2007MNRAS.377.1696M

Low-luminosity active galactic nuclei (AGNs) are perceived to be radio loud and devoid of a `big blue bump', indicating a transition from a radiatively efficient, geometrically thin, accretion disc in high-luminosity AGNs, to a geometrically thick, radiatively inefficient accretion flow at low luminosities and accretion rates. I revisit the issue of the spectral energy distributions (SEDs) of low-luminosity AGNs using recently published, high angular resolution data at radio, ultraviolet (UV) and X-ray wavelengths, for a sample of 13 nearby galaxies with low-ionization nuclear emission-line region (LINER) nuclei. I show that, contrary to common wisdom, low-luminosity LINERs have significant non-stellar UV flux, and UV/X-ray luminosity ratios similar, on average, to those of Seyfert 1 nuclei ∼10<sup>4</sup> times more luminous. The α<sub>ox</sub> index that quantifies this ratio is in the range between -0.8 and -1.4, and is below the extrapolation to low luminosities of the relation between α<sub>ox</sub> and UV luminosity observed at higher luminosities. In terms of radio loudness, most of the LINERs are indeed radio loud (or sometimes even `super radio loud') based on their radio/UV luminosity ratios, when compared to the most luminous quasars. However, the entire distribution of radio loudness has been shown to shift to higher radio/UV ratios at low AGN luminosities. In the context of this global shift, some LINERs (the majority) can be considered radio quiet, and some (from among those with black hole masses ≳10^8.5M_☉) are radio loud. The SEDs of these low-luminosity (∼10⁴⁰erg/s) AGNs are thus quite similar to those of Seyferts up to luminosities of ∼10⁴⁴erg/s, and there is no evidence for a sharp change in the SED at the lowest luminosities. Thin AGN accretion discs may therefore persist at low accretion rates, in analogy to some recent findings for Galactic stellar-mass accreting black holes.