Mid-infrared properties of nearby low-luminosity AGN at high angular resolution

We present high spatial resolution mid-infrared (MIR) 12 μm continuum imaging of low-luminosity active galactic nuclei (LLAGN) obtained with VLT/VISIR. Our goal is to determine whether the nuclear MIR emission of LLAGN is consistent with the existence of a dusty obscuring torus, the key component of the unification model for AGN. Based on available hard X-ray luminosities and the previously known tight correlation between the hard X-ray and 12 μm luminosities, we selected a sample of 17 nearby LLAGN without available VISIR N-band photometry. Combined with archival VISIR data of 9 additional LLAGN with available X-ray measurements, the dataset represents the bulk of southern LLAGN currently detectable from the ground in the MIR. Of the 17 observed LLAGN, 7 are detected, while upper limits are derived for the 10 non-detections. This increases the total number of AGN detected with VLT/VISIR to more than 50. All detections except NGC 3125 appear point-like on a spatial scale of ~0.35″. The detections do not significantly deviate from the known MIR-X-ray correlation but exceed it by a factor of ~10 down to luminosities <10⁴¹ erg/s with a narrow scatter (σ = 0.35 dex, Spearman rank ρ = 0.92). The latter is dominated by the uncertainties in the X-ray luminosity. Interestingly, a similar correlation with a comparable slope but with a normalization differing by ~2.6 orders of magnitude has been found for local starburst galaxies. In addition, we compared the VISIR data with lower spatial resolution data from Spitzer/IRS and IRAS. By using a scaled starburst template spectral energy distribution and the polycyclic aromatic hydrocarbon (PAH) 11.3 μm emission line, we were able to restrict the maximum nuclear star-formation contamination of the VISIR photometry to ≲30% for 75% of the LLAGN. Exceptions are NGC 1097 and NGC 1566, which may possess unresolved strong PAH emission. Furthermore, the MIR-X-ray luminosity ratio is unchanged over more than 4 orders of magnitude in accretion rate within the uncertainties. These results are consistent with the existence of the dusty torus in all observed LLAGN, although a jet or accretion disk as origin of the MIR emission cannot be excluded. Finally, because the MIR-X-ray correlation holds for all LLAGN and Seyferts, this is a very useful empirical tool for converting between the MIR and X-ray powers of these nuclei.

Based the ESO observing programs 083.B-0536 and 086.B-0349.Appendices are available in electronic form at http://www.aanda.org