Unveiling the Ionisation Continuum in Low-Luminosity AGN
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The accretion disc of low-luminosity active galactic nuclei (LLAGN) is predicted to disappear at low Eddington ratios (Lbol/Ledd < −3), while high-angular resolution observations of these nuclei have revealed a steep power law optical/UV continuum with α ∼ 2 (Fν ∼ ν −α ), characteristic of synchrotron emission. However, hydrogen absorption makes it impossible to probe this continuum shortwards of ∼ 1000 Å, and thus the possible contribution of an accretion disc cannot be fully discarded. Still, this information is encoded in the fine structure lines produced by the AGN ionised gas, whose relative intensities are sensitive to the shape of the ionising continuum and therefore can be used to probe the presence of the accretion disc. In this work we have combined diffraction-limited high-angular resolution observations from the Hubble Space Telescope in the optical/UV range and the Very Large Telescope in the near- to mid-infrared (IR) range for a sample of 10 LLAGN. These direct measurements of the accessible part of the LLAGN continuum have been combined with photo-ionisation simulations using Cloudy and mid-IR fine-structure lines observed by Spitzer, to infer the shape of the missing continuum below ∼ 1000 Å. This approach allowed us to reconstruct the primary ionising spectra and the disc contribution in prototypical LLAGN nuclei such as M87, NGC 1052, or the Sombrero galaxy. The IR emission line properties of the LLAGN in our sample can be reproduced by a power law continuum extending from the UV to the X-ray range, with a typical index of 0.35 . α . 0.55, with a negligible or no contribution at all from the accretion disc to the ionisation. These accretion discs must be truncated or in a cool state in order to explain the lack of ionising photons. The power law component dominating the extreme UV continuum in LLAGN is associated with inverse Compton emission from reprocessed synchrotron jet emission by the black hole corona. This is supported by photo-ionisation predictions for jet models applied to three of the LLAGN in our sample.