So finally, we turn to the study of quasars and other active galactic nuclei.
They represent very interesting phenomena and play important role in modern
cosmology. To define them, these are energetic events
in the course of certain galaxies. And we believe that energy is being
released because of the accretion on black holes that sit there and the loss of the
binding energy for material to get there produces the great release of energy from,
that is observed. There are empirical classification schemes
that describe the properties of active nuclei, and we'll go through these
shortly. And more recently, there have been an
invocation scheme design to explain how various aspects of observe phenomenology
may be really the same thing but observed from different angles.
One interesting about quasars is that they evolved very strongly in time.
As we go deeper in redshift, the numbers increase dramatically between here now and
say redshift of 2 by about a factor of thousand per pummeling volume.
Today, maybe 1 3rd of all galaxies show some sort of non-thermal activity in their
cores due to the creation onto a black hole.
Most of that is fairly luminosity. Maybe one galaxy in 10 million locally has
really a quasar like luminosity, but many have much weaker but still active nuclei.
And as we already hinted at, when we talked about elliptical galaxies, we now
think that most large galaxies that is non-[UNKNOWN] galaxies do harbor a super
massive black hole in their centers, and their formation is closely tied to
formation galaxies themselves. So, here is a cartoon version of what we
think an active nucleus looks like. In the middle, there is a massive black
hole. Surrounding it is an accretion disk of
material that has fallen down the potential well and it settle into this
because it still has some angular momentum even though its lost much energy, from
which it little bit slowly into the black hole.
The energy that's been released comes from binding energy of material that came from
large distances to the nearest proximity of the black hole.
Usually, the black holes seem to be rotating.
And if there is any magnetic field involved, that will create a jet of
material perpendicular to the accretion disk.
The jet comes because the magnetic field gets wound tight and accelerates the
ionized gas and electrons through relativistic speeds, essentially as a
cosmic accelerator. Surrounding the central engine, from which
we see continuum emission, is a region of broad emission line clouds.
Those are clouds of gas that they're moving at speeds of maybe the order of 1
10th of the speed of light because of the depotential well near the black hole and
they have characteristic emission lines. And larger yet is a narrow line emission
region of gas clouds that are being ionized by emssion from the central engine
and move with speeds that are more characteristic of galaxian potential
wells. And finally, we think that there is an
obscuring torus or disk of dust hiding the central engine from the view from the
side. Probably the most important global
characteristic of active nuclei is that they seem to emit energy at all different
energies from radio through gamma rays. And unlike stars that tend to have thermal
emission, usually focused over more or less one octave of frequency space, active
nucleus span a much broader range of frequencies.
Also, they tend to emit a higher energy than most stars.
So, ultraviolet and hard X, and X-rays are really good ways to find quasars since
stars generally do not emit much in these violent regions.
Because of that, the colors of quasars which will be logs of the flux ratio of
different bond passes tend to be very different from those of stars and that
provides a means of finding. This is also very useful when we look at
very high redshifts where the blue light in the rest frame is absorbed by the
intergalactic hydrogen. One characteristic thing about active
nuclei is that they do have emission lines, strong emission lines in their
spectra which comes from ionized gas. The gas is ionized by ultraviolet and
X-ray emission from the central AGN. These objects can reach phenomenal
luminosities up to maybe 10 to the 15th solar luminosities, a galaxy like the
Milky Way might have a luminosity of the order of 10 to the 10th or 10 to the 11th
solar luminosities. Quasars can be thousands of times more
luminous and yet that luminosity comes from, from a region that's smaller than
the solar system. They also show a strong variability of all
different wave lengths and the size of the small region dictates the times scales in
which it's going to happen. And if a regency is, of the order of light
hours across, then emission can vary on scales of hours.
And even with the most modern state-of-the-art techniques, the central
engines are still unresolved. Even for the nearest active galaxies.
So, lot of what we know about these objects is inferred from variety of their
physical properties that we observe that come from larger radii.
And because they tend to be far away, with high redshifts, they do not move very much
in time. Whereas, if you have a long enough
baseline observations, you can see most start in the Milky Way having some proper
motion. So, all of these characteristics have been
used as means to discover active galactic nuclei.
Most notably, it was the, the properties of the broad-band spectral energy
distribution. Here, we have, on a log, log diagram, what
spectra of quasars might look like from radio into the gamma rays.
And the first thing you notice is that they seem to be more or less flat.
There is a roughly equal amount of energy being contributed on broad range
frequencies. Very much unlike stars with black body
emission that seems to usually span about factor of two in wavelength as opposed to
many orders of magnitude that is shown here.
The different components are labeled here. They come from different sources of
emission. Some are thermal from say, hot accretion
disks, some are non-thermal from synchrotron electrons, or from the jet.
We will talk about those in more detail later.
And this is what the typical quasar spectrum looks like there is a strong
broad continuum and superposed on it are strong emission lines.
They're broad emissions lines which are Doppler broadened by the motion of the
clouds, which can be thousands of kilometers per second.
But also, narrow emission lines, which come from clouds moving further away from
the black hole, and mostly is probing the potential of the host galaxy and not the
black hole itself. We will also notice that many of these
lines come from highly ionized species of ions.
The notation of roman numeral after the elements name indicates how many electrons
have been removed, minus 1. So, Carbon I will be at, at, will be
neutral carbon, and Carbon IV is carbon that has lost three electrons.
Magnesium II is magnesium that's lost one electron and so on.
The square brackets indicates so called forbidden or semi-forbidden transitions.
