Optical Spectra of Extragalactic Radio Sources

Extract

Extragalactic radio sources can be grouped spectroscopically into those that show absorption lines produced by a stellar population (e.g. the nearer radio galaxies, and some more distant ones, usually of D or E type) and those that show only lines (usually in emission) produced by low-density gas (e.g. QSO’s and many N-type galaxies). If absorption lines from stars are present, one can estimate a minimum age from the stellar population, and perhaps a mass for the system from the stellar velocity dispersion. If no spectroscopic evidence on the stellar population is available (and this includes evidence from narrow-band filter photometry), then we have much less hold on the problems of age and evolutionary state. Absence of stellar absorption lines does not necessarily mean that stars are absent, but simply that their light, if present, does not contribute appreciably to the overall optical spectrum.