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We report the detection and successful modeling of the unusual 9.7\mum Si--Ostretching silicate emission feature in the type 1 (i.e. face-on) LINER nucleusof M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, wedetermine the feature in the central 230 pc of M81 to be in strong emission,with a peak at ~10.5\mum. This feature is strikingly different in characterfrom the absorption feature of the galactic interstellar medium, and from thesilicate absorption or weak emission features typical of galaxies with activestar formation. We successfully model the high signal-to-noise ratio IRSspectra with porous silicate dust using laboratory-acquired mineral spectra. Wefind that the most probable fit uses micron-sized, porous grains of amorphoussilicate and graphite. In addition to silicate dust, there is weak PAH emissionpresent (particularly at 11.3\mum, arising from the C--H out-of-plane bendingvibration of relatively large PAHs of ~500--1000 C atoms) whose characterreflects the low-excitation AGN environment, with some evidence that small PAHsof ~100--200 C atoms (responsible for the 7.7\mum C--C stretching band) in theimmediate vicinity of the nucleus have been preferentially destroyed. (abstractcontinues)

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