“The way the iron line is distorted.. . tells us that the mass concentration [re- sponsible for the altered spectra] is so high that it can only exist in the form of a black hole,” he says. Hasinger estimates that by searching for the characteristic ironemission lines, XMM-Newton can de- tect several hundred previously un- known black holes.

The iron spectra also contain informa- tion that enables astronomers to mea- sure the distance to a black hole in a galaxy too faint or dusty for its visible- light spectra to be detected.

Astronomers can use fluctuations in the intensity of the iron emissions to es- timate the mass of the black hole. XMM- Newton only detects such variability in the iron spectra in far brighter, nearby galaxies. Future X-ray observatories are being designed to record fluctuations from more distant galaxies, which hail from a time when the universe was much younger, Hasinger says.

Using this technique to weigh super- massive black holes that existed at dif- ferent times, astronomers could directly determine how rapidly these beasts grow and if they were significantly small- er in the past, says Andrew C. Fabian of the University of Cambridge in England.

Other techniques described last month, while more speculative, may even- tually enable astronomers to measure the masses of black holes going back to the era when galaxies first formed. These methods require the presence of quasars.

In one technique, researchers moni- tor the gas clouds that orbit a super- massive black hole and its quasar. If the quasar varies its brightness, so d o the surrounding clouds. An observer detects the variation in the quasar first, and the time delay indicates the distance between the clouds and the black hole. Combining this information with the speed of the clouds, as- tronomers can determine the black hole’s mass, note Karl Gephardt and John Kormendy of the University of Texas at Austin. Other researchers have recently confirmed this method by weighing black holes of known mass in nearby galaxies.

A second method relies on the obser- vation that the brightness of a gas cloud reflects its distance from the quasar. From this relationship and the velocity of the clouds, several teams of astronomers have begun calculating the mass of near- by black holes. If the techniques can be applied to the most distant quasars, as- tronomers will be able to probe the growth of black holes in the early uni- verse.

“We have entered a golden age in ob- servational studies of black hole ac- cretion and growth,” says Abraham Loeb of the Harvard-Smithsonian Cen- ter for Astrophysics in Cambridge, Mass. “The next decade is likely t o be exciting. ” -R. Cowen

JANUARY 6,2001

Heating, simulations get the drop on drips The behavior of dripping fluids may

seem of more concern to plumbers than to Ph.D.s. Yet studies of how drops elon- gate and break loose help scientists un- derstand phenomena ranging from atomic fission to cell division (SN: 7/30/94, p. 79). Drip research also advances droplet-re- lated applications such as ink-jet printing and depositing DNA on biochips. Now, two new studies are extending scientists’ understanding of drips.

A new drop-formation experiment in Texas demonstrates a way to prevent dripping when a layer of a dense fluid, such as oil, is placed above a lighter flu- id, such as air. The not-yet-published findings demonstrate that a temperature difference can suppress the breakdown of fluid layers into droplets.

Called the Rayleigh-Taylor instability, this heavily studied type of breakdown occurs in such diverse settings as seawa- ter mixing, supernova explosions, and laser-driven nuclear fusion. The new re- sults may also apply to everyday prob- lems such as dripping of industrial coat- ings or wet ceilings.

In the other study, of dripping faucets rather than ceilings, Indiana researchers have for the first time used a computer t o simulate a sequence of hundreds of drips rather than just one drip. From those simulations, they predict that slow- ly opening a faucet to a certain flow rate produces one dripping pattern. Taking the alternate path-of gradually opening the tap wide and then closing it back down to the same flow rate-may yield a different pattern, the simulations show.

In the Texas study, John M. Burgess and his colleagues at the University of Texas in Austin spread a thin layer of vis- cous silicone oil on the bottom of a clear, sapphire disk and then suspended the disk above an uncoated disk. They next cooled the top disk and heated the bot- tom one.

Despite the unstable arrangement of oil over air, no droplets formed as long as a modest temperature difference of i .. 0

i .. 0 C m

C > m n E d

The simplified computer model reveals an effect seen in experiments but never before simulated. The sequence of drips ti-om a faucet alternates between two sizes.

about 15°C was maintained, the scien- tists report in an upcoming issue of PHYSI-

Burgess explains that the liquid sur- face warms wherever gravity pulls it closer to the heat source. Since heat re- duces surface tension, the tendency to form a drop grows. However, cold in- creases surface tension, so thinner, cool- er regions quickly draw liquid away from budding drops and reverse the sagging. “Very small changes in surface tension are able to counteract these very small changes in surface shape,” he says.

By examining both gravity and heat, the Texas study “enables us to under- stand how two important things work t e gether” to affect the Rayleigh-Taylor in- stability, says Leo P. Kadanoff of the University of Chicago. The findings may also prove useful to “paint designers and lubrication specialists . . . interested in maintaining the thicknesses of their films,” he speculates.

In the other drip study, Bala Ambra- vaneswaran and his coworkers at Purdue University in West Lafayette made the leap to multiple-drip simulations by sketching individual drips as a line of dots, with each dot carrying certain properties, such as local radius. Simula- tions of a single, fully rendered drip re-

quire a day to compute. The Pur- due simplification slashed the time to only a few minutes, says study leader Osman A. Basaran. His team reported its findings in the Dec. 18,2000 PRL.

Unlike the surprisingly pathde- pendent patterns produced by the model, periodic variations in drop size and other behaviors that turned up in the simulation had previously been seen in ex- periments. They indicate the sim- ulations’ validity, Basaran says. Compared with experimental data and moreexact computations, the streamlined computer models are

CAL REVIEW LETTERS (PRL).

A liquid sags into the air beneath it, unless heat accurate to within 1 percent, he from below halts drop formation. adds. -l? Weiss

SCIENCE NEWS, VOL. 159 7