At the centers of galaxies, supermassive black holes, millions to billions times the mass of our Sun are normally found. The black holes are often hidden by a thick doughnut shaped ring of dust and gas known as a torus. Observations done to date conclude that these tire-like cloaking rings are created from the native material found near the center of a galaxy.
The black hole at the center of a galaxy named NGC 1068 is observed by the Atacama Large Millimeter/submillimeter Array (ALMA). The data obtained now suggests that the torus of dust and gas is actually formed by material flung out of the black hole’s accretion disk. Discovering this cosmic fountain of cold gas and dust could change our understanding of how black holes affect their host galaxy and possibly the intergalactic medium.
Jack Gallimore, an astronomer at Bucknell University in Lewisburg, Pennsylvania, and lead author on a paper published in Astrophysical Journal Letters, compares the black hole to an engine that is powered by material falling in on it from a compressed disk of dust and gas. Like an engine, a black hole also emits exhaust. Astronomers have now discovered that the exhaust is the likely source of the torus of material around the black hole. Optical telescopes are effectively unable to observe the region around the galaxy’s supermassive black hole due to the torus.
Messier 77, as NGC 1068 is also known, is a striped spiral galaxy about 47 million light years away from Earth. It lies in the direction of the constellation Cetus. The galaxy has an active galactic nucleus at its center. The nucleus consists of a supermassive black hole that is being fed by an accretion disk – a thin, rotating disk of gas and dust. Material spiraling toward the central black hole becomes superheated and glows bright with ultraviolet radiation. The outer reaches of the disk are considerably cooler and radiate more appreciably in infrared light and the millimeter-wavelength light that can be detected by ALMA.
An international team of astronomers used ALMA to look deep into this region and found a scattering of cool clouds, consisting of carbon monoxide, rising from the outer portion of the accretion disk. These clouds are moderately ionized by the energy from the hot inner disk and this enables them to stick to powerful magnetic field lines that wind around the disk.
The carbon monoxide clouds are accelerated centrifugally along the magnetic field lines, much like water being flung out of a rapidly rotating garden sprinkler. They reach extremely high speeds – roughly 400 to 800 kilometers per second (nearly 2 million miles per hour). As this is much faster than the rotational speed of the outer accretion disk (nearly three times as fast), the clouds are flung further out into the galaxy.
Due to their speed, the clouds reach escape velocity. They are then thrown out from both sides of the disk in a cone-like spray. Gallimore notes that by using ALMA, the team has seen for the first time that the gas that is thrown out hides the black hole, rather than the gas falling in. He concludes that these results indicate that the general theory of an active black hole is oversimplified.