The local space density of active galactic nuclei (AGNs) has bearing on a number of issues in extragalactic astronomy, including the fraction of galaxies hosting massive black holes, the cosmological evolution of quasars, and the contribution of AGNs to the cosmic X-ray background. It is therefore of fundamental importance to establish the extent and nature of nuclear activity in nearby galaxies. This contribution summarizes recent efforts to survey nearby galactic nuclei, discusses complications regarding the interpretation of the results, and presents a variety of fresh observational perspectives that help toward reaching a coherent understanding of nuclear activity in nearby galaxies.
Optical surveys find that a large fraction of nearby galaxies have nuclei that emit weak emission lines with a spectrum unexpected for photoionization by normal stars. Heckman (1980) identified low-ionization nuclear emission-line regions (LINERs) as a major constituent of the extragalactic population, particularly among early-type galaxies. The optical spectra of LINERs broadly resemble those of traditional AGNs such as Seyfert nuclei, except that they have characteristically lower ionization levels. These findings were strengthened by a number of subsequent studies, as reviewed by Ho (1996).
The latest and most sensitive survey of this kind was completed by Ho et al. (1997a, and references therein) using the Hale 5 m telescope at Palomar Observatory. Long-slit spectra of exceptional quality were taken of the nuclear region of a magnitude-limited (BT 12.5 mag) sample of 486 northern ( > 0°) galaxies that constitutes an excellent representation of the typical nearby galaxy population. The spectra are of moderate resolution (full-width at half maximum [FWHM] ~ 100-200 km s-1) and cover two regions of the optical window (4230-5110 Å and 6210-6860 Å) containing important diagnostic emission lines. The main results of the Palomar survey are the following. (1) AGNs are very common in nearby galaxies (Fig. 1). At least 40% of all galaxies brighter than BT = 12.5 mag emit AGN-like spectra. The emission-line nuclei are classified as Seyferts, LINERs, or LINER/H II-region composites, and most have very low luminosities compared to traditionally studied AGNs. The luminosities of the H emission line range from 1037 to 1041 ergs s-1, with a median value of ~ 1039 ergs s-1. (2) The detectability of AGNs depends strongly on the morphological type of the galaxy, being most common in early-type systems (E-Sbc). The detection rate of AGNs reaches 50%-75% in ellipticals, lenticulars, and bulge-dominated spirals but drops to 20% in galaxies classified as Sc or later. (3) LINERs make up the bulk (2/3) of the AGN population and a sizable fraction (1/3) of all galaxies. (4) A significant number of objects show a faint, broad (FWHM 1000-4000 km s-1) H emission line that qualitatively resembles emission arising from the conventional broad-line region of ``classical'' Seyfert 1 nuclei and QSOs.
Figure 1. Detection rate (left) and number distribution (right) of AGNs as a function of Hubble type in the Palomar survey. ``Type 1'' AGNs (those with broad H) are shown separately from the total population (types 1 and 2).
Radio observations provide further support for the prevalence of nuclear activity (see the contribution of E. Sadler in these proceedings). Weak radio cores with powers of 1019-1021 W Hz-1 at 5 GHz are found in ~ 50% of nearby elliptical and S0 galaxies (Sadler et al. 1989; Wrobel and Heeschen 1991). Where information is available, the cores have relatively flat spectral indices and nonthermal brightness temperatures (Slee et al. 1994), and the optical spectra of most of these sources are classified as LINERs (Sadler et al. 1989; Ho 1998a).