Nonstandard Central Engines in Nearby Galaxies

We argue that nearby galaxy nuclei contain massive black holes that are fueled by low radiative efficiency accretion flows. 1. Nuclear Activity in Nearby Galaxies A significant fraction of local galaxies exhibit signs of nuclear activity in the form of emission-line nuclei classified as Seyferts or LINERs. According to the Palomar spectroscopic survey of nearby galaxies, 43% of all northern galaxies brighter than BT = 12.5 mag are active (Ho, Filippenko, & Sargent 1997), albeit at a level substantially weaker than traditionally studied AGNs such as classical Seyfert nuclei and quasars. The sheer abundance of low-luminosity AGNs (LLAGNs), especially LINERs which make up two-thirds of the population, compels us to give them proper attention when considering AGN issues of a statistical nature. We do not yet have a full understanding of the physical origin of LINERs. Particularly thorny are the narrow-line objects (type 2 LINERs and transition objects), where in some cases the AGN signature can be either ambiguous or absent (Ho 2001a; Barth 2001). On the other hand, the affiliation of type 1 objects (those with detectable broad emission lines) with nonstellar processes seems quite secure. This contribution highlights some of the salient features of type 1 LLAGNs and implications we might draw concerning the nature of their central engines. 2. Notable Characteristics of Low-luminosity AGNs We begin by listing some of the observational properties that are unique to LLAGNs. When observing these sources, it should be noted that one must exercise caution to obtain nuclear fluxes — the quantities most pertinent to the AGN and most analogous to observations of quasars. In general the central source is sufficiently weak that it is completely overwhelmed by emission from the host galaxy. This applies to virtually all wavelengths. In order to reliably quantify the nuclear fluxes, one needs observations with good sensitivity, and more crucially, high angular resolution (generally ∼<1 ). (1) Accretion luminosities and Eddington ratios. LLAGNs are not only intrinsically faint, but more importantly, their accretion luminosities are low relative to their Eddington luminosities. Ho (2002) used the nuclear X-ray luminosities of a large sample of nearby galaxies to estimate their nuclear bolometric luminosities. Figure 1a shows that Lbol is generally higher in Seyferts