We'll now talk about unification models for active galactic nuclei.
They unify a lot of the observed [inaudible] in what's li-, what's somewhat
simpler picture of what's going on. The, the basic idea is that the structure
of the active nucleus is as follows. There is of course the big, black hole in
the middle that provides binding energy. There is an accretion disc of material
that falls in from which a lot of continuum luminosity will come from.
The ultraviolet radiation from the accretion disc will ionize the gas clouds
near the black hole. The broad emmision line clouds which are
close to it and moving fast, and then the narrow emmision line clouds which are
little further away. But somewhere in plain of the accretion
disc or close to it, there should be a torus of dust or maybe a warped disk which
obscures the central engine itself from the equatorial plane, more or less.
And does if you look perpendicular to it, you will see right into the central engine
and you'll see broad line clouds, the continuum source.
But if you look from the side you may only see the narrow emission line region.
Because it's much bigger physically than the other two, and it's only partly hidden
by the dust. This picture explains a remarkable number
of observations. And in this regard, if you'll look closer
to the axis of the system, you'll see broad lines in continuum.
So those would be Seyfer type on our traditional quasars or broad emission line
galaxies. If you look more from the torus side,
you'll see the Seyfert 2's, or narrow emission line objects, or narrow emission
line radio galaxies. So at some level, this is almost certainly
correct, with a, in a broad brush sense. There could be, however, some genuine
differences, even within a given orientation.
And that's subject of some research today, but big picture seems to be pretty much in
place. So let's look at this in a little more
detail. The, probably the most secure unification
is for the Seyfert galaxies themselves. Recall that Seyfert ones are those where
we see the bright continuum and broad emission line.
Seyfert twos you only see the narrow emission lines.
So look at the picture like this. If you look from the side.
You will only see the narrow emission line clouds.
You will, the broad line region will be hidden.
The contiuum will be hidden. But the light from the continuum and broad
emission lines will be scattered from free electrons or dust clouds.
Above or below the accretion disk and some of that would be scattered in the
direction of the torus. That is, if we're looking from the side
we'll still see some of that scattered light and so that opens up a severity of
detecting the hidden emission from reflected radiation.
But how can we tell? The key feature here is that, that
reflected radiation would be polarized. And if you take spectrum of a Seyfert 2
galaxy but using polarization filter, you will see that in polarized light broad
emission lines make an appearance. So that's the reflected broad emission
line light from electrons and or dust. This directly pretty much shows that the
unification picture is. Largely correct, at least for Seyfert
galaxies. But by extension you also apply to more
luminous active nuclei like quasars. It took little while to discover type two
quasars but we do have them now. Here is the same case of nearby Seyfert
galaxy and you see 1068, the picture on the right shows polarization vectors
superposed on Hubble Space Telescope image.
One could take images through polarizing filters and by rotating them find the
orientation polarization in their length is the relative fraction of polarized
light compared to the total. And you can see that there is very nice
systematic behavior of like polarized orthagonal to what likes the principle
axis of the system. In addition to polarization we should be
able to see dual cones of ionized gas. The gas along the torus plane will be
shielded from ionizing radiation. Accretion disk.
But the gas in the opening angles will see it, and it will become more ionized.
And indeed Hubble Space Telescope images of some of the nearby Seyfert galaxies
show ionization cones of ionized gas, just like this picture predicts.
Here is a more detailed portrait of the Seyfert galaxy, NGCN68, which shows the
ionization cones as well as the radio observations, so there is a radio jet that
goes in the same direction as the opening angle of the obscuring torus.
And so in the simplest sense the AGN unification scheme works as follows.
The obscured, active nuclei, Seyfert 2's or type 2 quasars or narrow lined radio
galaxies, in the un-obscured one of Seyfert 1's, regular quasars, or but are
not that radio loud. And blazars are a special type where you
look straight down the radio jet and that creates lot of the observed phenomena.
There are, however, some very low luminosity active nuclei where there is no
obscuration and yet there is no broad emission line region.
So there, more complex things are going on, and there are theoretical models that
explain those. One big question is, why are some of them
radio loud and others are not? And the explanation almost certainly has
to do with the spin of the black hole, and the basic picture is that the rotating
ones have preferred axis which can serve as axis along which magnetic field is
wound, from which radio jet can emerge. So the radio louds would have relatively
low spin parameters, defined here in the lower right corner.
We'll talk a little more about this in the next module.
And the radio quiet ones have a spin that's much less than a unit is required
for a, for a highly rotating black hole. This does explain a lot observed
phenomenon. But then the question then is also how did
black holes actual acquire angular momentum.
And that's not as simple as it seems because the material that gets acreted
that causes black holes to grow Has to be at almost the radial orbits.
Otherwise it will simply miss the black hole, be in some elliptical orbit and
never get in. So most of the material has to fall right
on a radial orbit because rip black hole is such a small target and therefore it
will have no angular momentum to speak of so.
Black holes cannot acquire much if any angular momentum through accretion.
So, then how did they get it? Well, one possibility is mergers of black
holes. And we think that, that does happen
through hierarchical structure formation. And then two black holes would orbit each
other, eventually they would merge because of the loss of energy through gravitaional
waves. But their orbital angular momentum will
then be absorbed by the resulting black hole, and that one will be spinning in the
same direction of the two were going around each other.
Next we will address the question of how does the, energy really gets generated and
how does it get out?