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\\mum Si--O\nstretching silicate emission feature in the type 1 (i.e. face-on) LINER nucleus\nof M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we\ndetermine the feature in the central 230 pc of M81 to be in strong emission,\nwith a peak at ~10.5\\mum. This feature is strikingly different in character\nfrom the absorption feature of the galactic interstellar medium, and from the\nsilicate absorption or weak emission features typical of galaxies with active\nstar formation. We successfully model the high signal-to-noise ratio IRS\nspectra with porous silicate dust using laboratory-acquired mineral spectra. We\nfind that the most probable fit uses micron-sized, porous grains of amorphous\nsilicate and graphite. In addition to silicate dust, there is weak PAH emission\npresent (particularly at 11.3\\mum, arising from the C--H out-of-plane bending\nvibration of relatively large PAHs of ~500--1000 C atoms) whose character\nreflects the low-excitation AGN environment, with some evidence that small PAHs\nof ~100--200 C atoms (responsible for the 7.7\\mum C--C stretching band) in the\nimmediate vicinity of the nucleus have been preferentially destroyed. (abstract\ncontinues)\n