AGN Unification model

A long time ago, in a galaxy
far far away…

The central super massive black hole had lured a large amount of
gas on to itself to enable the restart of the accretion mechanism, thereby
beginning to emit copious amounts of energy out into the universe,
the likes of which is possible through very few other process and is
thus seldom seen in the universe. In fact, it was so luminous, that it
was observed by a species who call themselves as humans, living on a
small blue planet they call the Earth. The story of how the humans studied
and understood this phenomena is a nice story, worthy of atleast one telling.

The emission from this galactic nuclei, was at first thought to be a nebula (the term which humans use to describe interstellar gas and dust) by the humans, but at this time, they were pretty much limited in the technology they possessed to view and study such phenomena accurately. The person who first observed this event was called Pierre Mechain, who was the one who classified it as a nebula, and passed this information to a fellow human Charles Messier who was making a catalogue of such astronomical sources. This object, thus came to be known as Messier 77, or just M77 (though in more recent catalogs, this object is also referred to as NGC 1068). Messier, being slightly smarter, classified this object as a star cluster, which was also backed up by another renowned astronomer of the day, William Herschel. When humans launched their space telescope, christened Hubble Space Telescope, they took the following image of M77. However, proper attention began to be given to this phenomena in the years

M77

from 1900 onwards in the Earth calendar. This was accentuated when earthlings began to do spectroscopic measurements of astronomical sources, which involves resolving the light coming from these stars into various frequency bands in the electromagnetic spectrum. They observed that this particular source exhibited strong and broad emission lines in its spectrum, much much higher than those exhibited by other sources. This is how they distinguished such sources from other ordinary sources. Thus, based on this criteria, and many others as well, humans found a method to classify such sources into different categories, which we will now investigate. Based on the position of these objects in their galaxies, these objects were collectively coined as Active Galactic Nuclei, or AGNs. (To read about how these supermassive black holes produce energy, read this article written by the same earthling who is publishing this article on our behalf: Accretion disks and accretion mechanism )

The first category they created was based on the band of frequency that is visible to the human eye, i.e. from 400 nm to 700 nm. Many quasars observed in the optical domain had an unusually blue color. This prompted them to look for such objects with a very blue (U – B) color index. These however, showed a pronounced “quietness” in the radio domain in the electromagnetic spectrum. Thus, the label of radio quiet quasars, or quasi-stellar objects (QSOs) fell to these objects. However, it turned out that these radio quiet objects do also emit in the radio domain, which is detected when the instruments used for detection are sensitive enough to detect it. Thus, nowadays, humans use the term QSO to cover both radio-quiet QSOs and quasars. Incidentally, QSOs are the most luminous objects observed in the universe, often outshining the host galaxies by factors as high as a thousand. Thus, QSOs appear as point sources in optical images.The QSOs with lower luminosities than above were spotted and resolved using the Hubble space telescope (HST).

An example of a QSO: QSO 1229 + 204

The next category of AGNs is what is called Seyfert Galaxies. These objects are much less luminous than the QSOs. On optical images, these appear as spiral galaxies with exceedingly bright cores, exhibiting the characteristic strong and broad emission lines. Seyfert galaxies can be distinguished further into two base categories, Seyfert 1 and Seyfert 2. Seyfert 1 galaxies exhibit both broad and narrow emission lines in their spectrum. Seyfert 2 galaxies, on the contrary exhibit only the narrower emission lines, but these “narrow” lines are much broader than those exhibited by normal galaxies. However, there are galaxies lying in between as well, such as for example Seyfert 1.3 or Seyfert 1.8, which exhibit broader emission lines but which are much less prominent than those exhibited by the Seyfert 1 galaxies. The M77 above is an example of a Seyfert galaxy. The illustration of such narrow and broad lined galaxies is given in the figure below:

Seyfert 1 : NGC 1275 Exhibits both narrow and broad emission lines in the optical spectrum of 1275. The lower panel is a zoom in of the upper panel to show some of the finer features.

 

Seyfert 2: NGC 1667
Strong narrow emission lines are observed, along with absorption lines of the host galaxy

The next category devised was of Radio Galaxies. This category consisted of elliptical galaxies with an active nucleus. In an analogous manner to the Seyferts, these are also sub-divided into two categories, namely broad line radio galaxies (BLRG) and narrow line radio galaxies (NLRG) based on the observed width of the emission lines. In principle, the two types of radio galaxies can be thought of as the radio-loud versions of the Seyfert counterparts, but with a different morphology of the host galaxy. Apart from this optical classification of the radio galaxies, they are also classified on the basis of their radio morphologies, i.e. based on the FR1 and FR2 criteria.

