Free Astronomy Magazine July-August 2016

SPACE CHRONICLES ful telescopes on Earth or in space,” Barth said. “ALMA has the revolu- tionary ability to observe disks of cold gas around supermassive black holes at small enough scales that we can clearly distinguish the black hole's influence on the disk's rota- tional speed.” The ALMA observations reveal de- tails of the disk's structure on the order of 16 light-years across. They also measure the disk's rota- tion well within the estimated 80 light-year radius of the black hole's “sphere of influence” – the region where the black hole's gravity is dominant. Near the disk's center, ALMA observed the gas traveling at more than 500 kilometers per second. By comparing these data with sim- ulations, the astronomers calculat- ed that the black hole at the cen- ter of NGC 1332 has a mass 660 mil- lion times greater than our Sun, plus or minus ten percent. This is about 150 times the mass of the black hole at the center of the Milky Way, yet still comparatively modest relative to the largest black holes known to exist, which can be many billions of solar masses. ALMA's close-in observations were essential, the researchers note, to avoid confounding the black hole measurement with the gravita- tional influence of other material – stars, clouds of interstellar gas, and dark matter – that comprises most of the galaxy's overall mass. “This black hole, though individually massive, accounts for less one per- cent of the mass of all the stars in the galaxy,” noted Barth. “Most of a galaxy's mass is in the form of dark matter and stars, and on the scale of an entire galaxy, even a giant black hole is just a tiny speck in the center. The key to detecting the influence of the black hole is to observe orbital motion on such small scales that the black hole's gravitational pull is the dominant force.” This observation is the first demonstration of this capability for ALMA. Astronomers use various techniques to measure the mass of black holes. All of them, however, rely on tracing the motion of ob- jects as close to the black hole as possible. In the Milky Way, power- ful ground-based telescopes using adaptive optics can image individ- ual stars near the galactic center and precisely track their trajecto- ries over time. Though remarkably accurate, this technique is feasible only within our own Galaxy; other galaxies are too distant to distinguish the motion of individual stars. To make similar measurements in other galaxies, astronomers either examine the aggregate motion of stars in a galaxy's central region, or trace the motion of gas disks and mega-masers − natural cosmic radio sources. Previous studies of NGC ALMA measures mass of black hole with extreme precision S upermassive black holes, some weighing millions to billions of times the mass of the Sun, dom- inate the centers of their host gal- axies. To determine the actual mass of a supermassive black hole, astron- omers must measure the strength of its gravitational pull on the stars and clouds of gas that swarm around it. Using the Atacama Large Millime- ter/submillimeter Array (ALMA), a team of astronomers has delved re- markably deep into the heart of a nearby elliptical galaxy to study the motion of a disk of cold interstel- lar gas encircling the supermassive black hole at its center. These ob- servations provide one of the most accurate mass measurements to date for a black hole outside of our Galaxy, helping set the scale for these cosmic behemoths. To obtain this result, Aaron Barth, an astronomer at the University of California, Irvine (UCI) and his team used ALMA to measure the speed of carbon monoxide gas in orbit around the black hole at the center of NGC 1332, a massive elliptical galaxy approximately 73 million light-years from Earth in the direc- tion of the southern constellation Eridanus. “Measuring the mass of a black hole accurately is very chal- lenging, even with the most power- by ALMA Observatory