Current Theories of Black Holes

Black Hole Anatomy

• We believe that we have some understanding of the structure near a massive black hole. The main components are the black hole, accretion disk, broad line region, molecular torus, narrow line region, and the jet. The sketch below shows each of these components.
  
  
  
  
• Black Hole: We've talked about this and understand it completely (?).
• Accretion Disk: This is caused by the conservation of angular momentum since we have to bring spinning material down into a very small region (a figure skater on speed - a lot of speed!).
• Molecular Torus: This is in the shape of a doughnut. We believe that we have seen one in NGC4261.
  
  
  
  Here is an artist's conception of what it would look like if we lived in the torus.
• Broad Line Region (BLR): This is the area where gas clouds exist but can barely hold themselves together due to their great speeds and high density. The cloud are moving several thousand km/s.
• Narrow Line Region (NLR): Since they are further away from the BH, these clouds are moving at more moderate speeds of a few hundred km/s.
• Jet: As material is funnelled down the accretion disk, some will go into the BH but most gets flung out along the jets. It is very hard to get material to go directly into the black hole.
  
  
• As we move closer to the black hole, everything gets hotter and more violent, and therefore the radiation becomes more energetic. At the outer edge of the accretion disk we see optical light mainly but the inner edge gives off mainly x-rays. Soon, new observations may allow us to see the x-rays on the inner edge.
  
  
• The situation is similar for stellar mass black holes. The diagram below is what we think the structure is if you had a star orbiting near to a black hole. Winds from the star fall onto the accretion disk at a hot spot.
  
  
  
  
• If you try to feed a black hole with a star or a gas cloud, the material will almost always end up in an accretion disk, no matter what the mass is of the black hole.

Theoretical Models

• There has been a long history in trying to come up with a complete theoretical understanding of the detailed structure near a black hole. The name that most people give for this is the UNIFICATION MODEL, but there really isn't a solid understanding of the physics.
• The unification model is designed to explain all aspects of supermassive black holes. The general class of objects that these black holes are in are called Active Galactic Nuclei (AGN). They come in a variety of configurations: for example, observing an AGN edge-on will look very different than observing it face-on (so the jet is pointing at you). But the unification model is supposed to explain all of this.
• The bottom line is that we really don't understand yet, and it will likely take a few years of x-ray observations to settle the debate.

Theory of the BH/Sigma correlation

• Sigma is a measure of how fast the stars are moving in the galaxy (also called the velocity dispersion).
• The sigma is measured far away from the black hole where it doesn't have any influence. For example, in our galaxy if we magically took the black hole out of the center, our Sun (and us) wouldn't care at all. We are too far away from it and it has too small a mass.
• We have found that the black hole mass is about 0.2% of the galaxy bulge mass - a very small quantity.
• Thus, how does such a small thing in the middle of the galaxy end up correlating so well with the rest of the galaxy. The belief is that they are related during their formation process.
• The observation evidence:
  • a tight correlation
  • pure disk galaxies do not have black holes (we are fairly sure about this), but pure disk galaxies make bulge galaxies which do have black holes, so where did the black holes come from?
  • the correlation is better with sigma than with mass. Sigma is a measure of how much the galaxy contracted.
• Given this evidence, it appears that it is the process of contraction that underlies the correlation - i.e., when material is driven down into a small radius, it appears that it does two things: makes a bulge (hot) system and makes a black hole.
• Thus, it appears that black holes and bulges form at the SAME time. To be more correct I should say that most of the mass growth for the black hole and for the galaxy was governed by the same process.
• It is possible (and required in some theories) that a seed black hole (with a mass with a few hundred solar masses) be present for this to happen. If this is the case, and since we have found a black hole in every galaxy, then one can argue that a seed black hole may be necessary for galaxy formation!
• The detailed physics of what determines the correlation is still very uncertain but most people feel that it is a FEEDBACK mechanism from the black hole on the galaxy. For example, as we pile more and more mass on the black hole, the energetics increase and we end up with stronger jets and winds. These push the infalling material out and we stop the growth of both galaxy and black hole.
• However, it is still a huge puzzle as to how such a small component can have such a significant effect.
• It's not clear when we will understand the dominant process. In a few years we will have measurements of hundreds of black holes, but that will likely only provide limited information. What we need is measures of black holes in young systems or those that are just forming. Thus, we need to look at distant objects, requiring us to shift from using resolved (rotation curves, proper motions, etc.) to unresolved (variability studies for example) observations.

