Less than 200,000 times the mass of the Sun
Smaller than the Sun
Some black holes are big and bold, making it fairly easy for astronomers to find them. Others, however, are so small and feeble that finding them takes more time, effort, and cleverness.
Astronomers had thought that the core of NGC 4178, a spiral galaxy that is a member of the Virgo Cluster, was black-hole free. Observations of the galaxy's infrared light, however, hinted otherwise. Follow-up work at other wavelengths confirmed the black hole's existence. It is one of the puniest "supermassive" black holes yet detected in the core of a galaxy, though, weighing in at less than 200,000 times the mass of the Sun. (By comparison, the black hole at the center of the Milky Way is about four million solar masses, while the largest black holes yet discovered are more than 10 billion solar masses.)
Astronomers observed NGC 4178 with Spitzer Space Telescope, which sees the universe at infrared wavelengths. The Spitzer observations revealed a bright glow at the galaxy's center. The glow was consistent with observations of supermassive black holes in other galaxies, which is produced as energy from a disk of hot gas encircling the black hole warms clouds of dust in the galaxy's heart.
The astronomers then observed the galaxy with Chandra X-Ray Observatory, a space-based X-ray telescope. They also reviewed older observations of NGC 4178 made by the Very Large Array, a network of radio telescopes in New Mexico. The X-ray and radio observations supported the idea of a supermassive black hole at the center of the galaxy, although a small one.
By combining the infrared, radio, and X-ray observations, the astronomers concluded that the black hole is quite small; only a handful of smaller central black holes have yet been seen. Vast clouds of gas dust surround the black hole. The gas and dust are pouring into the accretion disk around the black hole at a high rate, "feeding" the black hole and producing a lot of energy. Some of this energy has poked holes in the surrounding clouds, allowing some of the accretion disk's X-rays to escape into space. So while the black hole is small, it is growing in a hurry as it feeds off the environment around it.
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This document was last modified: June 24, 2016.
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