Black Holes

Black holes are the most extreme manifestation of the force of gravity. When enough matter, whether from the remnants left at the end of the life of a massive star, or gas at the center of a galaxy, is compressed into a small enough volume, the force of gravity (the mutual attraction, pulling everything together) becomes so strong that no other force can match it. The matter is pulled together and eventually becomes so compact and dense that its gravitational force becomes so strong that nothing can escape it, not even light. At that point it is said to have formed a black hole, and the point of no return --- the distance around it from which nothing can escape --- is called the event horizon. Scientists at KIPAC are working to understand many aspects of black holes: how they formed, how they grew, the exact processes by which energy is released as material falls into them, and the process by which some black holes are able to launch jets.

Black holes with masses comparable to that of the Sun (or, stellar mass black holes) are scattered through the Milky Way and neighboring galaxies, and are formed when the most massive stars come to the ends of their lives. In addition, supermassive black holes, a million to a billion times more massive than the Sun, sit in the centers of galaxies. This includes Sagittarius A*, which is found at the heart of our own Milky Way galaxy. Black holes are difficult to observe directly, since no light can escape from within the event horizon. Fortunately, when matter falls into a black hole, it becomes superheated and produces an intense source of light before it crosses the horizon. 

Out of Darkness... Light

We can observe stellar mass black holes in our galaxy when they feed on material from a companion star in an X-ray binary. When gas falls into a supermassive black hole at the center of a galaxy, it lights up active galactic nuclei (AGN) or quasars, the most powerful continuous sources of light we can see from the farthest reaches of the Universe. On top of impressive light shows, some black holes are able to launch streams of material into jets. Jets travel close to the speed of light; some can even span great distances, reaching far out of their host galaxies. KIPAC scientists use observations of black holes and their jets taken by X-ray telescopes (Chandra, Swift, XMM-Newton and NuSTAR), the Fermi gamma-ray telescope, and a wide range of optical and radio telescopes. These observations are then coupled with theoretical models and computer simulations to map out and understand the extreme environments around various types of black holes.

The X-rays that are emitted from a corona of energetic particles close to the black hole shine down on the accretion disk of gas spiralling in, allowing KIPAC astronomers to map out the extreme environment just outside the event horizon. In radio galaxies and radio-loud AGN, jets that can tap into the energy of a spinning black hole emanate vast distances out of the host galaxy, carrying vast amounts of kinetic energy into the surrounding gas. At the same time, powerful winds carry energy out into the galaxy. When jets point toward Earth, the AGN is called a blazar. Blazars are strongly variable in all observable bands of the electromagnetic spectrum, and simultaneous observations with instruments sensitive to different wavelengths of radiation are critical for studying them. KIPAC scientists study the gamma-ray emission from blazars to understand jets and their relation to the black holes and the accretion disks, as well as the contributions blazars have made to the evolution of the Universe as a whole.

Monsters Lurking in the Centers of Galaxies

The total energy output from the supermassive black holes in AGN and quasars is comparable to the total energy holding the constituent stars of their galaxies together. This means that the supermassive black holes in the centers of galaxies must have played an important role in the formation and growth of the galaxy, through a process known as AGN feedback. As galaxies grew bigger from the gas falling into them from outside, and the supermassive black holes at their centers grew with them, the energy output from the black holes would have been able to push gas away from the galaxy, slowing its growth, and controlling the formation of stars throughout the galaxy.

KIPAC scientists study how the supermassive black holes in the centers of galaxies interact with their surroundings. By studying not just the processes by which energy is released by black holes, but how AGN evolve over time, and the population of supermassive black holes that are found in different galactic environments, they are working to piece together how black holes grew with their host galaxies and the role they played in governing the growth of structure in our Universe.