April 2, 2001


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BERKELEY, CA ó  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 oldest and most distant supernova brings us information from an era when stars and galaxies were closer together and expansion was still slowing due to gravity.  Now the universe is accelerating, but that didnít begin until the universe was more than half its present age.

"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 didnít 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.

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