Observations of Quasars and Active Galaxies

Seyfert galaxies

The first hint of strange activity in galaxies came from the spectrum of NGC 1068. NGC 1068 was found to have broad and narrow emission lines in its nucleus, produced by highly ionized gas .

  • Broad emission lines: fast motions (1000 - 5000 km/s)
  • Narrow emission lines: slower motions (~500 km/s)
  • High ionization: strong energy source




NGC 1068 is an example of a Seyfert galaxy. We classify Seyfert galaxies by their emission line shapes.
  • Seyfert 1: both broad and narrow lines
  • Seyfert 2: narrow lines only


Spectrum of a Seyfert 1 galaxy:
 



The beginning of taxonomy in the study of active galaxies...

Seyfert galaxies are typically spiral galaxies, and often have nearby companions. While the spectrum of the galaxy shows stellar light, the spectrum of the nucleus shows a complicated spectrum, dominated by synchrotron radiation.


 

Radio galaxies


As radio astronomers mapped the sky, they found several bright radio sources associated with distant galaxies. One of the brightest was the radio source Cygnus A.

Cygnus A is a double-lobed radio source. Also notice the jets from the center. At the center sits a giant elliptical galaxy, at a distance of ~ 200 Mpc. At this distance, the lobes are separated by more than 100 kpc, and they have a radio luminosity of 1045 erg/s, 106 times greater than the radio luminosity of normal galaxies.

Radio galaxies, unlike Seyferts, are generally hosted by elliptical galaxies, not spirals. Seyferts are also radio quiet.

M87, the central elliptical in Virgo, also is a radio galaxy. It is close enough that we can examine its center in detail, where we can actually see the jet in the optical:

Quasars

Some radio sources were not found to be associated with obvious nearby galaxies. Instead, astronomers only found star-like objects, with very bizarre spectra -- they had extremely broad emission lines -- like Seyfert 1 and radio galaxies -- but which could not be associated with any known elements or molecules. The name "quasi-stellar radio source" or quasar was given to these objects.

In 1963, Maarten Schmidt realized that the emission lines from the quasar 3C 273 were actually from a very well-known element: hydrogen. They were, however, Doppler shifted to much redder wavelengths than expected, indicating (by Hubble's law) that the object was very far away.

An aside: redshifts

At large recession velocities, astronomers typically talk about the redshift of a galaxy, given by

Small z: low velocity -- nearby object. Large z: high velocity -- distant object.

Most galaxies we see pretty pictures of have z<0.05.

3C 273 had a redshift of z=0.158, and has a recessional velocity which was ~ 15% of the speed of light! This puts 3C 273 at a distance of ~ 450 Mpc.

Shortly thereafter, 3C 48 was found to have a redshift z=0.367, putting it 900 Mpc away.

Quasars were some of the most distant objects ever discovered. Yet they are relatively bright. This means the must be extremely luminous: 103 - 105 times more luminous than the entire Milky Way.

How are Seyferts (1 and 2), radio galaxies, and quasars related? What about blazars, BL Lac objects, OVVs , etc? A complicated taxonomy -- what's going on?