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.
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.
Theory of the BH/Sigma correlation
- 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.
Which came first: black holes, galaxies, or both at the same time?
- 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
- 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.
However, we still lots of data to analyse and much theory to explore so the
best thing to do is to wait and see.
- 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.
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
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
- 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
- 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.