6.2. Low Luminosity AGN
Most massive black holes in the local universe are accreting at rates that are much lower than those found in AGN. If the accretion is so small as to render the black hole undetectable, the galaxy is termed a quiescent galaxy. Slightly higher accretion rates will lead to the classification as a low luminosity AGN (LLAGN).
The nature of black hole accretion when the accretion rates are very low is a topic of active research. It was realized by several authors that when the accretion rate is low (relative to the Eddington rate), an accretion disk may switch into a hot, radiatively-inefficient mode (Ichimaru 1977; Rees 1982; Narayan & Yi 1994; Narayan & Yi 1995). In essence, the plasma becomes so tenuous that the timescale for energy transfer from the protons to the electrons (via Coulomb interactions) becomes longer than the inflow timescale. The energy remains as thermal energy in the protons (which are very poor radiators) and gets advected through the event horizon of the black hole. These are the so-called Advection Dominated Accretion Flows (ADAFs). ADAFs are to be contrasted with `standard' radiatively-efficient accretion disks in which the disk remains cool and geometrically thin all of the way down to the black hole (Shakura & Sunyaev 1973; Novikov & Thorne 1973). Broad iron line studies of LLAGN provide a potentially important probe of the physics of accretion when the accretion rate is low - the iron line traces only the radiatively-efficient portions of the disk since ADAFs are far too hot to produce fluorescent iron line emission.
Observationally, LLAGN have proven difficult to study due to the fact that they are X-ray faint. In addition, their X-ray spectra are typically complex with non-nuclear spectral components (such as starburst regions and/or thermal emission from hot gas) rivalling the nuclear component (e.g. see Ptak 1997). One of the best studied LLAGN resides in the nearby galaxy NGC 4258 (M 106). A short ASCA observation of this galaxy hinted at the presence of an iron line (Makishima et al. 1994). However, it took a deep ASCA observation to unambiguously detect the line and allow a detailed study (Reynolds, Nowak & Maloney 2000). It was found that the line in NGC 4258 is fairly weak (with an equivalent width of about 100 eV) and narrow (with a FWHM of less than 22000 km s-1). Reynolds et al. (2000) argue that this line does indeed originate from the accretion disk, implying that the X-ray emitting corona has a size greater than 100 GM / c2. The contrast between the iron lines found in NGC 4258 and its higher luminosity Seyfert cousins is consistent with an ADAF scenerio for LLAGNs. However, the observational results are not yet conclusive. If the iron line seen in NGC 4258 comes from material not associated with the accretion disk (such as a distant torus that is misaligned with the almost edge-on accretion disk so as not to obscure the central engine from our view), then the data are consistent with the presence of a ``Seyfert-like'' broad iron line. See Reynolds et al. (2000) for further details.
While it is significantly more luminous than NGC 4258, the well studied active nucleus in the galaxy NGC 4051 is also often classified as a LLAGN. This object display a classic relativistic iron line indicating the presence of a radiatively-efficient accretion disk in this object (Guainazzi et al. 1996). Wang et al. (1999) have recently discovered interesting temporal variability in this iron line which displays opposite trends to the variability found in MCG-60-30-15 - both the equivalent width and energy width of the line positively correlate with the source flux.