2.7. The Spectral Energy Distributions of LINERs

Luminous AGNs generally display a fairly ``universal'' spectral energy distribution (SED) (e.g., Elvis et al. 1994). The SED from the infrared to the X-rays, roughly flat in log F - log space, can be represented by an underlying power law ( 1, where F ^-) superposed with several distinct components, the most prominent of which is a broad UV excess. This so-called big blue bump is conventionally interpreted as thermal emission from an optically thick, physically thin accretion disk (Malkan and Sargent 1982). As multiwavelength data for LINERs and other low-luminosity AGNs become more readily available, we can begin to piece together their SEDs. Comparing SEDs of AGNs of various luminosities might yield clues to physical processes that depend on luminosity.

The SEDs of the low-luminosity AGNs share a number of common traits, and yet they differ markedly from the SEDs of luminous AGNs (Ho 1998b). To illustrate this point, Figure 5 normalizes the SEDs of seven low-luminosity objects (mostly LINERs) with the median SED of radio-loud and radio-quiet AGNs from Elvis et al. (1994). Several features are noteworthy. (1) The optical-UV slope is quite steep. The power-law indices for the seven low-luminosity objects average < > 1.8, whereas in luminous AGNs 0.5-1. (2) The UV band is exceptionally dim relative to the optical and X-ray bands; there is no evidence for a big blue bump component. Indeed, the SED reaches a local minimum somewhere in the far-UV or extreme-UV region. The mean value of _ox, the two-point spectral index between 2500 Å and 2 keV, is ~ 0.9, to be compared with < _ox > = 1.2-1.4 for luminous Seyferts and QSOs. (3) There is tentative evidence for a maximum in the SED at mid-IR wavelengths. (4) The nuclei have radio spectra that are either flat or inverted. (5) All sources, including the three spiral galaxies in the sample, qualify as being radio-loud. This finding runs counter to the usual notion that only elliptical galaxies host radio-loud AGNs. (6) The bolometric luminosities of the sources range from L_bol = 2 x 10⁴¹ to 8 x 10⁴² ergs s^-1, or ~ 10^-6-10^-3 times the Eddington rate for the black hole masses estimated for these objects.

Figure 5. Interpolated SEDs of seven low-luminosity AGNs (solid lines) normalized to the 1 keV luminosity of M81. The median SEDs of radio-loud (dotted line) and radio-quiet (dashed line) AGNs of Elvis et al. (1994), normalized in the same way, have been overplotted for comparison. From Ho (1998b).

The overall characteristics of these nonstandard SEDs can be explained by models of ``advection-dominated accretion flows'' (ADAFs; see Narayan et al. 1998 for a review). The accretion flow equations admit a stable advection-dominated, optically thin solution when the accretion rate falls to very low values ( 10^-2 _Edd). Under these conditions, the low density and low optical depth of the accreting material lead to inefficient cooling, and the resulting radiative efficiency is much less than the canonical value of 10%. This accounts for the low observed luminosities. Moreover, the predicted SEDs of ADAFs look radically different from the SEDs of optically thick disks but provide a good match for the observations of low-luminosity AGNs (Ho and Narayan 1998).