Active Galactic Nuclei (AGN)

Reading: Chapter 25: Active Galactic Nuclei

Galaxies are clusters of stars, gas, dust, and ???. Their visible light is produced primarily by stars , that is, by hot dense balls of gas. The light from normal galaxies is said to be stellar (thermal) in origin. The emission produced by active galactic nuclei (AGNs) is different in character. It is not produced by stars, it is referred to as nonstellar (nonthermal).

We return to the emission mechanism later.

I. Families of AGNs

a. Seyferts (housed in spiral galaxies)

In 1943, Carl Seyfert discovered normal looking spiral galaxies which had extremely bright nuclei. Seyferts produce most of their energy in small (based on the observed variability in the light emission) nuclei. They emit in the optical like normal galaxies (i.e., with stellar characteristics) but produce most of their energy in the radio and IR (with nonstellar characteristics). In the spectra of Seyferts, one finds very strong emission lines which indicate that material is moving in the cores of the galaxies at speeds of ~ 1,000 km per second. The emission from a Seyfert galaxy is dominated by radiation from the intense nucleus, which can be 10,000 times as bright as the nucleus of the Milky Way and, overall, can be 10 times as bright as the entire Milky Way

b. Radio Galaxies (often times housed in elliptical galaxies)

Two general classes, compact (core-halo galaxies) systems and extended (lobe radio galaxies) systems.

Compact radio galaxies.

In compact (core-halo) galaxies, the luminosity is dominated by the emission from a bright nuclear region (of size a few light years) with weaker emission coming from a larger extended halo (contained within the optical image of the galaxy). As typical of AGNs, there are jets emanating from the nuclear region.

M87

Lobe Radio (extended) galaxies.

In extended galaxies, there is a nuclear region but the emission is dominated by that from large extended radio lobe regions at the ends of jet(s) emanating from the nuclear region. These lobes can be large, sometimes as big millions of light years. The structures can be huge, as large 3 million light years from end-to-end (as large as the Local Group of galaxies)! The emitting lobes sit at the ends of the jets which emanate from the central nuclear region. Connection to central source is necessary because at the rate the lobes radiate they would very quickly burn themselves out (run out of energy) and stop shining. Given the large distances of the lobes from the nucleus, it must take the jets millions of years to reach the lobes.

Cygnus A

Centaurus A

3C219

Hubble Space Telescope (HST) pictures of radio galaxies.

c. Quasi-Stellar Objects (QSOs)

In 1963 Maarten Schmidt first identified Quasars (quasi-stellar radio source). Most Quasars are radio-quiet however (in that they emit much more optical radiation than radio radiation). Quasars are extremely bright in the optical and higher energies and appear star-like on photographic plates (===> their more appropriate name, Quasi-Stellar Object QSO). I will tend to use QSO in this class.

The nature of QSOs was initially mysterious because their spectra defied interpretation. The first QSO, 3C48 was discovered in 1960. The famous QSO 3C273 was discovered in 1962. Maarten Schmidt was the first to recognize and solve the problem,

The emission lines in QSOs were strongly redshifted, much more so than typical galactic sources. Schmidt found that 3C273 was receding from the Milky Way at 16 % the speed of light or 48,000 km per second. 3C48 was moving even faster at 37 % the speed of light. Recall that the Sun moves around the center of our Galaxy at a speed of ~ 220 km per second. This suggested that QSOs were at cosmological distances from us (based on Hubble's Law) and QSOs were among the most energetic objects in the Universe; the QSO 3C273 is 1,000 times brighter in the optical than is the Milky Way.

Most QSOs cannot be associated with normal galaxies, because of their great brightnesses and distances. However, several QSOs are located within normal galaxies prompting one to believe that all QSOs are housed in normal galaxies.

II. Comparison of the Energy Outputs of AGNs and Normal Galaxies

Object            Visible           Radio        Infrared     X-ray

Normal Galaxy        2             0.000001         0.1       0.0001
Radio               2-10           0.01-100         0.1      <0.01
Seyfert of N-type    2            0.001-100        1,000      0.01-10
QSOs (3C273)       1,000           1-1,000        10,000      100-??

The above luminositites are in units of 10¹⁰ L(Sun). Normal galaxies predominantly produce visible light (with a fair amount of IR), while AGNs produce disproportionately large amounts of radio, IR, and x-rays.

III. Continuity of AGN Properties

<-Radio Galaxies->

          <---------Seyferts--------->

                   <----------------------QSOs----------------------->


                    Relative Distance From Milky Way

The above shows that although AGNs have unusual (and odd) properties, they appear to be one class of object which ranges from not so outlandish properties to amazing properties.

