OPTICAL EMISSION-LINE GAS IN THE NUCLEI OF NORMAL GALAXIES : IMPLICATIONS FOR NUCLEAR ACTIVITY.

Abstract

ABSTRACT The properties of the optical emission-line gas in normal galactic nuclei are reviewed to investigate how common nuclear activity is in normal galaxies. In H II region nuclei, the weakness of the lines from highly ionized species reflects the lack of photons of requisite ionization energy produced by the exciting stars, and the presence of a nuclear spectrum is strongly correlated with a relatively hot stellar population. The gas in Seyferts is modeled as heated and ionized by a central source of high-energy photons, and is shown to be in a highly disturbed state. Evidence for the two models explaining LINER spectra, photoionization by a dilute but energetic continuum source (where nuclear activity is nearly universal), and shock-wave heating (where nuclear activity may not necessarily be manifested), is presented. A Space Telescope spectroscopic and imaging survey of these nuclei will help clarify many of these issues.