Anomalous Silicate Dust Emission in the Type 1 Liner Nucleus of M81

Abstract

We report the detection and successful modeling of the unusual 9.7 μm Si-O stretching silicate emission feature in the type 1 (i.e., face-on) LINER nucleus of M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we determine the feature in the central 230 pc of M81 to be in strong emission, with a peak at ∼10.5 μm. This feature is strikingly different in character from the absorption feature of the galactic interstellar medium, and from the silicate absorption or weak emission features typical of galaxies with active star formation. We successfully model the high signal-to-noise ratio IRS spectra with porous silicate dust using laboratory-acquired mineral spectra. We find that the most probable fit uses micron-sized, porous grains of amorphous silicate and amorphous carbon. In addition to silicate dust, there is weak polycyclic aromatic hydrocarbon (PAH) emission present (particularly at 11.3 μm, arising from the C-H out-of-plane bending vibration of relatively large PAHs of ∼500-1000 C atoms) whose character reflects the low-excitation active galactic nucleus environment, with some evidence that small PAHs of ∼100-200 C atoms (responsible for the 7.7 μm C-C stretching band) in the immediate vicinity of the nucleus have been preferentially destroyed. Analysis of the infrared fine structure lines confirms the LINER character of the M81 nucleus. Four of the infrared H 2 rotational lines are detected and fit to an excitation temperature of T ∼ 800 K. Spectral maps of the central 230 pc in the [Ne ii] 12.8 μm line, the H 2 17 μm line, and the 11.3 μm PAH C-H bending feature reveal arc-or spiral-like structures extending from the core. We also report on epochal photometric and spectroscopic observations of M81, whose nuclear intensity varies in time across the spectrum due to what is thought to be inefficient, subEddington accretion onto its central black hole. We find that, contrary to the implications of earlier photometry, the nucleus has not varied over a period of two years at these infrared wavelengths to a precision of about 1%.

The Role of the Accretion Disk, Dust, and Jets in the Ir Emission of Low-Luminosity Active Galactic Nuclei

“…The best-fitting model (Figure 4) provides a reasonable description of the SED of NGC 3998, and the strength of the silicate emission features is comparable to those observed in the Spitzer spectrum. More sophisticated treatment of grain chemistry/size/structure and/or radiative transfer effects may be necessary to improve the match between the observed and modeled feature profiles (e.g., Nikutta et al 2009;Smith et al 2010). The V-band optical depth of the model, τ V = 33, means that the AGN is heavily obscured in the optical.…”

Sub-parsec-scale dynamics of a dusty gas disc exposed to anisotropic AGN radiation with frequency-dependent radiative transfer

“…They argued that this result could be explained by the predominance of larger grains in AGN tori. Other interesting discussion about dust properties in AGNs are found in Lutz et al (2002); Maiolino & Natta (2002); Smith et al (2010); Xie et al (2014).…”

The 10 μm band in amorphous MgSiO3: the influence of medium-range structure, defects and thermal processing

“…The A&A 545, A60 (2012) 10 μm profiles observed for AGN silicates (e.g. Strum et al 2005) often differ significantly from the ISM profile, with varying peak wavelengths between 10 and 11.5 μm attributed to additional dust components (Markwick-Kemper et al 2007), or large porous grains (Smith et al 2010) due to grain growth by coagulation (Maiolino et al 2001).…”