The Nuclear Infrared Emission of Low-Luminosity Active Galactic Nuclei

Abstract

We present high-resolution mid-infrared (MIR) imaging, nuclear spectral energy distributions (SEDs) and archival Spitzer spectra for 22 low-luminosity active galactic nuclei (LLAGN; L bol 10 42 erg s −1 ). Infrared (IR) observations may advance our understanding of the accretion flows in LLAGN, the fate of the obscuring torus at low accretion rates, and, perhaps, the star formation histories of these objects. However, while comprehensively studied in higher-luminosity Seyferts and quasars, the nuclear IR properties of LLAGN have not yet been well-determined. We separate the present LLAGN sample into three categories depending on their Eddington ratio and radio emission, finding different IR characteristics for each class. (I) At the low-luminosity, low-Eddington ratio (log L bol /L Edd < −4.6) end of the sample, we identify "host-dominated" galaxies with strong polycyclic aromatic hydrocarbon bands that may indicate active (circum-)nuclear star formation. (II) Some very radio-loud objects are also present at these low Eddington ratios. The IR emission in these nuclei is dominated by synchrotron radiation, and some are likely to be unobscured type 2 AGN that genuinely lack a broad line region. (III) At higher Eddington ratios, strong, compact nuclear sources are visible in the MIR images. The nuclear SEDs of these galaxies are diverse; some resemble typical Seyfert nuclei, while others lack a well-defined MIR "dust bump". Strong silicate emission is present in many of these objects. We speculate that this, together with high ratios of silicate strength to hydrogen column density, could suggest optically thin dust and low dust-to-gas ratios, in accordance with model predictions that LLAGN do not host a Seyfert-like obscuring torus. We anticipate that detailed modelling of the new data and SEDs in terms of accretion disk, jet, radiatively inefficient accretion flow and torus components will provide further insights into the nuclear structures and

Nuclear obscuration in LINERs

“…However, the slope of this correlation is steeper (∼1.26) than previously found (∼1.06, see Asmus et al 2011). This excess in mid-IR luminosity for faint LINERs has already been found by Mason et al (2012). They argue that this discrepancy could be due to optically thin material that obscures the inner parts of the AGN because most of them showed silicate features in emission.…”

“…It is expected to be particularly relevant for LINERs where the AGN is faint. In this case the non-AGN contribution might be very strong at mid-IR wavelengths, dominating the entire emission (Mason et al 2012).…”

“…The inclusion of LINERs in this correlation allows it to be validated at lower luminosities (L X (2−10 keV) < 10 42 erg s −1 , see also Mason et al 2012). The final linear correlation for AGNdominated spectra is log(L X ) = (−12.34 ± 0.05) + (1.26 ± 0.01) log(νL ν (12 µm)), (2) which is very significant (r = 0.92 and P(null) = 2 × 10 −28 ).…”

“…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). The confirmation of such a trend for a large sample might confirm the Compton-thick nature of a large portion of LINERs.…”

“…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.…”