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 version (reload to restart lecture animations) from Chandra, http://chandra.harvard.edu/resources/animations/blackholes.html

Seyfert Galaxies --

Carl Seyfert first discovered a type of AGN. He noticed them because they appeared to have stars superposed on their centers, which were really the bright glow right around the black holes. The stellar appearing cores had emission lines implying very hot sources were exciting them with strong ultraviolet light.

We now know this ultraviolet output comes from the accretion 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. (from APOD, V. Veckman, http://antwrp.gsfc.nasa.gov/apod/ap040908.htm) l

The spectra of their nuclei showed extremely strong and broad emission lines; the broad lines imply that the gas producing them must be moving at a large range of velocities.

Although there isn't full agreement, one of the best ideas 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, http://www.iac.es/gabinete/noticias/2001/nov20.htm. The broad profiles are then due to the high velocity of stars caught deep in the gravitational potential of the black hole.

Luminosity of these galaxy centers alone can be 100 times that of the entire Milky Way. They are unusually bright in x-rays and infrared. Some have rapidly varying light outputs, changing substantially in a week or a month.

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)

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" which extend way beyond the visible galaxy in some cases:

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, http://www.hq.eso.org/outreach/press-rel/pr-2000/phot-05-00.html)

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

Visible 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:

• These "stars" had peculiar colors being much brighter in the ultraviolet than normal stars and redder at longer wavelengths
• Spectra were even stranger with very strong and broad emission lines but which occurred at wavelengths that did not seem to match up to known elements. The shapes and strengths of the lines, but not their wavelengths, were reminiscent of those from Seyfert galaxies.

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!.

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.

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.