On careful observations, humans noticed that some of the QSOs exhibited extreme variation in their optical luminosities. These luminosities can vary violently over a period of just a few days. This led to their labeling as Optically Violent Variables (OVVs). Besides this, these objects showed polarization in the detected optical radiation, usually of a few percent (whereas normal QSOs exhibited below 1%). OVVs are also strong radio emitters, and their radiation in other bands also varies strongly, with the amplitude increasing and time-scale decreasing as one moves to the higher end of the frequency spectrum.

Another peculiar class of AGNs observed exhibited a similar variability as OVVs, but failed to exhibit any strong emission and absorption lines. These objects are called as BL Lacs, in honor of the prototype the humans found in the source BL Lacertae. BL Lacs also exhibit polarization in their radiation, analogous to OVVs. The optical luminosity of BL Lacs can vary by several magnitudes if observed over sufficiently long intervals. Also, humans noted that if observed in the epochs of low luminosity, emission lines are sometimes observed, and thus BL Lacs appear to be OVVs. Hence they have now taken to calling BL Lacs and OVVs together as blazars. All blazars are radio sources. Blazars also show highly energetic and strongly variable . An example of this variability is shown in the following figure; it shows the variability of 3C279 in X-ray and , with photon energies above 100 MeV. Notice that on a timescale of a few days, the luminosity varies by approximately 10.

But, after all this hullabaloo over the different classification schemes, things were going to be massively simplified for the humans. Upon deeper analysis of the phenomena, it was understood that all the different types of the observed AGNs are a symptom of the angle at which these sources are viewed. This idea is clearly illustrated by the following figure:

What the human “physicists” learned that the structures around the central black hole play an important part in what type of AGN is observed. The central super massive black hole was estimated by them to be surrounded by, along with an accretion disk, a dusty torus, as shown in the figure. It also turned out that quite a few AGNs were accompanied by a “jet” which consisted of very highly energetic charged particles, accelerated to velocities near to that of the speed of light. These jets can be “one-sided” which renders the opposite side with a spectrum of subdued luminosity.

Prominent radio jets generated by 3C175

When viewed from this “weaker” side, we get the Seyfert and radio quiet QSOs. In a similar manner, depending on the viewing angle different classes of AGNs are obtained, which is nicely represented in the diagram.

Thus, the efforts of many humans
culminated in their proper understanding
of the phenomena called as AGNs.
And the story goes on as humans
try to understand many such phenomena
surrounding them ………………….

---

Sources:

Extragalactic Astronomy and Cosmology – Schneider

NASA, NRAO

Wikipedia

Accretion discs and the Accretion mechanism

How one of the most efficient energy production mechanism in the Universe works.

 

An accretion disk is a structure formed by the orbital motion of diffuse objects around a central body. The central body can be anything from young stars (called protostars), to neutron stars and also black holes. However, the most efficient energy is produced in the accretion disks surrounding black holes, particularly the super-massive black holes (SMBH) which are resident at the center of almost all the galaxies in the universe. When these SMBHs are accreting matter onto themselves, they release such tremendous amounts of energy that they often outshine their host galaxies. In such cases, these galaxies are called active galaxies, and the SMBHs, together with the accretion disk are called as the active galactic nuclei (AGN). As an example of the energy output in the case of AGNs, a subclass of them, called Quasars, release, typically, energies of the order  . An example of such an AGN is given in the following image taken from the Hubble Space Telescope:

An active galaxy with a visible jet from the AGN

 

So how are such high energies generated in the accretion disks surrounding AGNs ? Lets take a look:

The Principle of Accretion:

The total principle can be stated most compactly as follows:

“The (gravitational) potential energy is converted to rotational kinetic energy as the gas falls closer to the black hole (due to collisions between particles), which is then converted into heat via friction between the particles. This heat is then radiated away as electromagnetic radiation.”

