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Low-luminosity active galactic nuclei: are they UV faint and radio loud?

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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 ∼104 times more luminous. The αox 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 αox 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 ≳108.5 M⊙) are radio loud. The SEDs of these low-luminosity (∼1040 erg s−1) AGNs are thus quite similar to those of Seyferts up to luminosities of ∼1044 erg s−1, 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.

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