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

TL;DR

This study reports the detection of an unusual 9.7μm silicate emission feature in the nucleus of M81, modeled with porous silicate dust, revealing insights into dust composition and PAH presence in low-luminosity AGN environments.

Contribution

First detection and modeling of anomalous silicate emission in M81's nucleus using laboratory mineral spectra, highlighting dust properties in a LINER galaxy.

Findings

Silicate emission peak at ~10.5μm differs from typical absorption features.

Best fit with micron-sized, porous amorphous silicate and graphite grains.

Presence of large PAHs and evidence of small PAH destruction near the nucleus.

Abstract

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)