Harvard logo Smithsonian logo Harvard-Smithsonian Center for Astrophysics
Publications and Public Affairs

Page 2


HUBBLE PINPOINTS DISTANT SUPERNOVAE TO MEASURE DECELERATION OF UNIVERSE

Peering halfway across the universe to analyze light from exploded stars which died long before our Sun even existed, NASA's Hubble Space Telescope has allowed astronomers to determine that the expansion of the cosmos has not slowed perceptibly since the initial impetus of the Big Bang and, thus, should continue to balloon outward indefinitely.

Reporting their preliminary observations January 8 at the winter meeting of the American Astronomical Society in Washington, a large international team of scientists represented by Peter Garnavich of the CfA, concluded there is insufficient matter in the cosmos to provide the gravity necessary to halt its infinite expansion.

If these early conclusions are supported by additional observations, the lack of any significant deceleration since the initial conditions also means the universe could be as much as 15 billion years old. This would clearly establish the universe as truly older than the oldest stars, thus resolving the potential paradox caused by earlier estimates favoring a younger universe.

These results are based on unprecedented distance measurements to supernovae so far away they allow astronomers to determine if the universe was expanding at a faster rate long ago. The most distant supernova seen is approximately halfway back to the Big Bang, and thus exploded about 7.7 billion years ago. The two others studied each exploded approximately 5 billion years ago, or just before our own Solar System formed.

"We cannot make much of a conclusion from the single farthest supernova we've seen," says Garnavich. "But, when we average it with several others, we find, to a 95 percent level of confidence, that the density of matter is insufficient to halt the expansion of the universe."

"In other words, we'd bet $100 against your $5 that the universe isn't bound by matter--dark matter, bright matter, matter that clusters, or matter that's spread out," says team member Robert Kirshner, also of CfA.

Because supernovae are the brightest optical events in the universe, they are ideal candidates to use as yardsticks for measuring vast cosmic distances. The supernovae studied by Hubble, belonging to a class called Type Ia, are considered reliable distance indicators because there is a direct link between their intrinsic brightness and rate of dimming following their explosions. Although this class of supernovae has been sought since the 1950s, astronomers had to wait for Hubble's sharp vision to identify objects far enough away to provide evidence for the deceleration of the universe.

Because Hubble observations had to be scheduled long in advance, a ground-based search for candidate supernovae was made by the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii. Spectroscopic observations were made at the Keck Observatory, also on Mauna Kea, and at the joint Smithsonian-Arizona MMT, to measure the supernovae's redshifts, which indicate relative distances from Earth.

Hubble was then used for follow-up observations using discretionary time provided by Space Telescope Science Institute (STScI) director, Robert Williams. Hubble made five observations each of the three supernovae initially targeted. The observations were separated by about a week, to allow time for the supernovae to dim so astronomers could plot the required light curves.

The search for the "deceleration parameter"--which is fundamental to estimating the age of the universe and its ultimate fate--has been pursued by cosmologists for nearly a half-century. The researchers caution that their findings are preliminary and that a sample of many more supernovae is needed either to yield a value for the density of matter in space to within ten percent or to refine estimates of the universe's deceleration. They plan to use Hubble's new infrared capabilities to study supernovae at even greater redshifts.

The results of the study were published in the February 1, 1998, edition of The Astrophysical Journal Letters. The full HST team, in addition to Garnavich and Kirshner, includes, P. Challis (CfA), J. Tonry (UHawaii), R.L. Gilliland (STScI), R.C. Smith (UMich), A. Clocchiati (CTIO), A. Diercks (UWash), A.V. Filippenko (UCB), M. Hamuy (UAriz), C.J. Hogan (UWash), B. Leibundgut (ESO), M.M. Phillips (CTIO), D. Reiss (UWash), A.G. Riess (UCB), B.P. Schmidt (MSSSO), J. Spyromilio (ESO), C. Stubbs (UWash), N.B. Suntzeff (CTIO), and L. Wells (UAriz).


THE BLACK HOLE AT THE CENTER OF THE MILKY WAY

High-resolution observations with the Very Long Baseline Array (VLBA) radio telescope show that the unusual radio source called Sagittarius A* (Sgr A*) seems to be a massive black hole anchored at the very center of our Milky Way galaxy.

These results, in conjunction with other work, were presented January 7 at the American Astronomical Society meeting in Washington, DC, by Mark Reid of the CfA on behalf of an international team of astronomers, including Anthony Readhead of Caltech, Rene Vermeulen from the Netherlands, and Robert Treuhaft of the Jet Propulsion Laboratory.

Because of its similarity to the active nuclei of other galaxies, astronomers have long suspected that Sgr A*, an extremely bright, point-like source of radio emission, could be a massive black hole. However, the total power emitted by Sgr A* is comparatively low, less than that emitted by some rare interacting stars. Thus, based on the strength of its emissions, Sgr A* does not have to be a very massive object.

Recently, the motions of stars very close to Sgr A* were measured by a group led by Andreas Eckart and Reinhard Genzel of Germany's Max-Planck-Institut fur Extraterrestrische Physik. They found extremely fast motions, some exceeding 1000 km/second, which would require a total mass nearly three million times that of the Sun centered at the position of Sgr A* and within a region of space only about 100 times larger than our Solar System.