Comment--Because of their large luminosities and small sizes, it was initially hoped (by some researchers) that QSOs were not at the distances suggested by their redshifts. If they were actually closer, then their estimated luminosities would decrease and many problems would be eased. However, based on several arguments (e.g., association with galaxies in some cases, gravitational lensing, and reasonable explanations for super-luminal motion) the issue is probably settled.

Comment--There are more AGNs at large distances than in our local neighborhood. Recall that due to the finite speed of light (300,000 km per second), the light from distant objects takes a long time to reach us. This means that the light we receive today from a distant object, left that object many years in the past. That is, we look back in time as we peer to large distances in the Universe.

===> AGNs were more common when the Universe was young than today.

IV. Understanding the Nature of AGNs

To summarize the lecture up until now, the basic features of AGNs with which we must contend are:

• Exceedingly high brightnesses; they are up to 1,000 times brighter than a normal galaxy
• Small sizes; they are much smaller than normal galaxies, less than light-months to light-years in size
• Nonthermal emission; they do not shine like stars

Nonthermal Emission

Much of the radiation from AGNs is thought to be due to a process known as synchrotron emission. Synchrontron emission is produced when an energetic electron moves (spirals) through a region which contains a magnetic field.

AGN Energy Engines

The principal issues are the high brightnesses and small sizes of AGN. We therefore need an exceedingly efficient way to produce energy. A natural energy engine is a black hole.

Black Holes

A black hole is an object whose escape speed is equal to c, the speed of light ===> light cannot escape from the object and the object appears black!

Small Size

They are very small having radii of
radius = 3 (M/M[Sun]) kilometers

(The radius of a nonrotating black hole is named the Schwarzschild radius in honor of the man who first worked out the theory.) For comparison, a normal star like the Sun has a diameter of 1,400,000 kilometers which is over 200,000 times larger! Even a black hole which is as massive as 10⁹ M(Sun) has a diameter of only 6x10⁹ kilometers ~ 40 AUs, the size of our Solar System! Even very massive black holes are small enough to hide in AGNs.

Energy Production

If I stand at the top of the Empire State Building and drop a penny, what happens? Initially, the penny is stationary, but after I release it, the gravitational pull of the Earth accelerates the penny downward. The penny thus gains energy. When it hits the ground, it releases the energy it gained by punching a hole into the current or through some other action. The above is a nice way to gain some useful energy.

A similar engine could work in AGNs. A black hole energy engine is more efficient than nuclear fusion in the following sense.

• When a matter particle meets its anti-matter twin, it annihilates and produces energy. 100 % of the mass goes into energy. We define the efficiency of annihilation as 1.

• In a nuclear fusion reaction, a small amount of the mass of the fusing nuclei is converted into energy. For typical reactions, around 0.7 % of the mass is converted into energy. The efficiency of nuclear fusion is less than 0.01!

• When material falls onto a black hole, the amount of energy it gains and can convert into energy is the same as if 10 % to 100 % of its mass were converted into energy. Accretion onto a black hole is 10 to 100 times as efficient as a nuclear fusion reaction in producing energy.

The rate at which a black hole produces energy then depends upon the rate at which material falls onto it. A supermassive black hole must eat roughly 1 star like the Sun every 10 years to power low energy Seyferts to roughly 10 stars like the Sun every year to power the brightest QSOs.

Black Hole Engines in AGNs

In the cores of AGNs, a massive 10⁸ to 10⁹ M(Sun) may live and the stellar density is very high. Occassionally, a member of this beehive of stars passes too close to the central black hole. Tidal forces can tear these unfortunate stars apart. The debris then slowly spirals into the black hole. As we noted earlier, the black hole must eat 10 or so stars per year. The material gains and releases energy as it spirals onto the black hole. For example,

The tidal force weakens as the black hole eats stars (increases in mass). This occurs because the diameter of the black hole increases as it eats stars. When the mass of the black hole reaches 10⁸ to 10⁹ M(Sun) (or so), the tidal force is not strong enough to rip the passing stars apart. As a result, unless the star actually runs into the black hole (which is not likely), the star will simply pass the black hole by. The weakening of the tidal force stops the flow of material onto the black hole and the AGN shuts off.

V. Comparison to Reality

• Is there evidence for accretion disks in AGNs?

• Is there evidence for supermassive black holes in normal galaxies? Yes, there is ample evidence.