ScienceShot: Weighing Distant Black Holes

Give astronomers a few hours, and they can tell you the mass of any black hole within 244 million light-years. That's possible with a new technique that involves measuring the impacts of the light-sucking objects on clouds of molecular gas circling the centers of their home galaxies. In a paper published online today in Nature, researchers report that they have used the method to weigh the supermassive black hole at the heart of NGC 4526, which lies 53 million light-years away. Previously, researchers have estimated black hole masses by modeling their gravitational effects on individual stars or on clouds of electrically charged gases, but the motions of those objects are typically much more random than the motions of clouds containing uncharged gases. In the new work, scientists training their radio telescopes on NGC 4526 specifically measured the motions of gas clouds (depicted in purple, and superimposed on a Hubble Space Telescope image) containing carbon monoxide, a gas commonly found in such clouds. The motions of those clouds, as observed at a particular wavelength of microwave radiation, indicate that the black hole that resides there is about 450 million times the mass of our sun. Using the new technique with radio-telescope arrays poised to come on line soon, scientists should—working at the same resolution and sensitivity of today's instruments—be able to readily estimate in a matter of hours the masses of black holes inhabiting the centers of galaxies up to 244 million light-years away from ours. That's a volume of space that contains tens of thousands of galaxies, including hundreds that host substantial clouds of molecular gas, the researchers note.

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