Astronomers studying a giant black hole have found that it is surrounding by vast clouds of cold dust that nobody expected to see. Observations made with the European Southern Observatory’s (ESO) Very Large Telescope Interferometer (VLTI) in Chile showed that, instead of being arranged in a huge torus (doughnut shape) swirling into the black hole, it fills the areas above and below the torus region, and is being blown outwards.
Supermassive Black Holes (SMBH), containing the mass of millions or even billions of stars, are found at the centre of almost all galaxies. According to theory, when large clouds of gas or dust pass too close to an SMBH, they get drawn in and spun into an accretion disk, spiralling inwards and getting heated to such high temperatures that they emit ultraviolet or even X-ray light. Some of the material gets ejected at extremely high speeds in jets aligned with the poles of the Black Hole, via methods that astrophysicists are still trying to understand. The intense energy of the disk and the jets combine to create some of the most luminous sources of radiation in the universe, and such systems are called Active Galactic Nuclei (AGNs). AGNs are surrounded by toroids of hot dust, and it was believed that this was where most of their infrared energy came from.
The SMBH studied in these observations lies at the core of a galaxy in the constellation Centaurus (The Centaur), which is designated NGC 3783. It is a barred spiral galaxy about 137 million light years away, and is one of 47 members of its local galactic group.
The VLTI observations revealed that, while the hot dust toroid is where theory predicts, and has a temperature between 700 and 1000 degrees Celsius, it is dwarfed by the much larger clouds of cold dust streaming away from the SMBH above and below the toroidal region. It seems likely that the dust is being blown away by radiation pressure from the AGN, but it is not clear how all these processes interact. Current models will have to be updated to include the cold dust streams if we are to understand what’s happening in the cores of these active galaxies.
The dust in these clouds is a mix of silicate and graphite grains. Graphite dust is made from the same stuff as soot from a candle flame, although the grains are about a tenth of the size of those produced by the candle.
The VLTI is formed from a combination of the four 8.2-metre VLT Unit Telescopes, or the four moveable 1.8-metre VLT Auxiliary Telescopes. It makes use of a technique known as interferometry, in which sophisticated instrumentation combines the light from several telescopes into one observation. Although it usually does not produce actual images, this technique dramatically increases the level of detail that can be measured in the resulting observations, comparable to what a space telescope with a diameter of over 100 metres would measure.