Harvard-Smithsonian Center for Astrophysics|
The CfA Almanac Vol. XIII No. 1, March 2000
SUMMIT MEETING This stunning image of the panoply of telescopes atop the ancient volcano Mauna Kea on the island of Hawaii shows the first two antennas in the Submillimeter Array (SMA) now under construction. (Antennas are at lower left, with the SMA control building directly to their right; and, continuing right, are the James Clerk Maxwell Telescope and the Caltech Submillimeter Telescope. Immediately above the SMA: The new Japanese Subaru Telescope and the twin Keck Telescopes.) The SMA project reached an important milestone in September 1999: obtaining "fringes" from several celestial sources by the two antennas. By pointing, tracking, and observing 230 Ghz radiation from Mars, Venus, Saturn, and Jupiter on September 29, the SMA team achieved the submillimeter equivalent of "first light" and took a critical step toward the final success of the project. Upon completion, the SMA will consist of eight, 6-meter-diameter, movable antennas capable of being positioned by a wheeled transporter to create an interferometer of various configurations. The SMA will be used to study a host of astronomical objects and phenomena, emitting in the submillimeter range, the band of radiation between radio and infrared waves. Using advances in antenna fabrication, detector design, and computer support, the SMA will be able to probe the murky clouds of the Milky Way where stars are born, peer into the hearts of exploding galaxies, study cool faint objects of our own Solar System, and explore other questions in astronomy. The SMA is a collaborative project between SAO and the Institute of Astronomy and Astrophysics of the Academia Sinica of Taiwan. (Photo by Antony Schinckel)
An international team of radio astronomers, including scientists from the CfA, used the National Science Foundation's Very Long Baseline Array (VLBA) to make the most accurate measurement of the distance to a faraway galaxy. Their direct measurement calls into question the precision of distance determinations made by other techniques, including those made earlier by a team using the Hubble Space Telescope.
The radio astronomers measured a distance of 23.5 million light-years to the galaxy NGC 4258 in Ursa Major. "Ours is a direct measurement, using geometry, and is independent of all other methods of determining cosmic distances," said Jim Herrnstein, a former graduate student at the CfA who had joined the National Radio Astronomy Observatory (NRAO) in Socorro, NM. The team says their measurement is accurate to within less than a million light-years, or four percent. The galaxy is also known as Messier 106 and is visible with amateur telescopes.
Herrnstein, along with James Moran and Lincoln Greenhill of CfA, Phillip Diamond of England's Jodrell Bank facility, Makato Inoue and Naomasa Nakai of Japan's Nobeyama Radio Observatory, Mikato Miyoshi of Japan's National Astronomical Observatory, Christian Henkel of Germany's Max Planck Institute for Radio Astronomy, and Adam Riess, another former CfAer now at the Space Telescope Science Institute, announced their findings at an American Astronomical Society meeting last year.
The astronomers used the VLBA to measure directly the motion of gas orbiting what is generally agreed to be a supermassive black hole at the heart of NGC 4258. The orbiting gas forms a warped disk, nearly two light-years in diameter, surrounding the black hole. The gas in the disk includes water vapor, which, in parts of the disk, acts as a natural amplifier of microwave radio emission. The regions that amplify radio emission are called masers, and work in a manner similar to the way a laser amplifies light emission.
Determining the distance to NGC 4258 required measuring motions of extremely small shifts in position of these masers as they rotate around the black hole. This is equivalent to measuring an angle one ten-thousandth the width of a human hair held at arm's length. "The VLBA is the only instrument in the world that could do this," says the CfA's Jim Moran.
"This work is the culmination of a 20-year effort at the Harvard-Smithsonian Center for Astrophysics to measure distances to cosmic masers," according to Irwin Shapiro, CfA director. Collection of the data for the NGC 4258 project was begun in 1994 and was part of Herrnstein's Ph.D dissertation at Harvard University.
Previous observations with the VLBA allowed the scientists to measure the speed at which the gas is orbiting the black hole, some 39 million times more massive than the Sun. They did this by observing the amount of change in the wavelength of the radio waves caused by the Doppler effect. The gas is orbiting at a speed of more than two million miles per hour.
The orbiting disk of gas is almost edge-on as viewed from Earth. The astronomers obtained the orbital speeds and the positions of the masers in the disk by measuring the Doppler shift of the masers at the disk's sides, where the gas is moving almost directly away from the Earth on one side and toward the Earth on the other. Measurements of the different orbital speeds at different distances from the black hole, made in 1994, allowed them to determine the mass of the black hole. These measurements required the great resolving power, or ability to see fine detail, of the VLBA.
The newest observations were focused on maser "spots" on the near edge of the disk, where orbital motion shifts their position in the sky, though by an extremely small amount. The VLBA, however, was able to detect this extremely small movement, called "proper motion" by astronomers. This motion was detected by observing the galaxy at 4- to 8-month intervals over more than three years.
"By knowing the speed at which the gas is orbiting and then measuring its motion across the sky, we can use plain old trigonometry to calculate the distance," says Lincoln Greenhill. He added, however, that "you need a bit of luck to be able to do this. So far, we know of only 22 galaxies with water masers in their nuclear regions that also are relatively nearby. Then, the geometry of the disk, relative to Earth, has to be right to allow us to make such a measurement."
The VLBA measurement of NGC 4258's distance differs significantly from the distance to that galaxy determined through HST observations of Cepheid variable stars. Using such stars, a team of astronomers made preliminary estimates of the distance to NGC 4258 as either 27 or 29 million light-years, depending on assumptions about the characteristics of this type of star in that galaxy. Other Cepheid-based galaxy distances were used to calculate the expansion rate of the universe, called the Hubble Constant.
"This difference could mean that there may be more uncertainty in Cepheid-determined distances than people have realized," says Moran. "Providing this directly-determined distance to one galaxy--a distance that can serve as a milestone--should be helpful in determining distances to other galaxies, and thus the Hubble Constant and the size and age of the universe."
The VLBA is a system of ten radio-telescope antennas, each 25 meters (82 feet) in diameter, stretching some 5,000 miles from Mauna Kea in Hawaii to St. Croix in the U.S. Virgin Islands.