Flashes from the Past: Echoes from Ancient Supernovae (Forwarded)

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FOR IMMEDIATE RELEASE: Wednesday, December 21, 2005

RELEASE NO: NOAO 05-12

Flashes from the Past: Echoes from Ancient 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 a nearby galaxy 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.

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 looks 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," said astronomer Armin Rest of the
National Optical Astronomy Observatory, lead author of a paper on the
discovery in the December 22, 2005, issue of Nature. "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?"

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 traveling 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," said
Christopher Stubbs of the Harvard-Smithsonian Center for Astrophysics
(CfA), co-author of the paper and principal investigator for the
SuperMACHO program. "These light echoes give us that possibility."

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," said 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," explained Nicholas Suntzeff of NOAO. "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."

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," Suntzeff added. "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."

Two high-resolution color graphics to illustrate this result are available
at
http://www.noao.edu/outreach/press/p...12.html#images

Other co-authors of the Nature paper are Knut Olsen and Chris Smith
(CTIO); Jose Luis Prieto (Ohio State University); Douglas Welch (McMaster
University, Ontario); Andrew Becker and Gajus Miknaitis (University of
Washington); Marcel Bergmann (Gemini Observatory); Alejandro Clocchiatti
and Dante Minniti (Pontifica Universidad Católica de Chile); and, Kem
Cook, Mark Huber and Sergei Nikolaev (Lawrence Livermore).

Part of the National Optical Astronomy Observatory, CTIO is operated by
the Association of Universities for Research in Astronomy Inc. (AURA),
under a cooperative agreement with the NSF.

More information is available at the SuperMACHO Light Echoes site,
http://www.ctio.noao.edu/supermacho/lightechos/

[NOTE: Additional high-resolution artwork is available at
http://www.cfa.harvard.edu/press/pr0539image.html ]