Active Galactic Nuclei

Key points: What powers an active nucleus; accretion and accretion disks; variability and its significance

All large (massive) galaxies have black holes (of a million to a billion times the mass of the sun) in their cores. The most direct evidence is from spectra. For the galaxy to the right, the spectrum shows a line shifted on the nucleus first far to the blue, then far to the red, due to a large range of Doppler shifts from the huge mass. HST spectrum showing a nuclear black hole
NGC 4261, a galaxy with a large disk of gas in its nucleus  

As we saw on a small scale for the Galactic Center, when matter falls into the accretion disks around these black holes, a lot of energy is created. In some cases, this is happening on a large scale and continuously, creating an "active nucleus."  Its power is derived from the fact that mass falling into a black hole can release a total energy of E = mc2, and although reasonable efficiencies reduce this to maybe 0.1 mc2, it is still far more than can be released in fusion reactions in stars, 0.007 mc2.

Despite their huge energy outputs, 100 times that of the entire Milky Way, many active nuclei have rapidly varying light outputs, changing substantially in a week or a month.

animation of marching band out of synch because of sound  delay The rapid changes are evidence for the sources being very small. For example, if its output varies substantially in one month, the source is likely to be no more than a light month in size! Here, we illustrate the principle by comparison to a marching band. Suppose the band marks time to the drummer to the left in the picture. Sound comes to us directly from her, and at the same speed it makes its way across the field to the band members at the right. They start playing as soon as they hear it, but when their sound reaches us it is "behind" by the time it took to cross the field! (ugh - this is why some bands sound so bad during football halftime!) From the delay, we could deduce that the band is about a half sound second across. (animation by G. Rieke)

Accretion Disks --

Active galaxies were first noticed because they glowed brightly from a tiny source at their centers. These stellar appearing cores had emission lines implying very hot sources were exciting them with strong ultraviolet light.

NGC 5728, showing cones of ionized light where UV escapes

We now know this ultraviolet output comes from the accretion disk. It is the material in orbit around the black hole that gets heated to a very high temperature by its friction within the disk. See how it lights up a cone of glowing gas in the galaxy to the left. Only the cone of ultraviolet light can escape from the cavity in the accretion disk where the black hole lies; in other directions, the light is absorbed by the disk. (From STScI, modified by G. Rieke) The artist's concept below shows how the accretion disk lets out just the cones of ultraviolet light.buttonbook.jpg (10323 bytes)

(from APOD, V. Veckman,

lartist's concept of AGN accretion disk with cones of UV

The spectra of these nuclei show extremely strong and broad emission lines; the broad lines imply that the gas producing them must be moving at a large range of velocities, including very high ones as it feels the strong tug of the black hole.

artist's concept of black hole tearing matter from star Although there isn't full agreement, one idea is that these lines come from stars that are too close to the hot black hole and are having their outer layers stripped away and ionized to create the bright lines, as in this artist's concept From IAC, The broad profiles are then due to the high velocity of stars caught deep in the gravitational potential of the black hole.
animation of flying into an active nucleus The behavior of the active nucleus can take different forms, as we discuss below. However, the general pattern is illustrated in this imaginary space ship journey through an active galaxy. (adapted by G. Rieke, based on Mullard Space Science Laboratory animation) Here is a similar, slicker versionen00500_1.jpg (18578 bytes) (reload to restart lecture animations) from Chandra,

Active nuclei take fantastic forms:

1.) Radio Galaxies

Both ellipticals and spirals can emit at radio wavelengths but the most powerful radio galaxies are ellipticals. They fall into two broad categories:

1) galaxies with radio emission from the nucleus only

2) galaxies with "lobes" or "jets" that extend way beyond the visible galaxy in some cases:

Sketch of radio jets and lobes

picture of radio galaxy Centaurus A Centaurus A, in the southern sky and only about 3.5 million parsecs away, is the closest example of a radio galaxy. Here it is in visible light. (From European Southern Observatory,

The visible (below with the HST instrument WFPC2) reveals little more than extensive clouds of newly formed stars and dust, but in the near infrared (with NICMOS) we see the bright nucleus through all the dust. The larger-scale mid-infrared picture to the right shows the dusty remains of a spiral galaxy that has been swallowed up by the elliptical one that accounts for most of the mass of the system. (from J. Keene, JPL/Caltech,

Centaurus A nucleus, near infrared Centaurus A in mid-infrared showing trapezoidal remains of spiral galaxy falling in
Here is a composite picture combining radio, HI (hydrogen), infrared, visible and X-ray, (from ESO/WFI (op;tical) MPIfR/ESO/APEX/A. Weiss et al. (submillimeter)), NASA/CXC/CfA/R. Kraft et al. (X-ray) and SSC/JPL-Caltech/ J. Keene (infrared)
animation of Centaurus A at many wavelengths Here are individual frames at different wavelengths. The radio frame shows the jets emerging from the active nucleus, particularly their outer parts expanding into intergalactic space. In hydrogen, we see a huge disk of material surrounding the nucleus perpendicular to the jets. In the infrared, we see the warm dust tracing the remains of the spiral galaxy that has recently been swallowed by the large elliptical. Visible light shows the stars, and some of the dust from the spiral galaxy. In the X-rays, we see the inner parts of the jets from the active nucleus.buttonbook.jpg (10323 bytes)

The powerful jets emerging from active nuclei are another indication of the influence of their accretion disks; presumably the jets are accelerated by the dynamo of rapid motions where the disk approaches the black hole and are kept narrow by a force field, like magnetism, generated by the spinning of the accretion disk.

