Supermassive black hole
Artist impression of the center of galaxy NGC 1068, which docks an actively feeding supermassive black hole. Originating from the black hole's outer accretion disk, ALMA spotted clouds of cold molecular gas and dust. This material is being accelerated by magnetic fields in the disk, reaching speeds of around 400 to 800 km/s . As material gets expelled from the disk and goes on to mask the region around the black hole from optical telescopes on Earth and as such the black hole is cloaking itself behind a veil of its own exhaust. (Image Credit: NRAO/AUI/NSF; D. Berry / Skyworks)
Space

Black Hole Concealed by its Own Exhaust

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.

NGC 1068
ALMA image of the central region of galaxy NGC 1068. The torus of material harboring the supermassive black hole is highlighted in the pullout box. This region, which is approximately 40 light-years across, is the result of material flung out of the black hole’s accretion disk. The colors in this image represent the motion of the gas: blue is material moving toward us, red moving away. The areas in green are low velocity and consistent with rotation around a black hole. The white in the central region means the gas is moving both toward and away at very high speed, the conditions illustrated in the artist impression. The outer ring area is not related to the black hole and is more bound to the structure of the central 1,000 light-years of the host galaxy. Credit: Gallimore et al.; ALMA (ESO/NAOJ/NRAO); B. Saxton (NRAO/AUI/NSF)

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.

This research can be read on ArXiv and in the paper titled “High-velocity bipolar molecular emission from an AGN torus,” and is published in peer-reviewed Astrophysical Journal Letters Journal.