What is the nature of this extraordinary mass concentration? Is Sgr A* a gigantic black hole, or simply an unusual group of stars? One way to determine this is to measure the motion of Sgr A* itself. If it is a massive black hole, it should stay anchored to the center of the Milky Way. On the other hand, if it is a single star (or small group of stars), then, like other stars in its vicinity, Sgr A* should be moving very rapidly.

The results presented at the AAS meeting include images of Sgr A* with the Very Long Baseline Array (VLBA), the National Science Foundation-supported array of radio telescopes which spans the USA from Hawaii to the U.S. Virgin Islands. These observations provided enough resolution to see Sgr A* move by many diameters in one year. After tracking Sgr A* for two years, Reid and his collaborators found that most of its apparent motion could be attributed to the Sun's orbit about the center of the Milky Way. (Although it takes over 200 million years for the Sun to completely circle the Milky Way, the effects of its orbital motion can be detected in only 10 days by VLBA observations!)

After correcting for solar effects, the remaining motion of Sgr A* is less than 20 km/sec, even slower than the speed at which the Earth orbits the Sun. This result confirms similar measurements made with less intrinsic accuracy, but over a longer time period, using the Very Large Array by Don Backer of UC Berkeley and Dick Sramek of the National Radio Astronomy Observatory. Such a low speed rules out the option that Sgr A* is any single star, or even a small group of stars.

From the upper limit on the motion of Sgr A*, the astronomers conclude that its mass is certainly larger than a few thousand or, more likely, a few million, times that of the Sun. The results are totally consistent with the theory that Sgr A* is a massive black hole anchoring the center of the Milky Way.


TeV GAMMA RAY ASTRONOMY: NEW LIGHT
FROM THE DARK NIGHT SKY

Trevor C. Weekes of the Fred Lawrence Whipple Observatory, and leader of the international Whipple Observatory Gamma-Ray Collaborative, was recipient of the American Astronomical Society's Bruno Rossi Prize for High Energy Astrophysics, recognizing his key role in the development of very-high-energy gamma-ray astronomy. Weekes presented the Rossi Prize lecture the morning of January 8, in the Ballroom of the Washington Hilton. A summary of that talk follows:

Some of the most exciting results in astrophysics in the last few decades have come from that broad area of research classified as "high-energy astrophysics." Since this covers the study of X rays and gamma rays (both of which are strongly absorbed by the Earth's atmosphere), it is generally assumed that high-energy astrophysics can ONLY be pursued from space. It is surprising, therefore, that one of the newest disciplines of high-energy astrophysics, involving the study of very-high-energy gamma rays, should be possible using a ground-based technique.

Developed by a group of scientists at a number of institutions, but centered at the Smithsonian Institution's Fred Lawrence Whipple Observatory at Amado, Arizona, the so-called, "atmospheric Cherenkov imaging technique" uses large optical cameras to detect the effects of the interaction of very-high-energy gamma rays with the Earth's atmosphere and hence determine their origin. The sensitivity of this technique complements high-energy gamma-ray space telescopes, such as the EGRET aboard the Compton Gamma Ray Observatory.

The new technique gives the first glimpse of a violent universe as seen in photons a million times the energy of a photon of light. As expected, these very-high-energy photons come not from ordinary stars or galaxies, but from cosmic sources undergoing explosive emission processes.

The dominant sources of this energy in the galaxy are supernova remnants, the expanding debris that results from the catastrophic disintegration of dying stars. The strongest source and the first detected was the Crab Nebula, which dominates the sky at almost every waveband of photon energy. Although it exploded in 1054 AD, the Crab is still a dynamic object, with the ongoing injection of relativistic particles from a rapidly rotating pulsar at its center. Three other supernova remnants have been detected with similar energies, all in the southern hemisphere.

The most surprising results have come from the study of extragalactic objects known as "Active Galactic Nuclei." These sources are the cores of large galaxies and are believed to contain massive black holes. For reasons unknown, these black holes are associated with large jets of relativistic particles; and, in "blazars" (a sub-class of active galactic nuclei), these jets happen to be directed towards the Solar System. These are the strongest known sources of high-energy gamma rays, and the study of their gamma-ray signals (which are highly variable) is providing a new perspective on the dynamics of the jets. By looking directly down a jet, an observer is effectively looking down the bore of the cosmic cannon and witnessing the same kinds of interactions as seen in the beam of a man-made particle accelerator.

The Whipple Observatory group has also observed the shortest flare ever seen in gamma rays and some of the highest energies ever recorded from a blazar (Markarian 421). Earlier this year, the Whipple Observatory group discovered an extraordinary outburst from another blazar (Markarian 501), which lasted for more than six months. Fluctuations in the flares seen on short time-scales (less than 30 minutes), coupled with the high energies, place severe constraints on the possible emission mechanisms.

Based on the success of the Whipple technique, more than seven groups world-wide have built similar telescopes; and, very-high-energy gamma-ray astronomy has become one of the most active and productive disciplines in high-energy astrophysics. The Whipple Observatory group itself plans to build an array of telescopes in Arizona (the VERITAS Project) which will greatly increase observing sensitivity and, ultimately, offer new insights on the most powerful, most energetic, most violent processes in the Universe.

Previous Page			Next Page

Last modified on