Black holes are the powerhouse of galaxies. They are typically millions of times more massive than the Sun, yet occupy no space at all (this is a topic for an entire other post). Black holes exert a tremendously large gravitational pull, since gravity is the strongest for massive, small objects. One common misconception about black holes is that they suck material in. This just isn’t the case. If the Sun were replaced with a black hole with the same mass, the Earth would happily orbit this black hole without being pulled in (of course, the lack of sunlight would be bad for all life on Earth). For the most massive black holes, found in the center of many galaxies, there is nearby gas and dust that orbits the black hole at very close distances. This structure is known as an “accretion disk”, since the matter within the disk eventually falls into the black hole.
In a paper titled, “Ionization structure and Fe K alpha energy for irradiated accretion disks”, X. L. Zhou, Y. H. Zhao and R. Soria explore the structure of these accretion disks. Astronomers can trace the amount of energy in a system by examining the spectrum of an object. If there are a lot of high energy photons zipping around, many elements will exhibit ionized states. The more energy, the higher amount of ionization. At very high energies, such as those found in the X-ray portion of the spectrum, only the elements with a large amount of electrons are still able to be ionized. One of these elements is Iron, which is found all over the Universe, from distant galaxies, to the Sun, to your red blood cells. Zhou and the rest of the authors calculated the amount of ionization you would expect for black holes of a certain mass and distance above the accretion disk midplane. By comparing their predictions to observations, they can infer the mass of the black hole and configuration of the accretion disk.
These type of studies can help astronomers study the interactions between black holes and disks, and how they impact the formation and evolution of galaxies just like our own Milky Way.