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Post Info TOPIC: Ancient LMC Supernovae


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Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is seen here through a narrow filter that transmits only the red light of hydrogen atoms.
Ionised by energetic starlight, a hydrogen atom emits the characteristic red H-alpha light as its single electron is recaptured and transitions to lower energy states. As a result, this image of the LMC seems covered with shell-shaped clouds of hydrogen gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing hydrogen clouds are known as H II (ionised hydrogen) regions.

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Credit: John P. Gleason

This high resolution mosaic view was recorded in 6 segments, each with 200 minutes of exposure time. Itself composed of many overlapping shells, the Tarantula Nebula, is the large star forming region near centre left.
A satellite of our Milky Way Galaxy, the LMC is about 15,000 light-years across and lies a mere 180,000 light-years away in the constellation Dorado.

-- Edited by Blobrana at 23:29, 2006-05-18



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The colours highlighted on the mosaic of the Large Magellanic Cloud are light emitted by hydrogen (red), oxygen (green), and sulphur (yellow), while light from individual stars has been subtracted.
Visible in the above image are many small planetary nebulas pushed out by low mass stars, large emission nebula of ambient interstellar gas set aglow by massive stars, and huge gaseous supernova remnants cast off by massive stars exploding.



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N49 Supernova remnant

This false-colour image shows infrared (red), optical (green), and X-ray (blue) views of the N49 supernova remnant. This object, the remains of an exploded star, has million-degree gas in the centre, with much cooler gas at the outer parts of the remnant. While astronomers expected that dust particles were generating most of the infrared emission, the study of this object indicates that much of the infrared is instead generated in heated gas.

Images forming this composite were taken with three orbiting Observatories. The infrared image was taken with the Spitzer Space Telescope's Multiband Imaging Photometer for Spitzer (MIPS) at a wavelength of 24 microns. The optical image was taken with the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) of hydrogen emission. The X-ray image was taken with the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer.

Credit: NASA (SSC/HST/CXC), U.Illinois (R.Williams & Y.-H.Chu)



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In recent years, a number of ground-based optical and radio surveys of the Large and Small Magellanic Clouds have become available. New composite images of optical, radio, infrared, ultraviolet and X-ray wavelengths are giving astronomers at the University of Illinois at Urbana-Champaign a clearer picture of the birth, life and death of massive stars, and their effect on the gas and dust of the interstellar medium surrounding them.

From their birth to their death, massive stars have a tremendous impact on their galactic surroundings. While alive, these stars energise and enrich the interstellar medium with their strong ultraviolet radiation and their fast stellar winds. As they die, shock waves from their death throes inject vast quantities of mechanical energy into the interstellar medium and can lead to the formation of future stars.

"Comparing images at different wavelengths lets us create a more complete picture, rather than seeing only a few features in isolation. Using multi-spectral data sets, we can examine the physical structure of the interstellar medium and study the conditions that lead to star formation" - You-Hua Chu, chair of the astronomy department at Illinois.

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This false-colour image shows infrared (red), optical (green), and X-ray (blue) views of the large star-forming complex N51. The warm ionised gas is shown in green, the hot ionised gas is in blue, and the proto-stars are primarily in red. This colour image reveals the relative position of the expanding shell N51D and the recently formed proto-stars, allowing astronomers to determine whether the star formation is triggered by pressure from hot gas or by compression by a passing shock wave.
The infrared image was taken with the Spitzer Space Telescope's Infrared Array Camera (IRAC) at a wavelength of 8 microns. The optical image of hydrogen emission was taken as part of the Magellanic Cloud Emission-Line Survey (MCELS) with the Curtis-Schmidt Telescope at Cerro Tololo Inter-American Observatory in Chile. The X-ray image was taken with the European Space Agency's satellite, XMM-Newton, using its European Photon Imaging Camera (EPIC) camera.
Credit: NASA/SSC/MCELS/ESA/U.Illinois (Y.-H. Chu and R. A. Gruendl)

Massive stars interact with the interstellar medium in many ways. Their fast stellar winds and supernova blasts can sweep up the surrounding medium into expanding shells filled with hot gas.

