Galactic centers as dying quasars.

In recent years expert opinion has been converging on the belief that quasars are related to galaxies. Quasars radiate energy at rates comparable to those of galaxies, but look as compact as stars. Opinion generally has it that a quasar is something extremely energetic that happens in the center of a galaxy.

Much of the evidence for this belief has involved findings of starlike qualities in the luminous matter that surrounds some quasars: that the spectrum of its light looks like the spectra of stars, and, in at least one instance, that a supernova happened there (SN: 4/6/85,p. 212). Now there is evidence that could complement this, namely that the centers of rather ordinary-looking galaxies appear to be weak or dead quasars.

Such evidence is coming out of a survey by Alexei V. Filippenko and Wallace L.W. Sargent of the Palomar Observatory of California Institute of Technology. They are looking for evidence of low-luminosity galactic nuclei of the Seyfert 1 type. Seyfert galaxies seem to be an intermediate stage between ordinary "normal" galaxies and quasars. They have nuclei that are more energetic than "normal" but not quite as strong as quasars, and Seyfert nuclei are definitely surrounded by galaxies. Many astrophysicists now lump Seyferts, quasars and certain other highly energetic objects into the single category, active galactic nuclei (AGNs).

Filippenko and Sargent are engaged in a survey of bright, nearby galaxies to determine whether any of them are dwarf Seyfert 1 nuclei, that is, whether they show in a milder form the same sort of spectral qualities characteristic of well-known objects of the type. Results covering the first 75 galaxies in the survey appear in the ASTROPHYSICAL JOURNAL Supplement Series (Vol. 57, p. 503). Filippenko and Sargent conclude: "these data suggest that the faint end ... of AGNs is much more populated than was previously believed. Given the paucity of nearby [quasars] and the growing evidence that they reside in the nuclei of galaxies, it is likely that most of the objects we observed were [quasars] in the distant past."

The evidence is mainly the appearance in the spectra of many of these objects of bright resonant emissions (emission lines) similar to those of known quasars, particularly certain wavelengths characteristically emitted by hydrogen. The sample included some known Seyfert 1 nuclei for comparison and a number of objects known for their emission lines, collectively called liners, which seemed likely candidates for AGNs. Filippenko and sargent hope that a survey of at least 600 galaxies will eventually be completed.