They're called that way because they're never observed in terrestrial lab
conditions but they are observed in space. And that means that they usually require
very low pressure and low density of the gas, and very high temperatures which are
easily obtained in cosmic plasma's. But they're very hard to achieve in
laboratory. So, since the, their discovery in early
1960s, there have been surveys to find quasars in active, active, other active
nuclei in a systematic fashion in order to study them and to use them as cosmological
probes. At first, radiation was used this is how
quasars were discovered. More recently, optical surveys based on
colors are the dominant way of finding quasars.
The one thing to keep in mind that each of these methods has its own power and its
own limitation, and it's own selection effects.
And to really get a complete picture of active galactic nucleus population in the
universe, we have to actually approach this as a very panchromatic problem and
try to complete surveys on a variety of different wavelengths.
X-rays are also a very productive way to find them.
Stars are sometimes X-ray sources, but not nearly as luminous as these active nuclei.
And the great majority of X-ray sources in the sky outside of plane of the Milky Way
are active nuclei at some larger redshifts.
Nowadays, we have sky surveys that span full range of frequencies, from radio
through gamma rays. And combining them together provides means
of separating hese objects which have spectral energy distributiosn very much
unlike stars from stars. Whereas, pictures that just look like
stars but they're something very different.
Today, there are well over 100,000 quasars that have been confirmed by actual
spectroscopy, most of them coming from the Sloan Digital Sky Survey.
But there is over a million of additional highly likely quasars that are being
selected through their colors and haven't just had spectra taken yet.
These come from large modern digital sky surveys.
Early on, surveys were not nearly as large and they might have discovered tens or
hundreds of quasars, and they still serve the purpose.
The first surveys tend to be the tend to be done from Palomar Observatory and they
usually denoted with some acronym like P, something where P stands for Palomar, for
example, PG is for Palomar Green, after the astronomer who did the survey, PC is
for Palomar CCD based survey, PSS for Palomar Sky Survey and so on.
Many other observatories have participated in, in those enterprise, notably the
University of Michigan and, and Case Western Reserve University have done
extensive surveys for emission line objects as well as Byurakan Observatory in
Armenia. There have been many compilations of
quasars discovered in any which way, from radio through gamma rays, through X-rays.
And two catalogs that have been used through literature from Hewitt and
Burbidge and Veron and Veron-Cetty. However today, those heterogeneous
compilations are superseded by much more complete homogeneous, statistically well
understood examples. In addition to that, added partial overlap
there are about 1 million radio sources catalog so far.
Most of those radio sources in the sky are active galactic nuclei, not all some of it
comes from star formation. And those in our galaxy, of course, come
from things like Super Nova and unsourced star forming regions.
But outside the plane of the Milky Way, the great majority of them are from active
nuclei. Radio sources may or may not have other
visible manifestations of central activity, sometimes only radio is
detected. But no obvious signatures invisible or
even accurate. Some of the more famous radius surveys
include those from the Green Bank Observatory in US, from parks in Australia
from the VLA which are called NVSS and FIRST, and many others.
An interesting question to ask is, how many quasars are there?
And so, if you count them per unit area in the sky going down to the limits of
traditional sky series of the order of 20th magnitude, maybe going a little
deeper, 22nd magnitude, there are of the order 100 quasars per square degree.
And if you go to the very limit of observations, which of course we only do
on very small areas, we can infer that there are some tens of millions of quasars
observable in the in the, within our horizon.
Or roughly speaking, there is one 1 uasar for every 1,000 galaxies integrated over
all different directions. I mentioned the Sloan Quasar Survey and
that was component of the larger Sloan Digital Sky Survey.
They imaged sky in five filters that covered all the order of one half of the
sky, maybe a little less. And using ratios of fluxes in these
filters, they can separate quasars from stars.
Quasars don't look like stars that's why they are called quasars.
That stands for quasi-stellar object. But, of course, they look very different
once you take the spectra. This shows couple of the color, color
plots of star-like objects found in Sloan Sky Survey.
And the big locus that you see in the middle are mostly stars, galactic stars.
But all the dots standing away from that locus tend to be quasars that are a
different redshift. So, by putting boundaries in a color space
like that and then following up spectroscopically, one can find numerous
quasars. And that's how most of them are actually
done. Here are some additional plots.
Here, in order to avoid crowding by individual dots the density contours in
color space apply for most of the stars. And then the outlaying stars are plotted
as dots. Quasars that are being conformed
spectroscopically are indicated as colored points, and their color is representative
of the redshift as you can see. Now, colors are always defined as blue
minus red magnitude, and so the lower left corner corresponds to bluer colors the
upper right to the redder colors. You can see that quasars tend to be bluer
than most stars but sometimes they overlap with some kinds of stars.
The other major survey was done by Anglo-Australian telescope as part of
their redshift survey and is now called 2QZ survey.
That one detected couple tens of thousands of new quasars in smaller areas than the
Sloan. But nevertheless, provided another useful
sample for statistical studies. And here is ih, an interesting diagram.
What they have done here is they plotted all quasar spectra as individual rows in
this image, sorted in redshift. And so, the luminous ridges that you see
correspond to the strong emission lines. And as you increase the redshift, they
move to the red, as they should, because of the redshift.
And the new lines come in that are normally ultra-violet and not observable
and divisible, such as Lyman-Alpha or Carbon 4 and so on.
So, it's a nice illustration of how that there's a smooth distribution and how
redshift actually changes the appearance of the spectra.
Next, we will talk about classification of the active galactic nuclei.