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Combined image of NGC 1332 shows the central disk of gas surrounding the supermassive black hole at the center of the galaxy. New ALMA observations traced the motion of the disk, providing remarkably precise measurements of the black hole's mass: 660 million times the mass of our Sun. The main image is from the Carnegie-Irvine Galaxy Survey. The box in the upper left is from the Hubble Space Telescope and shows the galaxy's central region in infrared light and the dusty disk appears as a dark silhouette. The ALMA image, upper right box, shows the rotation of the disk, enabling astronomers to calculate its mass. The red region in the ALMA image represents emission that has been redshifted by gas rotating away from us; the blue represents blue-shifted gas rotating toward us. The range of colors represent rotational speeds up to 500 kilometers per second. Credit: A. Barth (UC Irvine), ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble; Carnegie-Irvine Galaxy Survey.
Supermassive black holes, some weighing millions to billions of times the mass of the Sun, dominate the centers of their host galaxies. To determine the actual mass of a supermassive black hole, astronomers must measure the strength of its gravitational pull on the stars and clouds of gas that swarm around it.
Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of astronomers from Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University, University of California Irvine and other universities has delved remarkably deep into the heart of a nearby elliptical galaxy NGC 1332 to study the motion of a disk of cold interstellar gas encircling the supermassive black hole at its center. These observations 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.
The ALMA observations reveal details of the disk's structure on the order of 16 light-years across. They also measure the disk's rotation 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 simulations, the astronomers calculated that the black hole at the center of NGC 1332 has a mass 660 million 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 gravitational influence of other material – stars, clouds of interstellar gas, and dark matter – that comprises most of the galaxy's overall mass.
Astronomers use various techniques to measure the mass of black holes. All of them, however, rely on tracing the motion of objects as close to the black hole as possible. In the Milky Way, powerful ground-based telescopes using adaptive optics can image individual stars near the galactic center and precisely track their trajectories 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 1332 with ground- and space-based telescopes gave wildly different estimates for the mass of this black hole, ranging from 500 million to 1.5 billion times the mass of the Sun. The new ALMA data confirm that the lower estimates are more accurate.
Crucially, the new ALMA observations have higher resolution than any of the past observations. ALMA also detects the emission from the densest, coldest component of the disk, which is in a remarkably orderly circular motion around the black hole.
Many past black hole measurements made with optical telescopes, including the Hubble Space Telescope, focused on emission from the hot, ionized gas orbiting in the central regions of galaxies. Ionized-gas disks tend to be much more turbulent than cold disks, which leads to lower precision when measuring a black hole's mass. As noted by the authors of this study, ALMA can map out the rotation of gas disks in galaxy centers with even sharper resolution than the Hubble Space Telescope. This observation demonstrates a technique that can be applied to many other galaxies to measure the masses of supermassive black holes to remarkable precision.
Aaron Barth et al., 2016, Astrophysical Journal Letters, 822L, 28
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