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An exploding star dubbed SN 1997ff, caught once on
purpose and twice by accident by NASA’s Hubble Space Telescope, is the
oldest and most distant Type Ia supernova ever seen, according to a
recent analysis by the Department of Energy’s National Energy Research
Scientific Computing Center (NERSC) at Lawrence Berkeley National
Laboratory.
Berkeley Lab astrophysicist Peter Nugent, a member of the team led by
Adam Riess at the Space Telescope Science Institute that studied the
distant supernova, used an IBM SP supercomputer to perform the analysis
at NERSC, the world’s largest unclassified supercomputing center.
Nugent says that the serendipitous discovery of the more than
11-billion year old supernova is important for several reasons.
“This supernova is consistent with the cosmological model of an
accelerating universe, a universe mostly filled with dark energy,” Nugent
says. “It argues against the notion that observations of distant Type Ia
supernovae may be systematically distorted by intervening gray dust or
the chemical evolution of the universe.”
Moreover, says Nugent, “the supernova is so ancient that it allows us to
glimpse an era when matter in the universe was still relatively dense and
expansion was still slowing under the influence of gravity. More recently
the dark energy has begun to predominate and expansion has started to
speed up.”
The Supernova Cosmology Project and the High-Z Supernova Search Team,
the two international groups of astronomers and physicists who discovered
the accelerating expansion of the universe, use Type Ia supernovae as
“standard candles” to measure cosmological parameters. Type Ia spectra
and light curves (their rising and falling brightness over time) are
all nearly alike, and they are bright enough to be seen at very great
distances.
With a redshift (or z) of about 1.7, says Nugent, “supernova 1997ff is
some 11.3 billion years old, much older — and much fainter — than the
previous record of z equals 1.2, which corresponds to an age of about
9.8 billion years old.” He adds that a supernova at redshift 1.7 “is
too far away to have been visible from the surface of the Earth. Only
a space-based telescope could have found it.”
SN 1997ff was first found, on purpose, by Ron Gilliland of the Space
Telescope Science Institute and Mark Phillips of the Carnegie Institute
of Washington, during the last week of December, 1997. Gilliland and
Phillips turned the Hubble Space Telescope on the same patch of sky
recorded in the renowned Hubble Deep Field of typical galaxies, looking
for bright spots which, after spurious or doubtful signals had been
rigorously eliminated, might prove to be supernovae. They found two
good candidates.
Gilliland and Phillips asked Nugent to help them determine what these
discoveries implied for the rate at which high-redshift supernovae
might occur in the universe as a whole. Their report, published in
1999, suggested that one of their two candidates, SN 1997ff, was
probably a Type Ia with a redshift greater than z = 1.32. Because
it had been observed in only one range of frequencies, however, the
uncertainties were too great to use the supernova for cosmological
estimates.
At high redshifts, much of an astronomical object’s characteristic
spectrum is shifted into the infrared. Without additional infrared
observations, no useful cosmological information could be derived
from SN 1997ff, nor could its type be positively identified. It
seemed unlikely that anyone had made such observations.
Enter serendipity. Gilliland learned that only 25 days after his and
Phillips’s observation, Rodger Thompson of the University of Arizona
had begun studying a small portion of the Hubble Deep Field with NICMOS,
an instrument aboard the space telescope that makes images in the near
infrared. Although Thompson had not been looking for supernovae, many
of his images accidentally included SN 1997ff and its host galaxy.
“Twenty-five days later may seem like a long time, but highly
redshifted objects are moving away from us so fast that time dilation
is large,” Nugent remarks. “At a redshift of 1.7, three and a half
weeks in our frame of reference is only about nine days of elapsed
time for the supernova itself.”
Six months later another set of infrared images of the same region,
made by Mark Dickinson of the Space Telescope Science Institute, caught
the now greatly faded supernova and its host galaxy once again. Nugent
learned of Dickinson’s work in the summer of 1999 and met with him at
the American Astronomical Society meeting the following year.
Once more, luck had provided a missing piece of the puzzle: by
digitally subtracting the new image of the host galaxy from images
made when the supernova was bright, Nugent proposed, much of the
remaining uncertainty about the supernova and its host could be
eliminated.
Intrigued by the accumulating data, Adam Riess queried Nugent in July
of 2000 about doing cosmology on an unnamed supernova at a redshift
“around 1.65.” There was only one such supernova; soon Riess and
Nugent were collaborating. “Adam had the monumental task of reducing
the observed NICMOS infrared data,” said Nugent, “while I concentrated
on comparing the reduced data to known supernovae and various sets of
cosmological parameters.”
Among the numerous calculations Nugent performed at NERSC in
communication with Riess, one of the most telling was a set of plots
seeking the best fit to parameters that included supernova type,
redshift, distance, and the evolution of the light curve. They
determined that SN 1997ff was almost certainly a Type Ia supernova
at a redshift of 1.7, first seen eight days after it exploded.
“Now we could do the cosmology,” Nugent says.
The conclusion that the expansion of the universe is accelerating is
based on the observation that Type Ia supernovae at redshifts greater
than 0.5 are dimmer — and thus farther away — than their redshifts
would suggest if the universe were coasting, or if expansion were
slowing under the influence of gravity.
“But SN 1997ff is so far away, and thus so old, that it brings us
information from an era when stars and galaxies were closer together
and expansion was still slowing due to gravity,” Nugent says. “Now
the universe is accelerating, but that didn’t begin until the
universe was more than half its present age.”
Thus SN 1997ff supports the model of a universe consisting of about
one third matter and ordinary energy and about two thirds “dark
energy,” which acts to overcome gravity. SN 1997ff argues against
alternative explanations of the observed relationship between
brightness and redshift of Type Ias.
Most important, says Nugent, SN 1997ff proves that while the most
distant supernova currently cannot be seen from ground telescopes, they
can be observed from space — and they can provide vital information
about the most basic cosmological questions, including, perhaps, the
nature of the dark energy itself.
“The results from SN 1997ff are one of the best arguments for the SNAP
satellite,” Nugent says. SNAP — for SuperNova/Acceleration Probe —
has been proposed to address just these kinds of questions. SNAP would
fly a 2-meter telescope and employ a CCD camera far larger and more
sensitive than any previous astronomical imager, especially in the
near infrared.
Adam G. Riess, Peter E. Nugent, and 12 of their colleagues, including
representatives of both the High-Z Supernova Search Team and the
Supernova Cosmology Project, are the authors of “A glimpse of the
epoch of deceleration from the highest redshift supernova observed,”
which will soon appear in the Astrophysical Journal.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California.
Additional information:
* SNAP satellite proposal
* NERSC
[NOTE: Images supporting this release are available at
http://oposite.stsci.edu/pubinfo/PR/2001/09/pr-photos.html]
