Let us now address the empirical Classification of Active Galactic Nuclei
based under observed properties. In case you're wondering, the background
image here are Darvin's finches. So we can classify active nuclei according
to their information. First of all whether or not they're Radio
loud. This was how first quasars were
discovered. Within the Radio loud category they come
in two varieties, so-called Fanaroff-Riley types one and two which I'll define in a
bit. And many of the Radio quiet ones are not
really Radio silent. They're just not as Radio loud as those
that we call Radio loud. Now according to the optical spectrum we
can divide them by whether or not they show broad emission lines.
They always do show narrow emission lines and those that have broad emission lines
are called type 1 versus those that are called, with narrow emission lines are
called type 2. They all does we talk about Seyfert
galaxies of type 1 or type 2 and also quasars of type 1 and type 2.
Luminosity is an obvious physical property to look at and there we can follow from
the lower luminosity objects such as Seyfert galaxies to the higher ones like
quasars. There are also special subtypes, for
example, the so called Blazars or BL Lac's which are, Radio loud quasars where we're
looking right down the relativistic jet and that causes strong variability.
So these classification schemes are all very emperical and largely parallel and
they do reflect some of the, intrinsic physical characteristics of these sources
but not necessarily always. And, some depend on the orientation from
which we're looking at. Seyferts first they are essentially low
luminosity equivalent of quasars and they've been noted over 100 years ago as
luminous nuclei of some nearby galaxies. And even had spectra that nobody
understood at the time. This was largely neglected until the
1940's where Carl Seyfert, after whom these galaxies are now named, really did
the first systematic study, but even then, it wasn't clear what was going on.
And somehow that was neglected by the time Quasars came on the scene and, even though
quasar spectra look like Seyfert galaxy spectra only more so.
Somehow that connection was not made at first.
Here are a couple pictures of Seyfert galaxy seem based on they tend to be
Spiral galaxies. And they have a very luminous, bright
nucleus which in saturates look like a really bright star smack in the middle of
a spiral galaxy. Nearby maybe 10% of early type spirals
contain these nuclei, a smaller fraction for the later type spirals.
They tend not to have much in terms of radio emission but they can have some.
And they also have moderate x-ray emission just like luminous quasars do.
Quasar themselves out shine their horse galaxies by great factor, maybe by a
factor of 1000. And this is why they're called Quasars.
Quasi stellar objects. Even with the Hubble Space Telescope it's
sometimes difficult to discern their host galaxies because the central source is so
bright. But nevertheless sometimes we can see
that. There are sometimes Spiral galaxy hosts,
but more often they tend to be in Elliptical galaxies.
Also these images often show morphology that's characteristic of galaxy mergers
that we've seen earlier. Tidal bridges, and tails and so on.
And we believe that this is not an accident, that Quasar activity is fueled
by galaxy encounters in a similiar way as the starbursts can be triggered.
So here are the spectra of Seyfert one Galaxies and Quasars are very similar to
this. They have both Narrow and Broad emission
lines. Broad emission lines tend to correspond to
more standard transitions like balmer lines of hydrogen.
Narrow emission lines tend to correspond to ionized gas that is not easily observed
in laboratory conditions, for example doubly ionized oxygen lines, 5007 axtron
are very prominent and almost impossible to do the lab, and you can see that there
are other prominent elements neon carbon, magnesium, and so on.
Type 1 Seyfert Galaxies also have bright continuum we can see right down towards
the Accretion disc. In the Narrow line of type 2 Seyfert
Galaxies the continuum is almost invisible but we still see emission lines.
Now it is interesting to think like you can see emission lines from star forming
regions as well, where young stars ionize the gas.
So how can we tell them apart? And the answers from atomic physics that
you can form line ratios that correspond to low and high ionized states.
For some transitions, you have to ionize gas very heavily, a lot of high-energy
photons. And sometimes stars, even the most
luminating stars don't make photons of that energy.
But yet, active nuclei do. And so in, in diagrams that show these
kinds of Line ratios. You can draw a boundary that separates
those separate, those powered by star formation versus those powered by
Accretion to black holes. And this is how we can tell them apart.
Radio galaxies were the first active nuclei that were really discovered and
from the very beginning of radio astronomy.
