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

Abstract

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 <10 41 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.

The dependency of AGN infrared colour-selection on source luminosity and obscuration

“…4.1), resulting in high completeness levels. With decreasing redshifts, however, X-ray observations and optical spectroscopy are able to detect more low-luminosity AGN, sources that will probably not dominate the total IR SED of a galaxy, resulting either from IR-luminous obscured starformation in the galaxy (Rigopoulou et al 1999;Veilleux et al 2009;Nordon et al 2013) or from the complete absence of a dust torus (e.g., Ho 2008;Asmus et al 2011;Burlon et al 2011;Plotkin et al 2012). The former plays a significant role, since IR-selected AGN have mostly been found in star-forming hosts (Hickox et al 2009;Griffith & Stern 2010).…”

“…These findings imply that dust-free clouds at <R sub frequently represent the bulk of the X-ray obscuration. As such, the AGN-induced dust emission will be relatively weak or non-existent especially in lowluminosity AGN (Ho 2008;Asmus et al 2011;Plotkin et al 2012), but high-luminosity cases also exist where the IR excess is not as extreme as expected (Comastri et al 2011). …”

“…Such conditions, however, are frequently not found when studying galaxies hosting low-luminosity AGN, where the host galaxy light dominates instead (Treister et al 2006;Cardamone et al 2008;Donley et al 2008Donley et al , 2012Eckart et al 2010;Petric et al 2011). High resolution imaging can probe the very centre of a system and avoid this problem (e.g., Asmus et al 2011), but it is only applicable to galaxies in the very local Universe. Also, there is evidence that IR selection is biased towards unobscured AGN Stern et al 2005;Donley et al 2007;Cardamone et al 2008;Eckart et al 2010;Mateos et al 2013).…”

Nuclear obscuration in LINERs

“…Mendoza-Castrejón et al 2015). Subarcsecond resolution studies claim a tight correlation between rest frame luminosities νL ν (12 µm) and L X (2−10 keV) for Type-1 and Type-2 Seyferts Gandhi et al 2009;Asmus et al 2011;Masegosa et al 2013;González-Martín et al 2013). Imaging data at mid-IR wavelengths have shown that some Compton-thick LINER candidates might fall off this relation, with L 12 µm higher than expected for their L X (Mason et al 2012;Masegosa et al 2013).…”

“…Nowadays we know that around 75−90% of LINERs show evidence of AGNs using multi-wavelength information (González-Martín et al 2006, 2009aDudik et al 2009;Younes et al 2011;Asmus et al 2011;Mason et al 2012) 1 . This is also confirmed from X-ray and UV variability studies (Maoz et al 2005;Hernández-García et al 2013.…”