"The expanding shells produce conditions that may start a new wave of star births. The combination of X-ray, optical and infrared observations allow us to determine whether the pressure of the hot gas or compression by a passing shock wave is responsible for triggering star formation" - Robert Gruendl, an Illinois astronomer who uses Spitzer Space Telescope observations to search for proto-stars.

In related work, Illinois astronomer Rosa Williams has added data from a new wavelength regime to her growing database on stellar graveyards in the Magellanic Clouds. Comparing infrared images obtained with the Spitzer Space Telescope, Williams explored the distribution of matter caught in the expanding shells of supernova remnants.

"We expected significant infrared emission to be generated by dust particles. Instead, most of the emission from these remnants came from heated gas" - Rosa Williams.

Strong ultraviolet radiation from nearby star-forming regions may have ionised the gas and torn apart the dust particles consisting of hydrocarbon molecules.

"Other dust particles could have been shattered by shock waves from the supernova. We are investigating the nature and amount of dust in regions surrounding the supernova remnants to see whether the deficiency in dust is inherent in the environment or created by the remnant" - Rosa Williams

Chu, Gruendl and Williams will present their latest findings at the American Astronomical Society meeting in Washington, D.C., on Wednesday (Jan. 11, 2006).



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Known as the N44 superbubble complex, this nebula is dominated by a vast bubble about 325 by 250 light-years across. A cluster of massive stars inside the cavern has cleared away gas to form a distinctive mouth-shaped hollow shell. While astronomers do not agree on exactly how this bubble has evolved for up to the past 10 million years, they do know that the central cluster of massive stars is responsible for the cloud's unusual appearance. It is likely that the explosive death of one or more of the cluster’s most massive and short-lived stars played a key role in the formation of the large bubble.

"This region is like a giant laboratory providing us with a glimpse into many unique phenomena. Observations from space have even revealed x-ray-emitting gas escaping from this superbubble, and while this is expected, this is the only object of its kind where we have actually seen it happening" - Sally Oey of the University of Michigan, who has studied this object extensively

One of the mysteries surrounding this object points to the role that supernova explosions (marking the destruction of the most massive of the central cluster's stars) could have played in sculpting the cloud.

"When we look at the speed of the gases in this cloud we find inconsistencies in the size of the bubble and the expected velocities of the winds from the central cluster of massive stars. Supernovae, the ages of the central stars, or the orientation and shape of the cloud might explain this, but the bottom line is that there’s still lots of exciting science to be done here and these new images will undoubtedly help" - Philip Massey, Lowell Observatory.

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Field of View: 9.8' x 5.5'
Orientation: Rotated 51 Degrees East of North
The intricate and colourful nebulae are produced by ionised gas [1] that shines as electrons and positively charged atomic nuclei recombine, emitting a cascade of photons at well-defined wavelengths. Such nebulae are called "H II regions", signifying ionised hydrogen, i.e. hydrogen atoms that have lost one electron (protons). Their spectra are characterized by emission lines whose relative intensities carry useful information about the composition of the emitting gas, its temperature, as well as the mechanisms that cause the ionisation. Since the wavelengths of these spectral lines correspond to different colours, these alone are already very informative about the physical conditions of the gas.

This image provides one of the most detailed views ever obtained of NGC 1929 nin the Large Magellanic Cloud, a satellite galaxy to the Milky Way, located some 150,000 light-years away and visible from the Southern Hemisphere. The images captured light of specific colours that reveal the compression of material and the presence of gases (primarily excited hydrogen gas and lesser amounts of oxygen and “shocked” sulphur) in the cloud.
Multiple smaller bubbles appear in the image as bulbous growths clinging to the central superbubble. Most of these regions were probably formed as part of the same process that shaped the central cluster. Their formation could also have been "sparked" by compression as the central stars pushed the surrounding gas outward. Our view into this cavern could really be like looking through an elongated tube, which lends the object its monstrous mouth-like appearance.