But first, he wasn't clear what his radio sources are, but then soon enough in
1950's thanks to Baden, Glovsky and others optical counterparts have been found for
some of the more luminous radio sources. And some of them corresponded to peculiar
looking galaxies like Centaurus A. And the vertical of A which is now Fornax
M87. Sometimes they're just some NGC Elliptical
galaxy and what's shown here in color is the contrast of the Radio emission or
intensity image of Radio emission superimposed on an optical image.
You can see that often times there are two lobes and there is a host galaxy right in
the middle. Now we know that this comes because a
central, the nucleus of the galaxy itself contains a central engine and has
relativistic jet that pumps these lobes. The lobes are powered by synchrotron
emission and from the measurements of their brightness and some simple physics,
it was inferred very early on that they contained phenomenal amounts of energy
something like10 to the 60th or 10 to the 61 ergs in highly ionized plasma.
So the question was, what deposited that energy in the Radio lobes?
What's powering them? And it was understood very early on that
black holes can be a viable mechanism to do that.
Here is a more Modern Radio Map of Cygnus A, one of the classical powerful Radio
sources, and you can see there is a Point like nucleus that does coincide with the
center of the galaxy. There is a very clear Jet on both sides
powering the Lobes. And these instabilities as the gas is
being accelerated out of the active nucleus region encounters.
Say, gas in the cluster that, surrounds this galaxy.
So, many instabilities and so on. So we can study the morphology of Radio
emission and learn more about physics of what goes on.
There is a simple morphological Classification for Radio Sources, and
those are done by two radio astronomers, called Fanaroff and Riley, and type 1
sources tend to have prominent jets, they're brighter in the middle than to the
sides and type 2 are low dominated sources and sometimes you don't see the Jets at
all order central nucleus at all. They do correspond to physically distinct
states FR1 types are probably younger radius sources, they're just beginning to
pump the Radio lobes. On the other hand they can be also older
but they couldn't develop Radio-louds because of their environment.
Bl Lacs or Blazars are a subtype of radio-loud quasars.
Bl Lac is a name for a variable star in fact quasars or other Blasars have been
spotted in 1920s and they were given names of variable stars.
Because nobody knew that they were a new phenomenon of nature.
There's about a dozen of them that have variable star names but BL Lacertae is the
proper type and, so, the whole subclass is now called Blazars which is BL Lac and
quasar. Sometimes you can say look you could have
gone the other way called them clocks but that's not how didn't catch.
While the spectra are dominated by strong blue continuum.
And this is the continuum that's from the accretion disk and the jet itself and,
it's been Doppler-boosted, since the jet is moving at relativistic speed towards us
and completely outshines the rest of the central engine, the broad line region, and
so on. Sometimes the continuum goes down and then
we can suddenly see the usual quazar spectra.
So Blazars are essentially cosmic accelorators and we are in their beam, and
that makes them useful for variety of studies that we'll mention later.
They're also very variable. This is why they're confused with variable
stars. Variability in their case comes from
instabilities in the jet. Shock waves or the jet is encountering gas
around the host galaxy and so on. And they're Doppler boosted, exe-
amplified. Was a relativistic motion of plasma
towards us. Our quasars in general do vary and most of
that variation is due to the changes in the accretion as the fuel is dropping down
onto the black hole. That's a differnt type of variablilty.
In addition to that there's variability that's due to the relativisitc jets.
Some. Some types of quasars have mixed or bolt
usually those we call optically violent variables, changed by more than 10% in
scope of, of a night or less and can change by factors of tens depending on
which frequency. Sometimes those variations are correlated
between different wavelengths, say optical and radio and gamma, and sometimes they're
not. These correlations or absence thereof,
tell us something about physics of what goes on inside these objects.
Now what we'd really like to have is a pure Physical Classification of active
nuclei. And the things that we can think about
would be the black hole mass itself. The Accretion rate which is what gives the
luminosity and the Angular momentum of the black hole.
These things are not directly observable, but they can be inferred from various
observations and there will be a luminosity sequence in this space with
quasars and powerful radio sources being high luminosity end, Seyferts being a low
luminosity end. The high luminosity sources certainly
corresponding to objects with higher Accretion rates and probably larger black
hole masses. And there is also this distinction between
Radio-quiet and Radio-loud, and this is probably where Angular Momentum of the
black hole comes in. But more about that later.
Next we will talk about Unification schemes for active nuclei that put lot of
these phenomenological observations into a fairly well-unified picture.