Credit: NASA/JPL-Caltech
Digging for Holes
Supermassive black holes, with masses of millions to billions of times the mass of our Sun, are thought to reside at the centers of most, if not all, galaxies. These supermassive black holes can play an important role in the evolution of their host galaxies through accretion of nearby material which can produce an outpouring of radiation and high-energy particles. Why supermassive black holes form at the centers of galaxies, and how they grow, are questions at the forefront of modern astrophysics. Central supermassive black holes are commonly surrounded by a donut-shaped torus of gas and dust. In some cases, the torus is viewed face-on from earth, which means that the observer on earth can get a good view of the central black hole, and the accretion disk which surrounds it. In others, the torus is viewed edge-on, which obscures the central black hole from direct view from earth. Understanding the overall importance of these central black holes to the fate of their host galaxies, and the Universe, requires that we determine the characteristics of the unobscured central black holes in addition to the unobscured ones. Finding them can be difficult. The image above illustrates how an obscured central black hole might appear in different wavelengths of the electromagnetic spectrum. Upper left shows an infrared image, where the IR emission comes from hot dust in the torus heated by the radiation from the central black hole's accretion disk. Upper right shows a visible-band image, where most of the visible light is blocked by the torus. Lower left shows a low-energy X-ray band image where the low-energy X-rays are also blocked by the torus. The most direct way to study obscured central black holes is in the high-energy X-ray band, since high-energy X-rays can penetrate the enormous depths of gas and dust in the torus.
Using archival data from NASA's Infrared Astronomical Satellite (IRAS), astronomers identified hundreds of galaxies showing evidence of excess infrared emission from hot dust. They then used NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), a hard X-ray imaging space observatory, to confirm the presence of an obscured supermassive black hole in the center of the galaxy by its tell-tale hard X-ray emission. This study showed that about 35% of supermassive black holes are effectively hidden from us by the gas and dust in the torus that surrounds them.
Published: April 14, 2025
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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 14-Apr-2025 11:37:42 EDT