The images used to produce the colour composite were obtained with the Gemini Multi-object Spectrograph (GMOS) at the Gemini South Telescope on Cerro Pachón in Chile. The colour image was produced by Travis Rector of the University of Alaska Anchorage and combines three single-colour images to produce the image.



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Ancient LMC Supernovae

A team of astronomers has found faint visible echoes of three ancient supernovae by detecting their centuries-old light as it is reflected by clouds of interstellar gas hundreds of light-years removed from the original explosions.

Located in the nearby Large Magellanic Cloud in the southern skies of Earth, the three exploding stars flashed into short-lived brilliance at least two centuries ago, and probably longer. The oldest one is likely to have occurred more than six hundred years ago (610 and 410 yrs).
The light echoes were discovered by comparing images of the Large Magellanic Cloud (LMC) taken years apart. By precisely subtracting the common elements in each image of the galaxy and looking by eye to see what variable objects remain, the team looked for evidence of invisible dark matter that might distort the light of stars in a transitory way, as part of a sky survey called SuperMACHO.

This careful image analysis also revealed a small number of concentric, circular-shaped arcs that are best explained as light moving outward over time, and being scattered as it encounters dense pockets of cool interstellar dust. Team members then fit perpendicular vectors to the curves of each arc system, which were found to point backwards toward the sites of three supernovae remnants, which were previously known and thought to be relatively young.

"Without the geometry of the light echo, we had no precise way of knowing just how old these supernovae were. Some relatively simple mathematics can help us answer one of the most vexing questions that astronomers can ask-exactly how old is this object that we are looking at?" - Astronomer Armin Rest of the National Optical Astronomy Observatory (NOAO), lead author of a paper on the discovery in the December 22, 2005, issue of Nature.

Just as a sound echo can occur when sound waves bounce off a distant surface and reflect back toward the listener, a light echo can be seen when light waves travelling through space are reflected back toward the viewer-in this case, the Mosaic digital camera on the National Science Foundation's Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile.
This technique can be extended to famous supernovae in history.

"Imagine seeing light from the same explosion first seen by Johannes Kepler some 400 years ago, or the one recorded by Chinese observers in 1006. These light echoes give us that possibility" - Christopher Stubbs of the Harvard-Smithsonian Centre for Astrophysics (CfA), co-author of the paper and principal investigator for the SuperMACHO program.

In principle, astronomers can split the light echo into a spectrum to investigate what type of supernova occurred.

"We have the potential with these echoes to determine the star's cause of death, just like the archaeologists who took a CT scan of King Tut's mummy to find out how he died" - co-author Arti Garg of CfA.

Astronomers can also use supernova light echoes to measure the structure and nature of the interstellar medium. Dust and gas between the stars are invisible unless illuminated by some light source, just as fog at night is not noticeable until lit by a car's headlights. A supernova blast can provide that illumination, lighting up surrounding clouds of matter with its strobe-like flash.

"We see the reflection as an arc because we are inside an imaginary ellipse, with the Earth at one focus of the ellipse and the ancient supernovae at the other. As we look out toward the supernovae, we see the reflection of the light echo only when it intersects the outer surface of the ellipse. The shape of the reflection from our vantage point appears to be a portion of a circle" - Nicholas Suntzeff of NOAO.

An unusual aspect of the arcs is that they generally appear to move much faster than the speed of light. This does not violate the cosmic speed limit, which states that any object cannot move faster than the speed of light.

"What our telescopes see is the reflection moving, and not any physical object. It is also very exciting that our observations confirm the visionary prediction of Fritz Zwicky in 1940 that light from ancient supernovae could be seen in echoes of the explosion" - Nicholas Suntzeff.

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