchers could confirm a previously suspected offset between the ring's center and the star's location. This "wobble" in the dust's orbit is most likely caused by the unbalancing action of – so far undetected – massive planets likely to be orbiting within the ring. Furthermore, the image of the ring appears to be smudged out at its tips and reveals the presence of finer dust extending out beyond the main body of the ring.

For high resolution versions of the above image, click on the following links: Image only or Image with labels.
What caused the wheel of dust around HR 4796 A to run off its axis? The most plausible explanation is that the gravitational force of one or more planets orbiting in the gap within the ring must be tugging at the dust, thus unbalancing their course around the star in predictable ways. Computer simulations have already shown that such gravitational tides can shape a dust ring into eccentricity, and findings from another the eccentric dust ring around the star Formalhaut may be observational evidence for the process. Since no planet candidates have been spotted near HR 4796 A yet, the planets causing the dust ring to wobble are probably simply too faint to detect with current instruments. Nevertheless, the Subaru image allows scientists to infer their presence from their influence on the circumstellar dust.

The Subaru Telescope's near-infrared image is as sharp as the Hubble Space Telescope's visible-light image, thus enabling accurate measurements of its eccentricity. While Subaru Telescope's mirror is much larger than Hubble's, light from the HR 4796 A system must first pass through the turbulent air layers of Earth's atmosphere before Subaru's instruments can measure it. Subaru's adaptive optics system (AO188) allows it to correct for most of the atmosphere's blurring effects in order to take razor-sharp images. The application of an advanced image processing technique, angular differential imaging, to the data suppressed the star's bright glare and enhanced the faint light reflected from the ring so that it was more visible.

This image gives scientists more information about the relationship between a circumstellar disk and planet formation. Planets are believed to form in the disks of gas and dust that remain around young stars as the by-products of star formation. As the material is swept up by the newborn planets or blown out of the system by the star's radiation, such (primordial) disks soon disappear in a few tens of million years. Nevertheless, some stars are surrounded by a debris or secondary disk, mainly composed of dust long after the primordial disk should have dispersed. Collisions between small solid bodies ("planetesimals") left over from planet formation may continuously replenish the dust in these disks. The dust ring around HR 4796 A is such a debris disk and provides essential information for studying planet formation and possible formed planets in such debris disk systems.

References:

Refer to the following article for the published report of the results:
Thalmann et al., The Astrophysical Journal Letters, Volume 743, Issue 1, (2011)

Articles describing other results from the SEEDS project can be found at the following locations:
Direct Images of Disks Unravel Mystery of Planet Formation, February 17, 2011. (Subaru Telescope Press release: 17 Feb. 2011)
Discovery of an Exoplanet Candidate Orbiting a Sun-Like Star, December 3, 2009. (Subaru Telescope Press release: 3 Dec. 2009)

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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.

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