In a nutshell, my specialty is black holes. A black hole is, to put it simply, a single point in space where a certain amount of mass is sitting. If you think about it this already doesn't make any sense: even subatomic particles have a certain physical dimension, and it's completely nonsensical to think that a ton of them can all be at in the same exact point in space...indeed, we have no idea of what actually happens inside a black hole, but we do know that General Relativity, the most accurate theory of gravity we have been able to write, predicts both their existance and how they affect their surroundings; every time we've been able to test its predictions, they have been spot on. For a theory that is over 100 years old, that is really impressive.
The actual equation required to study a black hole is called Einstein's field equation, and it is the basic equation of all of General Relativity. It also looks really fucking scary:
Honestly i haven't had to work with it in about two years and i don't really remember how to solve it off the top of my head, i just posted it to brag. Fortunately, it is possible to grasp what a black hole does even using simple classical physics. Imagine a particle of mass m travelling at the speed of light (the highest possible speed achievable in nature). It's kinetic energy will be 1/2 m c^2. Now imagine it is moving inside the gravitational field of another mass M; its binding gravitational energy will be GmM/R We can equal the two, to find a specific radius at which the particle requries a velocity of exactly c to escape the bigger mass:
1/2 m c^2 = GmM/R, which gives us
R = 2 GM/c^2.
This radius is called the Schwarzschild radius, or event horizon. What it means is, if you have a point-like mass M and you pass at a radius closer than the Schwarzschild radius, even light can not escape, as the velocity required to leave is higher than c, which is the highest possible velocity anything in nature can achieve. The higher the mass M, the larger the radius. For an object with the same mass as the Sun, this radius is 3 km, which is considerably less than its actual size. In other words the Sun doesn't have a true event horizon, because it's not a point-like mass, but it is extended over a radius of about 700000 km. However, suppose you could somehow, someway compress the entire Sun within 3 km; in that case, it would have a Schwarzschild radius, turning it into a black hole. Black holes are ridicolously dense objects; they need to contain the entire mass of a star in a really, really small volume.
Two kinds of black holes
We know of two kinds of black holes in nature: stellar mass black holes, and supermassive black holes.
Stellar mass black holes as the name implies are about as heavy as a star, and they are born when a particular kind of star ends its life. The way a star functions is fairly simple in principle: they are big spheres of self-gravitating gas. In order for the star to be stable, some amount of internal pressure is required to keep the gas from falling on itself due to gravity. In a typical star, this is caused by the nuclear reactions happening in its core: the energy they release "pushes" outwards, balancing gravity. When a star isn't capable of nuclear reactions anymore however nothing opposes gravity, and the gas can collapse. In some situations, there is a quantum mechanical effect called degenarcy pressure that eventually stops the collapse and stabilizes the star, but sometimes even that isn't enough, and most of the gas in the star keeps falling towards the core. When this happens, a black hole is born.
We observe stellar mass black holes in a particular kind of system, called X-ray binaries. A good portion of stars aren't born alone, but they have a twin companion. If one of the two stars in the binary system becomes a stellar mass black hole, it can start devouring the outer layers of its companion because of its gravitational pull. As the gas falls onto the black hole (the technical term is, the gas is accreted by the black hole) it is heated to extreme temperatures, and ends up emitting very brightly in X-rays which some satellites like XMM-Newton, Chandra, NuStar and Swift can observe.
The other kind of black holes, which are the ones i work on, are called supermassive black holes. Their mass is between a few million and tens of billions times the mass of the Sun, which is way, way more than any single star could ever achieve. We aren't really sure where they come from, but we do know that they can't come from a stellar mass black hole that has accreted enough gas to grow to extreme masses. This is because a black hole can only eat so much mass at a time; there is a limit to accretion, called the Eddington limit. As gas is accreted, it needs to dissipate its energy in some way, otherwise its orbit will never approach the Schwarzchild radius. This is what originated the emission we observe from black holes. However, light can exert a force on matter, pushing it away. The more matter falls, the more light is emitted, the more matter is pushed away. Therefore, there is a limit to how fat a black hole can get in a given time. Supermassive black holes are observed up to very far distances, which corresponds to very short times after the Big Bang. This gives them a very short time to grow to their extreme masses, too short for their original mass to be close to a star's. Their origin is one of the great mysties of modern astrophyics. Almost every galaxy in the universe, inlcuding ours, hosts exactly one supermassive black hole in its center.
Active Galactic Nuclei
Supermassive black holes do not typically contribute to the light we see coming from a galaxy...they are in the middle of nowhere and light can't escape from them anyway, so why would they? About 1% of the time however something wierd happens: the core of the galaxy appears way brighter than it should be. In this case, we say that the galaxy hosts an active galactic nucleus, or AGN. Like in X-ray binaries, this happens when supermassive black holes have a disk of gas surrounding them; as the gas is accreted, it emits light. AGN accretion disks aren't as hot as those in X-ray binaries, so they emit mainly at optical and ultraviolet frequencies, rather than X-rays. The brightest, baddest AGN are called quasars, and they outshine their entire host galaxy: we only see the emission of the core region, despite the fact that it isn't much bigger than the solar system. We know the emission region is that small, and therefore the emission can only be produced by something as compact as a black hole, because of a simple argument. AGN are very variable sources; their luminosity can change drastically over years, months, days or even minutes in the most extreme cases. Because information can not propagate faster than the speed of light, if we see an object varying over a time t we can roughly estimate its size R: t = R/c. It can not be bigger, otherwise it would take longer to vary any property, such as luminosity.
In about 10% of the AGN, not all the gas falling onto the black hole crosses the event horizon. Some of it is instead launched away from the black hole in two jets of plasma which move at close to the speed of light. These jets are some of the largest structures observed in nature, as they can reach sizes that are tens of times the typical size of a galaxy. AGN jets are some of the brightest objects in the sky at basically every wavelength; they are observed as far away as 12 billion light years away from us.
AGN are tightly connected to the galaxy that hosts them; the two evolve together over time. On the one hand, the galaxy's gas needs to somehow be sent towards the central region in order to trigger accretion, which requires some kind of "traumatic" event happening to the host galaxy. Typically, orbits of stars and gas clouds in a galaxy are very stable, and it is not simple to figure out exactly how so much gas ends up in the central region. As far as we know, there are two kinds of events that can make this happen. The first is some kind of instability appearing in the galaxy itself, without any external trigger. This happens in spiral galaxies like the one in the image below, in the left galaxy: instead of following the galaxy's spiral arms, stars and gas enter peculiar orbits which appear as a bar-shaped structure near the center. Bars are inherently unstable, and they can channel any gas that enters them towards the center, where it will be swallowed by the central black hole. The second kind of trigger is mergers between two galaxies. Mergers can very easily disrupt the orbits of a galaxy's components, and again, the result is that the gas is sent towards the center region. This is how a galaxy can trigger an AGN. Once an AGN is triggered in turn it affects how its host galaxy evolves. Accretion and jets are extremely bright and violent phenomena, much more so than nuclear reactions; part of the energy they emit can be re-absorbed in the host galaxy, blowing away any gas cloud that could otherwise have formed new stars. This process is known as AGN feedback, and without it we couldn't explain the star formation history of the universe: without it, there would be too much star formation in any galaxy, too many young stars which are not observed in reality.
I won't go into detail of the mechanics of AGN and jets, which is what my thesis and future research is about, but if you have any questions ask away!