Next Contents Previous

12.3.3. Quasars

The optical properties of quasars may be summarized as follows

  1. They appear stellar on direct photographs, although in some cases there is a faint jet or wisp extending from the stellar object.

  2. They have large redshifts ranging up to z = 3.5.

  3. Assuming that the redshifts are cosmological and that the distance is given by the Hubble law with a Hubble constant, H, equal to 100 km/sec/Mpc, then the absolute optical magnitudes range from -22 to -26 or more, so that they are up to 100 times brighter than the most luminous galaxies at optical wavelengths.

  4. Often the optical emission is variable on time scales ranging from a few hours to a few years.

  5. The luminosity rises sharply toward the infrared, where most the radiated energy lies. There is also an excess of UV emission compared with galaxies, so that the presence of a large redshift causes the quasar to appear blue when measured by UBV photometry or when the color is estimated from the "red" and "blue" plates of the Palomar Sky Survey. For very large red shifts (z gtapprox 3), however, the color may appear neutral or even "red."

  6. The spectra show intense broad emission lines, with line widths corresponding to velocities up to 4000 km/sec. The most commonly observed lines are those of Lyalpha (lambda1216), CIV (lambda1549), CIII (lambda1909), MgII (lambda2798), OIII (lambda4363, lambda4959, lambda5007), and the Hydrogen Balmer series.

  7. Some quasars show narrow absorption lines, often with several sets of redshifts which are usually smaller than the emission-line redshift.

Next Contents Previous