Having said that, lets go into a bit more detail.
For every massive body in the universe, there is associated with it its very own gravitational field. The same is the case for the SMBH at the center of the galaxy. In this gravitational field, the accreting material (which is gas) is arranged into orbits around the black hole in an analogous manner to that of the solar system. The gas is arranged into a disk shape due to the intrinsic angular momentum of the central black hole, combined with collisions between the particles. This disk is perpendicular to the angular momentum vector of the black hole. This is illustrated with the following artist’s impression:

Thus, when the gas falls from outer orbits into the inner orbits, its gravitational potential is converted into kinetic energy. If this inflow of the gas is not stopped, then the gas will just fall into the black hole without radiating anything away.
However, the thing that stops this direct inflow of gas is the fact that the gas surrounding the black hole possess some amount of angular momentum. Thus, in accordance with the conservation of angular momentum, there is the formation of a “barrier” due to the angular momentum of the gas which prevents it from falling directly into the black hole.
Next, the force of friction can be assumed to be much smaller than the gravitational force produced by the black hole. Thus, the disk will rotate locally with Keplerian velocity, i.e.

where

Since a Kepler disk rotates differentially (different angular velocity for different radii R), the gas will be heated by internal friction. In addition, the friction will also cause a slight decrease in the rotational velocity of the particles, causing them to move inwards. The energy source for the heating of the gas is provided by this inward motion, i.e. conversion of potential energy to (rotational) kinetic energy, and then to heat through internal friction.

According to the Virial theorem , half of the potential energy will be released as the kinetic energy, which in this case is the rotational energy. The other half of the energy is available for conversion into heat.

To get a quantitative feel of the phenomena, lets do a bit of a “back of the envelope” type of calculation to see what results we may derive.

Temperature Profile of a geometrically thin, optically thick accretion disk:

When a mass m falls from a radius of to radius r, the change in its energy is:

where

According to Virial theorem, half of this energy is available to be converted into heat, i.e. . Thus, if we assume that this energy is emitted locally, then the luminosity can be given by:

[remember that luminosity is given by units J/s and that is how we get the accretion rate into the above equation]
For the stationary case,  is independent of radius, because otherwise, the matter would accumulate at some radius. Thus, equal amount of matter per unit time flows through any cylindrical radius.
If the disk is optically thick, the local emission corresponds to that of a black body. Thus, we can apply Stefan-Boltzmann law to relate the luminosity to the temperature of the accretion disk as follows:

with

Thus, for a thin ring between r and  , the luminosity can be given by

where the extra factor of 2 comes due to the fact that there are two sides to the ring so formed.
Now, equating the two expressions for the luminosity, we get the temperature profile as a function of the distance from the black hole, i.e. as a function of the radius:

A more detailed analysis also takes into account the effects due to friction, as well as the amount of energy used in heating up the gas itself. On taking into account all these extra considerations, the formula obtained is, apart from a numerical factor, the same as the one obtained above. Thus, the complete formula is:

which is valid for
where

Scaling the above equation with and using the fact that  (for this expression, see this), we get

Lets see what we can understand from this equation of the temperature profile of the accretion disk:

• First of all, the temperature profile of the disk does not depend on the detailed mechanism of the dissipation, because viscosity terms do not appear explicitly in this equation. This allows us to obtain quantitative predictions from the model of the geometrically thin, otically thick accretion disk. (Note that the exact source of the viscosity is still unknown. Molecular viscosity seems too small to fit the bill. The leading candidates are turbulent flows in the disk, magnetic fields produced by the SMBH, and hydrodynamic instabilities)
• The temperature of the disk increases inwards as    as expected. Thus, the emission spectra of the AGN is a combination of a number of different blackbodies consisting of rings with different radii at different temperature. Thus, the spectrum does not have a Planck shape, but a much broader energy distribution.
• For a fixed ratio  , the temperature increases with an increase in the accretion rate . This was again expected because the accretion rate was proportional to the locally emitted energy.
• For a fixed ratio   , the temperature of the accretion disk decreases with an increase in the mass of the black hole M. This is a bit unexpected, but has a feasible explanation to it. As the mass of the black hole increases, the tidal forces at any fixed radius are correspondingly lower, which in turn results in reduced amount of friction and in flow of gas, which in turn reduces the temperature. Most importantly, it implies that the maximum temperature in the accretion disks of AGNs is lower than the corresponding temperatures for neutron stars and stellar mass black holes. This is backed up by observations, in which neutron stars and stellar mass black holes are found to emit in the hard X-ray regions (hence called X-ray binaries), while the AGN thermal radiation peaks out at the UV range.

Thus, we have seen how accretion disks are used to convert the gravitational potential energy of the central body into kinetic, and then ultimately into electromagnetic energy. This analysis of accretion disks and their mechanism involves a few approximations, but is well suited towards describing many instances of this phenomena.

Sources:

Wikipedia
Scholarpedia
Extragalactic Astronomy and Cosmology – Peter Shneider