Observations of Quasars and Active Galaxies


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.

3C273 optical image and spectrum

(courtesy Bill Keel, UAlabama)

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, 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 measuring the wavelengths of their spectral lines. Redshift ("z") then measures how far those lines are shifted from their normal wavelength.

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

Most galaxies we see pretty pictures of are nearby with small redshifts of 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 ~ 650 Mpc.
Nothing had ever been observed this distant.

Shortly thereafter, another quasar, 3C 48 was found to have a redshift z=0.367, putting it ~1500 Mpc away.

(note: at these redshifts, the notion of distance becomes ill-defined, because of the curvature of space. we'll talk about that more when we talk about cosmology!)

Quasars were 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.

Seyfert galaxies

The first hint of strange activity in otherwise normal 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 Hercules A.

Hercules A: radio emission (pink) overlaid on optical.

Hercules A is a double-lobed radio source, where the lobes contain hot, ionized gas. Also notice the jets from the center. At the center sits a giant elliptical galaxy, at a distance of ~ 650 Mpc (z=0.155). At this distance, the lobes are separated by more than 250 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:

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