Most galaxies have black holes in their cores. Quite often, those huge black holes are pretty quiet, like the black hole in the core of our own Milky Way galaxy. But sometimes, huge amounts of gas and dust are drawn inward. This material becomes very hot and starts to glow, before it finally disappears into the black hole. Thus, these supermassive black holes become visible to telescopes here on Earth. The more gas and dust they gobble up, the brighter the galaxy’s core is shining.
Japanese astronomers have now discovered where this material comes from. They measured the amount of cold, dense clouds of molecular gas in the central regions of galaxies that have feasting black holes in their cores. Molecular gas doesn’t emit any visible light, so optical telescopes cannot see it. Instead, the Japanese astronomers studied existing observations from a number of millimeter-wave radio telescopes, including ALMA. These instruments can observe radiation from hydrogen cyanide – molecules that consist of one hydrogen atom, one carbon atom and one nitrogen atom.
Hydrogen cyanide can only exist in thick clouds of cold molecular gas. So by measuring the amount of hydrogen cyanide, the astronomers could determine how much dense molecular gas there is close to the galaxy’s core. They found that this is related to how gluttonous the black hole is. If there’s a lot of dense cold gas, the black hole is devouring a lot of material, like a cosmic Cookie Monster. If there’s less dense cold gas, the black hole is eating at a much slower pace. This suggests that a disk of dense molecular gas is feeding the black hole.
Clouds of molecular gas are also the birth places of new stars. So the astronomers expect that new stars are being born in the dense disks. When these stars reach the end of their brief lives, they explode as a supernova. The explosion causes a lot of turbulence in the surrounding gas. This may temporarily cause more material to fall into the black hole. So if the eating frenzy of the black hole varies over time, that may be the result of supernova explosions.
For this study, the astronomers used existing observations of a number of so-called Seyfert galaxies. Seyfert galaxies (named after American astronomer Carl Seyfert) are galaxies with an exceptional bright core. These ‘active galaxies’ are powered by very massive black holes that gobble up matter from their surroundings. The brightness of a Seyfert galaxy’s core is a good measure of the amount of gas that’s falling into the central black hole. Most of the galaxies are at distances of tens of millions of light-years.
The observations of the Seyfert galaxies were analyzed by Takuma Izumi, Nozomu Kawakatu and Kotaro Kohno – astronomers at the University of Tokyo and at Kure College in Hiroshima. They looked at existing observations that were carried out by ALMA and by two other arrays of radio dishes: the Plateau de Bure Interferometer in southern France and the Nobeyama Millimeter Array in Japan. The new results have been published in The Astrophysical Journal.