Which came first: black holes, galaxies, or both at the same time?

• Evidence for black holes first:
  • The number of quasars peaked about 10 billion years ago.
  • Most of the light from galaxies peaked about 6-8 billion years ago.
  • Thus, it appears that quasars (which are black holes) came before galaxies.
  • BUT, both measurements are tricky since we can't be sure that we are seeing all of the galaxy light and quasar light.
  • However, it is clear that there were black holes in existence in the early Universe since we see quasars as far out as we can look. In fact, these are some of the most energetic quasars which imply that they have very large black hole masses. So, if galaxies come first then we must then have already formed a very large galaxy which is difficult at these early times.
  
  
  
• Evidence for galaxies first:
  • We see some galaxies that are forming now in merging systems , but we don't see quasars in these systems (see Antennae Galaxy ).
  • Either, not every merger feeds a black hole (which would be difficult to avoid), or galaxies are formed before black holes.
  • Disk galaxies don't have black holes, but bulges do, and we think that some disk galaxies merge to form bulges.
  
  
  
• Evidence for formation at the same time:
  • The tight correlation suggests that it is concentration process that is important, and that is what governs both events.
  • We've never seen a bulge without a black hole, so it is unlikely that galaxies form first. Also, since disks may not have black holes then it is unlikely that black holes came first. So, we are left to conclude that they formed at the same time.

However, we still lots of data to analyse and much theory to explore so the best thing to do is to wait and see.

One of the best ways to study this is to look at galaxies and quasars at very early times. If we can probe to the era when we think either galaxies or black holes are forming, then we would be in a much better position to understand what is happening.

How to make a supermassive black hole

• There are two options: grow by accretion of surrounding material or build up of smaller ones.
• For theorists, it is easier to grow a black hole by accretion of material, either by a collapse of a massive gas cloud (during the galaxy formation) or a funnelling in of material during a merger or bulge growth (we can notice this in a galaxy merger simulations). Here is a sketch .
• The biggest obstacle here is that you need to cram an incredible amount of material down to a very small size ALL AT ONCE. Many theoriest have trouble with this aspect of it.
  
  
• To build up a black hole from smaller ones, you have to solve two problems. First, you still have to make a black hole. Second, you have to get them to come together.
• We know how to make solar mass black holes, but it is very hard to get that many in one spot such that they will come together. As in the case where Jupiter may eject smaller planets, so does a collection of black holes causes the system to evaporate. The individual black holes are ejected from the center (and even from the galaxy!) as they pass close to each other (a slingshot effect).
• The easier thing would be to use processes that forms increasingly larger black holes. For example, have ten 10-solar mass black holes come together to form one 100 solar mass object; then have 10 100-solar mass black holes come together, and so on and so on.
• Not only is this difficult since the timescales have to work out perfectly, but if that is how black holes grow then we should see black holes of all masses (and we don't yet).
• Also, if you grow black holes ONLY from mergering of smaller ones, then that leads to the problem that you never get quasars. Quasars are black holes that are actively feeding - so we know that at some times the black hole must be growing by eating gas, dust and stars. When two black holes merge, no light is emitted, which means that we would never see it. Since we do see quasars, we know that some mass (and maybe even most) is obtained during the accretion phase.
  
  
• Observations show that we have black holes over a million solar masses or under 20 solar masses, but we haven't found any in between yet.... until Chandra measured a 500 solar mass black hole. Here is the image .
• This result suggest that there may be many black holes in this mass range. Previous detections were not possible since they didn't have the spatial resolution in x-rays.
• Chandra and other future x-ray telescopes will provide significant information about the formation of black holes and their structure.
• Globular clusters may contain black holes, and if so, they would be natural targets for where the seed black holes come from. I had a press release about this in 2002 and in 2008.
  
  
• At the end of 2012, a new black hole was found that confused the picture of how black holes are related to the galaxy. Here is a press release about it.