Anomalous Silicate Dust Emission in the Type 1 LINER Nucleus of M81
  We report the detection and successful modeling of the unusual 9.7\mum 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\mum. 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 graphite. In addition to silicate dust, there is weak PAH emission
present (particularly at 11.3\mum, 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 AGN environment, with some evidence that small PAHs
of ~100--200 C atoms (responsible for the 7.7\mum C--C stretching band) in the
immediate vicinity of the nucleus have been preferentially destroyed. (abstract
continues)