2.) Quasars or QSOs or Quasi-stellar objects: How powerful can active nuclei get?

Quasars were first distinguished in the early 60s because some very bright radio sources were identified with starlike objects on photographs

3C279Visible light picture of the QSO 3C279 (From NASA Extragalactic Database)


 Of 3C273 (Also from NED)

Hubble's Law and the Nature of Quasars

Although they appeared to be stars, quasars were seen to be very peculiar ones if that was what they really were:

Maartin Schmidt The spectra of QSOs didn't remain mysterious for long.

Maarten Schmidt figured out the "mystery" of QSO spectra in 1963.(From California Institute of Technology)

QSOs are NOT made of mysterious elements.

QSOs do  have very LARGE redshifts -- some of them are moving at nearly 90% the speed of light.

The spectra had appeared strange because spectral lines produced in the ultraviolet had been shifted into the visible part of the spectrum. Astronomers were not used to seeing ultraviolet lines, much less seeing them in the visible spectral range!.

Highly redshifted quasar emission lines The lines from this quasar "should" all lie in the ultraviolet. However, if we multiply their usual wavelengths by 3.66, they all line up exactly at the right positions (shown by the red vertical lines).

This result is consistent with their being Doppler shifted, if the quasar is moving away from us at a large portion of the speed of light! We assign a wavelength shift of

z = (the change in wavelength)/(the 'normal' wavelength)

the observed wavelengths are just the normal ones multiplied by 1+z.

HST images of quasar host galaxies Needless to say, this idea was controversial for a while, in part because it required them to have HUGE energy outputs.

Some astronomers proposed that the quasars were "local" but for some reason had been given large velocities.

However, we can now take pictures, especially with HST, that show they all lie in the centers of galaxies and are therefore  indeed similar to scaled-up versions of Seyfert and radio galaxies.

It is now accepted that quasars are active galaxy nuclei that are so bright we can see them to great distances, and hence their lines are redshifted by huge amounts according to Hubble's Law. Under this interpretation, they are the most luminous single objects in the Universe.

HST image of 3C273 and its jet radio image superimposed on HST visible light one

Their properties resemble those of radio galaxies. For example, to the left (above) is an image showing the jet of 3C273 in visible light (From J. Bahcall, STScI), and to the right a radio image (Taken with MERLIN ), is superimposed on the visible light one. 3C273 has a jet similar to, but much bigger than, the one in M87.

Their light output has been seen to vary on timescales as short as a day.

===> this implies that the region producing emission from QSOs must be very small, as small as only a light day across (about 3 times the distance of Pluto)

===> that 10,000 times the luminosity of the Milky Way is coming from a region no bigger than the Solar System stretches the laws of physics. Only the energy generated by falling into a black hole can satisfy these requirements.

More about the nature of these extreme sources

M87 nucleus showing accretion disk and Doppler shifts We are now convinced that all active nuclei derive their energy from matter falling into very massive black holes.

In some cases, we can see a disk of gas in rapid orbital motion around the black hole, which is presumably the matter on the way to falling in and releasing huge amounts of energy. (from Hayden Planetarium, audiovisual archive)

animation of matter orbiting a black hole

The radio emission is produced by electrons moving rapidly (near the speed of light) in a magnetic field. When we see the jets in the infrared and in visible light, the emission appears also to be produced this way.

radio image with visible light image of disk added In this radio image of M87, the galaxy with the black hole and surrounding disk above, there is a narrow "jet" of high energy material coming out of the center. We have superimposed the visible light image of the galaxy nucleus to show how the jet is perpendicular to the disk. (From F. Owen et al.
HST view of the M87 jet We have an even clearer view in visible light, as shown in this HST image. (From STScI)
large scale deep radio image of M87 jet A large scale image of M87 shows how the particles from the jet leak out into intergalactic space. The jet images above are within the burned out region in the center.

(From NRAO, D. Finley,

animation of motions in the jet The bright knot in the "jet" shows shocks passing through. This animation is based on 6 years of data with the Hubble Space Telescope (Animation by G. Rieke, data from Biretta et al.)

It is not clear how such long and narrow structures are maintained, but it must involve large scale and strong magnetic fields. The motions in the jets appear to exceed the speed of lightlink to an extra topic -- M87 is typical, and the movement in the animation above works out to 6 times the speed of light. We still believe in the theory of relativity, that says nothing can really move faster than light, so we explain what we see in these jets as a kind of optical illusion.buttonbook.jpg (10323 bytes)

artist's concept of AGN black hole Here is how one artist imagines it might look close to the black hole, with a hot accretion disk and loops of hot gas confined by strong magnetic fields. From XMM Newton image gallery,

The extreme gravity of such massive blackholes may produce curious effects, if we ever acquire the technology to get close to them ribbon.jpg (3557 bytes)

Why do we study active galaxy nucleilink to a key question

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Far Side by Gary Larson, from Peter Barthel,

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